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TR.31.2.16 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:

ParameterDescription
ZIP f1 The zip code of the site location to determine the latitude and longitude and consequently the Ss and S1 factors. (ASCE 7-16 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.
Note: The mapped spectral response acceleration values for IBC 2018 are retrieved from dynamic data provided by the USGS when 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:

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 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, 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 7-16 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 7-16 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 7-15 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.31.2.14 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.

Exceptions 1 and 2 in Clause 11.4.8 of ASCE 7-16 are implemented.

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