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TR.31.2.13 IBC 2006/2009 Seismic Load Definition

The specifications of the seismic loading chapters of the International Code Council 2006 & 2009 code and the ASCE 7-05 (including Supplement #2) code for seismic analysis of a building using a static equivalent approach are available in the program. Depending on the definition, equivalent lateral loads will be generated in the horizontal direction(s).

General Format

There are two stages of command specification for generating lateral loads. This is the first stage and is activated through the DEFINE IBC 2006 LOAD command.

DEFINE IBC 2006 (ACCIDENTAL) LOAD
map-spec ibc06-spec
weight-data

Refer to Common Weight Data for information on how to specify structure weight for seismic loads.

Where:

map-spec = { ZIP f1 | LAT f2 LONG f3 | SS f4 S1 f5 }

Where:

ParameterDescription
ZIP f1 The zip code of the site location to determine the latitude and longitude and consequently the Ss and S1 factors. (ASCE 7-05 Chapter 22). 
LAT f2 The latitude and longitude, respectively, of the site used with the longitude to determine the Ss and S1 factors. (ASCE 7-05 Chapter 22).
LONG f3 The latitude and longitude, respectively, of the site used with the longitude to determine the Ss and S1 factors. (ASCE 7-05 Chapter 22).
SS f4 The mapped MCE for 0.2s spectral response acceleration. (IBC 2006/2009 Clause 1613.5.1, ASCE 7-05 Clause 11.4.1).
S1 f5 The mapped MCE spectral response acceleration at a period of 1 second as determined in accordance with Section 11.4.1 ASCE7-05
ibc06-spec = { RX f6 RZ f7 I f8 TL f9 SCLASS f10 (CT f11) (PX f12) (PZ f13) (K f14) (FA f15) (FV f16) }

Where:

ParameterDescription
RX f6 The response modification factor, R, for lateral load along the X direction, (ASCE Table 12.2.1). This is the value used for calculating Cs.
RZ f7 The response modification factor, R, for lateral load along the Z direction, (ASCE Table 12.2.1) This is the value used for calculating Cs.
I f8 Occupancy importance factor. (IBC 2006/2009 Clause 1604.5, ASCE 7-05 Table 11.5-1) 
TL f9 Long-Period transition period in seconds. (ASCE 7-05 Clause 11.4.5 and Chapter 22).
SCLASS f10 Site class. Enter 1 through 6 in place of A through F, see table below. (IBC 2006/2009 Table 1613.5.2, ASCE 7-05 Section 20.3)  
CTX f11 Optional Ct value in X-direction to calculate time period. (ASCE 7-05 Table 12.8-2). If specified, it is your responsibility to provide the value in the correct system of units. Refer to AISC 7-05 for values.
If the value of Ct is not provided, then the program computes the average value of the modulus of elasticity of the model, E a v g   =   E / M (where M is the number of members) and uses this to determine the structure type:
  1. Eavg < 4,000 ksi, the program uses a Ct for a moment-resisting concrete frame.
  2. Eavg > 10,000 ksi, the program uses a Ct for a moment-resisting steel frame.
  3. 4,000 ksi ≤ Eavg ≤ 10,000 ksi, the program uses a Ct value for "all other structural systems".
Note: It is your responsibility to ensure that the structure type used actually matches the description for the automatically determined structure when Ct not specified. Refer to the IBC/ASCE 7 code for detailed descriptions.

ASCE 7-05 also includes "Eccentrically braced steel frames". STAAD.Pro does not select this value automatically. For this structure type, you must specify Ct.

CTZ f12 Optional Ct value in Z-direction to calculate time period. (ASCE 7-05 Table 12.8-2). Refer to CTX for details.
PX f13 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. (ASCE 7-05 Table 12.8-2). 
PZ f14 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. (ASCE 7-05 Table 12.8-2). 
XX f15 Optional exponent value, x, in X-direction, used in equation 12.8-7, ASCE 7. (ASCE 7-05 table 12.8-2 p129). If the value of x is not provided, then the program computes the average value of the modulus of elasticity of the model to determine the structure type. Refer to CTX for details.
XZ f16 Optional exponent value, x, in Z-direction, used in equation 12.8-7, ASCE 7. (ASCE 7-05 table 12.8-2 p129). If the value of x is not provided, then the program computes the average value of the modulus of elasticity of the model to determine the structure type. Refer to CTX for details.
FA f17 Optional Short-Period site coefficient at 0.2s. Value must be provided if SCLASS set to F (i.e., 6). (IBC 2006/2009 Clause 1613.5.3, 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 2006/2009 Clause 1613.5.3, ASCE 7-05 Section 11.4.3).
Note: For additional details on the application of a seismic load definition used to generate loads, refer to GUID-6D9B5C48-9FFF-4548-BFB4-ACAE1E973743.

Implementation in STAAD.Pro

The seismic load generator can be used to generate lateral loads in the X & Z directions for Y up or X & Y for Z up. Y up or Z up is the vertical axis and the direction of gravity loads (See the SET Z UP command in TR.5 Set Command Specification). All vertical coordinates of the floors above the base must be positive and the vertical axis must be perpendicular to the floors. 

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 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.  

Table 1. Sections of IBC 2006 implemented and omitted in the program

Implemented sections of

IBC 2006/2009 (ASCE 7-2005)

Omitted sections of

IBC 2006/2009 (ASCE 7-2005)

11.4 12.8.4.1
11.5 12.8.4.3 and onwards
12.8

Additionally, Supplement #2 of ASCE 7-05—as referenced by IBC 2009—specifies a different equation to be used for the lower bound of the seismic response coefficient, which has also been implemented.

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

  1. The Time Period of the structure is calculated based on section 12.8.2.1 of ASCE 7-05 (IBC 2006/2009). 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. you 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 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. 
  7. The amplification of accidental torsional moment, as described in Section 12.8.4.3 of the ASCE 7-05 code, is not implemented.
  8. The story drift determination as explained in Section 12.8.6 of the ASCE 7-05 code is not implemented in STAAD.Pro.

Methodology

The design base shear is computed in accordance with the following equation (equation 12.8-1 of ASCE 7-05):

V = CsW

The seismic response coefficient, Cs, is determined in accordance with the following equation (equation 12.8-2 of ASCE 7-05):

Cs = SDS/[R/IE]

For IBC 2006, Cs need not exceed the following limits defined in ASCE 7-05 (equations 12.8-3 and 12.8-4):
  • Cs = SD1/[T⋅(R/I)] for T ≤ TL
  • Cs = SD1 · TL/[T2(R/I)] for T > TL

However, Cs shall not be less than (equation 12.8-5 of ASCE 7-05, supplement #2):

Cs = 0.044 · SDS · I ≥ 0.01

In addition, per equation 12.8-6 of ASCE 7-05, for structures located where S1 is equal to or greater than 0.6g, Cs shall not be less than

Cs = 0.5 · S1/(R/I)

For an explanation of the terms used in the above equations, please refer to the IBC 2006/2009 and ASCE 7-05 codes.

Example 1

DEFINE IBC 2006
LAT 38.0165 LONG -122.105 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

Example 2

The following example shows the commands required to enable the program to generate the lateral loads. Refer to TR.32.12 Generation of Loads for this information.

LOAD 1 (SEISMIC LOAD IN X DIRECTION)
IBC LOAD X 0.75
LOAD 2 (SEISMIC LOAD IN Z DIRECTION)
IBC LOAD Z 0.75