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ASCE7-05&-10 SEISMIC WIZARD

Monday 26 November 2012 10:02

ASCE7-05 Seismic Wizard Handbook page 2

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Disclaimer page 3

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Monday 26 November 2012 10:02

page 4 Table of Contents

ASCE7-05&-10 Seismic Wizard Handbook

Chapter 1 Introduction . . . . . . . . . . . . . . . 5

Chapter 2 Scope . . . . . . . . . . . . . . . . . 6Engineering Overview . . . . . . . . . . . . . . 6

Chapter 3 Limitations . . . . . . . . . . . . . . . . 7

Chapter 4 Theory and Assumptions . . . . . . . . . . . . . 9Definitions . . . . . . . . . . . . . . . . 9Seismic Assessment . . . . . . . . . . . . . . 9

11.6 - Seismic Design Category . . . . . . . . . . . . 9Seismic Assessment for SDC = A only . . . . . . . . . . . 9

11.7 - Base Shear Combination and Distribution of Force . . . . . . . . 9The Base Seismic Load Cases . . . . . . . . . . . . 10

Seismic Assessment for SDC = B-F . . . . . . . . . . . . 1012.8.2.1 - Approximate Fundamental Period . . . . . . . . . . 1012.2.1 - Seismic Force Resisting System . . . . . . . . . . . 1112.3.2 - Structural Irregularities . . . . . . . . . . . . 1112.7.2 - Effective Seismic Weight . . . . . . . . . . . . 1212.8.1 - Seismic Base Shear . . . . . . . . . . . . . 1212.8.3 - Vertical Distribution of Seismic Forces . . . . . . . . . . 1212.8.4 - Horizontal Distribution of Seismic Forces . . . . . . . . . 1212.3.4 - Redundancy Factor . . . . . . . . . . . . . 13The Base Seismic Load Cases . . . . . . . . . . . . 13

Chapter 5 References . . . . . . . . . . . . . . . . 14

Chapter 1 : Introduction ASCE7-05&-10 Seismic Wizard page 5ASCE7-05&-10 Seismic Wizard Handbook

Chapter 1 Introduction

This handbook describes the Fastrak Seismic Wizard, a component of Fastrak Building Designer which creates seismic load cases. These can then be combined together with other load cases as required and designed for within Fastrak Building Designer.

Fastrak Building Designer includes a seismic analysis and design capability which allows you to load and design a model in accordance with the ASCE7-05&-10 /IBC 2006 method of seismic assessment.

This very powerful tool has been developed to aid engineers in the seismic assessment of structures. You will find that the determination of Seismic Design Category (SDC), response modification coefficient (R), seismic base shear (V), along with other associated factors is rigorous but the final seismic load cases adopted are your responsibility.

Unless explicitly noted otherwise, all clauses, figures and tables referred to in this document are from ASCE 7-05&-10.

ASCE7-05&-10 Seismic Wizard page 6 Chapter 2 : ScopeChapter 2 Scope

Fastrak Building Designer, in the first instance, is targeted to address low and medium rise steel and composite buildings in low and medium seismic areas. To that end, seismic forces are determined using the Equivalent Lateral Force (ELF) method.

In the current version of the software, the Modal Analysis procedure for seismic analysis is beyond scope.

It should be noted that the software in assessing the ELF method does not assess building height (Table 12.2.1). The assessment of applicability of ELF for building height is the users responsibility.

Engineering Overview The basic engineering approach is to determine the Seismic Design Category, then the Seismic Base Shear on the structure. These are dependent upon many factors (including building use, location, site conditions, fundamental period and loading).

The Seismic Base Shear is then distributed to each level in the structure based upon height above the base. The net result is an inverted triangle of equivalent static lateral loads up the structure. Although these are applied as a series of loads on each floor, for visualization purposes they are represented graphically as a single point load and moment acting through the center of mass of that level. These equivalent static lateral forces are applied to the model during analysis.

It is these sets of forces considered independently in the X and Y directions with eccentricity that create an appropriate a number of seismic load cases to be used in combination with other loads.

The ELF approach approximates to the structure 1st mode of vibration in X and Y. If the structure is such that its first mode is significantly different then it has vertical or plan irregularity and the application of the equivalent lateral force method is restricted. The irregularities are user defined and not automatically determined by Fastrak Building Designer.

