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TR.31.2.15 IBC 2012 Seismic Load Definition

The specifications of the seismic loading chapters of the International Code Council 2012 code and the ASCE 7-10 code for seismic analysis of a building using a static equivalent approach have been implemented as described in this section. Depending on the definition, equivalent lateral loads will be generated in the horizontal direction(s).

General Format

Lateral loads generated by seismic loading are specified in two stages. The first is to define the IBC 2012 loading, as detailed here. The second is to include that specification in one or more load cases.

There are two stages of command specification for generating lateral loads. This is the first stage and is activated through the DEFINE IBC 2012 LOAD command. 

DEFINE IBC 2012 (ACCIDENTAL) LOAD
map-spec ibc12-spec 
(optional-weight-specs)
weight-data

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

Where:

ParameterDescription
map-spec This identifies the mapped spectral accelerations at a given location by either ZIP (postal) code, latitude and longitude, or direct input. The ZIP code and latitude & longitude options are for use in mainland US only: 
{ ZIP f1 | LAT f2 LONG f3 | SS f4 S1 f5 }
ibc12-spec This identifies the parameters needed to determine the lateral loading to be applied in a load case which references this definition. 
{ RX f6 RZ f7 I f8 TL f9 SCLASS f10 (CTX f11) (CTZ f12) (PX f13) (PZ f14) (XX f15) (XZ f16) (FA f17) (FV f18) }

The optional seismic weight option is provided for old files. It is recommended that instead the weights are defined using a one or more reference load cases of type MASS, (refer to TR.31.6 Defining Reference Load Types). If the older format is used, refer to TR.31.2 Definitions for Static Force Procedures for Seismic Analysis for the range of available weight commands that can be used.

The ACCIDENTAL option is used to include an additional torsional moment taken as the lateral load is applied at a horizontal eccentricity as 5% of the building dimension at each level. This option should not be used if the option to include the natural torsion in the application of seismic load (refer to TR.32.12.2 Generation of Seismic Loads for additional details).

map-spec parameters:

ParameterDescription
ZIP f1 The zip code of the site location to determine the latitude and longitude and consequently the Ss and S1 factors. (ASCE 7-10 Chapter 22). 
LAT f2 The latitude and longitude, respectively, of the site used with the longitude to determine the Ss and S1 factors. (ASCE 7-10 Chapter 22).
LONG f3 The latitude and longitude, respectively, of the site used with the longitude to determine the Ss and S1 factors. (ASCE 7-10 Chapter 22).
SS f4 The mapped MCE for 0.2s spectral response acceleration. (IBC 2012 Clause 1613.5.1, ASCE 7-10 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 ASCE7-10.

ibc12-spec parameters:

ParameterDescription
RX f6 The response modification factor, R, for lateral load along the X direction, (ASCE 7-10 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 7-10 Table 12.2.1) This is the value used for calculating Cs.
I f8 Occupancy importance factor (IBC 2012 Clause 1604.5, ASCE 7-10 Table 11.5-1). 
TL f9 Long-Period transition period in seconds (ASCE 7-10 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 2012 clause 1613.3.2, ASCE 7-10 Section 20.3) 
CTX f11 Optional Ct value in X-direction to calculate time period. (ASCE 7-10 Table 12.8-2). If specified, it is your responsibility to provide the value in the correct system of units. Refer to AISC 7-10 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 7-10 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 7-10 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 7-10 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 7-10 Table 12.8-2). 
XX f15 Optional exponent value, x, in X-direction, used in equation 12.8-7, ASCE 7. (ASCE 7-10 table 12.8-2). 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 7-10 table 12.8-2). 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 2012 Clause 1613.3.3, ASCE 7-10 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 2012 Clause 1613.3.3, ASCE 7-10 Section 11.4.3).
Note: For additional details on the application of a seismic load definition used to generate loads, refer to TR.32.12.2 Generation of Seismic Loads.

Implementation in STAAD.Pro

Note: Refer to AD.2007-11.3.8 IBC 2012 / ASCE 7-10 Seismic Loads for additional information on using this feature.

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-2012 code (except 1613.5.5) have been implemented. This section directs the engineer to the ASCE 7-2010 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 2012 implemented and omitted in the program

Implemented sections of

IBC 2012 (ASCE 7-10)

Omitted sections of

IBC 2012 (ASCE 7-10)

11.4 12.8.4.1
11.5 12.8.4.3 and onwards
12.8

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 7-10 (IBC 2012). 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 7-10 (IBC 2012). 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 7-10 (IBC 2012). 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 7-10. 

  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 7-10 for IBC 2012). 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 7-10 code, is not implemented.

  8. The story drift determination as explained in Section 12.8.6 of the ASCE 7-10 code is not implemented in STAAD.

Methodology

The design base shear is computed in accordance with the following equation (equation 12.8-1 of ASCE 7-10):

V = CsW

The seismic response coefficient, Cs, is determined in accordance with the following equation (equation 12.8-2 of ASCE 7-10):

Cs = SDS/[R/IE]

For IBC 2012, Cs need not exceed the following limits defined in ASCE 7-10 (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 7-10):

Cs = 0.044 · SDS · I ≥ 0.01

In addition, per equation 12.8-6 of ASCE 7-10, 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 2012 and ASCE 7-10 codes.

Example 1

DEFINE IBC 2012
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

Using SS and S1 for map-spec

DEFINE IBC 2018
SS 2.451 S1 0.882 –
I 1.25 RX 2.5 RZ 2.5 SCLASS 4 TL 12 FA 1 FV 1.5

Using Latitude and Longitude

DEFINE IBC 2018
LAT 34.0998 LONG -118.4128 -
I 1.25 RX 2.5 RZ 2.5 SCLASS 4 TL 12 FA 1 FV 1.5

Using ZIP Code

DEFINE IBC 2018
ZIP 90210 –
I 1.25 RX 2.5 RZ 2.5 SCLASS 4 TL 12 FA 1 FV 1.5
Parent topic: TR.31.2 Definitions for Static Force Procedures for Seismic Analysis
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