bridge seismic design csi sap 2000

7/26/2019 Bridge Seismic Design Csi Sap 2000

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Bridge Seismic Design


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CSiBridge2016

Bridge Seismic DesignAutomated Seismic Design of Bridges

AASHTO Guide Specification forLRFD Seismic Bridge Design

ISO BRG091415M14 Rev. 0Proudly developed in the United States of America September 2015


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Copyright

CopyrightComputers & Structures, Inc., 1978-2015All rights reserved.

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[email protected](for general information)[email protected](for technical support)
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DISCLAIMER

CONSIDERABLE TIME, EFFORT AND EXPENSE HAVE GONE INTO THE

DEVELOPMENT AND TESTING OF THIS SOFTWARE. HOWEVER, THE USER

ACCEPTS AND UNDERSTANDS THAT NO WARRANTY IS EXPRESSED OR

IMPLIED BY THE DEVELOPERS OR THE DISTRIBUTORS ON THE ACCURACY

OR THE RELIABILITY OF THIS PRODUCT.

THIS PRODUCT IS A PRACTICAL AND POWERFUL TOOL FOR STRUCTURAL

DESIGN. HOWEVER, THE USER MUST EXPLICITLY UNDERSTAND THE BASICASSUMPTIONS OF THE SOFTWARE MODELING, ANALYSIS, AND DESIGN

ALGORITHMS AND COMPENSATE FOR THE ASPECTS THAT ARE NOT

ADDRESSED.

THE INFORMATION PRODUCED BY THE SOFTWARE MUST BE CHECKED BYA QUALIFIED AND EXPERIENCED ENGINEER. THE ENGINEER MUSTINDEPENDENTLY VERIFY THE RESULTS AND TAKE PROFESSIONAL

RESPONSIBILITY FOR THE INFORMATION THAT IS USED.


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Contents

Foreword

Step 1 Create the Bridge Model

1.1 Example Model 1-1

1.2 Description of the Example Bridge 1-2

1.3 Bridge Layout Line 1-4

1.4 Frame Section Property Definitions 1-4

1.4.1 Bent Cap Beam 1-4

1.4.2 Bent Column Properties 1-5

1.4.3 I-Girders Properties 1-6

1.4.4 Pile Properties 1-7

1.5 Bridge Deck Section 1-8

1.6 Bent Data 1-8

1.7 Bridge Object Definition 1-11

1.7.1 Abutment Property Assignments 1-12

1.7.2 Abutment Geometry 1-15

1.7.3 Bent Property Assignments 1-15

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CSiBridg e Seismic Design

1.7.4 Bent Geometry 1-17

1.8 Equivalent Pile Formulation 1-17

1.9 Bent Foundation Modeling 1-18

1.10 Mass Source 1-19

Step 2 Ground Motion Hazard and Seismic Design Request

2.1 Overview 2-1

2.2 AASHTO and USGS Hazard Maps 2-1

2.3 Seismic Design Preference 2-3

2.4 Seismic Design Request 2-4

2.5 Perform Seismic Design 2-8

2.6 Auto Load Patterns 2-9

2.7 Auto Load Cases 2-10

Step 3 Dead Load Analysis and Cracked Section Properties

Step 4 Response Spectrum and Demand Displacements

4.1 Overview 4-1

4.2 Response Spectrum Load Cases 4-1

4.3 Response Spectrum Results 4-5

Step 5 Determine Plastic Hinge Properties and Assignments

5.1 Overview 5-1

5.2 Plastic Hinge Lengths 5-1

5.3 Nonlinear Hinge Properties 5-4

5.4 Nonlinear Material Property Definitions 5-7

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Contents

5.4.1 Nonlinear Material Properties Definitions for

Concrete 5-75.4.2 Nonlinear Material Properties Definitions for

Steel 5-9

5.5 Plastic Hinge Options 5-10

Step 6 Capacity Displacement Analyses

6.1 Displacement Capacities for SDC B and C 6-2

6.2 Displacement Capacities for SDC D 6-3

6.3 Pushover Results 6-7

Step 7 Demand/Capacity Ratios

Step 8 Review Output and Create Report

8.1 Design 01 D/C Ratios 8-2

8.2 Design 02 Bent Column Force Demand 8-2

8.3 Design 03 Bent Column Idealized Moment

Capacity 8-2

8.4 Design 04 Bent Column Cracked Section

Properties 8-3

8.5 Design 05 Support Bearing Demands Forces 8-3

8.6 Design 06 Support Bearing Demand

Displacements 8-4

8.7 Design 07 Support Length Demands 8-5

8.8 Create Report 8-5

Chapter 9 Caltrans Fault Crossing Seismic Bridge Design

9.1 Introduction 9-1

9.2 Fault Crossing Response Spectrum Loading 9-2

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CSiBridg e Seismic Design

9.3 Defining Fault Crossing Seismic Design Requests 9-5

9.4 Running Fault Crossing Seismic Design Requests 9-10

9.5 Creating a Seismic Design Report 9-11

9.6 Automatic Load Cases and Combinations 9-12

9.7 General Displacement Loading 9-14

9.7.1 Defining Load Patterns and Response Spectrum

Functions 9-15

9.7.2 Defining a Seismic Design Request 9-17

References

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Foreword

Over the past thirty-five years, Computer and Structures, Inc, has introduced

new and innovative ways to model complex structures. CSiBridge, the latest

innovation, is the ultimate integrated tool for modeling, analysis, and design of

bridge structures. The ease with which all of these tasks can be accomplished

makes CSiBridge the most versatile and productive bridge design package in

the industry.

Automated seismic design, one of CSiBridges many features, incorporates the

recently adopted AASHTO Guide Specification for LRFD Seismic Bridge

Design 2nd

Edition, 2011. The 2011 implementation in CSiBridge also satisfiesthe 2012 and 2014 interim revisions, which do not contain any changes that af-

fect the program. CSiBridge allows engineers to define specific seismic design

parameters that are then applied to the bridge model during an automated cycle

of analysis through design.

Now, users can automate the response spectrum and pushover analyses. Fur-

thermore, the CSiBridge program will determine the demand and capacity dis-

placements and report the demand/capacity ratios for the Earthquake Resisting

System (ERS). All of this is accomplished in eight simple steps outlined as fol-

lows:

1.

Create the Bridge Model

2. Evaluate the Ground Motion Hazard and the Seismic Design Request

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CSiBridge Seismic Design

3. Complete the Dead Load Analysis and evaluate the Cracked Section Prop-erties

4. Identify Response Spectrum and Demand Displacements

5. Determine Plastic Hinge Properties and Assignments

6. Complete Capacity Displacement Analysis

7. Evaluate Demand/Capacity Ratios

8.

Review Output andCreate Report

A detailed explanation of each of the steps is presented in the chapters that fol-

low. The example bridge model shown in the figure illustrates the CSiBridge

Automated Seismic Design features.

Schematic of the Eight Steps in the

Automated Seismic Design of Bridges using CSiBridge

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Foreword

In addition to AASHTO Bridge Seismic Design, CSiBridge provides the capa-

bility to perform Caltrans Fault-Crossing Seismic Bridge Design. This newseismic design procedure considers the more severe case where the rupture of a

seismic fault that crosses a bridge structure causes significantly different

ground displacements for the supports on either side of the fault. Most of the

concepts that apply to AASHTO Bridge Seismic Design also apply to the Cal-

trans Fault-Rupture case, with some new techniques introduced for this special

purpose. The details are provided in the last chapter of this manual.

Foreword vii


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STEP 1Create the Bridge Model

1.1 Example Model

This chapter describes the first step in the process required to complete a Seis-

mic Design Request for a bridge structure using CSiBridge. It is assumed the

user is familiar with the requirements in the program related to creating a

Linked Bridge Object. Only select features of the model development are in-

cluded in this chapter. The CSiBridge model used throughout this manual is

available and includes all of the input parameters.

