IBC 2006 This document illustrates the method of Seismic Load (Static Equivalent method) as per IBC 2006

THE IBC 2006 STATIC EQUIVALENT METHOD FOR SEISMIC

ANALYSIS

OBJECTIVE

In this session, we will briefly go over the theoretical basis for equivalent static

force procedures such as IBC. We will also learn how to define seismic

parameters and weights, combine gravity loads with lateral loads, view the

generated lateral loads, handle cases like tension-only bracing members, etc.

GENERAL OVERVIEW When a building is subjected to an earthquake, it undergoes vibrations. The

weights of the structure, when accelerated along the direction of the earthquake,

induce forces in the building. Normally, an elaborate dynamic analysis called

time history analysis is required to solve for displacements, forces and reactions

resulting from the seismic activity. However, codes like IBC provide a static

method of solving for those values. The generalized procedure used in those

methods consists of 3 steps

Step 1: Calculate Base Shear = Factor Cs * Weight W where "Cs" (seismic

coefficient) is calculated from terms which take into consideration the

Importance factor of the building, Site Class and soil characteristics, etc. W is

the total vertical weight derived from dead weight of the building and other

imposed weights.

Step 2: The base shear is then distributed over the height of the building as a

series of point loads.

Step 3: The model is then analyzed for the horizontal loads generated in step 2.

The input required in STAAD consists of 2 parts.

Part 1, which appears under a heading called DEFINE IBC LOAD contains the

terms used to compute "f" and "W" described in step 1. Part 2, which appears

within a load case, contains the actual instruction to generate the forces

described in step 2 and analyze the structure for those forces.

On a broad basis, the rules described in section 1613 of the ICC IBC-2006 code

(except 1613.5.5) have been implemented. This section directs the engineer to

Section 12.8 of the ASCE 7-2005 code. The specific section numbers of ASCE

7- those which are implemented, and those which are not implemented, are

shown in the table below.

The associated pages of the ASCE 7-2005 code are 115 thru 129.

Methodology

The design base shear is computed in accordance with the equations shown

below

The seismic response coefficient, Cs, is determined in accordance with the

following equation:

For IBC 2006, Cs need not exceed the following limits defined in ASCE 7-05:-

Cs shall not be less than

In addition, for structures located where S1 is equal to or greater than 0.6g, Cs

shall not be less than

For an explanation of the terms used in the above equations, please refer to IBC

2006 and ASCE 7-05 codes.

See the Appendix below: (press “control” button and click the link to go

directly to the appendix)

SPECIFYING THE INPUT IN THE STAAD FILE

There are 2 stages of command specification for generating lateral loads. This is

the first stage and is activated through the DEFINE IBC 2006 LOAD command.

For IBC 2006 using a known zip code, the command is thus:-

DEFINE IBC 2006 (ACCIDENTAL) LOAD

ZIP f11 RX f5 RZ f6 I f4 TL f15 SCLASS f7 (CT f8) (PX f9) (PZ f10) (K

f16) (FA f17) (FV f18)

Weight spec (See section 5.31.2.2 of the Technical Reference manual for

complete weight specification)

For IBC 2006 using a known longitude and latitude, the command is thus:-

DEFINE IBC 2006 (ACCIDENTAL) LOAD

LAT f12 LONG f13 RX f5 RZ f6 I f4 TL f15 SCLASS f7 (CT f8) (PX f9)

(PZ f10) (K f16) (FA f17) (FV f18)

Weight spec (See section 5.31.2.2 of the Technical Reference manual for

complete weight specification)

For IBC 2006 using specific SS and S1 values, the command is thus:-

DEFINE IBC 2006 (ACCIDENTAL) LOAD

SS f14 S1 f3RX f5 RZ f6 I f4 TL f15 SCLASS f7 (CT f8) (PX f9) (PZ f10)

(K f16) (FA f17) (FV f18)

Weight spec (See section 5.31.2.2 of the Technical Reference manual for

complete weight specification)

where, S1 f3 = the mapped MCE spectral response acceleration at a period of 1

second as determined in accordance with Section 11.4.1 ASCE7-05

I f4 = Occupancy importance factor. (IBC 2006, ASCE 7-05 Table 11.5-1)

RX f5 = The response modification factor for lateral load along the X direction,

(IBC 2006 ASCE Table 12.2.1)

RZ f6 = The response modification factor for lateral load along the Z direction,

(IBC 2006 ASCE Table 12.2.1)

SCLASS f7= Site class. Enter 1 through 6 in place of A through F, see table

below. (IBC 2006 ASCE 7-05 Section 20.3)

The Soil Profile Type parameter SCLASS can take on values from 1 to 6. These

relate to the values shown in Site Class Definitions Table in the following

manner:-

IBC

Class

SCLASS

value

A 1

B 2

C 3

D 4

E 5

F 6

CT f8 = Optional CT value to calculate time period. (IBC 2006 ASCE 7-05

Table 12.8-2).

