slabworks manual 1.07beta

SlabWorks™ 1.07beta

User Manual

Design of Stiffened Slab-on-Grade

Foundations on Expansive Clay Soil

(Issued August 17, 2010. - Copyright Eric Green 2005-2007)

SlabWorks™ Manual v1.07beta Table of Contents

Page ii ©Eric Green 2005-2010

END-USER LICENSE AGREEMENT

Use of the SlabWorks™ software is governed by the End User License

Agreement (“EULA”) that is included with the program.

Do not use the program without reading the EULA!

The EULA is provided in various forms, including PDF, DOC, HTML and RTF

formats. If you cannot find the EULA, or cannot read any of the provided

formats, contact the SlabWorks™ at [email protected] and you will be

provided with a copy in an accessible format.

SlabWorks™ Manual v1.07beta Beta Software Warning

Page iii ©Eric Green 2005-2010

BETA SOFTWARE WARNING

This Software Product is an unfinished test version and not a commercial

product. It precedes the official General Availability (GA) version that will be

released to the general public. You acknowledge that the Software Product is

expected to have faults and errors, does not have guaranteed accuracy and

reliability, can be inherently unstable, is not yet fully tested or supported and

that its use is expected to result, from time to time, in "crashes" and loss of

business information. We recommend against use of this Software Product in a

commercial or production environment. Tester is advised not to rely exclusively

on the Software Product for any reason. Implementation of beta software should

be limited to experienced users only in a staging or test environment.

The Software Product is made available to participants in the Beta Test

Program to allow them to exercise the product release and help to locate issues

that can be addressed before the official release. Beta Testers receive pre-release

software and any related documentation and are in a position to influence

changes with their comments and suggestions. Eric Green grants you, an

individual user, the right to use this beta test version of the Software Product

solely for the purpose of testing and evaluating the software and providing

feedback on its design, features and faults to Eric Green.


Page iv ©Eric Green 2005-2010

Table of Contents

1.0 Introduction ................................................................................................... 1

1.1 Design Methods .......................................................................................... 1

1.2 Quick Start ................................................................................................. 3

1.3 Feedback ..................................................................................................... 3

2.0 Using SlabWorks™ ........................................................................................ 4

2.1 Installing and Configuring SlabWorks™ .................................................. 4

2.2 Quick Start ................................................................................................. 6

2.3 Explanation of Menu Items ..................................................................... 10

3.0 Registering SlabWorks™ ............................................................................ 12

3.1 Why Register? .......................................................................................... 12

3.2 How Do I Register?................................................................................... 12

3.3 Entering Registration Code ..................................................................... 13

3.4 Is SlabWorks™ Free or Public Domain? ................................................. 15

3.5 Program Support ...................................................................................... 15

4.0 General Design Issues ................................................................................. 16

4.1 Reinforcing ............................................................................................... 16

4.2 Design Methods ........................................................................................ 16

4.3 The Geotechnical Report .......................................................................... 16


Page v ©Eric Green 2005-2010

4.4 General Design Guidance ........................................................................ 17

4.4.1 General Fill ....................................................................................... 17

4.4.2 Select Structural Fill ........................................................................ 18

4.4.3 Impermeable Perimeter Cap ............................................................ 18

4.4.4 Beam Bearing .................................................................................... 19

4.4.5 Corners .............................................................................................. 19

4.4.6 Ground Penetration .......................................................................... 20

4.4.7 Drainage ............................................................................................ 20

4.4.8 Tree Removal .................................................................................... 20

4.4.9 Landscaping Beds ............................................................................. 20

4.4.10 Trees .................................................................................................. 21

4.4.11 Plumbing Trenches ........................................................................... 22

4.4.12 Slab Reinforcing ................................................................................ 22

5.0 BRAB Design Method .................................................................................. 24

5.1 Background .............................................................................................. 24

5.2 Design Methodology ................................................................................. 24

5.3 SlabWorks™ Implementation – General ................................................ 27

5.4 SlabWorks™ Implementation - Bonded Reinforcement ......................... 28

5.5 SlabWorks™ Implementation - Post Tensioned Reinforcement ............ 29


Page vi ©Eric Green 2005-2010

5.6 Design Limitations ................................................................................... 30

5.6.1 General .............................................................................................. 30

6.0 WRI Method ................................................................................................. 31

6.1 Background .............................................................................................. 31


6.3 Effective PI ............................................................................................... 33

6.4 SlabWorks™ Implementation - General ................................................. 35

6.5 SlabWorks™ Implementation – Bonded Reinforcement ........................ 36

6.6 SlabWorks Implementation - Post-Tensioned Reinforcement ............... 39

6.7 WRI Limitations ....................................................................................... 40

6.8 SlabWorks™ Limitations ......................................................................... 43

6.8.1 Shear.................................................................................................. 43

6.8.2 Deflection ........................................................................................... 43

7.0 PTI Analysis ................................................................................................. 44

7.1 Background .............................................................................................. 44


7.3 Changes from 2nd Edition to 3rd Edition ............................................... 45

7.4 Changes to 3rd Edition in Addendum No. 1 ........................................... 45

7.5 SlabWorks™ Implementation – General ................................................ 47


Page vii ©Eric Green 2005-2010

7.6 SlabWorks™ Implementation – Post-Tensioned Reinforcing ................ 47

7.7 SlabWorks™ Implementation – Bonded PTI .......................................... 48

7.8 Design Limitations ................................................................................... 53

8.0 Input Mode ................................................................................................... 55

8.1 No Input Checking ................................................................................... 55

8.2 Input Tabs ................................................................................................ 55

Analysis Type Tab .............................................................................................. 57

8.3 Project Data Tab ...................................................................................... 59

8.4 Material Tab ............................................................................................. 60

8.5 Soils Tab ................................................................................................... 65

8.6 Post-Tensioning Tab ................................................................................ 69

8.7 Loads and Deflection Tab ........................................................................ 72

8.8 Bonded Reinforcement Tab...................................................................... 75

8.9 Bonded Reinforcement Options Tab ........................................................ 77

9.0 Output Mode ................................................................................................ 82

9.1 Print Setup ............................................................................................... 82

10.0 Moment of Inertia ........................................................................................ 84

10.1 Uncracked Moment of Inertia.................................................................. 84

10.2 Cracked Moment of Inertia ...................................................................... 84


Page viii ©Eric Green 2005-2010

10.3 Effective Moment of Inertia ..................................................................... 85

11.0 Verification and Validation ......................................................................... 86

12.0 Building Code Requirements ...................................................................... 87

12.1 International Building Code (2006)......................................................... 87

12.1.1 Foundation Design Requirements .................................................... 87

12.1.2 Drainage Requirements .................................................................... 88

12.2 International Residential Code (2003) .................................................... 88

12.3 ACI 318-02 – Building Code Requirements for Reinforced Concrete .... 89

13.0 Texas Section ASCE .................................................................................... 90

13.1 Background .............................................................................................. 90

13.2 Design Process .......................................................................................... 90

13.3 Slab-On-Grade Design ............................................................................. 91

13.4 Slabs-on-Grade with Piers ....................................................................... 92

SlabWorks™ Manual v1.07beta List of Figures

Page ix ©Eric Green 2005-2010

List of Figures

Figure 2-1 – Archive Extraction Menu .................................................................... 4

Figure 2-2: Save File Location. ................................................................................ 5

Figure 2-3: Main Menu Bar. .................................................................................... 6

Figure 2-4: Main Input Screen................................................................................. 6

Figure 2-5: Save-As Screen. ..................................................................................... 8

Figure 2-6: Typical Output Screen. ......................................................................... 9

Figure 3-1: Start-up splash screen. ....................................................................... 14

Figure 3-2: Registration screen. ............................................................................. 14

Figure 4-1: Possible Bathtub Effect of Fill. ........................................................... 18

Figure 4-2: Solution to Bathtub Effect of Fill........................................................ 19

Figure 4-3: Effect of Improper Landscaping. ........................................................ 21

Figure 5-1: Design Rectangles. .............................................................................. 25

Figure 5-2: Limiting Cases of Support for BRAB Slab Analysis. ......................... 26

Figure 5-3: BRAB Limiting Cases ......................................................................... 26

Figure 5-4: BRAB Design Equations. .................................................................... 27

Figure 5-5: BRAB Effective Load in Long Direction. ............................................ 27

Figure 5-6: BRAB Design Equations for PT Slab. ................................................ 30

Figure 6-1: WRI Basic Cantilever Length. ............................................................ 32

SlabWorks™ Manual v1.07beta List of Figures

Page x ©Eric Green 2005-2010

Figure 6-2: WRI Cantilever Adjustment Factor. .................................................. 33

Figure 6-3: WRI Design Equations. ....................................................................... 33

Figure 6-4: Over-Consolidation Coefficient ........................................................... 34

Figure 6-5: Slope of Natural Ground vs. Slope Correction Coefficient ................ 34

Figure 6-6: Effective Moment of Inertia ................................................................ 36

Figure 6-7: WRI Deflection Equations for PT Reinforcing. .................................. 39

Figure 6-8: WRI Design Equations for PT Reinforcing......................................... 40

Figure 7-1: WRI Design Equations for PT Reinforcing......................................... 51

Figure 7-2: Effective Moment of Inertia ................................................................ 51

Figure 8-1: Analysis Type Input Tab. .................................................................... 57

Figure 8-2: Project Data Input Tab. ...................................................................... 59

Figure 8-3: Material Data Input Tab..................................................................... 60

Figure 8-4: Geometry Data Input Tab. .................................................................. 62

Figure 8-5: Soil Data Input Tab. ............................................................................ 65

Figure 8-6: Post-Tensioning Data Input Tab. ....................................................... 69

Figure 8-7: Loads and Deflection Data Input Tab. ............................................... 72

Figure 8-8: Bonded Reinforcement Data Input Tab. ............................................ 75

Figure 8-9: Bonded Reinforcement Options Input Tab......................................... 77

Figure 9-1: Font Selection. ..................................................................................... 82

SlabWorks™ Manual v1.07beta List of Appendices

Page xi ©Eric Green 2005-2010

List of Appendices

Appendix 1 – BRAB Climatic Rating, Cw.

Appendix 2 – BRAB Support Index, C.

SlabWorks™ Manual v1.07beta 1.0 Introduction

Page 1 of 95 ©Eric Green 2005-2010

1.0 Introduction

Welcome to SlabWorks™, a software program for the analysis and design of

slab-on-grade foundations built on expansive and compressible soils.

SlabWorks™ implements all of the generally accepted foundation design

methods for slabs on expansive clay, including the PTI, WRI and BRAB design

methods. In addition, SlabWorks™ includes enhancements to the original design

methods, such as the design of foundations with bonded reinforcement using the

PTI design equations, and design of Post-Tensioned foundations using BRAB

and WRI design criteria.

SlabWorks™ is intended to assist the practicing profession structural engineer

design slabs-on-grade more efficiently, and to allow rapid examination of various

design options. This program is only authorized for use by and distributed to

licensed professional engineers. Any other use of this program is a violation of

the license agreement. To use all the features of the program, your version must

be registered, which is currently free of charge for qualified individuals.

While this program may be distributed free of charge by the author, it is not

public domain, and the copyright to the program remains the property of Eric

Green. You may not distribute this software or your registration number to other

users under any circumstances. Distribution of the software by anyone other

than Eric Green is a violation of Federal copyright law.

1.1 Design Methods

There are three generally recognized methods for the design of slabs-on-grade for

expansive soil. All three methods provide design equations to provide design

forces (shear and moment) and deflections. Some of the methods also provide

guidance on designing foundations to resist the calculated forces and limit

deflections. SlabWorks™ implements all three methods.



• PTI Analysis - This method is based on a set of equations derived by

regression analysis. The regression analysis was performed on the results

of a parametric finite element analysis of slabs on expansive soil that

considered different slab geometries, slab loads and soil parameters This

method considers both edge lift and center lift soil movement modes. The

manual provides explicit design guidance for post-tensioned foundations,

but acknowledges that the methods may also be used for the design of

foundations with bonded reinforcement.