Figure 1-1 3D View of Example Model

Example Model 1 - 1


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As described in the AASHTO Guide Specifications for LRFD Seismic Bridge

Design, the seismic design strategy for this bridge is Type 1 Design; a ductilesubstructure with an essentially elastic superstructure. This implies that the de-

sign must include plastic hinging in the columns.

1.2 Description of the Example Bridge

The example bridge is a three-span concrete I-girder bridge with the following

features:

Piles: 14-inch-diameter steel pipe pile filled with concrete. The concrete is re-

inforced with six #5 vertical bars with three #4 spirals having a 3-inch pitch.

Pile Cap: The bent columns are connected monolithically to a concrete pile capthat is supported by nine piles each. The pile caps are 13-0 x 13-0 x 4-0

Bents: There are two interior bents with three 36-inch-diameter columns.

Deck: The deck consists of five 3-3-deep precast I-girders that support an

8-inch-thick deck and a wearing surface (35 psf). The deck width is 35'-10"

from the edge-of-deck to edge-of-deck.

Spans: Three spans of approximately 60-0.

The abutments are assumed to be free in both the longitudinal and transverse

directions.

Figure 1-2 Example Bridge Elevation

1 - 2 Description of the Example Bridge


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STEP 1 - Create the Bridg e Model

Figure 1-3 Example Bridge Plan

Figure 1-4 Example Bridge BENT1 Elevation

Description of the Example Bridge 1 - 3


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1.3 Bridge Layout Line

The example model has three spans of approximately 60 feet each. The layout

line is defined using the Layout > Layout Line > New command and the

Bridge Layout Line Data form shown in Figure 1-5. The layout line is straight,

with no variation in elevation. The actual length of the layout line is 178.42 ft.

Figure 1-5 3D Bridge Layout Line Data

1.4 Frame Section Property Defini tions

Four frame section properties must be described by the user to develop the ex-

ample model. The four types of frame elements used in the example model

consist of a pile, bent cap beam, bent column, and precast concrete I-girder.

The section property definition for each of the elements is given in the subsec-

tions that follow.

1.4.1 Bent Cap Beam

The bent cap beams were defined using the Components > Type > Frame

Properties > Expand arrow command. The Add New Property button >

1 - 4 Bridge Layout Line


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Frame Section Property Type: Concrete > Rectangular was used to add the fol-

lowing concrete rectangle:

Figure 1-6 3D Cap Beam Section Property Definition

The material property used was 4000 psi. Note that the units shown in Figure

1-6 are in inches. (To check this, hold down the Shift key and double click inthe Depth or Width edit box. This will display the CSiBridge Calculator.)

1.4.2 Bent Column Properties

The bent columns were defined using the Section Designer option that can be

accessed using the Components > Type > Frame Properties > New > Other

> Section Designer command. The size and quantity of both the vertical and

confinement reinforcing steel were defined using the form shown in Figure 1-7.

Further discussion of the column section properties as they pertain to the plas-

tic hinge definitions is provided in Step 5.

Frame Section Property Definitions 1 - 5


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Figure 1-7 Bent Column Property Definition

1.4.3 I-Girder Properties

The I-girder properties were

input using inch units, as

shown in Figure 1-8. (Again,

check this by holding down

the Shift key and double

clicking in a dimension edit

box to display the CSiBridge

Calculator.)

Figure 1-8 Precast I-Girder

Properties

1 - 6 Frame Section Property Definitions


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1.4.4 Pile Properties

The piles were defined as 14inch-diameter concrete piles with six #9 vertical

bars (Components > Type > Frame Properties > New > Concrete > Circu-

lar command). The outer steel casings of the pile were found to increase in the

flexural stiffness of the piles by a factor of 2.353. This value was applied as a

property modifier to the pile section property. The pile will be added to the

bridge model as Equivalent Cantilever piles, as shown in Figure 1-9 and as

described in subsequent Section 1.8. Using this method, the pile is replaced by

a beam that has equivalent stiffness properties to that of the pile with the sur-

rounding soil.

Figure 1-9 Pile Properties

Frame Section Property Definitions 1 - 7


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1.5 Bridge Deck Section

The bridge deck section is 38.833 feet wide with a total of five

I-girders, as shown in Figure 1-10 (Components > Superstructure Type >

Deck Section > Newcommand). The parapets as well as the wearing surface

are not part of the bridge deck structural definition but will be added to the

bridge model as superimposed dead loads (SDEAD).

Figure 1-10 Bridge Deck Section Properties

1.6

Bent Data

The bents for the subject model have three columns, each with a cap beam

width of 38.25 feet. The Bridge Bent data form shown in Figure 1-11, which is

accessed using the Components > Substructure Item > Bents > New com-

mand, is used to input the number of columns and the cap beam width. Since

multiple columns are specified, the location, height and support condition for

each column needs to be specified using the Bent Column Data form.

1 - 8 Bridge Deck Section


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Figure 1-11 Bridge Bent Data

After the Modify/Show Column Databutton is used, the Bent Column Data

form shown in Figure 1-12 can be used to define the type, location, height, an-

gle and boundary conditions as well as the seismic hinge data for each bent

column.

Figure 1-12 Bent Column Data

For the seismic hinge data, RH Long and RH Trans are the relative clear

heights (from -1.0 to 2.0) from the base of the column to the point of contra-

Bent Data 1 - 9


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flexure under horizontal loading at the top of the bent, used to determine the

hinge lengths and positions for bridge seismic design. RH Long is for longitu-dinal loading (normal to the plane of the bent), and RH Trans is for transverse

loading (in the plane of the bent). Only concrete columns are affected. Steel

columns are not affected and use their own calculation. For each physical bent

column, the reference hinge property to be used at the top and bottom of the

column can be "Auto", "Auto Fiber", "None", and a list of user-defined hinge

properties. The reference hinge properties will only be used when the Concrete

or Steel Hinge Type is set to Auto: From Bent in the Bridge Seismic Design

Preference form, which is accessed using the Design/Rating > Seismic Design

> Preferences command. Under the case that the Hinge Type is Auto: From

Bent, if the reference hinge property is set to Auto, then the program will gen-

erate AASHTO/Caltrans hinges for concrete columns and FEMA 356 hingesfor steel columns; if the reference hinge property is set to user-defined hinge

property, then for the force-controlled type hinges, or the deformation con-

trolled type hinges with moment-rotation or force-displacement nonlinear

property types,

An important part of this example model is the inclusion of the foundation el-

ements. Although the foundations can be represented as Fixed, Pinned, or

Spring-Support restraints at the base of the columns, these have been explicitly

modeled in this example. It is important to note that when foundation objects

are part of the bridge model, the base of the bent column must not be re-

strained, but instead, connected to the foundation elements. Restraining the

base of the columns in the Bent Column Data form using Fixed or Pinned re-

straints would prevent the bridge loads from reaching the foundation. In this

example, a foundation spring (BFSP1) having no stiffness in any direction is

used as the Base Support data. After the foundations have been modeled and

connected to the bent column bases, support of the bent columns will be

achieved. The Foundation Spring Data form is shown in Figure 1-13. Access

this form by clicking the Foundation Spring Propertiesbutton on the Bridge

Bent Column Data form and then the Add New Foundation Springbutton on

the Define Bridge Foundation Springs form, or by using the Components >

Substructure Item > Foundation Springs > Newcommand.

1 - 10 Bent Data


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Figure 1-13 Bent Column Base Restraint Definitions

1.7 Bridge Object Definition

The Bridge Object Data form (click the Bridge > Bridge Object > Newcom-

mand) is used to define the complete bridge object, including the superstructure

and substructure. See Figure 1-14.

The seismic response of the bridge model will depend on the Earthquake Re-

sisting System (ERS). The user can define the types of support conditions at

the abutments and bents. The ERS will depend on the types of supports used at

the abutments and bents and the bearing properties that are used for each. If a

bearing has a restrained DOF, it will provide a load path that will act as part of

the bridge ERS. Abutments can be defined using bents as supports (this feature

was not used in the subject example).

Bridge Object Definition 1 - 11


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Figure 1-14 Bridge Object Data form

The span data is used to define the span lengths and bent locations. Cross dia-

phragms also can be included in a bridge model using the Modify/Show As-

signments > In Span Cross Diaphragmscommand and Modify/Showbutton.

