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) LOAD
map-spec ibc18-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:
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:
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.
Note: 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 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