In order to accommodate potential displacement of mass in rigid diaphragms, an eccentricity from the center of mass is applied to the story shear to consider potential torsional effects on the structure.

In addition, an assessment of seismic story drift is made from floor to floor and compared to permitted limits. During this process, assessment is made for the limits of the stability coefficient, .

< 0.1 - First-order (Elastic) analysis is acceptable. < 0.25 - P-delta effects must be taken into account, hence a second-order analysis is

required. > 0.25 - structure is unacceptable for seismic design using the ELF method.

Note In Fastrak Building Designer, when < 0.1, the program still undertakes a second-order analysis.

Chapter 3 : Limitations ASCE7-05&-10 Seismic Wizard page 7Chapter 3 Limitations

It is essential that you read and fully appreciate the following limitations in the software.

Documented limitations in the application of ASCE7-05&-10: 11.6. If S1 < 0.75, you have a choice as to whether to use the alternative method of using

only table 11.6.1 to determine the Seismic Design Category. In this case, it is up to you to verify that all the conditions of 11.6 are met.

11.7.3, 4 & 5. For structures of seismic design category A, Fastrak Building Designer does not consider the load path connections, connection to supports or the anchorage of concrete or masonry walls.

12.2.3.1 and 12.2.3.2. In the seismic wizard, Fastrak Building Designer permits you to select a single type of seismic force resisting system in each direction X and Y.

Should a structure have more than one type of system in a given direction, clause 12.2.3.2 advises you on the values of R and Cd that should be used. You are able to adjust the values of R and Cd manually to accommodate this situation. You should also verify that the structure complies with the other conditions of this clause.

Should a structure have more than one type of system on different levels again you need to make the relevant judgements as to whether and how this can be modelled correctly within Fastrak Building Designer.

Table 12.2-1. When using Table 12.2-1, Fastrak Building Designer does not do any verification of structural system or make any assessment of building height limits.

12.2.5.1. For dual systems there is a requirement to verify that moment frames carry at least 25% of the design seismic forces. Fastrak Building Designer does not perform this check and thus it is up to you to perform this verification.

12.2.5.2. Should you define a cantilevered column system in Fastrak Building Designer, it is up to you to ensure that the provisions of this clause are complied with.

12.2.5.3. Inverted pendulum structures are beyond scope of the current version. 12.2.5.4. This clause is not considered by Fastrak Building Designer, assessment of building

height is a user decision. 12.2.5.5. Special Moment Frames are not checked by Fastrak Building Designer for this

clause, you will need to make a separate assessment of this. 12.2.5.6. Ordinary and Intermediate Moment Frames are not checked by Fastrak Building

Designer for this clause, you will need to make a separate assessment of this. 12.2.5.7. Ordinary and Intermediate Moment Frames are not checked by Fastrak Building

Designer for this clause, you will need to make a separate assessment of this. 12.2.5.8. Ordinary and Intermediate Moment Frames are not checked by Fastrak Building

Designer for this clause, you will need to make a separate assessment of this. 12.2.5.9. Other Steel Intermediate Moment Frames are not checked by Fastrak Building

Designer for this clause, you will need to make a separate assessment of this. 12.2.5.10. Shear wall-frame interactive systems are beyond scope of the current version. 12.3.1. Fastrak Building Designer only models single or multiple rigid diaphragms or no

diaphragms. Fastrak Building Designer does not model flexible diaphragms directly.

ASCE7-05&-10 Seismic Wizard page 8 Chapter 3 : Limitations 12.3.1.2 says that rigid diaphragms are not permitted with any horizontal irregularity. Within Fastrak Building Designer, a structure with rigid diaphragms and horizontal irregularities will be permitted however, it is your responsibility to ensure that this combination is appropriate for the building being designed.

12.3.2 and Tables 12.3-1 & -2. In Fastrak Building Designer, you define all horizontal and vertical irregularities. Fastrak Building Designer does not verify any of them. The defined irregularities are then used in the consideration of whether a frame is suitable or not for the Equivalent Lateral Force Procedure. Since the software makes no consideration of any of the irregularities, they are set at your responsibility.