No cross diaphragms were used as part of the example model.

1.7.1 Abutment Property Assignments

Both the start and end abutment assignments are specified using the Bridge Ob-

ject Abutment Assignments form shown in Figure 1-15 (Bridge > Bridge Ob-

ject > Supports > Abutments). The abutment bearing direction can be as-

signed a bearing angle if skewed abutments are needed. Diaphragms can be

added to the abutment as well.

Abutments are modeled using an Abutment Property, which can be defined

using the command Components > Substructure Item > Abutments > New.

This can also be accessed by clicking the + button next to the AbutmentProperty option in the Substructure Assignment area of the Bridge Object

Abutment Assignments form. This brings up the Abutment Data form as shown

1 - 12 Bridge Object Definition


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in Figure 1-16. Note that a default abutment property is always created when-

ever the first bridge object is defined, and that is what is used for this example.

Figure 1-15 Abutment Assignments

Figure 1-16 Abutment Data

Abutments can alternatively be modeled using bents by selecting Bent Proper-

ty in the Substructure Assignment area of the Bridge Object Abutment As-

signment form. After that selection has been, an option is available to select theappropriate property definition from a list of previously defined bent proper-

ties, or to add a new one by clicking the + button.



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The substructure location data is critical because CSiBridge accounts for the

superstructure/substructure kinematics. The ends of the bridge deck will have atendency to rotate due to gravity loading. If the abutment bearings are re-

strained against translation at both ends of a bridge, outward reactions on the

bearings and deck moments can be induced as a result of these restraints. The

amount of outward thrust and the moment in the deck are a function of the

amount of rotation and distance from the deck neutral axis to the top of abut-

ment bearings. Therefore, the user should pay special attention to the substruc-

ture and bearing elevations as well as the bearing restraint properties. The user

also must keep in mind that the seismic resisting load path is dependent on the

restraint properties of the bearing at both abutments and bents.

For this example, only the vertical translation of the abutment bearings was set

to Fixed. All other abutment bearing components were set to Free since theabutment restraint was assumed to be free in the longitudinal and transverse di-

rections. See Figure 1-17 (display this form by clicking the + plus beside the

Bearing Property drop-down list on the Bridge Object Abutment Assignments

form and the Add New Bridge Bearingor Modify/Show Bridge Bearingbut-

ton on the Define Bridge Bearings form).

Figure 1-17 Abutment Bearing Properties

To help visualize the abutment geometry, the graphic shown in Figure 1-18 in-

cludes the values in the example model to define the location of the abutment

bearings and substructure. It should also be noted that the CSiBridge program

automatically includes the BFXSS Rigid Link when the bridge object is updat-

ed.



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1.7.2 Abutment Geometry

Figure 1-18 also shows the location of the BBRG1 action point. This is the lo-

cation where the bearing will translate or rotate depending on the bearing defi-

nitions.

Figure 1-18 Abutment Bearing Geometry

1.7.3 Bent Property Assignments

The bent property assignments are made using the Bridge Object Bent As-

signment form, shown in Figure 1-19 (Bridge > Bridge Object > Supports >

Bentscommand). Similar to the abutment property assignments, the bent prop-

erty assignments will include the bent directions, bearing properties, and sub-

structure locations.



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Figure 1-19 Bent Assignments form

For this example model, the bearing properties at the bents have fixed transla-

tion restraints in all directions but free restraints for all rotational directions.

See Figure 1-20 (click the + plus beside the Bearing Property drop-down list;

click theModify/Show Bridge Bearingbutton on the Define Bridge Bearings

form).

Figure 1-20 Bent Bearing Data



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1.7.4 Bent Geometry

The bent geometry is shown in Figure 1-21 for the input values used to define

the bearing and substructure elevations from the Bridge Object Bent Assign-

ment form (Figure 1-19).

Figure 1-21 Bent Support Geometry

Note that the BBRG2 connects to the center of the cap beam. The substructure

elevation is used to define the top of the cap beam. The action point of BBRG2is at Elevation -49.0.

1.8 Equivalent Pile Formulation

Although it is not required to include explicit foundation elements (foundations

can be modeled as fixed, pinned or partially fixed restraints at the base of the

columns), these were included as part of the example model. Foundations can

be modeled in many ways. Equivalent length piles were used with an equiva-

lent length of 5.1 feet to model the pile surrounded by soil, as described in Sec-

tion 1.4.4. The equivalent lengths were established using the equations shown

in Figure 1-22.

Equivalent Pile Formul ation 1 - 17


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Figure 1-22 Equivalent Pile Properties

After the lengths of the piles were known, the piles were connected to an area

object representing the pile cap. The cap was meshed at the top of the pile loca-

tions. The completed pile cap appears in Figure 1-23, which is shown using a

3D extruded view.

Figure 1-23 View of Bent Foundations

1.9

Bent Foundation Modeling

The next and critical step in the model definition is to connect the foundation to

the base of the bent columns. For this example, joint constraints were used as

illustrated in Figure 1-24. This method of connecting the column base to the

foundation preserves connectivity even when updating the linked bridge model.

1 - 18 Bent Foundation Modeling


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Figure 1-24 Bent Column Base Connectivity

1.10

Mass Source

The Mass Source definition is used to define the mass to be included in the

modal and response spectrum load cases. Mass and weight are treated separate-

ly in CSiBridge: mass is used for inertia in dynamic analysis, and weight isused for gravity loads.

By default, mass comes from the material mass density and any additional

mass assigned to joints, line objects, and area objects. However, you can use

the Mass Source command to specify that mass is to be computed from load

patterns, either in addition to or instead of the default mass.

Multiple Mass Sources definitions can be created for advanced dynamic analy-

sis. This is rarely necessary. For this example, a single Mass Source is defined

that uses the default mass plus mass from load patterns.

The command Advanced > Define > Mass Source opens the Mass Source

form in shown Figure 1-25. Here the default mass source already defined can

be seen. Clicking the Modify/Show button opens the Mass Source Definition

form shown in Figure 1-26.

Column-to-Foundation Connection

Mass Source 1 - 19


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STEP 2Ground Motion Hazard and Seismic Design Request

2.1 Overview

The ground motion hazard (response spectrum) can be determined by

CSiBridge by defining the bridge location using the latitude and longitude or

the postal zone. As an alternative, the user can input any user defined response

spectrum file. The site effects (soil site classifications) also are considered and

are part of the user input data.

2.2 AASHTO and USGS Hazard Maps

The recently adopted AASHTO Guide Specification for the LRFD Seismic

Bridge Design incorporates hazard maps based on a 1000-year return period.

When the user defines the bridge location by Latitude and Longitude,

CSiBridge creates the appropriate response spectra curve as follows:

Overview 2 - 1


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Figure 2-1 AASHTO/USGS Hazard Maps used to determine the Demand Response Spectrum

Figure 2-2 Response Spectrum Function Data form

2 - 2 AASHTO and USGS Hazard Maps


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STEP 2 - Ground Motion Hazard and Seismic Design Request

From the Response Spectrum Data form (Loads > Functions > Type > Re-

sponse Spectrum > New > NCHRP 20-07), the values for SDSand SD1 are de-termined by CSiBridge and reported. The SD1 value is used to determine the

Seismic Design Category (SDC). The SDC is used to determine the analysis

and design requirements to be applied to the bridge. For example, if the SDC is

A, no capacity displacement calculation is performed. If the SDC is B or C,

CSiBridge uses an implicit formula (see Section 4.8 of the AASHTO Seismic

Guide Specification). If the SDC is D, CSiBridge uses a nonlinear pushover

analysis to determine the capacity displacements.

