TR.31.2.17 IBC 2018 Seismic Load Definition
The specifications of the seismic loading chapters of the International Code Council 2018 code and the ASCE 7-16 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
There are two stages of command specification for
generating lateral loads. This is the first stage and is activated through the
DEFINE IBC 2018 LOAD command.
DEFINE IBC 2018 (ACCIDENTAL) LOADmap-spec ibc18-spec weight-dataRefer 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:
| 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 7-16 Chapter 22). |
| LAT f2 | The latitude of the site used to determine the Ss and S1 factors. (ASCE 7-16 Chapter 22). |
| LONG f3 | The longitude of the site used to determine the Ss and S1 factors. (ASCE 7-16 Chapter 22). |
| SS f4 | The mapped MCE for 0.2s spectral response acceleration. (ASCE 7-16 Clause 11.4.2). |
| S1 f5 | The mapped MCE spectral response acceleration at a period of 1 second as determined in accordance with Section 11.4.2 ASCE7-16. |
ZIP code or LAT/LONG
methods are used. An internet connection is required for this. If an internet
connection is not available, then the values should be input using the
SS and S1 parameters.ibc18-spec = { 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) }Where:
| Parameter | Definition |
|---|---|
| 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 2018 Clause 1604.5, ASCE 7-16 Table 11.5-1) |
| TL f9 | Long-Period transition period in seconds. (ASCE 7-16 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 2018 clause 1613.3.2, ASCE 7-16 Section 20.3) |
| CTX f11 |
Optional CT value in X-direction to calculate
time period. (ASCE 7-16 Table 12.8-2). If specified, it is
your responsibility to provide the value in the correct system of units.
Refer to AISC 7-16 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 7-16 also includes
|
| CTZ f12 |
Optional CT value in Z-direction to calculate
time period. (ASCE 7-16 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 7-15 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 7-16 Table 12.8-2). |
| XX f15 |
Optional exponent value, x, in X-direction, used
in equation 12.8-7, ASCE 7. (ASCE 7-16 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 7-16 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
2018 Clause 1613.3.3, ASCE 7-16 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
2018 Clause 1613.3.3, ASCE 7-16 Section 11.4.3).
|
Implementation and Methodology
Refer to TR.31.2.15 IBC 2012 Seismic Load Definition for details on the implementation of the IBC 2018 / IBC 2015 / IBC 2012 / ASCE 7-16 / ASCE 7-10 static seismic method.
In exception 1, for Site class E and Ss > 0.75, Fa value is taking the same for site class C. In this case, there is slight reduction in the seismic force generated.
In exception 2, for site class D, and S1 > 0.1, the program checks if the time period, T > 1.5Ts . If that is the case then Cs value is taken as 1.5×Cs obtained from eqn. 12.8-3 or 12.8-4 (based on the time period). In this case the seismic force increases. In this case but when T ≤ 1.5Ts , Cs as calculated by eqn 12.8-2 is used.
Example 1
DEFINE IBC 2018
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 45Example 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