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STAAD Commands and Input Instructions
Section 5
5-222
R = Force modification factor conforming to Table 4.9.1.B that reflects the
capability of a structure to dissipate energy through inelastic
behaviour.
STAAD utilizes the following procedure to generate the lateral
seismic loads.
1. User provides seismic zone co-efficient and desired "nrc-spec"
(1995) through the DEFINE NRC LOAD command.
2. Program calculates the fundamental period(T) of the structure
by
(a) finding out whether the structure being analysed is a
moment resisting frame made primarily of steel or of
concrete or it is a structure of any other type. Alternatively,
the software uses the optional parameter CT if provided.
The calculation is done per sentence 7(a) & 7(b) of section
4.1.9.1.
(b) using the Rayleigh method or using the optional parameters
PX , PZ –if provided. The stipulations of sentence 7(c) of
section 4.1.9.1 are also considered while calculating.
(c) taking the conservative value of T between those calculated
by methods (a) and (b) above.
3. Program finds out the value of Seismic Response Factor(S) per
table 4.9.1.A utilizing the values of T as calculated above and
the values of ZA & ZV input by the user.
4. Program calculates V per sentence 4 section 4.1.9.1. W is
obtained from the weight data (SELFWEIGHT, JOINT
WEIGHT(s), etc.) provided by the user through the DEFINE
NRC LOAD command. The weight data must be in the order
shown.
5. The total lateral seismic load (base shear) is then distributed
by the program among different levels of the structure per
applicable NRC guidelines like sentence 13(a) section 4.1.9.1.
There are 2 stages of command specification for generating lateral
loads. This is the first stage and is activated through the DEFINE
NRC LOAD command.