2.3 Seismic Design Preferences

Figure 2-3 Bridge Seismic Design Preferences form

The Design/Rating > Seismic Design > Preferences command accesses a form

that can be used to specify the design code, concrete hinge type, steel hinge

type and the hinge length option for all Seismic Design Requests. There arefour choices for the hinge type: Auto: AASHTO/Caltrans Hinge for concrete

and FEMA 356 hinge for steel, Auto: Fiber Hinge, Auto: From Bent and User-

assigned. The following hinge length options are provided: Use Longitudinal

Seismic Design Preferences 2 - 3


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Hinge Length, Use Transverse Hinge Length, Use Shortest Hinge Length (DE-

FAULT), Use Longest Hinge Length, and Use Average Hinge Length. Thelongitudinal and transverse hinge lengths are calculated based on the Seismic

Hinge Data specified in the Bridge Bent Column Data Form introduced in the

Section 1.6.

2.4 Seismic Design Request

Figure 2-4 Bridge Design Request form

The Design/Rating > Seismic Design > Design Request > Add New Request

command accesses a form that can be used to specify the name, check type,

loading and design request parameters for a Seismic Design Request. There are

two check types available: AASHTO Seismic Design and Caltrans Fault Cross-

ing. For the loading, the pre-defined response spectrum function (see Section

2.2) to be used for a specific Seismic Design Request should be selected for the

horizontal and/or vertical direction. None should be selected if no response

spectrum is to be included in either direction in the seismic design request. The

form is shown in Figure 2-4.

For this example, which is of AASHTO Seismic Design, clicking the Modi-

fy/Showbutton will display the Substructure Seismic Design Request Parame-

ters form, shown in Figure 2-5. A brief description of the parameters on that

form follows.

2 - 4 Seismic Design Request


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Figure 2-5 Seismic Design Parameters form

Item Substructure Seismic Design Request Parameter

1 Seismic DesignCategory(SDC) Option

The user can choose to have the SDC be selected by the program(i.e., Programmed Determined), or the user can impose a valuefor the SDC (i.e., User Defined). To impose a value, select it fromItem 4, the Seismic Design Category.

2 Seismic DesignCategory

If the user has opted to specify the Seismic Design Category inItem 3, the user must specify the Seismic Design Category here asB, C or D.

3 Bent Dis-placementDemand Factor

This is a scale factor. The bent displacement demands obtainedfrom the response-spectrum analysis are multiplied by this factor. Itcan be used to modify the displacement demand due to a dampingvalue other than 5%, or to magnify the demand for short-period

structures. This factor will be applied to all bents in both the longi-tudinal and transverse directions.

Seismic Design Request 2 - 5


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4 Gravity LoadCase Option

The user can specify which gravity load case is used to determinethe cracked section properties for the bent columns. The choicesinclude Auto-Entire Structure, Auto This Bridge Object, or UserDefined. As a default, all Dead and Super Dead loads are includedin the Auto-Entire Structure gravity load case.

5 Gravity LoadCase

If the User Option is selected for Item 6 Gravity Load Case Option,the gravity load case name must be selected here.

6 AdditionalGroup

If the Auto-This Bridge Object option is selected for Item 6 GravityLoad Case Option, an additional group can be included in thegravity load case. This item is required only when the gravity loadcase is program determined. It may include pile foundations andother auxiliary structures.

7 Include P-Delta If P-Delta Effects are to be included, the user needs to specify yeshere. P-Delta effects will cause a more abrupt drop in the pusho-ver curve results if an idealized bilinear hinge has been assigned tothe bent columns. It is recommended that an initial Seismic DesignRequest be performed before including the P-Delta effects to helpthe user understand the nonlinear behavior of the bents.

8 CrackedPropertyOption

The cracked section properties for the bent columns can be auto-matically determined by the program or they can be user defined.If program determined, the automatic gravity load case will be runiteratively. Section Designer will use the calculated axial force atthe top and bottom on the column to determine the cracked mo-ments of inertia in the positive and negative transverse and longi-tudinal directions. The average of the top and bottom columncracked properties will be applied as named property modifier sets

and the analysis will be re-run to make sure the cracked-modifiedmodel converges to within the specified tolerance.

9 ConvergenceTolerance

This value sets the relative convergence tolerance for the bent-column cracked-property iteration. This item is required only whenthe cracked-property calculation is program determined.

10 MaximumNumber ofIterations

This value sets the maximum number of iterations allowed for thebent-column cracked-property iteration. The first run is consideredto be the zero-th iteration. Usually only one iteration is needed.This item is required only when the cracked-property calculation isprogram determined.

11 AcceptUnconvergedResults

Specifies if the seismic design should or should not continue if thebent-column cracked-property iteration fails to converge. This itemis required only when the cracked-property calculation is program

determined.

12 Modal LoadCase Option

Specifies if the modal load case is to be determined by program orspecified by the user. The modal load case is used as the basis ofthe response-spectrum load case that represents the seismic de-sign. If program determined, the modal load case will use the stiff-

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ness at the end of the auto-gravity load case that includes thecracked property effects. If user-defined, the user can control theinitial stiffness, Eigen vs. Ritz, and other modal parameters by se-lecting user defined for Item 15 Modal Load Case.

13 Modal LoadCase

The name of an existing modal load case to be used as the basisof the response-spectrum load case. This item is required only ifItem 14 Modal Load Case Option is user-defined.

14 Type of Modes This is either Eigen or Ritz indicating the type of modes requested.

15 AdditionalNumber OfModes

The number of additional modes to consider beyond those auto-matically determined. This can be zero (default), positive, or nega-tive. The default number of modes is determined based on thenumber of bridge spans. The minimum number of modes is 12. Fora bridge object with more than two spans, 6 modes are added foreach additional span.

16 ResponseSpectrum LoadCase Option

Specifies if the response-spectrum load case is to be determinedby program or specified by the user. The response-spectrum loadcase represents the seismic demand. If program determined, thisload case will use the given response-spectrum function and modalload case. Acceleration load will be applied in the longitudinal andtransverse directions of the bridge object, and combined using the100% + 30% rule. If user-defined, the user can control the loadingor select SRSS as the method to account for directional combina-tions.

17 ResponseSpectrum LoadCase

The name of an existing response-spectrum load case that repre-sents the seismic demand. This item is required only if the re-sponse-spectrum load case option is user-defined.

18 ResponseSpectrum An-gle Option

Specifies if the angle of loading in the response-spectrum loadcase is to be determined by program or specified by the user. Ifprogram determined, the longitudinal (U1) loading direction is cho-sen to be from the start abutment to the end abutment, both pointslocated on the reference line of the bridge object. This item is re-quired only if the response-spectrum load case option is user-defined.

19 ResponseSpectrum

Angle

Angle (degree, from global X) that defines the direction of the re-sponse spectrum load case. This item is required only if the re-sponse spectrum load case is user-defined.

20 DirectionalCombination

The type of directional combination for the response spectrumanalysis

21 DirectionalScale Factor For absolute directional combination this is the scale factor used forthe secondary directions when taking the absolute sum. This istypically 0.3 if a 100/30 rule is to be applied. For CQC3 directionalcombination, this is the scale factor applied to the response spec-trum function in the second horizontal direction. This is typicallygreater than 0.5. For the SRSS directional combination the direc-

Seismic Design Request 2 - 7


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tional scale factor is normally 1.0.

22 FoundationGroup

If foundations are included and explicitly modeled, then the founda-tion objects need to be assigned to a group and that group needsto be identified here. This way the foundation objects will be includ-ed in the pushover load case. This item is required only if the seis-mic design category is D.

23 PushoverTargetDisplacementRatio

The target displacement is defined as the target ratio of Capacity/Demand for the pushover analyses. This item is required only if theseismic design category is D.

24 Bent FailureCriterion

The criteria to determine the bent failure. means the bent fails when the pushover curve slope becomesnegative. This item is required only if the seismic design category isD.

25 PushoverCurve DropTolerance

Relative decrease in base shear from the maximum that deter-mined the displacement capacity from the pushover curve.

2.5 Perform Seismic Design

It is not necessary to execute an analysis of the bridge model before running

the Seismic Design Request. To start the Bridge Seismic Design Request, use

the Design/Rating > Seismic Design > Run Seismiccommand. The Perform

Bridge Design form, which is shown in Figure 2-6, will be displayed. The De-

sign Nowbutton will start the seismic design process.