• BRAB Analysis - This method is based on an effective cantilever length

that is based on climate and plasticity index. The cantilever length is

assumed to increase linearly with slab length. This method considers both

edge lift and center lift soil movement modes, but derives one common set

of equations to provide design forces and deflections for both modes. The

BRAB manual provide explicit design guidance only for foundation

reinforced with bonded reinforcement. Post-tensioned foundations are not

discussed.

• WRI Analysis - This method is a modification of the BRAB method. It

assumes that cantilever length is primarily a function of plasticity index

and climate, and only loosely related to slab length. This method considers

both edge lift and center lift soil movement modes, but derives one

common set of equations to provide design forces and deflections for both

modes. The WRI manual indicates the method can be used to design

foundations reinforced with either post-tension and bonded reinforcement.

However, the manual does not give explicit design guidance for either

reinforcing system.



1.2 Quick Start

Running an analysis in SlabWorks™ consists of three steps:

• Input. Analysis data is input through the input menu. Input information

generally consists of foundation geometry, loads, reinforcing and soil

parameters.

• Analysis. The analysis is run through the analysis menu.

• Output. Output is automatically displayed after the analysis is run.

Output generally consists of allowable vs. actual moments, shears and

deflections. If the results are satisfactory to the design engineer, the

results can be printed for a permanent record. If the results are not

satisfactory, the engineer can return to the input menu to modify the

design.

1.3 Feedback

At SlabWorks™, we appreciate feedback. Complaints, complements and

suggestions will be accepted equally. We can be reached at

[email protected]. Include the word “SlabWorks” in your subject line

or your email will be ignored (we receive hundreds of spam e-mails a week). We

regret that we cannot offer direct user support at this time. We are considering

opening a web-based discussion board to provide user support. Let us know if

you would be interested in this type of forum

SlabWorks™ Manual v1.07beta 2.0 Using SlabWorks™


2.0 Using SlabWorks™

Running a SlabWorks™ analysis is as simple as inputting the appropriate

design data at the Input menu, then running the analysis at the Analyze

menu. Output is displayed automatically after the analysis is complete. Existing

files can be opened through the File menu.

2.1 Installing and Configuring SlabWorks™

SlabWorks™ is supplied as a compressed self-extracting archive file named

SW_Install.exe. Run this program to decompress the install files. By default, this

archive will decompress the install files at its current location (For example, if

the archive is located on the desktop, the install files will be decompressed onto

the desktop). When the self-extracting file is run, an option window will open

that allows you to select an alternate location for the install files. This is done by

clicking on the Browse button and selecting the alternate location.

Figure 2-1 – Archive Extraction Menu

Click on the Unzip button to decompress the install files. The compressed

archive file contains two files, setup.exe and SlabWorkscab. Run the program

Setup.exe and follow the on-screen directions for installation. Once the

installation is complete, the temporary install files Setup.exe and

SlabWorks.cab may be deleted.



Run the SlabWorks™ program from the start menu at Start-Programs-

SlabWorks or directly by running the SlabWorks.exe in the install directory.

This directory is “/Program Files/SlabWorks” unless changed by the user during

installation.

If you have obtained a registration code, it should be entered when the program

is first started. Directions for obtaining a registration Code can be obtained at

Chapter 3.2 How Do I Register? on Page 12, while directions for entering the

code can be obtained at Chapter3.3 Entering Registration Code on Page 13.

The first time you run the program, you must configure two program options for

proper operation. First, set the default location where you would like data files

to be stored. This can be over-ridden when saving files if desired. Setting this

option is done at the Help-Options-Save File menu. Select the save file

location from the menu and click OK .

Figure 2-2: Save File Location.

Second, the printer font must be selected. Setting the printer font and printing is

discussed in Chapter 9.1 on page 82.



2.2 Quick Start

Click the Input menu in order to enter the input mode:

Figure 2-3: Main Menu Bar.

The tabbed input screen will appear. The Analysis Type tab will always be the

first tab shown:

Figure 2-4: Main Input Screen



Using the options buttons on the Analysis Tab, select the analysis type,

foundation type, slab type and soil type. The options selected on this tab will

control the inputs that are available on the remaining input tabs. For example, if

the user selects Bonded Reinforcing, the inputs for a post-tensioned

foundation design will not be available.

After selecting the appropriate options from the Analysis Tab, click through the

remaining tabs and input the appropriate design information. Tabs are selected

by clicking the mouse on the tab. An in-depth discussion of the various input

fields found on each tab is found in Chapter 8.0 at page 55.

After all design data in input, click the OK button at the bottom of the input to

exit the input mode. If you click the Cancel button, all of the input data will be

lost.

After exiting the input mode, click on the File menu and then the Save

submenu in order to save the input data. To save data that has been modified

from a previous run under a new name, click the Save As submenu.



Figure 2-5: Save-As Screen.

Click in the File Name text box and type in the desired file name. Click Save .

After saving your file, the analysis can be run by clicking the Analysis menu

When you run an analysis, SlabWorks™ checks to ensure that all required

variables were input during the input phase. If any required variables were not

input, you will be directed to the empty input field when attempting to analyze

the design. SlabWorks™ does not check the input values to ensure they are

within a reasonable range. Remember: Garbage In – Garbage Out.

If all required variables are input, the program will run the analysis and the

results are automatically displayed in the Analysis Results window. The

Analysis Results window is customized depending on the type of analysis being



run (PTI, BRAB or WRI and Post-Tensioned or Bonded Reinforcing) and

contains three to five tabs containing the results.

The first two to four tabs contain the results of most design checks performed in

the SlabWorks™ analysis. The design checks are generally displayed as the

allowed (or required) value, actual value and percent allowable (or percent

required). If the percent allowable (or percent required) in unacceptable, the

results are displayed in red as shown below.

Figure 2-6: Typical Output Screen.

The last tab on the Output window is labeled Warnings. This tab lists all failed

design checks, plus warnings. Warnings are design checks that do not explicitly

violate the design code, but should be reviewed for approval by the design

engineer. An example of this would be a beam that is under-reinforced according

to ACI-318 code requirements. Design codes for slab-on-grade foundations do not

explicitly require minimum steel. Thus, the design engineer must decide if

minimum steel requirements are required for the design.



2.3 Explanation of Menu Items

This section provides a brief overview of the function of all menu items.

File Menu

The file menu is used to load and save project files, and to exit the program.

File – New

This menu clears all input fields.

File – Open

This menu is used to select a previously saved file

File – Save

This menu saves the current project under the current file name. If a file

name has not already been selected (the project is new), the user is required

to provide a file name prior to saving.

File - Save As

This menu saves the current project file under a new name. This is most

commonly used to save a copy of an existing project prior to making

modifications, in order that the original file if left unchanged.

File – Exit

Exit SlabWorks™



Analysis Menu

The menu starts the SlabWorks™ analysis of foundation. The output screen

is automatically displayed upon successful analysis. Printed output is

accessed from the output screen.

Help

The help menu is used to view the current registration and program version

number, and to set various program options.

Help – Options

The user can select the default file-save location and the default printer from

this menu.

Help - About

This menu displays the version number of SlabWorks™ and the current

registration (User Name, Company, PE number and PE registration state

and SlabWorks™ key).

SlabWorks™ Manual v1.07beta 3.0 Registering SlabWorks™


3.0 Registering SlabWorks™

3.1 Why Register?

Without a valid registration key, SlabWorks™ is fully functional except for the

length and width of the slab, which are fixed. Therefore, the program cannot be

used for design or analysis unless a registered version is obtained.

Registration is required in order that updated version of the software can

distributed to end users in the event that critical bugs or errors in the software

are found. Without registration, there is no way to insure that all users of the

program are notified.

3.2 How Do I Register?

There is currently no charge for registration. This may change in the future, and

updated versions may require the purchase of a license. The registered version of

SlabWorks™ is only available to licensed professional engineers (civil, structural

or geotechnical). SlabWorks™ is currently registered only to engineers, not

companies. Two methods of registration are available, e-mail and regular mail:

By e-mail: To register, you must send an e-mail to

[email protected]. The subject must line of the e-mail

must read ”SLABWORKS™ REGISTRATION”. In the body of the

e-mail includes only your name (First and Last), state of

registration, company name and PE number. We will only respond

to e-mails from the domain associated with the company that is

listed as your employer with the appropriate state board of

registration. For example, if the state board lists your employer as

XYZengineering, will we respond to e-mail from

Bob.Jones@XYZengineering, but will not respond to

[email protected]. If you do not have an email address



associated with your company’s domain name, or if the employer

listed with the state board is incorrect, you will not be able to

register the program by e-mail. Be prepared to wait as long as 3

weeks to obtain a registration code.

By regular mail: We will mail a registration code for the program

to the address listed with the state board of registration for the

associated PE number if a self-addressed, stamped envelope and a

letter containing your name (First and Last), company name, state

of registration, PE number and e-mail address is sent to:

Eric Green

6306 Oakburl Lane

Sugar Land, Texas 77479

The return address must be the same as the company address listed

with the state board of registration. Be prepared to wait as long as

4 weeks to obtain a registration code. If the address listed with the

state board is incorrect, you will not be able to register the program

using this method.

3.3 Entering Registration Code

Once you have obtained a registration code, register the program at the startup

splash screen by clicking on the Register button.



Figure 3-1: Start-up splash screen.

Enter the information in the registrations screen exactly as supplied and click on

the OK button. This information is case sensitive. The program will tell you if

the registration attempt was successful or unsuccessful. If the attempt was

unsuccessful, return to the registration screen by clicking the Register button

a second time and make sure the information entered is correct.

Figure 3-2: Registration screen.



3.4 Is SlabWorks™ Free or Public Domain?

No. While SlabWorks™ may be distributed to certain individuals without

charge, it is a copyrighted program, and all rights are reserved by the author,

Eric Green. You may not distribute your registration code or the program to

anyone else. To do so is a violation of Federal Copyright Law.

3.5 Program Support

Currently, the sole source of support for SlabWorks™ is this user manual and

the FAQ page on the SlabWorks™ website (www.slabongrade.net). No direct

support is available. A user forum (discussion board) is currently being

considered. Let us know if you would be interested in participating (or

moderating) as discussion board.

If you have questions or comments about the operation of SlabWorks™, drop us

an e-mail at [email protected]. We will read your e-mail, and if

appropriate, will address your comments/questions in the next version of the

manual and/or in the website FAQ.

SlabWorks™ Manual v1.07beta 4.0 General Design Issues


4.0 General Design Issues

4.1 Reinforcing

Three reinforcing options exist for the design of slabs-on-grade. These are

bonded reinforcing (conventional reinforcing), post-tensioning and hybrid

systems combining post-tensioning and bonded reinforcing. Any of these systems

can produce acceptable performance if properly design and constructed.

4.2 Design Methods

There are three generally accepted design methods available for the design of

slab-on-grade foundations on expansive clay. These are:

• BRAB

• WRI/CRSI

• PTI

With modification, any of the three methods can be used to design either

stiffened or uniform thickness slabs using post-tension reinforcement or

conventional bonded reinforcement.

4.3 The Geotechnical Report

The design engineer should obtain a geotechnical report prior to designing a

foundation on expansive clay. The geotechnical report should contain:

1. Recommendations for foundation system type. The geotechnical engineer

should indicate if the site is appropriate for stiffened slab-on-grade

construction. The report should differentiate between compliant slabs-on-

grade (where the superstructure loads are carried by a deep foundations such

as piers and the interior thin slab simply forms a barrier between the interior



and subgrade) and stiffened slabs-on-grade where is slab is free floating and

superstructure distress is limited by the stiffness of the foundation.

2. Fill requirements. The geotechnical engineer should clearly indicate if fill is

required for slab-on-grade construction. Fill properties and required

compaction should be specified.

3. Expected movement. The geotechnical engineer should indicate the potential

vertical movement at the site for both heave and settlement conditions.

4. Soil related foundation design parameters. The geotechnical report should

contain all soil design parameters required to design the foundation.

5. Effect of existing vegetation. The geotechnical engineer should discuss the

effect of removing existing vegetation.

