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:
Parameter | Definition |
---|---|
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:
|
ibc12-spec | This identifies the parameters needed to determine the lateral loading to be applied in a
load case which references this definition.
|
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:
Parameter | Definition |
---|---|
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:
Parameter | Definition |
---|---|
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,
(where M is the number of
members) and uses this to determine the structure type:
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 |
CTZ f12 |
Optional Ct
value in
Z-direction to calculate time period. (ASCE 7-10 Table 12.8-2).
Refer to |
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).
|
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-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.
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:
-
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.
-
The period is also calculated in accordance with the Rayleigh method. This is reported in the output as T.
-
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.
-
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.
-
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.
-
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. -
The amplification of accidental torsional moment, as described in Section 12.8.4.3 of the ASCE 7-10 code, is not implemented.
-
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):
The seismic response coefficient, Cs, is determined in accordance with the following equation (equation 12.8-2 of ASCE 7-10):
- 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):
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
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