Figure 2-6 Perform Seismic Design

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It is noted that for a designed request, when clicking the button "Delete Design

for Request", a message box asking to remove all the program-generated items,such as load cases, load patterns, group, generalized displacement will be

popped up. The Yes button will bring up another message box asking to re-

move all the program-generated hinges. The program-generated items can be

removed by clicking the button "Clean up Request" if they were kept when de-

leting the design results. Also if the same design request is selected to be de-

signed again when the model is locked, then a new set of the program-

generated items will be created and previous generated items will be kept;

when the model is unlocked, then the program will ask to remove the previous

program-generated items or to keep them.

2.6 Auto Load PatternsAfter the Bridge Seismic Design has been run, the user can review the load pat-

tern and load cases that CSiBridge has automatically generated by accessing

the Define Load Patterns form show in Figure 2-7 (Loads > Load Patterns

command).

Figure 2-7 Auto Load Patterns

Auto Load Pat terns 2 - 9


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2.7 Auto Load Cases

The reason for each of the auto load cases is explained in Step 7.

Figure 2-8 Auto Load Cases

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Step 3Dead Load Analysis and Cracked Section Properties

As shown in the schematic included in the Foreword, the third step begins with

the dead load analysis of the entire bridge model. The results of the dead load

analysis are then used to verify the analytical model. For concrete bent columns,

these results are used for the determination of the cracked section properties that

are then applied to the bent columns as frame section property modifiers. The re-

duced stiffnesses of the concrete bent columns will affect the response spectrum

and pushover analyses. The frame section property modifiers are defined sepa-

rately for each of the concrete bent and abutment columns as a named propertyset. The user can use the Section Designer program to observe the moment-

curvatures and I,crackedproperties for the various cross-sections (see also Step 5).

The calculation of the cracked section properties will be skipped for the steel

bent columns and thus no frame section property modifiers will be generated and

assigned to the steel bent columns.

Auto load patterns and auto load cases are produced by the program. The load

case, which has the default name, , is automatically developed by

CSiBridge as a single stage construction load case and is used to apply the

cracked section property modifiers to the columns. Figure 3-1 shows the Load

Case Data form for the GRAV load case (Analysis > Load Cases >

Type > All > New > Highlight GRAV > Modify/Show Load Case).

The auto load cases are not modifiable.

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Figure 3-1 Auto Stage Construction Load Case used to apply

Cracked Section Property Modifiers

As an option, the user can overwrite the cracked section property determined by

the program and instead, apply a user defined value. See Step 2 for the user op-

tions available in the Seismic Design Request.

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Step 4Response Spectrum and Demand Displacements

4.1 Overview

The seismic response of the entire bridge structure is analyzed by CSiBridge us-

ing the response spectrum function defined in Step 2. The number of modes used

by CSiBridge is automated and depends on the number of bridge spans. The user

should check the total mass participation to ensure that an adequate number of

modes are included in the modal analysis. The additional number of Modes can

be added to the auto-generated modal load case as the item 15 in Figure 2-5. Theresponse spectrum displacements are used by CSiBridge as the displacement de-

mands as defined in Section 4.4 of the AASHTO Seismic Guide Specification.

4.2 Response Spectrum Load Cases

For the case that no response spectrum function is assigned to the vertical direc-

tion loading in the seismic design request form shown in Figure 2-4, three re-

sponse spectrum load cases are automatically produced by CSiBridge:

RS_X, RS_Y and RS_XY. With a response

spectrum function assigned to the vertical direction loading, an additional re-

sponse spectrum load case RS_Z is automatically produced andRS_XYwill be RS_XYZ. For the case of no vertical load-

ing, the first two response spectrum load cases apply the dynamic loads along the

U1 and U2 directions. The U1 direction is defined as the longitudinal loading di-

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rection that is chosen to be from the start abutment to the end abutment, both

points located on the reference line of the bridge object. If the user wants to applya response spectrum load along a different axis, a directional overwrite is availa-

ble in the Substructure Seismic Design Request Parameters form (see Chapter 2).

Figure 4-1 U1 Direction Response Spectrum Load Case form

The third response spectrum load case uses a Directional Combination option of

ABS, with an ABS scale factor of 0.3. This response spectrum load case will

satisfy the AASHTO Seismic Guide Specification, Section 4.4, which requires

the response spectrum loads to be combined using the 100/30 percent rule in each

of the major directions. The single response spectrum load case,

RS_XY, envelopes the maximum response spectrum results for each

of the combinations 100/30 and 30/100. The Load Case Data form for the re-sponse spectrum load case RS_XY is shown in Figure 4-2.

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Step 4 - Response Spectrum and Demand Displacements

The modal damping coefficient is set to 5 percent, but this value can be modified

as necessary by the user in the Substructure Seismic Design Request Parametersform (Chapter 2).

Figure 4-2 ABS Response Spectrum Load Case form

To illustrate the ABS directional combination feature, the following BENT1 dis-

placements are summarized for example model MO_1C:

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Figure 4-3 BENT1 Displacements for the three

Auto-Defined Response Spectrum Load cases

Figure 4-4 Modal Load Case Definition

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Step 4 - Response Spectrum and Demand Displacements

4.3 Response Spectrum Results

Upon completion of the response spectra analysis, the displacements are tabulat-

ed for each bent. The displacements are calculated using Generalized Displace-

ments to account for the average cap beam displacements and the relative dis-

placement between the cap beam and foundation. The displacements for the ABS

response spectrum load case also are tabulated for each of the bearing active de-

grees of freedom. These can be viewed using the Home > Display > Show Ta-

blescommand to display the Choose Tables for Display form. Select the Design

Results for Bridge Seismic, Support Bearing Demands-Deformations item. These

displacements also can be displayed and animated on screen or read from the

quick report created using the Design/Rating > Seismic Design > Reportcom-

mand.

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Step 5Determine Plastic Hinge Properties and Assignments

5.1 Overview

For bridge structures having a Seismic Design Category (SDC) D the AASH-

TO Seismic Guide Specification requires that the displacement capacity be de-

termined using a nonlinear pushover analysis. This requires that the column

plastic hinge lengths and plastic hinge properties be determined for each col-

umn that participates as part of the Earthquake Resisting System (ERS).

In this step, the methodologies used to calculate the plastic hinge lengths and

properties will be explained. After the hinge properties have been determined,

the plastic hinges are assigned to the ERS columns. The automation of the plas-

tic hinge assignments will also be explained in this step.

5.2 Plastic Hinge Lengths

The plastic hinge lengths for the concrete bent columns used in the Seismic

Design Request is determined for the AASHTO Seismic Guide Specification,

Section 4.11.6, as follows:

For reinforced concrete columns framing into a footing, an integral bent cap,

and oversized shaft, cased shaft, the plastic hinge length, LPin inches, may be

determined as:

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0 08 0 15P ye blL . L . f d= + ,

where

L = length of column from point of maximum moment to the point of

moment contraflexure (in.),

yef = the effective yield strength of the longitudinal reinforcing (ksi), and

bld = the diameter of the longitudinal reinforcing (in.).

The hinge length is compared to the value for the minimum hinge length, de-

scribed as 0 3P ye blL . f d= , and the larger value is used.

Note that, the L values for concrete columns are specified in the Bridge Bent

Column Data Form (Section 1.6). Here a relative height, RH, is specified from

the bottom of the column to the point of contraflexure, separately for longitu-

dinal and transverse bending. Legal values are 1 RH 2, where RH = 0 is

the bottom of the clear height of the column and RH = 1 is the top. For the bot-

tom hinge:

= |RH|, subject to 2

For the top hinge:

= |RH 1|, subject to 2

This can be summarized in the following table:

Below Column Clear Height Above

RH -1.00 -0.75 -0.50 0.00 0.25 0.50 0.75 1.00 1.50 1.75 2.00

Bottom Hinge

L/H 1.00 0.75 0.50 0.50 0.50 0.50 0.75 1.00 1.00 1.00 1.00

Top Hinge

L/H 1.00 1.00 1.00 1.00 0.75 0.50 0.50 0.50 0.50 0.75 1.00

5 - 2 Plastic Hinge Lengths


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Step 5 - Determine Plastic Hinge Properties and Ass ignments

For the steel columns, based on the Section 4.11.8 in AASHTO Seismic Guide

Specification, the plastic hinge region is determined as the maximum of 1/8 ofthe clear height of a steel column or 1.5 times the gross cross-sectional dimen-

sion in the direction of bending.