4.4 General Design Guidance

When implementing one of the recognized foundation design methodologies (PTI,

BRAB or WRI), there are design considerations that are not discussed in these

design guidelines. Some of these design considerations include:

4.4.1 General Fill

In many developments, uncontrolled fill is placed over large areas, commonly

resulting from the excavation of roadbeds and general grading for site drainage.

This may occur several years before construction of buildings is started. The

foundation engineer must ensure that uncontrolled fill has not been placed on

the lot, or that the uncontrolled fill has been accounted for in the design. This

could be done be removing and replacing the uncontrolled fill, or perhaps

ensuring that the grade beams bear on undisturbed soil.



4.4.2 Select Structural Fill

Select structural fill can be used to reduce the amount of potential vertical

movement. If select fill is used, care must be taken to ensure the fill is properly

compacted and meets the requirements for select fill. This normally requires the

services of a geotechnical testing firm. Consider testing the physical properties of

the fill (such as PI and LL) after the fill is placed to ensure compliance with

specifications.

4.4.3 Impermeable Perimeter Cap

Movement of water into and out of the soil under the slab is the primary cause of

foundation movement. When the slab is built on select fill, the problem is

exacerbated because the select fill frequently extends outside of the building

perimeter and is relatively permeable compared to the native soils. This can

result in the select fill acting as a bathtub, exposing the underlying expansive

clay to large amounts of water (Figure 4-1).

Figure 4-1: Possible Bathtub Effect of Fill.



The solution to this problem is to install an impermeable cap of fat clay around

the perimeter of the building. Ensure that this cap provides proper drainage

away from the foundation (Figure 4-2).

Figure 4-2: Solution to Bathtub Effect of Fill.

4.4.4 Beam Bearing

Good practice indicates that the beams should bear on undisturbed native soils

or compacted structural fill. Foundations that are cast on improperly compacted

fill will experience differential settlement resulting from consolidation of the fill.

4.4.5 Corners

All three design methods treat the slab as a one-way system. However, at the

foundation corners, a biaxial state of bending occurs. In slabs with widely spaced

beams, the point of maximum moment at the corner may not cross a beam.

Additional beams or a diagonal beam running to the corner to the first beam

intersection should be considered in these areas.



4.4.6 Ground Penetration

The perimeter grade beam also serves as a vertical moisture barrier. The deeper

the grade beam penetrates into the soil, the more effective it will be in

stabilizing the soil moisture. At least a foot of penetration is generally

recommended.

4.4.7 Drainage

Proper drainage away from the foundation is important to maintain constant

soil moisture. The minimum slope that is generally accepted is 5% within 10 feet

of the building perimeter. This drainage must be established using impermeable

fill. The engineer should verify that local drainage is being taken care of by the

civil or landscape architects. If this is not the case, proper drainage must be

ensured on the structural drawings.

4.4.8 Tree Removal

Trees significantly alter the soil moisture balance of the soil, reducing the

equilibrium soil moisture in their vicinity. If a foundation is constructed over an

area where a tree was recently removed, the soil will gain moisture over time

and heave after the foundation is constructed. This effect is increased if the tree

is removed during a dry period and construction is started soon after the tree is

removed.

4.4.9 Landscaping Beds

Improperly constructed planting beds can result in saturated soil around the

perimeter of the building, even when the soil surface nominally has positive

drainage away fro the building (Figure 4-3). This problem is exacerbates if the

building is constructed on select fill extending under the planting beds. IN this

case, improperly constructed planting beds can act to inject water directly into

the select fill (Figure 4-1).



The engineer should discuss landscaping expectations with the architect/owner

to ensure that the effect of landscaping on the structure is fully understood.

Ideally, no planting beds will be located near the structure. However, this is

rarely possible. One solution is to line the bottom of the planting beds with a

moisture barrier or layer of impermeable fat clay. However, design of the

landscaping is outside of the scope of services of the structural engineer, and is

the responsibility of the landscape architect or owner.

Figure 4-3: Effect of Improper Landscaping.

4.4.10 Trees

Trees planted near a foundation can upset the soil moisture balance due to the

water demand of mature trees, especially during drought cycles. While it may

take a number of years before the tree gets large enough to cause structural

damage, this will eventually occur if the tree is close enough to the slab. In

general, the distance from the tree to the foundation must be at least half the

height of the tree, but the required distance varies with tree species.



The engineer should consider discussing landscaping expectations with the

owner and/or architect. If landscaping requirements dictates that trees must be

planted near the foundation, the engineer can recommend over-designing the

foundation to account for the effect of trees.

4.4.11 Plumbing Trenches

Plumbing trenches should be backfilled with compacted select fill in order to

prevent entry of moisture under the slab through void space in the trench

backfill material. Trenches should never be backfilled with sand of granular

materials. Consider requiring the use of a fat clay plug at the building

perimeter.

4.4.12 Slab Reinforcing

Overall structural performance of stiffened slabs is generally independent of the

performance of the thin slab in the areas between beams. This portion of the

foundation slab is generally intended only to acts as a separator between the

building and the soil below. However, if thermal or shrinkage cracking is noted

in these areas, many owners will perceive the foundation is in a failed state. This

is particularly important if the owner anticipates the use of tile or stone finishes.

Therefore, performance expectations with respect to slab cracking should be

discussed with the owner and architect prior to design.

In the past, many engineers have relied on the minimum temperature and

shrinkage steel requirements from ACI-318 (0.18% steel). These guidelines are

intended for elevated structural slab and are not applicable to slabs-on-grade.

This is discussed in the commentary to the latest version of ACI-318.

The engineer should instead refer to ACI 224 “Control of Cracking” for guidance

on controlling cracking of slabs-on-grade. Generally, cracking in stiffened slabs

is controlled with bonded reinforcement, and control joints are not used.



According to ACI 224, 0.50%-0.60% steel is required to control cracking with

steel alone. Control joints can be installed, with the control joints located mid-

way between the stiffening beams. Control joints near beams will not be effective

because the beams restrain the concrete from movement.

SlabWorks™ Manual v1.07beta 5.0 BRAB Design Method


5.0 BRAB Design Method

5.1 Background

The BRAB analysis is performed in accordance with the design methodology

presented in Criteria for Selection and Design of Residential Slabs-on-Ground

prepared for the Federal Housing Administration by the Building Research

Advisory Board in 1968 (referred to in this manual at the BRAB Report). The

Building Research Advisory Board was a Special Advisory Committee of the

Division of Engineering of the National Research Council. We recommend that

engineers wanting to design using this method obtain and read the full text of

the BRAB Report as it contains additional limitations, design requirements and

background information not discussed in this manual. The BRAB Report may be

obtained from the National Institute of Standards and Technology or the

National Academies Press at http://www.nap.edu.

5.2 Design Methodology

Similar to the other three design methods implemented in SlabWorks™, the

BRAB method is at it simplest level a method of predicting the maximum

moment, shear and deflection occurring in a slab-on-grade. Once these values

are established, the actual design of the beams is conventional and will not be

unfamiliar to anyone who has performed conventional reinforced concrete beam

design.

In the BRAB method, the foundation is designed in the following steps

• Determine dead and live loads

• Divide slab into overlapping rectangles (Figure 5-1: Design

Rectangles.Figure 5-1).

• Calculate imposed shear and moment in slab using BRAB equations.



• Select trial geometry (beam depth, width, spacing and reinforcement)

• Check allowable moment and shear against imposed moment and shear.

• Check actual deflection against allowable deflection.

Figure 5-1: Design Rectangles.

The BRAB report determines moments, shears and deflections in the slab by

assuming that the slab is only partially supported by the underlying soils. The

soil is assumed to be rigid in the areas of support (inelastic support conditions).

Two design conditions are considered, loss of support at the edges (center

lift/edge settlement) and loss of support at the interior (edge lift/center

settlement) (Figure 5-2).

The amount of slab support provided by the underlying soil is called the support

index, C, and is defined as the fraction of the foundation that is supported by the

soil. The support index is a function of the climatic rating (CW) and the effective

plasticity index of the soil (PIE). The higher the plasticity index, the higher the

potential for soil movement and the less support considered in the analysis. The

chart for determining the support index as a function of CW and PIE is shown in

Appendix 2.



Figure 5-2: Limiting Cases of Support for BRAB Slab Analysis.

Once the support index is found, moment, shear and deflection are found as

shown in Figure 5-3.

Figure 5-3: BRAB Limiting Cases

The BRAB report makes certain simplifying assumptions in order to reduce the

two load cases to one set of design equations for shear, moment and deflection.

Additionally, certain assumptions are made that reduce the uniform load in the

long direction relative to the short direction. Once these simplifications and



modifications are made, the actual design equations for shear, moment and

deflection are shown in Figure 5-4.

Figure 5-4: BRAB Design Equations.

The w in these equations in the effective uniform load (wE). In the short

directions, w is equal to wE. In the long direction, the effective load is calculated

using the equation in Figure 5-5.

5.04.04.1 ≥

−⋅=

S

L

EL

Lww

Figure 5-5: BRAB Effective Load in Long Direction.

5.3 SlabWorks™ Implementation – General

Several factors regarding the BRAB design method and the SlabWorks™

implementation must be considered by the design engineer when performing a

BRAB design analysis. Important considerations include:

1. SlabWorks™ implements only the Type III BRAB foundation, which is

defined as reinforced and stiffened.

2. The BRAB manual contains recommendation for the allowable deflection

based on the superstructure type. Recommended deflection limits are:



Superstructure Type Maximum Permissible

Deflection Ratio (∆/L)

Wood 1/200

Unplastered masonry or gypsum wallboard 1/300

Stucco or plaster 1/360

3. Tilting of the superstructure is not a design criterion in the BRAB method.

4. Deflection is calculated based on the creep modulus of concrete, which is

assumed to be 1.5x106 psi.

5. BRAB modifications suggested by the Texas Section of the ASCE must be

manually implemented by the user, as they are not automatically

implemented by SlabWorks™. Suggested modifications by TxASCE include:

5.1. Regardless of actual beam length, the analysis length shall be limited to

50 feet.

5.2. Use the maximum long-term creep factor as provided in ACI 318.

5.4 SlabWorks™ Implementation - Bonded Reinforcement

SlabWorks™ does not make any modifications of the BRAB method when

analyzing slabs with bonded reinforcing. However, the user should be aware of

the following assumptions made in the BRAB design method related to the

design of foundations with bonded reinforcement:

1. Concrete design in the BRAB method is done using working stress design.

1.1. An assumed internal moment arm length to beam depth of 0.865 is

used.

1.2. Allowable shear strength of concrete is assumed to be 75 psi.



2. Deflections are calculated using the fully cracked moment of inertia.

3. Compression steel is ignored in calculating the cracked moment of inertia

and moment capacity.

4. Effective PI must be calculated using method presented on page 65 of the

BRAB manual.

5.5 SlabWorks™ Implementation - Post Tensioned Reinforcement

The BRAB report considers only the design of slabs with bonded reinforcement.

However, once design moments and shears are calculated using the BRAB

design equations, the slab can be designed using post-tensioned reinforcing.

SlabWorks™ makes the following modifications to the BRAB design method

when performing analysis of post-tensioned foundations:

1. Design forces Mdesign, Vdesign and ∆design are determined using the equations

presented in the BRAB report (Figure 5-4). Deflection is calculated using the

uncracked moment of inertia.

2. Mallow and Vallow for the center lift and edge lift modes are determined using

the formula in the PTI design manual (Figure 5-6).

3. Post-tensioning forces act to decreases of increases deflection depending on

the eccentricity of the resultant post-tensioning force (above or below the

neutral axis). However, SlabWorks™ ignores the effect of post-tensioning

force on deflection in the BRAB analysis. This is done because the BRAB

equations calculate an equivalent deflection based on the full slab length.

Therefore, including the effects of post-tensioning would result in an

overestimation of the effect of post-tensioning force on deflection, as the

actual cantilever is significantly shorter than the full slab length.