PX f9 = Optional Period of structure (in sec) in X-direction to be used as

fundamental period of the structure. If not entered the value is calculated from

the code. (IBC 2006 ASCE 7-05 Table 12.8-2).

PZ f10 = Optional Period of structure (in sec) in Z-direction to be used as

fundamental period of the structure. If not entered the value is calculated from

the code. (IBC 2006 ASCE 7-05 Table 12.8-2).

ZIP f11 = The zip code of the site location to determine the latitude and

longitude and consequently the Ss and S1 factors. (IBC 2006, ASCE 7-05

Chapter 22).

LAT f12 = The latitude of the site used with the longitude to determine the Ss

and S1 factors. (IBC 2006, ASCE 7-05 Chapter 22).

LONG f13 = The longitude of the site used with the latitude to determine the Ss

and S1 factors. (IBC 2006, ASCE 7-05 Chapter 22).

SS f14 = Mapped MCE for 0.2s spectral response acceleration. (IBC 2006,

ASCE 7-05 Chapter 22).

TL f15 = Long-Period transition period in seconds. (IBC 2006, ASCE 7-05

Chapter 22).

K f16 = Exponent used in equation 12.8-7, ASCE 7. (IBC 2006, ASCE 7-2005,

table 12.8-2 page 129).

FA f17 = Optional Short-Period site coefficient at 0.2s. Value must be provided

if SCLASS set to F (i.e. 6). (IBC 2006, ASCE 7-05 Section 11.4.3).

FV f18 = Optional Long-Period site coefficient at 1.0s. Value must be provided

if SCLASS set to F (i.e. 6). (IBC 2006, ASCE 7-05 Section 11.4.3).

Example

There are 3 methods to define IBC 2006. The 3 methods are defined below.

Method 1:- Input Zip code

DEFINE IBC 2006

ZIP 92887 I 1.25 RX 2.5 RZ 2.5 SCLASS 4 TL 12 FA 1 FV 1.5

SELFWEIGHT

JOINT WEIGHT

51 56 93 100 WEIGHT 650

MEMBER WEIGHT

151 TO 156 158 159 222 TO 225 324 TO 331 UNI 45

Method 2:- Input Latitude and Longitude

DEFINE IBC 2006

LAT 33.8845 LONG -117.9274 I 1.25 RX 2.5 RZ 2.5 SCLASS 4 TL 12 FA 1

FV 1.5

SELFWEIGHT

JOINT WEIGHT

51 56 93 100 WEIGHT 650

MEMBER WEIGHT

151 TO 156 158 159 222 TO 225 324 TO 331 UNI 45

Method 3:- Input Ss and S1 values

DEFINE IBC 2006 ACCIDENTAL

SS 2.15822 S1 0.80585 I 1 RX 3 RZ 4 SCLASS 4 TL 12

SELFWEIGHT

JOINT WEIGHT

51 56 93 100 WEIGHT 650

MEMBER WEIGHT

151 TO 156 158 159 222 TO 225 324 TO 331 UNI 45

Steps used to calculate and distribute the base shear are as follows:

1. The Time Period of the structure is calculated based on section 12.8.1 of

ASCE 7-05 (IBC 2006). This is reported in the output as Ta.

2. The period is also calculated in accordance with the Rayleigh method. This is

reported in the output as T.

3. The user may override the Rayleigh based period by specifying a value for

PX or PZ (Items f7 and f8) depending on the direction of the IBC load.

4. The governing Time Period of the structure is then chosen between the above

two periods, and the additional guidance provided in section 12.8.2.1 of ASCE

7-05 (IBC 2006). The resulting value is reported as "Time Period used" in the

output file.

5. The Design Base Shear is calculated based on equation 12.8-1 of ASCE 7-05

(IBC 2006). It is then distributed at each floor using the rules of clause 12.83,

equations 12.8-11, 12.8-12 and 12.8-13 of ASCE 7-05.

6. If the ACCIDENTAL option is specified, the program calculates the

additional torsional moment. The lever arm for calculating the torsional moment

is obtained as 5% of the building dimension at each floor level perpendicular to

the direction of the IBC load (section 12.8.4.2 of ASCE 7-05 for IBC 2006). At

each joint where a weight is located, the lateral seismic force acting at that joint

is multiplied by this lever arm to obtain the torsional moment at that joint.