Calculated hinge lengths may be different for bending in the longitudinal or

transverse direction of the bents. However, each hinge can only have a single

hinge length in the model. Set the Hinge Length Option in the Bridge Seismic

Design Preferences Form as described in Section 2.3 to specify whether to use

the Longitudinal, Transverse, Longest, Shortest, or Average hinge length for a

given instance of the model. After performing a bridge seismic design with one

of these options, you can re-run the design with a different choice to see the ef-

fect.

After the hinge lengths and properties have been determined, the hinges are

placed on the bent columns at each end of the column at distances from each

end equal to 1/2 the hinge length, as shown below in Figure 5-1.

Figure 5-1 Hinge Locations

Plastic Hinge Lengths 5 - 3


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Figure 5-2 Hinge Locations

5.3 Nonlinear Hinge Properties

For a steel column, the CSiBridge Automated Seismic Design Request uses a

hinge property that is consistent with the FEMA 356. For a concrete column,

the Automated Seismic Design Request uses a hinge property that is consistent

with the AASHTO/CALTRANS idealized bilinear moment-curvature diagram,

as shown in Figure 5-3 (click the Display menu > Show Moment Curvature

Cure command on the Section Designer form). From the moment curvature

shown, the yield and plastic moments along with the I,crackedproperties can be

observed for a specific axial load, P. Note that this form is made available toallow users to better understand the effects of axial loads and fiber mesh lay-

outs on the frame member properties. The axial load values input on this form

are not used in the analysis and design of a model.

5 - 4 Nonlinear Hinge Properti es


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Figure 5-3 Moment Curvature Diagram

Typically, the axial loads in the bent columns change as the bent is pushed over

due to the overturning effects. Therefore, the yield and plastic moments will

change depending on the amount of axial load present in a particular column at

a particular pushover step. These effects are captured in the nonlinear hinge re-

sponses whenever P-M or P-M-M hinges are specified. For this reason, the Au-

tomated Seismic Design procedure assigns coupled P-M-M hinges to the bent

columns. The default settings are shown in Figure 5-4 (select the frame(s) to be

assigned a hinge, click Advanced > Assign > Frames > Hinges, select Auto,

click the Modify/Show Auto Hinge Assignments Databutton). The length ofthe plastic hinge also is calculated by CSiBridge when using the Automated

Seismic Design procedure.

Nonlinear Hinge Properti es 5 - 5


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Figure 5-4 Auto Hinge Assignment Data

Figure 5-5 Sample Hinge Data form

5 - 6 Nonlinear Hinge Properti es


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Upon completion of the Pushover Analysis, the Hinge Results can be traced.

This feature is explained in detail in Step 6.

5.4 Nonlinear Material Property Defini tions

The ductile behavior of a plastic hinge is significantly affected by the nonlinear

material property used to define the frame member receiving the hinges. The

material nonlinear properties must be defined using the Advanced Nonlinear

Material Data forms.

5.4.1 Nonlinear Material Property Definitions for Concrete

For concrete, the nonlinear material property data form appears as shown in

Figure 5-6 (Components > Type > Material Properties > Expand arrow >check the Show Advanced Properties check box > Add New Material > set

Material Type to Concrete > Modify/Show Material Properties button >

Nonlinear Material Databutton):

Figure 5-6 Nonlinear Material Data form for Concrete

Nonlinear Material Property Definitions 5 - 7


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Figure 5-7 Nonlinear Stress-Strain curves for Confined and Unconfined Concrete

Figure 5-8 Concrete Model - Mander Confined

5 - 8 Nonlinear Material Property Definitions


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5.4.2 Nonlinear Material Property Definitions for Steel

Similarly, for steel, the nonlinear material data form appears as show in Figure

5-9. The user can specify the parametric strain data, which includes the values

for the strain at the onset of hardening, ultimate strain capacity, and the final

slope of the stress-strain diagram.

Figure 5-9 Nonlinear Material Data form for steel

Nonlinear Material Property Definitions 5 - 9


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Figure 5-10 Nonlinear Stress-Strain Plot for steel

5.5 Plastic Hinge Options

Concrete column section properties can be defined for use in two ways such

that hinge properties can be assigned to them during the Automated Seismic

Design procedure. One method is to use the Section Designer and the other is

to define a rectangle or circle using the Components > Type > Frame Prop-erties > Newcommand and define a rectangular or circular shape. Internally,

CSiBridge will convert the rectangular or circular shapes into Section Designer

sections for the purposes of determining the hinge and cracked section proper-

ties. The advantage of using the Section Designer feature is that the user can

choose to have the hinge defined using fibers. This option is applied when the

user activates the Design menu > Fiber Layoutcommand from within Section

Designer and sets the Fiber Application to Calculate Moment Curvature Using

Fibers, as shown in the following form.

5 - 10 Plastic Hinge Options


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Figure 5-11 Plastic Hinge Fiber option

The fiber mesh also can be specified in this form. The mesh can be rectangular

or cylindrical depending on the shape of the column. Another advantage of us-

ing the Section Designer feature is that complex sections, similar to the one be-

low, can be handled.

Figure 5-12 Section Designer options

Plastic Hinge Option s 5 - 11


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Step 6Capacity Displacement Analysis

This step describes the automated procedure that CSiBridge uses to determine

the bridge seismic capacity displacements. The method used varies depending

on the Seismic Design Category (SDC) of a particular bridge. A flowchart that

describes when an implicit or pushover analysis is used to determine the capac-

ity displacements is shown in Figure 6-1:

Figure 6-1 Rectangular Beam Design

Displacement Capacities for SDC B and C 6 - 1


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Figure 6-2 Design Requirements for SDC

A B C D

Identification ERS Recommended Required Required

Demand Analysis Required Required Required

Implicit Capacity Required Required Required

Push Over Capaci-

ty

May be required

Support Width Re-

quired

Required Required Required

Detailing Ductility SDC B SDB C SDB D

Capacity Protec-tion

Recommended Required Required

Liquefaction Recommended Required Required

The user can overwrite the program determined SDC to enforce that a pusho-

ver analysis is used to determine the displacement capacity. The differences

between the implicit and pushover approaches are described in the following

sections.

6.1 Displacement Capacit ies for SDC B and C

For structures having reinforced concrete columns, the displacement capacities

for SDC B and C are found using the following equations. The AASHTO

Seismic Guide Specification equations are also noted.

For SDC B:

( )0.12 1.27 ln( ) 0 .32 0.12LC o oH x H = (4.8.1-1)

For SDC C:

( )0.12 2.32 ln( ) 1.22 0.12LC o oH x H = (4.8.1-2)

in which

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Step 6 - Capacity Displacement Analysis

o

o

Bx

H

= (4.8.1-3)

where,

Ho = Clear height of the column (ft)

B0 = Column diameter or width parallel to the direction of displace-

ment under consideration (ft)

= Factor for the column end restraint conditions

CSiBridge uses the relative heights RH Long and RH Trans of Seismic

Hinge Data specified in Bent Column Data form, shown in Figure 1-12, to de-

termine the factor :

RH 0 or RH 1: = 1.0

0 < RH 0.5: = 1 / (1 RH)

0.5 < RH < 1.0: = 1 / RH

For the bent columns that are not of Type 1 reinforced concrete, CSiBridge us-

es the same equations to determine the capacity. In this case, users may over-

write the SDC as D for a better solution, in which the capacity is determined

based on the pushover analysis results.