12.3.3.3. Fastrak Building Designer does not take any special consideration of elements supporting discontinuous walls or frames (horizontal irregularity 4 and/or vertical irregularity 4). Should either of these irregularities be set by you, Fastrak Building Designer will set the Equivalent Lateral Force method to be invalid and thus seismic load cases will not be created.

12.3.3.4. For structures in seismic design category D-F, certain irregularities increase the connection design forces in diaphragms and collectors and in the collector design itself. The design of all these items is beyond the scope of the current version of Fastrak Building Designer.

12.4.3 and Tables 12.3.1 & 2. All structures with horizontal irregularity 4 or vertical irregularity 4 are beyond the scope of the current version of Fastrak Building Designer. Therefore, no account is taken of situations where the over strength factor is required in combinations.

12.4.4. Fastrak Building Designer does not automatically handle the minimum uplift on cantilevers. It is up to you to apply the relevant loads in the model and to include them in the relevant combination for design of the cantilever.

12.7.3b. The overall story drift calculated by Fastrak Building Designer does not take account of any contribution from panel zone deformations.

12.8.1.3. This clause has not been implemented. 12.8.2.1. The alternative method for calculating Ta in structures not exceeding 12 stories

with concrete or steel moment resisting frames has not been implemented. 12.8.3. For a structure with bases at a number of levels, the heights used in equation

12.8-12 are based in the height from the base at the lowest level. 12.8.4.3. For SDC C, if type 1a or 1b torsional irregularity exists the torsional moment

should be amplified as per Eq 12.8-14. This is beyond the scope of the current version of Fastrak Building Designer.

Chapter 4 : Theory and Assumptions ASCE7-05&-10 Seismic Wizard page 9Chapter 4 Theory and Assumptions

This section describes the theory used in the development of the seismic procedure within Fastrak Building Designer and the major assumptions that have been made, particularly with respect to interpretation of ASCE7-05&-10 and IBC 2006. We consider it advisable that you are fully familiar with these publications before using the software.

DefinitionsSome definitions and standard terms:-

ELF Equivalent Lateral Force Method is the approach taken by Fastrak Building Designer to determine seismic design forces and drift.

Elastic analysis is a first-order linear static analysis.

P-delta analysis is a second-order analysis: The solver performs a linear static analysis, and then updates the stiffness matrix based on membrane forces derived from the initial displacements. A second linear static analysis is then performed using the modified stiffness matrix. This results in a full assessment of both P (structure) and P (member) forces in the structure.

Seismic AssessmentThe Seismic forces applied to the structure are dependent upon many factors (including building use, location, site conditions and fundamental period). They are determined in the software with the assistance of the Seismic Wizard.

To be able to run the Seismic Wizard you must have defined at least one dead loadcase and one live loadcase.

11.6 - Seismic Design Category As the first step in the above process you are required to enter the ASCE7 maximum considered earthquake ground motions for short (0.2 second) and 1 second periods and also the site class and occupancy. From this input the Seismic Design Category (SDC) is determined in accordance with section 11.6.

If S1 < 0.75, you have a choice as to whether to use the alternative method of using only table 11.6.1 to determine the Seismic Design Category. In this case, it is up to you to verify that all the conditions of 11.6 are met.

Seismic Assessment for SDC = A only

11.7 - Base Shear Combination and Distribution of Force If SDC = A, the next stage is to specify a base shear combination. This should include all the dead loads that you want to include in the seismic design. From this combination the dead load at each floor is determined.

ASCE7-05&-10 Seismic Wizard page 10 Chapter 4 : Theory and AssumptionsNote This base shear combination is used to develop the seismic design loading and is not used in any analysis of the structure.

Vertical Distribution of Seismic Forces

If SDC = A, the lateral seismic force, Fx, at any floor is obtained from eqn 11.7-1:

Fx = 0.01wx

Where wx is the proportion of total effective seismic weight located or assigned at level x. It is determined within a structure in one of two ways.

within the length of a column, at each level all horizontal incoming members are considered and the vertical loading from these members are summed and considered as the vertical loading in the level at that column. This includes the self weight of columns, shear walls and bearing walls from above that level.This is repeated across the level. w the total load at the level is the total of all these column loads.

at the top of a column, the total vertical load in the column top taken as the vertical loading at that location. The self weight from columns,shear walls and bearing walls below that level are NOT included.