Allowable defection

( )

uncrackedIE

LLC

L ⋅⋅

⋅⋅−⋅≤

∆

48

1'3ω

PT Allowable Stress:

A

Pff

fdUserDefinef

ff

rCv

Ct

CC

⋅+⋅≤

⋅≤

⋅≤

2.07.1

45.0

'

'

'

PT Design Stress (edge lift)

beams

v

r

t

rc

r

t

rt

A

Vf

e

P

S

M

A

Pf

e

P

S

M

A

Pf

max

max

max

=

++=

−−=

PT Design Stress (center lift)

beams

v

r

t

rc

r

t

rt

A

Vf

e

P

S

M

A

Pf

e

P

S

M

A

Pf

max

max

max

=

−+=

+−=

Figure 5-6: BRAB Design Equations for PT Slab.

5.6 Design Limitations

5.6.1 General

There are no known limitations for the use of the BRAB design method, or the

SlabWorks™ implementation of the BRAB design method.

SlabWorks™ Manual v1.07beta 6.0 WRI Method


6.0 WRI Method

6.1 Background

WRI analysis and design is performed in accordance with the design

methodology presented in Design of Slab-On-Ground Foundations - A Design,

Construction & Inspection Aid for Consulting Engineers, prepared by Donald

Snowden for the Wire Reinforcement Institute and the Concrete Reinforcing

Steel Institute. This guide was first published in 1981. An update was published

in 1996. The update did not change the design method in any material fashion.

The document is available for free at http://www.wirereinforcementinstitute.org.

The WRI method is a variation of the BRAB design method, and uses the same

support index and assumed support conditions presented in the BRAB report.

The major modification is the use of a cantilever length adjustment factor and

the use of the cantilever lengths to directly calculate shear, moment and

deflection.


Similar to the other three design methods implemented in SlabWorks™, the

WRI manual provides a method of predicting the maximum moment, shear and

deflection occurring in a slab-on-grade. However, diverging from the other design

manuals, the WRI manual does not provide any design guidance beyond

development of design loads and deflections. For example, the manual does not

define a maximum allowable deflection (although L/480 is suggested), does not

define maximum beam spacing and does not define minimum steel ratios.

Similar to the BRAB report, the WRI method determines moments, shears and

deflections in the slab by assuming that the slab is partially supported by the

underlying soils. The soil is assumed to be rigid in the areas of support (inelastic

support conditions), and the interface is assumed to transfer both tensile and



compressive stresses. Two design conditions are considered (Figure 5-2), loss of

support at the edges (center lift/edge settlement) and loss of support at the

interior (edge lift/center settlement).

The design cantilever used by WRI is based on the same support index used by

BRAB (Appendix 2). The support index is a function of the climatic rating and

the effective plasticity index of the soil. However, in the WRI method the support

index directly establish the cantilever length independent of slab length.

Therefore, the WRI method predicts larger cantilevers in short slabs and shorter

cantilevers in long slabs relative to the BRAB method.

Figure 6-1: WRI Basic Cantilever Length.

The cantilever length is then reduced by a factor K, which is based solely on slab

length. This allows for a shorter cantilever length as the slab length decreases.



Figure 6-2: WRI Cantilever Adjustment Factor.

Once the support index is found, the design moment, shear and deflection are

found as:

Figure 6-3: WRI Design Equations.

6.3 Effective PI

In the WRI design method, the effective PI is first calculated as a weighted,

effective PI using the BRAB methodology (Page 65 of the BRAB manual). The



Effective PI is then modified by two correction factors, an over-consolidation

coefficient (Figure 6-4) and a slope correction factor (Figure 6-5).

The derivation of these correction factors is not discussed in the WRI manual. If

the user wishes for these adjustment factors to be ignored in the analysis, an

unconfined compressive strength of 6000 psi and a slope of 0% should be

entered. This provides coefficients of 1.0 for both correction factors.

Figure 6-4: Over-Consolidation Coefficient

Figure 6-5: Slope of Natural Ground vs. Slope Correction Coefficient



6.4 SlabWorks™ Implementation - General

Several factors regarding the WRI design method and the SlabWorks™

implementation must be considered by the design engineer when performing a

BRAB design analysis. Important design considerations that are applicable to

both bonded and post-tensioned reinforcement include:

1. Texas ASCE Modifications – If selected by the user, SlabWorks™ implements

the modifications suggested by the Texas Section of the ASCE. These

modifications include:

1.1. Regardless of actual beam length, the analysis length is limited to 50

feet. This has the effect of capping the maximum cantilever length.

1.2. The minimum design length LC is increased by a factor of 1.5 with a

minimum design length of 6 feet

2. WRI/BRAB Cantilever – The program can make calculations using cantilever

lengths calculated using the BRAB method or the WRI method. Generally

speaking, the BRAB method will provide shorter cantilevers in small

foundations and longer cantilevers in large foundations relative to the WRI

method.



3. Soil Bearing Strength – SlabWorks™ checks soil bearing based on the

projected area of the grade beams.

4. The WRI manual recommends an allowable deflection of 1/480 regardless of

superstructure type.

6.5 SlabWorks™ Implementation – Bonded Reinforcement

For bonded reinforcing, design of the foundation generally proceeds using

conventional reinforced concrete ultimate strength design methods in accordance

with ACI-318. However, the SlabWorks™ implementation provides several

options not included in the WRI Manual. Factors regarding the WRI design

method and the SlabWorks™ implementation that must be considered by the

design engineer include:

1. Effective/Cracked Moment – The program automatically calculates deflection

in both the cracked and uncracked conditions. When calculating the cracked

deflection, SlabWorks™ allows the use of the fully cracked or effective

moment of inertia (per ACI-318). The effective moment is equal to:

cr

a

crg

a

cre I

M

MI

M

MI ⋅

++⋅

=

33

1

Figure 6-6: Effective Moment of Inertia

2. Compression Steel – The WRI manual is silent in regards to consideration of

compression steel in calculating bending strength. If selected by the user,



SlabWorks™ will include compression steel in bending strength calculations.

Including compression steel can make a significant difference in the cracked

moment of inertia.

3. Compression Steel and Cracked Moment of Inertia - The WRI manual is

silent in regards to consideration of reinforcing steel in calculating the

moment of inertia. By default, SlabWorks™ considers the compression steel

in calculating the moment of inertia. Tension steel is always included in

calculating the cracked moment of inertia, as the cracked moment would

otherwise be zero.

4. Reinforcing Steel and Uncracked Moment of Inertia - By default,

SlabWorks™ considers both the tension and compression steel in calculating

the uncracked moment of inertia.

5. Slab Reinforcing - If selected by the user, SlabWorks™ will include slab

reinforcing when calculating the uncracked moment of inertia, cracked

moment of inertia and bending strength. Slab steel will be transformed in

cracked and uncracked moment of inertia calculations only if the appropriate

check-boxes are unselected (see 3 and 4 above). Slab steel will be included in

the bending strength calculations as compressions steel only if the

appropriate check boxes are unselected (see 2 above).

Only the portions of the slab steel in the effective flange width is included in

the appropriate calculation. If the user chooses to ignore beam flanges (see 6



below), including slab steel in the calculations will not have a significant

effect in the bending strength of moment of inertia unless the slab is heavily

reinforced.

6. Include Flanges in Bending Strength–SlabWorks™ will calculate bending

strength and deflection of the slab by considering the contribution of the

beams alone (ignoring the slab per BRAB), or will include the contribution of

the effective flange per ACI-318 recommendations. Slab steel in the flanges is

considered as compression and tension reinforcing steel if the appropriate

check boxes are selected.

7. Factored/Service Loads – SlabWorks™ can calculate deflection and strength

using factored or service loads. Strength calculations are normally done using

factored loads and deflections are normally calculated using service loads.

The PTI load factor is only used for PTI design using bonded reinforcement,

and is not applicable to WRI analysis.



6.6 SlabWorks Implementation - Post-Tensioned Reinforcement

In its design examples, the WRI manual considers only bonded reinforcement.

However, the method is also applicable to post-tensioned foundation, and the

manual includes a formula to estimate the required beam depth for a post-

tensioned slab-on-grade. The formulas for design moment, design shear and

deflections are independent of foundation reinforcement type.

Factors regarding the WRI design method and the SlabWorks™ implementation

which must be considered by the design engineer when analyzing a post-

tensioned foundation include:

1. Mdesign and Vdesign are calculated using the equations provided in the WRI

report (Figure 6-3 on page 33).

2. Once design moments and shears are calculated using the WRI design

equations, analysis of the post-tensioned slab generally follows the methods

presented in the PTI manual for design of the post-tensioned stiffened slab

with the listed exceptions:

2.1. The effect of compression and tension steel, if present, is not

considered in calculating the moment of inertial.

2.2. The calculation of deflection uses the WRI formula but includes the

effect of post-tensioning forces per the PTI method.

IE

eP

IE

LL

C

e

C

C

CL⋅⋅

⋅⋅−

⋅⋅

⋅⋅=∆

24

2'4 βω

IE

eP

IE

LL

C

e

C

C

EL⋅⋅

⋅⋅+

⋅⋅

⋅⋅=∆

24

2'4 βω

Figure 6-7: WRI Deflection Equations for PT Reinforcing.



2.3. Mallow and Vallow for the center lift and edge lift modes are determined

using the design formulas in the PTI design manual.

PT Allowable Stress:

A

Pff

fdUserDefinef

ff

rCv

Ct

CC

⋅+⋅≤

⋅≤

⋅≤

2.07.1

45.0

'

'

'

PT Design Stress (edge lift)

beams

v

r

t

rc

r

t

rt

A

Vf

e

P

S

M

A

Pf

e

P

S

M

A

Pf

max

max

max

=

++=

−−=

PT Design Stress (center lift)

beams

v

r

t

rc

r

t

rt

A

Vf

e

P

S

M

A

Pf

e

P

S

M

A

Pf

max

max

max

=

−+=

+−=

Figure 6-8: WRI Design Equations for PT Reinforcing.

6.7 WRI Limitations

Several limitations and cautions must be considered by engineers using the WRI

design methods. These include:

1. The WRI method is a modification of the BRAB method. It is important to

note that the WRI method is empirically derived from the experience of Mr.

Snowden. As noted in the forward to the 1981 report:

This procedure was developed by Walter L. Snowden, P.E.,

Consulting Engineer, of Austin, Texas, over a period of 15 years. It is

empirically derived by observing slab perforce and writing or



modifying equations to give results which approximate the

foundations which had been found to give satisfactory results.

2. The WRI manual includes the statement

Variations from the BRAB Report #33 were developed to maintain a

reasonable ratio between the cost of the slab-on-ground and the

value of the house it supported.

Given the increase in the average cost of homes over the past twenty-five

years, and the engineer may decide that the price-benefit ratio of 1981 is not

appropriate for 2005.

3. The derivation of the over-consolidation coefficient and a slope correction

factor is not discussed in the WRI manual.

4. The WRI manual does indicate if deflection should be calculated using the

cracked, uncracked or effective moment of inertia. It is implied that cracked

moment of inertia should be used for bonded reinforcement, and uncracked

for post-tensioned reinforcement. Use of the fully cracked moment of inertia

is most likely conservative for foundation designs where the cracking moment

is at near to or greater than the imposed bending moment.

5. For purposes of calculating deflections, the WRI manual does not indicate if

the moment of inertia should be calculated using only beam widths, or if

beam flanges should be included.

6. For purposes of calculating deflection, the WRI manual does not explicitly

state if the cracked moment of inertia be calculated using the normal or creep

modulus of concrete. The formulas derived in the WRI manual for

approximating the required beam depths for the uncracked condition assume

the creep modulus of elasticity. This is consistent with general practice for

design of structures under long-term load.



7. The formulas provided in the WRI manual for preliminary sizing of beam

depth based on deflection criteria must not be used for design. First, the

formula given in the WRI manual for approximating the required beam depth

for a post-tensioned foundation is wrong. The formula from the WRI manual

is:

3533

B

LMd C

⋅⋅=

The correct formula, assuming an uncracked section and ignoring the flanges,

is:

3

4

533

B

LMd C

⋅

⋅⋅=

Thus, the WRI formula overestimates the required beam depth by 59% and

thus overestimates the required I by about 400%. Since this sizing formula

ignores the beam flanges, it is even more conservative if final design

calculations include the beam flanges, which would be typical for post-

tensioned design.