The following example shows the commands required to enable the program to

generate the lateral loads. Users may refer to Section 5.32.12 of the Technical

Reference Manual for this information.

Example

LOAD 1 (SEISMIC LOAD IN X DIRECTION)

IBC LOAD X 0.75

LOAD 2 (SEISMIC LOAD IN Z DIRECTION)

IBC LOAD Z 0.75

TO SPECIFY IBC 2006 FROM THE GUI

The static equivalent method for performing dynamic analysis per the IBC 2006

code has been implemented in STAAD.Pro 2007 Build 02. This option can be

accessed from the General | Load page as explained below.

Description When the General | Load Page is selected, the right hand side of

the screen will display the following if no load cases exist in the model

Definitions contains the options through which one creates the “Define” block

of data required to create wind load cases, seismic load cases like IBC and

UBC, moving load cases and time history load cases. When the tree view is

expanded, it will look as shown below

Select Seismic Definitions and click on Add. In the dialog box that comes up,

select IBC 2006 from the drop-down list.

In this dialog box, we can specify the various parameters as described below

Include Accidental Load Check this box to calculate the accidental torsion

component described in section 12.8.4.2 of ASCE 7-05.

Parameter

The various parameters for the IBC 2006 code, such as the Occupancy

Importance factor IE, Response modification factors RX and RZ, spectral

response accelerations SDS, SD1 and S1, etc., are described in detail in Section

5.31.2.6 of the STAAD Technical Reference manual. After specifying the

values for the parameters, click on the Add button.

We will see that the Load dialog box has now been updated

Next, we should define the structural weights for calculating Base Shear. After

highlighting the expression LAT 38.0165……, click on Add. A new dialog box

titled Add New Seismic Definitions will come up.

Self weight

This is the self weight of the structure.

Joint Weights

These are the concentrated weights acting at one or more joints. Member

Weights Distributed and concentrated weights acting on member spans are

specified through this option. After clicking the Member weight button, the

Member weight dialog box appears, as shown below

Select the Concentrated or Uniform load type from the Loading Type drop

down list. Enter the intensity of the distributed weight or magnitude of the

concentrated weight as the case may be, along with the location of the load.

Element Weights

If the structural model consists of plate elements representing entities like floor

slabs, the pressure loads on those slabs can be considered for weights

calculation for lateral load generation per UBC/IBC/other codes. This is done

with the help of the Element Weights option. Its parameters include the

magnitude of the uniform pressure, and the elements they are applied on. Since

it is a weight, it is a quantity without a sign.

Reference Load

Reference Load cases which are described in AD.2007-1001.1.12 of the

STAAD.Pro 2007 Software Release Report can be referred to using this option.

Loads which are specified under Reference Loads can be used as weights for

IBC.

Floor Weights

In many situations, a user may decide not to include the structural slabs in

his/her analytical model. Hence, the model may be solely the skeleton framing

system consisting of the beams, columns and bracing members.

Under these circumstances, the loads which act on the slab can no longer be

applied on the structure using the ELEMENT PRESSURE options. This is

because there are no elements to represent the slab. So, an alternative is to apply

the load using the FLOOR LOAD option. It is described in detail in section

5.32.4 of the Technical Reference manual.

Within a UBC/IBC/other codes definition, the FLOOR WEIGHT is the

counterpart for the FLOOR LOAD just as MEMBER WEIGHT is the

counterpart for a MEMBER LOAD, and an ELEMENT WEIGHT is the

counterpart for an ELEMENT LOAD.

Its parameters are hence very similar to what are found in a normal FLOOR

LOAD definition. XRANGE, YRANGE and ZRANGE options allow the user

to narrow in on panels at specific regions of the building. The pressure value is

provided as a quantity without sign because it is contributing to the overall

weight - a numerically positive term.

Once we finish specifying the seismic definitions, we can start adding load

cases.

To do that, select Load Cases Details and click on Add.

In the dialog box that appears, provide a Title and click on Add.

We can add more load cases in this manner.

To add load items to our first load case, keep the expression 1: SEISMIC IN

THE X DIRECTION highlighted and click on the Add button.

Here, enter the Factor, Direction, etc. and click on Add

The Load dialog box will show the new load item

We can continue adding other load items to this load case in a similar fashion.

Once the analysis is performed you may be able to view the results in the output

file as shown below.

Appendix :1

Regards

*******************

Nihesh.N

STAAD Technical Support

Bentley Systems

TEL: 714-974-2500 ex 5484

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