Note that a missing component of loading in a roof plane occurs where a sloping roof joins a column part way up its height. The Fastrak model does not consider this situation currently.

Horizontal Distribution of Seismic Forces The lateral seismic force is applied at the center of mass of each story.

For SDC = A, you are not required to cater for accidental torsion and hence no additional eccentricity has to be applied.

The Base Seismic Load CasesFor seismic and drift combinations, the following base load cases are defined for each direction +/- X/Y

SDC A Seismic ASCE7-05/-10 +X Seismic ASCE7-05/-10 -X Seismic ASCE7-05/-10 +Y Seismic ASCE7-05/-10 -Y

Note The remaining stages of the seismic assessment listed below only apply to SDC=B-F and are not relevant to SDC = A.

Seismic Assessment for SDC = B-F

12.8.2.1 - Approximate Fundamental PeriodThe software determines Ct and x for the chosen structure type from Table 12.8-2.

Chapter 4 : Theory and Assumptions ASCE7-05&-10 Seismic Wizard page 11The approximate fundamental period, Ta, for each direction X and Y is then determined using Eqn 12.8-7. You are able to override Ta by directly supplying the fundamental period, T in each direction. In this case the software checks that the values you supply do not exceed the upper limit on calculated period using the coefficient obtained from table 12.8-1.

12.2.1 - Seismic Force Resisting System You need to specify the basic seismic force resisting system (SFRS) for each orthogonal direction. Table 12.2-1 is then used to determine the various coefficients and factors for design: R (response modification coefficient - for base shear); 0 (system over-strength factor - for design forces) and Cd (deflection amplification factor - for seismic design story drift).

12.3.2 - Structural Irregularities The structure is assessed for irregularities to ensure it is valid to proceed with Equivalent Lateral Force procedure. All structure irregularities are user defined. The software does not determine any of them automatically.

SDC B SDC C SDC D SDC E SDC FPlan Irregularity (Table 12.3-1)

1a) Torsional Irreg ELF No ELFa

a. beyond current scope.

No ELF No ELF No ELF

1b) Extreme torsional irreg ELF No ELFa No ELF No ELF No ELF

2) Re-entrant corners ELF ELF ELF b

b. ELF only permitted if T < 3.5Ts where Ts = SD1/SDS

ELF b ELF b

3) Diaphragm discontinuity ELF ELF ELF b ELF b ELF b

4) Out of plane offset No ELFa No ELFa No ELFa No ELFa No ELFa

5) Non parallel systems ELF ELF c

c. use 100/30 orthogonal combination in lateral forces.

ELF c ELF c ELF c

Vertical Irregularity (Table 12.3-2)1a) Soft story irreg d e

d. Exception - all story drift ratios > 130%, no tors effect need considering, no story drift for top two stories.e. Exception - ignore for 1 story (all SDC) and 2 story (SDC A-D).

ELF ELF No ELF No ELF No ELF

1b) Extreme soft story d e ELF ELF No ELF No ELF No ELF

2) Mass irreg d e ELF ELF No ELF No ELF No ELF

3) Vertical geometric discontinuity ELF ELF No ELF No ELF No ELF

4) In-plane discontinuity in vert lat resisting elements No ELF

a No ELFa No ELFa No ELFa No ELFa

5a) Weak story ELF ELF ELF b No ELF No ELF

5b) Extreme weak story f

f. Exception - limited to 2 stories or 30ft high unless weak story able to carry 0 x design force. (The height taken is a user issue).

ELF ELF No ELF No ELF No ELF

ASCE7-05&-10 Seismic Wizard page 12 Chapter 4 : Theory and Assumptions12.7.2 - Effective Seismic Weight

W is the effective seismic weight as per 12.7.3. This is determined from a base shear combination that you create from the available load cases.

Note This base shear combination is used to develop the seismic design loading and is not used in any analysis of the structure.

12.8.1 - Seismic Base Shear

The seismic response coefficient Cs is calculated using a number of simple formulae given in Eqns 12.8-2, -3, -4, -5, and -6.