The formula for bonded reinforcement assumes that the require beam depth

for the cracked condition is 59% of the required beam depth for the uncracked

condition. Thus, the cracked moment of inertial is assumed to be about 21%

of the uncracked moment of inertia. This may be a reasonable estimate for

the lightly reinforced sections used in most slab-on-grade foundations.

However, in some cases this formula may be overly conservative, especially if

beam flanges are used in the final calculation.

8. No guidance is given for maximum beam spacing.

9. No guidance is given for checking soil-bearing strength. Common practice

suggests using the projected areas of the beams to calculate bearing pressure.



The latest PTI Manual (3rd Edition) includes the effective flange width of the

beams to calculate bearing pressure.

6.8 SlabWorks™ Limitations

6.8.1 Shear

For WRI design, SlabWorks™ calculates the shear strength of concrete beams

using the full depth of the beam rather than the distance from the extreme

compression fiber to centroid of tension steel. This is slightly non-conservative.

However, shear rarely controls slab design.

SlabWorks™ allows use of the full shear capacity of the concrete without shear

reinforcing. This is specifically allowed by ACI-318 for slabs and footings.

6.8.2 Deflection

When calculating the uncracked deflection of a conventionally reinforced slab,

SlabWorks™ only considers the effective flange with of the T-Beam in

calculating I. For a post-tensioned slab, the full slab with is used.

SlabWorks™ Manual v1.07beta 7.0 PTI Analysis


7.0 PTI Analysis

7.1 Background

PTI analysis is performed using the methods presented in Design and

Construction of Post-Tensioned Slabs-on-Ground (3rd Edition including

Addendum No. 1) published by the Post-Tensioning Institute and commonly

referred to as the PTI Manual. This method in turn in based on the research

published in Development of a Design Procedure for Residential and Light

Commercial Slabs-on-Ground Constructed Over Expansive Soils written by W.K.

Wray in 1978. This design method is based on a regression analysis of multiple

foundation designs analyzed using a finite element analysis program.

We recommend that the user obtain and read the full text of the PTI manual as

it contains additional limitations and design requirements not discussed here.

The manual be obtained from the Post-Tensioning Institute at http://www.post-

tensioning.org. To truly understand the limitations of the design method it is

necessary to review Dr. Wray’s Dissertation. A copy of Dr. Wray’s dissertation

can be obtained from ProQuest Dissertations & Theses at http://www.umi.com.

The dissertation order number is 7909251.


Similar to the other three design methods implemented in SlabWorks™, the PTI

method is at it simplest level a method of predicting the maximum moment,

shear and deflection occurring in a slab-on-grade. Once these values are

established, the actual design of the beams is conventional and will not be

unfamiliar to anyone who has performed post-tensioned concrete design.

The most important design values used in the PTI method are ym and em, which

represent the expected differential movement and the expected distance over

which the movement will occur. Design is very sensitive to these inputs.



The PTI manual present technique for designing slabs that are reinforced

against temperature and shrinkage cracking (BRAB type II slabs) and slabs that

are structurally reinforced and stiffened (BRAB type III slab). SlabWorks™

currently implements only the Type III design but considers both expansive and

compressible soils, using post-tension or bonded reinforcing.

7.3 Changes from 2nd Edition to 3rd Edition

Several significant changes were made between from the 2nd to 3rd editions of the

PTI Manual. These include:

1. Bearing Pressure – The 3rd edition allows bearing pressure to be calculated

using a bearing width equal to the beam width plus 16 times the slab

thickness for interior beams and 6 times the slab thickness of perimeter

beams. This is the formula used by ACI-318 for flange contribution to

bending strength.

2. Deflections – The 3rd edition calculates a minimum moment of inertia

(stiffness) rather than calculating the actual deflection. In some cases there is

a large difference between the two criteria.

3. Post-Cracking Behavior – The 3rd edition requires a minimum post-

cracking strength of the section. Allowable post-cracking strength is can be

provided by PT reinforcement or bonded reinforcement, and design moments

are only 90% of the service moment (No load factors required). The examples

in the manual use the full slab width rather than effective flange with when

checking the positive moment capacity. SlabWorks™ used the same design

method.

7.4 Changes to 3rd Edition in Addendum No. 1

Several significant changes were made in Addendum No. 1 to the 3rd Edition.

These include:



1. Post Cracking Strength – The 3rd Edition required that the post-cracking

strength of the slab-section equal 90% of the uncracked strength. Addendum

No. 1 reduces this requirement to 50% of the uncracked strength.

2. Stiffness – Addendum No. 1 reduced the required uncracked slab stiffness

by 33% relative to the required stiffness required by the 3rd Edition.

3. Shear Strength - Under the 3rd Edition, the contribution of the concrete to

shear capacity was equal to 1.7 times SQRT(f’c). Addendum No. 1 increased

the contribution of the concrete to 2.4 time SQRT(f’c).

7.5 Changes to 3rd Edition in Addendum No. 2

Several minor changes were made in Addendum No. 2 to the 3rd Edition. As

listed in the addendum, these are:

1. Slab Shape Factor - Sections 3.8 and 4.5.1 have been rewritten to clarify

that a designer has options that may be considered when the slab shape

factor exceeds 24.

2. Rib Spacing - Section 4.5.2.1 has been revised to permit rib spacings less

than 6 feet and to clarify what spacing shall be used to compute moments and

shears.

3. Rib Width - Section 4.5.2.3 has been revised to limit rib widths to a range of

6 to 14 in.

4. Uniform Thickness Conversion - Section 6.12 has been revised to clarify

that when converting to an equivalent uniform thickness foundation, the

conformant ribbed foundation need not satisfy the cracked section provisions.

5. The Modulus of Elasticity of Soil, Esoil - in the equation for β has been

set as a constant = 1,000. Because this is the only location that the variable,



Esoil appears in this manual, the definition for Esoil has been dropped from

the List of Symbols and Notation in Appendix A.1.

7.6 SlabWorks™ Implementation – General

1. Perimeter Loads – The PTI manual suggest that when loads vary on the

perimeter (ratio of largest to smallest exceeds 1.25) that the maximum loads

should be used for center lift design and the smallest load for edge lift design.

For expansive soils, SlabWorks™ uses the maximum perimeter load to

calculate moment, shear and deflection in the center lift design mode. The

maximum perimeter loads is also used to calculate shear in the edge lift

design mode. The minimum perimeter load is used to calculate moment and

shear for the edge lift design mode. Maximum perimeter loads are used to

calculate shear for the edge lift design mode because this is more

conservative than using the minimum perimeter loads.

For compressible design, only the maximum perimeter load is used in the

calculations. The “Minimum Perimeter Load” field is ignored. This may or

may not be conservative depending on magnitude of the maximum and

minimum perimeter loads. Therefore, the user should run the SlabWorks™

analysis twice, once with the true maximum perimeter load and once with

the minimum perimeter loads input into the “Maximum Perimeter Load”

field.

The PTI manual suggest the foundation design for compressible soil also be

designed for some nominal differential movement under the center lift design

mode using the design equations for expansive clay.

7.7 SlabWorks™ Implementation – Post-Tensioned Reinforcing

General design factors that must be considered by the design engineer when

performing a SlabWorks™ analysis using the PTI design method include:



1. Slab Loads – The PTI regression analysis assumes the loads on the slab at 40

psf live load, 15 psf dead load for partitions and 50 psf dead load for slab

weight. SlabWorks™ allows the use of alternative dead and live loads for

purposes of calculating soil bearing stress through the “Superstructure

Weight” field. However, the PTI assumed loads of 105 psf total loads will be

used for shear, moment and deflection analysis of the slab regardless of the

superstructure weight input by the user. Note that if the uniform dead plus

live load is significantly greater than 90 psf this may invalidate the

assumptions in the use of the PTI procedure.

2. Texas ASCE - If selected by the user, SlabWorks™ implements the

modifications suggested by the Texas Section of the ASCE. These

modifications are applicable to post-tensioned foundations only and include:

2.1. Minimum residual prestress is 100 psi.

2.2. Eccentricity must be less than 5-inches.

2.3. The maximum tensile stress at service loads must be less than 4(f’c)1/2

unless additional bonded reinforcement equal to 0.33% of the gross

beam section is provided.

2.4. The PTI em and ym must take into account the presence of trees and

other environmental effects.

7.8 SlabWorks™ Implementation – Bonded PTI

As noted in the PTI manual, the PTI method was not developed specifically for

post-tensioned reinforcement, and can be used to analyze foundations



constructed with bonded reinforcement. However, the method does assume that

the sections will remain uncracked. Therefore, in order to analyze a slab with

bonded reinforcement, some modifications to the PTI method are required.

Generally, SlabWorks™ analyzes bonded slabs by calculating an equivalent

beam depth that provides an uncracked stiffness equal to the cracked stiffness of

the actual foundation geometry. This depth is then used in the PTI formulas to

calculate design shears, design moments and deflection/stiffness.

Factors that must be considered by the design engineer when analyzing a

foundation with bonded reinforcement using the PTI design method include:

1. Strength Analysis and Factored Loads – The PTI regression analysis was

developed using service loads. Therefore, SlabWorks™ uses the PTI

equations to calculate moment and shear using service loads, and then

multiplies the resultant by the specified load factor to calculate the design

moment and design shear. The load factor should be between 1.4 (all dead

load) and 1.6 (all live load), depending on the relative ratio of live load to

dead load (This load factor is an average load factor, which should give a total

factored load equal to the requirements of ACI-318/ASCE 7).

2. Deflection and Factored Loads – SlabWorks™ can calculate deflection using

factored or service loads. Deflections are normally calculated using service

loads. Using a load factor for the deflection calculation has the effect of

adding an additional factor of safety.



3. Effective Beam Depth – The regression in the PTI analysis use beam depth

as an input, and the model used to develop the equations assumes that the

section remains uncracked. With bonded reinforcement, the section may

crack, reducing the modulus of inertia and increasing deflections.

SlabWorks™ accounts for this by calculating an effective beam depth based

on the imposed moment and calculated moment of inertia. The effective

depth can be calculated in one of three ways:

3.1. Actual Depth – Actual beam depth is used in regression equation to

determine shear, moment and deflection. Use of this option implies

that the section remains uncracked. Deflection will be underestimated

if actual the bending moment exceeds the cracking moments.

3.2. Fully Cracked Moment of Inertia – Beam depth used in the regression

equation is the beam depth that results in a moment of inertia equal to

the fully cracked moment of inertia of the actual section. This method

ensures that the stiffness considered in the PTI regression equations is

the same as the actual cracked stiffness. Because M = F(dE) = F(M),

this value is found iteratively:

Step Calculation Description

1 Mi = F(dA) PTI Regression equation givens

bending moment as a function of beam

depth. For the first iteration the actual

beam depth is used.



2 IC,i = F(Mi) Cracked moment of inertia is found

based on applied bending moment and

actual beam dimensions.

3 dE,i = F(IC,i) Effective depth is found as the depth

that given a moment of inertia equal to

the cracked moment of inertia.

4 Mi+1 = F(dE,i) PTI Regression equation gives bending

moment as a function of effective beam

depth.

5 Check convergence. If solution has not

converged, go to step 2.

Figure 7-1: WRI Design Equations for PT Reinforcing.

3.3. Effective Moment of Inertia – Beam depth used in the regression

equation is the beam depth that results in a moment of inertia equal to

the effective moment of inertia of the actual section. The beam depth is

found iteratively as shown in Figure 7-1. Effective moment of inertia is

calculated per ACI-318:

cr

a

crg

a

cre I

M

MI

M

MI ⋅

++⋅

=

33

1

Figure 7-2: Effective Moment of Inertia

4. Compression Steel – If selected by the user, SlabWorks™ will include

compression steel in bending strength calculations. Including compression

steel can make a significant difference in the cracked moment of inertia.