6.2 Displacement Capacit ies for SDC D

When the Seismic Design Category for a bridge structure is determined to be

SDC D or the user overwrites the SDC as D, CSiBridge uses a pushover analy-

sis in accordance with the AASHTO Seismic Guide Specification, Section

4.8.2 to determine the displacement capacities. This requires that CSiBridge

actually perform several pushover analyses, depending on the number of bents

that are part of the Earthquake Resisting System (ERS). Each bent is analyzed

in a transverse and longitudinal direction local to the specific bent. For the ex-

ample bridge used in this manual, there are three spans with two interior bents.Bents can be used as abutment supports so it is possible to have additional

bents participating as part of the ERS. But, for the example bridge, there are

two interior bents. This means that a total of four pushover analyses are needed

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to determine the displacements capacities for each bent in each of the trans-

verse and longitudinal directions.

To perform multiple pushover analyses on a single bridge model, CSiBridge

uses several nonlinear single-staged construction load cases.

For the example bridge, the four separate pushover load cases are named as fol-

lows:

PO_TR1

PO_LG1

PO_TR2

PO_LG2

The SDReq1 is the name provided by the user to identify a particular seismic

design request.

TR denotes Transverse and LG denotes Longitudinal.

The is added to the beginning of each auto load case name

to distinguish the load cases that are automatically provided by CSiBridge from

user defined load cases.

Figure 6-3 shows the nonlinear single-staged construction load case for theBENT1 transverse direction.

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Figure 6-3 BENT1 Transverse Pushover Load Case

The user can not modify this load case because it is defined automatically. The

PO_TR1 load case starts from the end of the initial nonlinear loadcase named, bGRAV.

The bGRAVload case is shown in Figure 6-4 and is needed to iso-

late the bents from the rest of the bridge model and to apply the cracked section

property modifiers as well as apply the dead load.

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Figure 6-4 BENT1 Application of Property Modifiers andDead Loads to BENT1

The load pattern used to apply the lateral pushover loads or displacements to

BENT1 is named, PO_TR1. A 3D view of the PO_TR1

loads is shown in Figure 6-5. The magnitudes of these loads are based on the

reactions from the superstructure.

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Figure 6-5 BENT1 Pushover Load Pattern for the Transverse Direction

6.3 Pushover Results

After the pushover analyses have run, the capacity displacements are automati-

cally identified as the maximum displacement of the pushover curve just before

strength loss (negative slope on the pushover curve) for each of the pushover

runs.

The pushover results can be viewed using the Home > Display > More >

Show Static Pushover Curvecommand. An example output is shown in Fig-

ure 6-6 for the BENT1 transverse and longitudinal pushover load cases.

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Figure 6-6 Display of BENT1 Pushover Curves

6 - 8 Pushover Results


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Step 7Demand/Capacity Ratios

After the demand displacement (Step 4) and displacement capacity (Step 6)

analyses have been completed, CSiBridge computes the ratio of the De-

mand/Capacity displacements and reports these values in the Seismic Design

Report. The table of D/C ratios can be viewed using the Home > Display >

Show Tablescommand, and then selecting Design Data > Bridge > Seismic

Design data > Table: Bridge Seismic Design 01 Bent D-C. The subject ta-

ble will appear similar to the table shown in Figure 7-1:

Figure 7-1 D/C Displacment Ratios

In the table shown, all four D/C ratios are reported, namely, the transverse and

longitudinal directions for each bent (the example model has two bents). Note

that the Generalized Displacement name also is reported. Generalized dis-placements are used to average the top of bent displacements and to determine

the relative displacements between the bent cap beam and the foundation. The

generalized displacement definition is automatically defined by CSiBridge and

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can be viewed using the Advanced > Define > Generalized Displacements

command.

7 - 2 Demand/Capacity Ratios


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Step 8Review Output and Create Report

This step describes the two methods of viewing the seismic design results. The

first way to review the results is to use the Home > Display > Show Tables

command. The second way is to create a report using the Orb > Report > Cre-

ate Reportcommand.

The entire list of output tables for the Bridge Seismic Design includes the follow-

ing:

The seven Bridge Seismic Design tables are described in the sections that follow.

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8.1 Design 01 D-C Ratios

The Demand/Capacity ratios are summarized for each bent in each direction.

Values less than 1.0 indicate that an adequate capacity exists for a given bent and

direction for the ground motion hazard used in the seismic design request. Values

greater than 1.0 indicate an overstress condition.

8.2 Design 02 Bent Column Force Demand

A summary of the bent column seismic demand forces are tabulated.

8.3 Design 03 Bent Column Idealized Moment Capacity

The idealized column plastic moments are calculated and tabulated. The axial

load P represents the demand axial load. The idealized plastics moments are de-termined using the associated axial load value, P. This table is for concrete col-

umns only.

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Step 8 - Review Outpu t and Create Report

8.4 Design 04 Bent Column Cracked Section Properties

A summary of the cracked property modifiers that get applied to each of the bent

columns is tabulated. This table is for concrete columns only.

8.5 Design 05 Support Bearing Demand Forces

The forces in the bearing due to the seismic loads are presented in the following

table. All bearings at the abutments and bents that are found to resist seismic

forces are included in the subject table.

Design 04 Bent Column Cracked Section Properties 8 - 3


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8.6 Design 06 Support Bearing Demand Deformations

The deformations for all bearings at the abutments and bents that resist seismic

loads are tabulated and reported.

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8.7 Design 07 Support Length Demands

The support lengths are calculated from the bearing displacements and represent

the amount of displacement normal to a specific bent or abutment.

8.8 Create Report

A single command can be used to create a report using the Design menu >

Bridge Design > Create Seismic Design Reportcommand. Several representa-

tive pages of the report that can be created using the previously noted report re-

quest are included in the following pages. Theses have been excerpted from a 30

page summary report that CSiBridge writes as a Microsoft Word document.

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8 - 6 Create Report


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Create Report 8 - 7


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8 - 8 Create Report


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Chapter 9Caltrans Fault Crossing Seismic Bridge Design

9.1 Introduction

Automated seismic design for bridges crossing seismic faults is available in

CSiBridge. The methodology is based on the following references:

Rakesh K. Goel and Anil K. Chopra, Analysis of Ordinary Bridges Crossing

Fault Rupture Zones, Research Conducted for the California Department of

Transportation Contract No. 59A0435 Earthquake Engineering Research

Center University of California at Berkeley February 2008 Report No.

UCB/EERC-2008/01.

Rakesh K. Goel and Bing Qu, Analysis of Bridges Crossing Fault-Rupture

Zones: Step-By-Step Procedure for SAP Implementation.

Caltrans, Ordinary Bridges that Cross Faults, Bridge Design Aids 14-6.

Caltrans, Analysis of Ordinary Bridges that Cross Faults, Memo to

Designers 20-8, January 2013.

In summary, this seismic design procedure considers the rupture of a seismicfault that crosses a bridge structure such that the supports are subject to

significant ground displacements that are different for the supports on either side

of the fault.

Introduction 9 - 1


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CSiBridge - Caltrans Fault-Crossing Seismic Bri dge Design

The displacement demand on the structure is calculated by performing a

nonlinear static analysis for the full displacement of the fault and adding to thatthe results of a generalized response-spectrum analysis for the inertial forces due

to this non-uniform ground motion.As an aside, by using this method, a fault

which occurs near the bridge but does not cross the bridge will produce the same

demand as using standard response-spectrum analysis with uniform ground

acceleration.

The displacement capacity of the bents is determined by performing nonlinear

static pushover analysis of the isolated bents in the longitudinal and transverse

directions. This is the same procedure used in CSiBridge for automated

AASHTO LRFD seismic bridge design.

Finally, a demand/capacity ratio is computed for each bent in the longitudinal

and transverse direction and reported in a table. A report can be generated that

provides a description of the bridge structure, and the seismic demands and

capacities for one or more fault-crossing cases.

Caltrans Fault-Crossing Seismic Bridge Design follows the same general

approach as AASHTO LRFD bridge seismic design, differing primarily in the

specification of the Seismic Design Request. For this reason, you should be

familiar with details of the AASHTO procedure described in the earlier in this

manual before proceeding further.