The seismic base shear, V is then determined from Eq 12.8-1 using Cs and W.

12.8.3 - Vertical Distribution of Seismic Forces

The vertical distribution of seismic forces is calculated as per Eqns 12.8 -11 & 12.

One force, w is calculated for and is applied to each level.

w the proportion of total effective seismic weight located or assigned to a level is determined within a structure in one of two ways.

within the length of a column, at each level all horizontal incoming members are considered and the vertical loading from these members are summed and considered as the vertical loading in the level at that column. This includes the self weight of columns, shear walls and bearing walls from above that level.This is repeated across the level. w the total load at the level is the total of all these column loads.

at the top of a column, the total vertical load in the column top taken as the vertical loading at that location. The self weight from columns, shear walls and bearing walls below that level are NOT included.

Note that a missing component of loading in a roof plane occurs where a sloping roof joins a column part way up its height. The Fastrak model does not consider this situation currently.

12.8.4 - Horizontal Distribution of Seismic Forces

Inherent torsion exists at any floor if the center of mass is eccentric to the center of rigidity.

If rigid diaphragms exist, accidental torsion is catered for by adding in an eccentricity of (+ and -) 5% of the dimension of the structure perpendicular to the line of action of the lateral load.

Note To review graphically the centers of mass or rigidity within the model pick Select / Show / Alter State, and then pick Center of Mass or Center of Rigidity from the dialog.

Chapter 4 : Theory and Assumptions ASCE7-05&-10 Seismic Wizard page 1312.3.4 - Redundancy FactorA redundancy factor, is determined in each orthogonal direction, dependant on the SDC.

SDC B & C: = 1.0 SDC D, E & F; = 1.3*

*you can use 1.0 if the conditions a or b are met in 12.3.4.2

The Base Seismic Load CasesFor seismic and drift combinations, the following base load cases are defined for each direction +/- X/Y and +/- eccentricity

SDC B-F - For normal situations

For X direction Seismic ASCE7-05/-10 +X, +ecc Seismic ASCE7-05/-10 +X, -ecc Seismic ASCE7-05/-10 -X, +ecc Seismic ASCE7-05/-10 -X, -ecc

For Y direction Seismic ASCE7-05/-10 +Y, +ecc Seismic ASCE7-05/-10 +Y, -ecc Seismic ASCE7-05/-10 -Y, +ecc Seismic ASCE7-05/-10 -Y, -ecc

SDC B-F - With out of plane offset plan irregularityAs above for normal situations, plus an additional 4 new loadcases for each of the above to account for 30% of the load in the orthogonal direction at +/- eccentricity.

ASCE7-05&-10 Seismic Wizard page 14 Chapter 5 : ReferencesChapter 5 References

1. ASCE/SEI 7-05. Minimum Design Loads for Buildings and Other Structures. ASCE, 2006.

2. ASCE/SEI 7-10. Minimum Design Loads for Buildings and Other Structures. ASCE, 2010.

3. IBC 2006. International Building Code. The International Code Council, 2006.

Chapter 5 : References ASCE7-05&-10 Seismic Wizard page 15

ASCE7-05&-10 Seismic Wizard page 16 Chapter 5 : References

ASCE7-05&-10 Seismic Wizard HandbookChapter 1 IntroductionChapter 2 ScopeEngineering Overview

Chapter 3 LimitationsChapter 4 Theory and AssumptionsDefinitionsSeismic Assessment11.6 - Seismic Design Category

Seismic Assessment for SDC = A only11.7 - Base Shear Combination and Distribution of ForceVertical Distribution of Seismic ForcesHorizontal Distribution of Seismic Forces

The Base Seismic Load Cases

Seismic Assessment for SDC = B-F12.8.2.1 - Approximate Fundamental Period12.2.1 - Seismic Force Resisting System12.3.2 - Structural Irregularities12.7.2 - Effective Seismic Weight12.8.1 - Seismic Base Shear12.8.3 - Vertical Distribution of Seismic Forces12.8.4 - Horizontal Distribution of Seismic Forces12.3.4 - Redundancy FactorThe Base Seismic Load Cases

Chapter 5 References