5. Compression Steel and Cracked Moment of Inertia - By default, SlabWorks™

considers the compression steel in calculating the moment of inertia. Tension



steel is always included in calculating the cracked moment of inertia, as the

cracked moment would otherwise be zero.

6. Reinforcing Steel and Uncracked Moment of Inertia - By default,

SlabWorks™ considers both the tension and compression steel in calculating

the uncracked moment of inertia.

7. Slab Reinforcing - If selected by the user, SlabWorks™ will include slab

reinforcing when calculating the uncracked moment of inertia, cracked

moment of inertia and bending strength. Slab steel will be transformed in

cracked and uncracked moment of inertia calculations only if the appropriate

check-boxes are unselected (see 4 and 5 above). Slab steel will be included in

the bending strength calculations as compressions steel only if the

appropriate check boxes are unselected (see 3 above).

Only the portions of the slab steel in the effective flange width is included in

the appropriate calculation. If the user chooses to ignore beam flanges (see 6

below), including slab steel in the calculations will not have a significant

effect in the bending strength of moment of inertia unless the slab is heavily

reinforced.

8. Include Flanges in Bending Strength–SlabWorks™ will calculate bending

strength and deflection of the slab by considering the contribution of the

beams alone (ignoring the slab per BRAB), or will include the contribution of

the effective flange per ACI-318 recommendations. Slab steel in the flanges is



considered as compression and tension reinforcing steel if the appropriate

check boxes are selected.

9. Uniform Thickness Slab – SlabWorks™ does not implement the PTI uniform

thickness analysis for bonded reinforcement.

7.9 Design Limitations

Several limitations must be considered when using the PTI design method.

1. Beam depths in the long and short directions must be the same. While the

foundation can be constructed with the beams in one direction deeper than

the beams in the other direction, the foundation must be analyzed assuming

that the depth of all beams is equal to the shallower depth.

2. The PTI manual recommends calculating the effect of subgrade friction on

post-tensioning stress using only the weight of the slab. This assumes that all

slab shortening occurs prior to additional load being placed on the slab. In

fact, some shortening due to thermal of shrinkage effects may occur after

additional loads are placed on the building. For this reason, the user may

want to define the subgrade friction assuming some portion of the total

service loads are present. This can be accomplished by increasing the

coefficient of subgrade friction.

3. The primary shortcoming of the PTI design method is restraint-to-shortening

(RTS) cracking. RTS cracking normally results because interlock between the

grade beams and subgrade prevents the post-tensioning cables from placing

the structure in full compression. Therefore, areas may exist in the center of



the slab in which no compression exists. If no bonded reinforcement is

present, which is the normal design condition for post-tensioned slabs-on-

ground, these areas are un-reinforced.

Historically, it is assumed that RTS cracking does not affect foundation

performance because all deflection is assumed to occur in the beta zone. The

design engineer should consider past experience in determining if this

assumption is valid in the area where the slab is being constructed. Forensic

evaluation of foundations has frequently shown deflection outside of the beta

zone.

In order to minimize the occurrence of RTS cracking, one can specify a higher

minimum residual port-tensioning stress (as recommended by the Texas

Section of ASCE), or assume a higher coefficient of subgrade friction.

As tile floor finishes have become more popular, RTS cracking has become a

bigger problem due to reflective cracking of floor finishes. This potential

problem should be discussed with the architect so that appropriate measures

can be taken to minimize floor finish cracking, such as the use of anti-

fracture or cleavage membranes under the floor tile.

SlabWorks™ Manual v1.07beta 8.0 Input Mode


8.0 Input Mode

The input screen has nine tabs that allow data input appropriate to the analysis

methodology selected by the user. Input fields that are not valid for the selected

analysis type will not be shown.

8.1 No Input Checking

Input values are NOT checked to ensure values are within acceptable limits for

the design methodology selected by the user. Ensuring the adequacy of the input

values in the responsibility of the user.

8.2 Input Tabs

All input is made on one of nine input tabs. The input tabs are:

1. Analysis - Select analysis type (PTI/BRAB/WRI, PT/Bonded,

Expansive/Compressive).

2. Project - Input project description.

3. Materials - Input material data

4. Geometry - Input slab geometry

5. Soils - Input soil data.

6. Post-Tensioning - Input post-tensioning data

7. Loads and Deflection - Input loads and allowable deflections.

8. Bonded Reinforcement - Input bonded reinforcement data

9. Bond Reinforcement Options - Bonded reinforcement calculation

options



For each tab, this manual discusses the recommendations of the WRI, BRAB,

PTI, ACI and ASCE if applicable. These recommendations are not coded into

SlabWorks, and it is the responsibility of the user to select the appropriate

design values.



Analysis Type Tab

The tab is used to select the design method, reinforcement type, slab type and

soil type. Since available input fields vary with the analysis type, this tab should

be completed prior to any additional data input.

Figure 8-1: Analysis Type Input Tab.

1. Analysis Method

1.1. PTI - Post Tensioning Institute (2nd and 3rd edition)

1.2. BRAB - Building Research Advisory Board

1.3. WRI - Wire Reinforcing Institute



2. Reinforcing Type

2.1. Post-Tensioned - Slab is reinforced with post-tensioning tendons.

Supplemental bonded reinforcement may also be included.

2.2. Bonded Reinforcement - Slab is reinforced with bonded reinforcement

only.

3. Slab Type

3.1. Stiffened Slab - Slab contains perimeter and interior grade beams.

3.2. Uniform Thickness Slab - Slab does not contain grade beams or down

turned edges.

4. Soil Type

4.1. Expansive Soil - Slab is built on expansive soils.

4.2. Compressible Soil - Slab is built on compressive soils.

5. Use Texas ASCE Modifications – The Texas Section of ASCE recommend

certain modifications in the BRAB, WRI and PTI analysis. These

recommendations are discussed in Chapter 13.0. Not all ASCE

recommendations are hard coded into SlabWorks™. Check the method

specific section of the manual for information on how SlabWorks™

implements the Texas ASCE modifications for each specific design method.



8.3 Project Data Tab

This data is not used in the analysis but allows to user to identify the project.

This data will be reflected in the output file.

Figure 8-2: Project Data Input Tab.



8.4 Material Tab

Figure 8-3: Material Data Input Tab.

1. Concrete

1.1. f'C - Compressive strength of concrete in psi.

1.2. EC - Creep modulus of concrete in psi. Creep modulus is used to

calculate long-term deflection of the slab under permanent loads.

1.2.1. ACI - Recommends using a creep modulus of Ec/2 to determine

long-term deflections. Slightly lower factors may be used if the

ratio of steel reinforcing is considered. ACI recommends



33*(wC)^0.5*sqr(f'C), equal to 57,000*sqr(f'C) for normal weight

concrete.

1.2.2. BRAB - Requires 0.5*f’c, assumed to be equal to 1,500,000 psi.

1.2.3. PTI – Recommends half the nominal modulus.

1.2.4. WRI – Equations for estimating beam depth use 1,500,000 psi.

No specified requirements.

1.3. Unit weight of concrete - For normal weight concrete, the unit weight

is normally assumed to be145 to 150 pcf. This value should include the

weight of steel.

1.4. Allowable tensile stress - Taken as a multiple of the square root of F'c.

1.4.1. ACI – Recommends a value of 7.5 for purposes of calculating

cracked sections in normal weight concrete.

1.4.2. PTI - Recommends 6.0. However, this is based on working stress

design. For ultimate stress design a higher values is

appropriate.

1.4.3. Texas ASCE - For the PTI method, Texas ASCE recommends 4.0

without use of bonded reinforcement and 6.0 with 0.33% bonded

reinforcement top and bottom (0.66% total).

2. Conventional Steel

1.5. Es - Elastic modulus of steel in psi. ACI - Recommends using

29,000,000 psi.

1.6. fsyb - Yield strength of beam reinforcing steel in psi.

1.7. fsys - Yield strength of slab reinforcing steel in psi.



Geometry Tab

All input fields are shown below. In operation, only the input field relevant to

the current analysis type will be shown:

Figure 8-4: Geometry Data Input Tab.

1. Rectangle designation - This is for user reference only and is not used in the

analysis.

2. Slab thickness - Thickness of slab in inches

2.1. BRAB – Recommended slab thickness is 4 inches. If slab thickness is

increased to 5 inches, slab steel can be reduced.



2.2. PTI – For a uniform thickness slab, the minimum thickness slab is 7.5

inches unless a continuous perimeter stiffening slab is used (See

4.2(C)(2)(b)).

2.3. WRI – No limits.

3. Slab length - Length of the slab in the long and short direction.

4. Number of beams - Number of grade beams in the long and short direction.

4.1. BRAB - The maximum beam spacing under BRAB is 15 feet, and the

recommended spacing is 9 to 12 feet. Beam spacing should be as equal

as possible. When beam spacing is not equal, grade beam reinforcing

steel and beam width should be apportioned is relationship to the

beam spacing (See BRAB page 18). Beams should be continuous and

should "dead-end" into the perimeter beam.

4.2. PTI – See 7.0 of this section.


5. Depth of beams - Beam depth in long and short directions. Analysis assumes

all beams are the same depth. If beam depths vary, use the smallest depth for

the solution.

5.1. BRAB – No limits.

5.2. PTI - The beam depth must be at least 12", and at least 7" greater than

the slab thickness. Beam depths must be the same.


6. Width of beams - Beam width in long and short directions. Analysis assumes

all beams are the same width. If actual beam widths vary, the average beam



width can be used for an approximate solution. Otherwise, use the minimum

beam width.

6.1. BRAB - Recommends a beam width between 8 and 14 inches.

6.2. PTI - Requires a beam width between 8 and 14 inches. PTI states that

while wider beams can be used (for example when required for

bearing), the additional width cannot be considering in the flexural

analysis.


7. Beam Spacing (PTI Only) - Maximum beam spacing in the PTI analysis is 17

feet, and the minimum spacing is 6 feet. Smaller spacing may be used in

construction but cannot be considered in the analysis. If the ratio between the

largest and smallest beam spacing exceeds 1.5, use 0.85*Smax, otherwise use

the average beam spacing.



8.5 Soils Tab

Figure 8-5: Soil Data Input Tab.

1. PTI Analysis Only – PTI recommends that all soil values be supplied as part

of a soils investigation report submitted by the Geotechnical engineer.

1.1. Es - Soil modulus of elasticity in psi. PTI suggests using 1000 psi if the

actual value is unknown.

1.2. yme - Maximum differential soil movement, edge lift mode (inches).

1.2.1. If yme is calculated using the provisions of the 3rd edition of the

PTIO manual, various assumptions about the site must be

made, including assumptions about irrigation and tree removal.



The design engineer should review the assumptions used by the

geotechnical engineer to the extent reasonably possible and

verify that they are appropriate for the project.

1.2.2. The method for predicting yme in Appendix A.3 of the 2nd edition

of the PTI manual does not consider non-climatic factors such as

drainage, slopes, cut/fill sections, soil conditions at time of

construction and vegetation. If any of these factors are present,

the designer should consider increasing yme above the value

predicted from purely climatic considerations.

1.3. ymc - Maximum differential soil movement, center lift mode (inches).

1.3.1. If ymc is calculated using the provisions of the 3rd edition of the

PTIO manual, various assumptions about the site must be

made, including assumptions about irrigation and tree removal.

The design engineer should review the assumptions used by the

geotechnical engineer to the extent reasonably possible and

verify that they are appropriate for the project.

1.3.2. The method for predicting ymc in Appendix A.3 of the 2nd edition

of the PTI manual does not consider non-climatic factors such as

drainage, slopes, cut/fill sections, soil conditions at time of

construction and vegetation. If any of these factors are present,

the designer should consider increasing yme above the value

predicted from purely climatic considerations.

1.4. eme - Edge moisture variation distance, edge lift mode (inches).

1.4.1. It should be noted that the chart in appendix A.3 of the 2nd

edition of the PTI manual is back calculated from successful

foundations, represents only a climatic 10-year return period



and doe not consider any non-climatic factors (drainage or

vegetation). These charts should not be used for design without

consideration of these factors.

