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[anandrshah9]

Hello friends and seniors,

I would like to contribute in the current discussion of seismic forces in
STAAD specifically with 2 notes.

1 we can apply E.Q. from two ways. One is calculation of E.Q. forces ( base
shear required as per IS 1893-2002 manually and applying each storey shear
at each floor from out side in program with rigid diaph. Action using slave
rigid master or slave fx fz master. Or we can specify necessary data in
programe to calculate total base shears by itself. But in later case I have
noticed that if we don’t give time period px and pz along with necessary
data in edit file, then total base shear calculated by programe is not
matching with manual calculation as per code.
2 And the second thing, I have noticed is distribution of seismic forces
(storey shears ) at different level and in lateral load resisting elements
is different in both the cases , when we applied from outside with slave
rigid master or slave fx fz master on each rigid floor or we give necessary
data in programe to calculate and distribute itself. And even I have tried
to match it with manual approximate design like cantilever , portal or
substitute frame method, but that is also not matching.

These are the problem I have faced, and looking forward a  solutions.

Regards,

Anand Shah
(Dalal Mott Macdonald, Ahmedabad)

[bhumika]

Hi Anand,

your second approach is very much close to the actual one that is acting on
the structure. but that depends upon how it distribute forces to the
columns.

One another way to define the static earthquake force is apply earthquake
force as a joint weight. For this you have to determine the distribution
factor for each column in two horizontal directions according to it's size.
and then this distribution factor is multiplied with the total weight of the
structure. Of course total weight of the structure is approximate.

I think this is the better way to assign seismic load than the Master slave
specification.

Regards,
Bhumika

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