9.2

Fault-Crossing Response-Spectrum Loading

Step 4 of the AASHTO Seismic Bridge Design procedure is replaced by

specifying the expected location, magnitude, and direction of fault displacement

that will occur at the bridge site, as well as the associated response-spectrum

loading that is associated with this motion. The fault displacement is specified

directly as part of the Seismic Design Request, as described later. First the

response-spectrum functions must be specified.

One or more response-spectrum functions should be defined that characterize the

dynamic response to the ground motion. The response-spectrum functions shouldbe chosen as appropriate for near-fault behavior. Different functions may be

chosen for the directions parallel to the fault, normal to the fault, and vertical. It

is important to understand how these functions will be used.

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Two types of response-spectrum functions may be applied in each direction:

A standard response-spectrum function representing uniform ground

acceleration

A fault-displacement response-spectrum function representing the vibration

due to non-uniform ground displacement

Both types of response-spectrum loading may be applied in a given direction,

although usually only one or the other would be.

Standard response-spectrum functions are applied to uniform ground motion

represented as a translational acceleration in a single direction. For purposes of

comparison with fault-rupture function, this loading can be viewed as a rigid-

body translation of the entire structure an arbitrary distance u0, multiplied by the

mass at each joint, and multiplied by g/u0, to create a force load acting on the

structure. Here gis the gravitational constant in the same units as the

displacement u0. Note that the units of the load are Length x Mass x

Length/Time^2 / Length = Force.

For ground motion caused by the rupture of a fault at a bridge structure, the

loading is calculated by first determining the deflected shape of the structure due

to linear static application of the ground motion. For example, consider a fault

that crosses the structure between two bents, and experiences a slip of 2u0in the

direction parallel to the fault. The supports on one side of the fault move a

distance of u0 transversely to the left, and on the other side of the fault by a

distance of u0transversely to the right. Each support moves a distance of u0from

its initial position.

The deflected shape of the structure due to this loading reflects the ground

motion, but is moderated by the flexibility of the foundation, bents, bearings, and

superstructure. This deflected shape is called the quasi-static deflection, and is

used to calculate the seismic load that will be used with the response-spectrum

function. An example is shown below for a three-span structure with a transverse

fault slip occurring within the first span.

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The response-spectrum load is calculated as a set of forces acting at each joint

that are given by the product of the deflections at the joint (in any direction),

multiplied by the mass at that joint, and multiplied by g/u0. Note that this loading

may have components in any direction. For example, fault motion transverse to

the bridge may cause forces in the vertical and longitudinal directions as well as

the transverse direction.

While the actual fault motion may be a net slip of 2u0, the response-spectrum

loading is applied to a displaced shape caused by motion of +1 and -1 unit

displacements on either side of the fault, due to the use of the scale factor g/u0.

This follows the method of Goel, Chopra, and Qu. Note that this approach has

the benefit that if the fault does not cross the bridge, the entire structure moves as

a rigid body for a distance u0, and the loading is then identical to the case of

uniform acceleration.

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9.3 Defining Fault-Crossing Seismic Design Requests

Define one or more Seismic Design Requests, using the command shown below.

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Click Add New Request, which brings up the following form:

Choose the bridge object to which this request will apply, and select the Check

Type to be Caltrans Fault Crossing, which changes the form as shown below.

Enter the loading data as follows:

Choose Planar fault definition. General Displacement Loading will be

described later in this document.

Enter the station where the fault crosses the layout line that was used to

define the bridge object.

Enter the orientation of the fault. Default is perpendicular to the layout line

at the crossing station. You may enter a bearing, such as N30E or

S251933.45W, or a skew angle relative to Default, such as 30 or -45.

For each direction of displacement loading to be considered simultaneouslyin this Design Request, enter the displacement magnitude and the

corresponding response-spectrum function.

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o See Step 3 above for more information about response-spectrum

functions.o The displacement is the actual movement, u0, of each support on

either side of the fault. In other words, it is half the total slip of the

fault. The default is 0.5m.

o For vertical motion on an inclined fault, enter the components in the

vertical and horizontal-normal directions.

For each direction of uniform acceleration loading (no slip), enter the

corresponding response-spectrum function.



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The displacement demand will be calculated by applying the specified fault

motions as follows:

(1) The specified fault displacements will be applied together in a nonlinear-

static displacement load case.

(2) The fault-displacement response-spectrum loads, if any, will be applied

together in a response-spectrum load case.

(3) The uniform-acceleration response-spectrum loads, if any, will be applied

together in a separate response-spectrum load case.

(4) The two response-spectrum load cases will be combined in a load

combination.

(5) The final demand result is obtained by combining the displacement load case

(1) with the response-spectrum load combination (4).

Note that if the specified displacement is too large, the nonlinear-static load case

will not converge and the seismic design request will not complete. This

indicates that the structure does not have sufficient ductility to resist the specified

loading, and further calculation not warranted. For many structures, the amount

of displacement that can be accommodated is quite small. If you cannot get

convergence with the desired value, try a smaller value so that you can determine

the capacity of the structure.

By default, the different directions of loading in the response-spectrum load cases

are combined by absolute sum. This may be changed to the SRSS or CQC3method in the design request parameters, see below. This same method will be

used to combine the two response-spectrum load cases together in the response-

spectrum load combination. If CQC3 is chosen, it actually applies only to the

uniform-acceleration load case (3), and SRSS will be used for the fault-

displacement load case (2) and for the response-spectrum load combination (4).

In any case, the final combination (5) of the nonlinear-static displacement load

case and the response-spectrum load combination will always use absolute sum.

Additional detail is provided in topic Automatic Load Cases and Combinations

below.

Additional parameters may be specified for this Design Request by clicking onthe Modify/Showbutton, bringing up the form as shown below. These

parameters are optional and do not usually need to be changed.

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Most of these parameters are identical to the parameters for the AASHTO Seismic

Design Request, and are described in the CSiBridge Seismic Analysis and Design

manual. A few of these parameters of particular interest for fault-crossing analysis

are described here:

The Seismic Design Category (1) has the same meaning as it does for

AASHTO seismic design, but it is not automatically determined from the

response-spectrum functions. Due to the severity of fault-crossing motion,

category D is probably most appropriate, meaning that the capacity will be

determined by nonlinear static pushover analysis. However, if you just want

to study the seismic demand due to fault crossing and dont care about

capacity, setting the category to be less than D will speed up the analysis. Type of Modes (13) is fixed to be Ritz. This is superior to using Eigen modes

for ground displacement loading, which tends to excite higher frequency

modes than acceleration loading.



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Directional Combination (17) may be Absolute, SRSS, or CQC3. Goel and

Qu recommend using Absolute for fault-crossing analysis.

9.4 Running Fault-Crossing Seismic Design RequestsAfter defining one or more Seismic Design Requests, these can be run using the

command shown below:

Using the buttons on the right, select the Design Requests that you would like to

run, and set their action to Design. Then click the Design Nowbutton.

We recommend unlocking the model before using this command, which will

delete all prior results. You may wish to save the model under a new name before

doing this. We also recommend simultaneously running all design requests of

interest at the same time, although this is not required.

After clicking the Design Nowbutton, CSiBridge will create and run multiple

load cases for each Design Request to calculate the demands and capacities. The

results of some cases are used to create additional cases, so the analyses are

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performed in a sequence of runs. These load cases are described later in this

document.

You will know when the seismic design is done when a table of final results is

presented, or a message is produced in case the Design Requests were unable to

complete for any reason.

9.5 Creating a Seismic Design ReportUse the command below to create a Bridge Seismic Design Report that includes

a description of the bridge object, the seismic loading, and the demand and

capacity results.

This report will be written to an .RTF file that can be opened in Microsoft Word

for viewing, editing, and printing.

The same data can be viewed within CSiBridge using the command Home >

Display > Show Tables.

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Results are presented for both displacement and force demands, as well as

capacities. While the displacement demands may be meaningful for design

purposes, the force demands should be used only for reference purposes, since

they superpose the linear respon

bridge seismic design csi sap 2000

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