1.5. emc - Edge moisture variation distance, center lift mode (inches).

1.5.1. It should be noted that the chart in appendix A.3 of the 2nd

edition of the PTI manual is back calculated from successful

foundations, represents only a climatic 10-year return period

and doe not consider any non-climatic factors (drainage or

vegetation). These charts should not be used for design without

consideration of these factors.

1.6. Allowable bearing strength - Allowable bearing strength of soil in psf.

Bearing is calculated only on projected area of grade beams. Slab area

does not contribute to bearing.

1.7. Subgrade Modulus (Ks) – This value is used to find the stress in the

slab under concentrated loads. Recommended values are shown in the

table below.

1.8. Cp - This value is used to find the stress in the slab under concentrated

loads. Recommended values are shown in the table below

Type of Subgrade Ks (lb/in3) Cp

Lightly compacted, high

plastic, compressible soil.

4 2.35

Compacted, low plastic

soil.

40 1.34

Stiff, compacted, select

granular or stabilized fill.

400 0.74



2. BRAB and WRI Analysis

2.1. PI - Effective Plasticity Index (PI). The effective PI is a weighted

number that considers the variation of PI with depth. The top 15 feet

of soil are considered. The weight factor for the top five feet is 3.0, the

middle five feet is 2.0, and the bottom five feet is 1.0. If the highest PI

soil is at the surface, the effective PI is equal to the highest PI. See

page 65 of the BRAB manual for a complete discussion.

2.2. Cw - Climatic Rating. This factor is indicative of the intensity of the

dry-moisture cycle. The higher the climatic rating the more stable the

moisture balance. Taken from BRAB Figure 1, page 38. It is important

to note that the climatic rating indicates potential shrink-swell

resulting from natural climatic cycles only. Certain man-made site-

specific conditions (drainage, vegetation, irrigation, leaks) may

increase the intensity of the moisture cycle beyond its natural limits

2.3. qu - Unconfined compressive strength of soil in psf.

3. WRI Analysis Only

3.1. Site slope - Site slope in %. Used to calculate site slope correction factor

3.2. Allowable bearing strength - Allowable bearing strength of soil in psf.

Bearing is calculated only on projected area of grade beams. Slab area

does not contribute to bearing.



8.6 Post-Tensioning Tab

Figure 8-6: Post-Tensioning Data Input Tab.

1. Slab Tendons

1.1. Specify average prestress - SlabWorks™ calculates the required

number of tendons based on minimum residual prestress (psi) and

maximum tendon spacing. Increasing the residual prestress will

reduce the possibility of experiencing restrain to shortening (RTS)

cracking.

1.1.1. PTI – Recommends a minimum residual prestress of 50 psi.

1

2

3

4

5

6

7

8



1.1.2. Texas ASCE - Recommends a minimum residual prestress of

100 psi.

1.2. Maximum tendon spacing - Maximum allowable tendon spacing in

feet. PTI recommends that maximum spacing should following local

practice. This value is normally in the range of 5 to 6 feet.

1.3. Specify number of tendons - SlabWorks™ calculates residual prestress

based on a specified number of tendons. This option is generally used

to evaluate existing designs.

2. Subgrade friction

2.1. PTI - Friction loss is calculated using the PTI methodology.

2.2. User defined - User specifies total friction is kips. In this event, the

coefficient of subgrade friction is not required.

3. Tendon Strength and Size

3.1. Strength - Ultimate strength of post-tensioning tendons in psi. 270 ksi

tendons are the most commonly used today.

3.2. Tendon size - Select tendon diameter or specify cross-sectional area. All

tendons must be the same size, which is good design practice to avoid

construction mistakes. If the tendon size varies, specify the smallest

tendon used.

4. Stress loss - This is the post-installation loss of tension force in the tendons,

measured in psi. This varies with the type of tendon. With low-relaxation

steel, the loss is typically assumed to be 15,000 psi.



5. Post-tension stress - This is the initial jacking force on the tendon as a

percent of ultimate strength. Tendons are most commonly jacked to 70% of

the ultimate strength.

6. Number of beam tendons (bottom) - Number of tendons located in the bottom

of each grade beam. Beam bottom tendons are not required but will decrease

the eccentricity and in some cases will allow for fewer total tendons. All grade

beams must have the same number of tendons.

7. Tendon cover – Distance from the face of the concrete to the centroid of the

tendon. All tendons must have the same cover. If tendon cover varies, specify

the greatest amount of cover to obtain the conservative solution.

8. Coefficient of subgrade friction - This is the coefficient of friction between the

slab and subgrade. This value is used to determine the amount of post-

tensioning that is required to overcome friction and is only used if the user

does not specify the frictional force (See 2.0 above). The remaining tension is

assumed to compress the slab. PTI recommends 0.75 for slabs on

polyethylene and 1.0 for slabs cast directly on a sand base.

Since stiffened slabs are interlocked into the soil with the grade beams, the

friction force calculated is an equivalent friction force rather than an actual

friction force. Most of the resistance to slab shortening is from soil bearing an

compression at the side of the grade beams.



8.7 Loads and Deflection Tab

Figure 8-7: Loads and Deflection Data Input Tab.

1. PTI Analysis Only

1.1. Perimeter load – Uniform service (unfactored) perimeter line load in

plf. The PTI regression analysis was run for perimeter loads ranging

from 600 plf to 1500 plf. The PTI procedure may not be valid for loads

outside of this range. If perimeter line loads vary significantly

(Mmax/Mmin > 1.25) , PTI requires that the slab be designed for center

lift using the largest perimeter line load, and for edge lift using the

lowest perimeter line load.



1.2. Concentrated Load – Line load in lb/ft. This load is analyzed in both

the long and short directions.

1.3. Allowable deflection, edge lift - Allowable deflection under edge lift

conditions.

Material Allowable

Wood Frame 480

Stucco or Plaster 720

Brick Veneer 960

Concrete Masonry

Units

1920

Prefab Roof Trusses 2000

1.4. Allowable deflection, center lift - Allowable deflection under center lift

conditions.

Material Allowable

Wood Frame 240

Stucco or Plaster 360

Brick Veneer 480

Concrete Masonry

Units

960

Prefab Roof Trusses 1000

2. BRAB and WRI – The BRAB and WRI manuals both recommend a total load

of 200 psf for single story wood-framed houses. WRI suggest 275 for a two

story house and 350 for a three story house.

2.1. Uniform Live Load – Average uniform live load in psf.

2.2. Uniform Dead Load – Average uniform live load in psf.



2.3. Allowable deflection - BRAB recommends 1/200 for wood finish, 1/300

for unplastered masonry or gypsum wallboard and 1/360 for stucco or

plaster. WRI recommends 1/480.

2.4. Dead Load Include Slab Weight – Unless this box is checked,

SlabWorks™ will automatically calculate the weight of the slab based

on the input geometry and concrete density, and add it to the specified

uniform dead load.



8.8 Bonded Reinforcement Tab

Figure 8-8: Bonded Reinforcement Data Input Tab.

1. Area of steel in top of beams (in^2) - Area of steel in the tops of the grade

beams, per beam (negative steel). All beams are assumed to have the same

amount of steel.

1.1. PTI – No guidance.

1.2. BRAB - No supplemental steel is required if the slab steel is capable of

calculated moments. Minimum steel ratio is 0.3%.

1.3. WRI - No guidance.



2. Area of steel in bottom of beams (in^2) - Area of steel in the bottoms of the

grade beams, per beam (positive steel). All beams are assumed to have the

same amount of steel.

2.1. PTI - No guidance.

2.2. BRAB - Minimum steel ratio is 0.3%.


3. Beam steel top cover (inches) - Distance from centroid of the top steel to the

top surface of the concrete beam. All beams are assumed to have the same

cover.

4. Beam Steel bottom cover (inches) - Distance from centroid of bottom steel to

the bottom surface of the concrete beam. All beams are assumed to have the

same cover.

5. Slab steel (in^2/ft) - Area of slab steel. Slab steel cover is assumed to be the

same as the top beam steel.

5.1. PTI - No guidance.

5.2. BRAB - Slab steel is placed in the top third of the slab. If the

maximum beam spacing is 12 feet or less, the slab steel shall be #3

bars each way at 12 inches o.c. If beam spacing exceeds 12 feet, slab

steel shall be #3 bars at 10 inches o.c. If a 5 inch slab is used, a 12 inch

bar spacing can be used regardless if beam spacing.


5.4. ACI – See Section 12.3 on page 89.



8.9 Bonded Reinforcement Options Tab

Figure 8-9: Bonded Reinforcement Options Input Tab.

1. Load Factors (WRI and Bonded PTI Analysis)

1.1. Use ACI factored load for strength calculations - Ultimate strength

design requires the use of factored loads in calculating flexural and

shear strength of beams. However, if desired, unfactored loads can be

used in the analysis. This would have the net effect of lowering the

factor of safety for flexural and shear design. For the WRI analysis, the

program will use the larger of 1.2DL+1.6LL or 1.4DL. Since the PTI

method uses a perimeter line load that combines dead and live loads,

the load factor must be specified by the user and must consider the net

1

2

3

4

5

7

6



contribution of live and dead load to the total load. In general, dead

load comprises the majority of the total load and the appropriate load

factor typically will be 1.4 to 1.5.

For the PTI method the contribution of the slab weight and uniform

loads are integrated into the regression equations and cannot be

factored. Therefore, SlabWorks™ calculates the shear and moment

using unfactored loads, and then multiplies the result by the load

factor in order to obtain the factored shear and moment.

1.2. Use ACI factored load for deflection calculations - Deflections are

normally calculated using unfactored (service) loads. However, factored

loads can be used if desired by the user. This has the net effect of

providing a factor of safety for the deflection design criteria. Note that

the PTI, BRAB and WRI methods all use service loads to calculate

deflections (providing no factor of safety).

2. Bonded PTI – Beam Depth - The PTI method assumes an uncracked section,

and the moment of inertia is represented in the regression equations by the

beam depth. When using bonded reinforcement, the section will be cracked if

the bending moment exceeds the cracking moment. Thus, the PTI analysis

may not be accurate for a slab with bonded reinforcement if the actual beam

depth is used in the regression equations presented in the PTI manual. To

address this issue, SlabWorks™ provides the user three options regarding

the beam depth used in the calculation. A complete discussion of the various

methods of calculating the moment of inertia in a reinforced concrete beam is

found in Section 10.0 on page 84.

2.1. Use full beam depth – The actual beam depth is used in the PTI design

equations. This implies that the applied moment is less than the

cracking moment and the beams are therefore uncracked.



2.2. Use effective beam depths from ACI-318 effective I - SlabWorks™

calculates an effective beam depth that results in a section with an

uncracked moment of inertia equal to the effective moment of inertia of

the actual (full depth) section. The effective moment of inertia is

calculated using the ACI-318 effective moment equation shown in

Figure 6-6 on page 36.

This analysis approach is based on the concept of equivalent stiffness,

where the section used to calculate design moments and shears has the

same stiffness as the cracked section. Actual section properties are

used to calculate shear and bending strength once the design moment

and shear are calculated.

2.3. Use effective beam depths from fully cracked I - SlabWorks™

calculates the effective beam depth that results in a section with an

uncracked moment of inertia equal to the fully cracked moment of

inertia of the actual (full depth) section.

This analysis approach is based on the concept of equivalent stiffness,

where the section used to calculate design moments and shears has the

same stiffness as the cracked section. Actual section properties are

used to calculate shear and bending strength once the design moment

and shear are calculated.

3. Compression Steel (WRI and Bonded PTI)

3.1. Ignore compression reinforcement in strength calculations – All

compression reinforcement is ignored in calculating the flexural

strength of the section.



3.2. Ignore compression reinforcement in deflection calculations (do not

transform when calculating I) – All compression reinforcement is

ignored in calculating the uncracked and cracked moment of inertia.

4. Ignore all reinforcing steel in uncracked moment of inertia calculations

(tension and compression steel) - All reinforcing steel is ignored when

calculating the uncracked moment of inertia.

5. Use Flanges (WRI and Bonded PTI)

5.1. Use beams and flanges per ACI 318 in bending strength and deflection

calculations.

5.2. Use beams only (BRAB Method) – Flanges are ignored in bending

strength and deflection calculations.

6. Use slab steel in calculations (WRI and Bonded PTI) - Slab steel is included

in all calculations where appropriate. All steel in the effective flange is

included in the analysis unless the user as indicated that flanges are to be

ignored, in which case only slab steel within the width of the beam is

considered. Inclusion of slab steel is over-ridden when appropriate. For

example, if the user has indicated that compression steel is to be ignored,

then slab steel in the compression zone is ignored even when the use slab

steel check box is set.

7. Fully Cracked vs. Effective Moment of Inertia (WRI Only) – The WRI manual

use the fully cracked moment to calculate deflection. This option allows the

use to calculate the deflection using the more realistic ACI-318 method of

effective moment of inertia. A complete discussion of the various methods of

calculating the moment of inertia in a reinforced concrete beam is found in

Section 10.0 on page 84.



7.1. Use fully cracked moment for deflection calculation - In calculating the

deflection in the cracked condition, SlabWorks™ uses the fully cracked

moment of inertia.

7.2. Use Effective Moment of Inertial for deflection (ACI-318 9.5.2.3) - In

calculating the deflection in the cracked condition, SlabWorks™ uses

the effective moment of inertia as defined by ACI-318 The effective

moment of inertia is calculated using the ACI-318 equation shown in

Figure 6-6 on page 36.

SlabWorks™ Manual v1.07beta 9.0 Output Mode


9.0 Output Mode

Printed output is accomplished by clicking the Print button located at the

bottom of the analysis screen.

9.1 Print Setup

If the proper font is not selected in the option screen, the printed output will not

format properly. The font is selected from the Help-Options-Printing Menu. Most

systems already have “Courier New” installed, and the program is set up to work

best with this font.

Figure 9-1: Font Selection.

Any fixed width font such as a courier font or a line printer font will work.

SlabWorks™ installs the “Bitstream Verra Sans Mono” font in case no other

fixed width fonts are already installed on the system. This font will produce

SlabWorks™ Manual v1.07beta 9.0 Output Mode


acceptable printed output. If interested, experiment to find a font you like.

However, all proportional fonts will format improperly.

SlabWorks™ Manual v1.07beta 10.0 Moment of Inertia


10.0 Moment of Inertia

SlabWorks™ provides the option to use the effective moment of inertia, the

cracked moment of inertia or the uncracked moment of inertia for most

deflection calculations.

10.1 Uncracked Moment of Inertia

The uncracked moment of inertia for slabs-on-grade has historically been

calculated without consideration for the effect of reinforcing steel. This was

because steel is typically present in low ratios, and it is more difficult to

calculate the modulus of inertia and include the effect of the steel.

SlabWorks™ will include the effects of reinforcing steel in calculating the

moment of inertia if desired by the user. This transformation is more effective in

the design of foundation because the assumed modulus of elasticity of the

concrete is the creep modulus, which is typically considered to be only 50% of the

normal modulus.

10.2 Cracked Moment of Inertia

The cracked moment of inertia is only meaningful when tension steel is present,

as an implicit assumption in the calculation is that the concrete has no tensile

strength. Therefore, without the presence of tension steel the cracked moment

would be zero.

Because the steel is considered in calculating the cracked moment, this can

result in a cracked moment larger than the uncracked moment is heavily

reinforced beams, if the steel is ignored in calculating the uncracked moment.

This is not an error, but merely a reflection of the fact that calculation of the

moment of inertia without considering the reinforcing steel is a conservative

method.

SlabWorks™ Manual v1.07beta 10.0 Moment of Inertia


10.3 Effective Moment of Inertia

Generally, the bending moment in a beam is not constant across the length.

Additionally, flexural cracking (and subsequent reduction in moment of inertia)

occurs only intermittently along the beam. Therefore, the using the fully cracked

moment of inertia to calculate deflection will overestimate the actual deflection.

ACI 318 allows the use “effective moment” that will give more accurate results

when used in deflection equations. The ACI 318 equation is:

cr

a

crg

a

cre I

M

MI

M

MI

++

=

33

1

The ACI-318 formula for effective moment of inertia is based on ACI Committee

435 report “Deflections of Reinforced Concrete Flexural Members,” published in

the Journal of the American Concrete Institute, June 1966. For cantilever beams

the effective moment of inertia is calculated at the support, which is the point of

maximum moment. Thus, for cantilever beams, the effective moment of inertia is

most likely conservative (lower than the actual value).

The use of the effective vs. fully cracked moment of inertia for calculating the

deflection in slabs on grade has not been discussed in the literature. However,

its use in analyzing elevated structural slabs is fully accepted. In a lightly

reinforced slab, the use of the effective moment may be non-conservative as the

slab may form a hinge at the crack. In a moderately reinforced beam, use of the

fully cracked moment of inertia to calculate deflection is almost certainly

conservative. The ultimate decision whether to use the cracked or effective

moment of inertia remains with the engineer of record.

SlabWorks™ Manual v1.07beta 11.0 Verification and Validation


11.0 Verification and Validation

SlabWorks™ includes the following input files in the “Verification” subdirectory.

These problems can be compared to the referred problems for verification and

validation purposes.

File Name Description

WRI Manual Example.sog WRI TF 700-R-03 design example (Appendix

B), Bonded Reinforcing

BRAB Example #1 BRAB Report No. 33 (Page 185), bonded

reinforcement.

BRAB Example #2 BRAB Report No. 33 (Page 225), bonded

reinforcement.

PTI2 Appendix A.6 PT.sog PTI 2nd Edition Appendix A.6, PT

Reinforcement.


Reinforcement.


Reinforcement.


Reinforcement.


Reinforcement.


Reinforcement.

If you are aware of other published design examples, we will add these examples

to the verification and validation package on request.

SlabWorks™ Manual v1.07beta 12.0 Building Code Requirements


12.0 Building Code Requirements

12.1 International Building Code (2006)

12.1.1 Foundation Design Requirements

The International Building Code (IBC) contains requirements for the design of

foundations on expansive clay. The requirements for foundations on expansive

clay are found in section 1805.8. Section 1805.8 allows for four design

approached for dealing with expansive clay:

1. Removal of expansive soil to depth to a depth “sufficient to ensure a

constant moisture content in the remaining soil.”

2. Stabilization of soil “by chemical, dewatering, presaturation or

equivalent techniques. “

3. Use of slab-on-grade of mat foundation designed and constructed in

accordance with the WRI/CRSI, PTI design methods or rational

method accounting for soil-structure interaction, deformed shape of the

soil support, and plate action of the slab in center lift and edge lift

conditions.

4. Compliance with 1805.8.1.

Section 1805.8.1 deals with the design of deep foundation, and generally requires

that the foundation be designed to resist the effects of volume change such that

any structural movement does not interfere with the “usability and

serviceability of the structure.”

1805.8.1 Foundations. Footings or foundations placed on or within

the active zone of expansive soils shall be designed to resist

differential volume changes and to prevent structural damage to the

supported structure. Deflection and racking of the supported



structure shall be limited to that which will not interfere with the

usability and serviceability of the structure

Foundations placed below where volume change occurs or below

expansive soil shall comply with the following provisions:

1. Foundations extending into or penetrating expansive soils

shall be designed to prevent uplift of the supported structure.

2. Foundations penetrating expansive soils shall be designed

to resist forces exerted on the foundation due to soil volume

changes or shall be isolated from the expansive soil.

12.1.2 Drainage Requirements

The IBC code also contains requirements for drainage and grading in Section

1803.3 and 1805.3.4. Adherence to these requirements will minimize the possible

of foundation failure due to poor drainage. Section 1803.3 generally requires a

5% slope ways fro the building perimeter for a distance of 10 feet. Section

1805.3.4 presents minimum foundation elevations, stating:

1805.3.4 Foundation Elevations. On graded sites, the top of any

exterior foundation shall extend above the elevation of the street

gutter at point of discharge of the inlet of an approved drainage

device a minimum of 12 inches plus 2 percent.

12.2 International Residential Code (2003)

Section R403.1.8 of the International Residential Codes IRC requires that

foundation on expansive clay “be designed in accordance with Section 1805.8 of

the International Building Code.” The code also allows the local building official

to approve alternative systems “which have performed adequately in soil

conditions similar to those encountered at the building site.”



12.3 ACI 318-02 – Building Code Requirements for Reinforced Concrete

As discussed in the commentary, ACI 318-05 minimum steel requirements

(0.18%) for elevated slabs are not applicable to slabs-on-grade. Instead, refer to

ACI 224 “Control of Cracking” for directions on controlling cracking of slabs-on-

grade. Generally, cracking is controlled with a combination of steel and control

joints. According to ACI 224, 0.50%-0.60% steel is required to control cracking

with steel alone.

SlabWorks™ Manual v1.07beta Appendices


13.0 Texas Section ASCE

13.1 Background

The Texas Section of the American Society of Civil Engineers has published a

recommended practice (RP) intended to supplement the design provisions

presented in the various national design standards (PTI, WRI and BRAB). The

stated rationale of this document is:

National building codes have general guidelines, which may not be sufficient for

the soil conditions and construction methods in the State of Texas. The purpose

of this document is to present recommended practices for the design of

residential foundations to augment current building codes to help reduce

foundation related problems.

The RP defines expansive soil as soils with a weighted plasticity index (BRAB

method) of greater than 15 or a maximum potential volume change of 1 percent

or greater. A free copy of the RP can be obtained at http://www.texasce.org.

This section describes the report as it relates to the implementation of PTI, WRI

and BRAB design methods. We recommend that the user obtain and read the

full text of the RP as it contains additional limitations and design requirements

not discussed here. The RP can be obtained directly from the Texas Section of

the ASCE from their website.

13.2 Design Process

The RP requires that foundation design consist of four steps:

• A site-specific geotechnical investigation completed by a geotechnical

engineer.

• A foundation design by a licensed engineer.



• Construction phase observation by the engineer of record or an approved

delegate.

• Issuance of a compliance letter by the engineer of record.

13.3 Slab-On-Grade Design

The RP allows design of slabs-on-grade using four methods, BRAB, PTI and WRI

and finite element analysis. The RP provides specific modifications and guidance

for the use of each method. Except as noted, these modifications are

implemented by SlabWorks™ if selected by the user. Finite element analysis is

not discussed in this section as it is not implemented in SlabWorks™.

BRAB – BRAB modifications are not implemented in SlabWorks™. See the

Section 5.0 for discussion.

• Regardless of actual beam length, the analysis length shall be limited to

50 feet.

• Use the maximum long-term creep factor as provided in ACI 318.

PTI Method

• Minimum residual prestress is 100 psi.

• Eccentricity must be less than 5-inches.

• The maximum tensile stress at service loads must be less than 4(f’c)1/2

unless additional bonded reinforcement equal to 0.33% of the gross beam

section is provided.

• The PTI em and ym must take into account the presence of trees and other

environmental effects.



WRI

• Regardless of actual beam length, the analysis length shall be limited to

50 feet.

• The minimum design length LC shall be increased by a factor of 1.5 with a

minimum design length of 6 feet.

13.4 Slabs-on-Grade with Piers

The RP provides specific guidance for the design of slabs-on-grade with piers.

This type of system is commonly used in many areas of the country. The RP

requires:

• The slab must be design as a slab-on-grade for purposes of heave and as

an elevated structural slab for purposed of settlement.

• Elevated structural slabs must be designed in accordance with applicable

building codes (this is generally construed as indicating the design must

meet the requirements of ACI-318 and associated standards).

• The piers shall not be rigidly attached to the grade beams unless the

foundation system is specifically designed to resist uplift forces (this

implies that uplift forces must be supplied as part of the geotechnical

investigation).



Appendices



Appendix 1 – BRAB Climatic Rating, Cw.



Appendix 2 – BRAB Support Index, C.

slabworks manual 1.07beta

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