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# Etabs dynamic analysis (more understanding)

## Etabs dynamic analysis (more understanding)

(OP)
Hi all

I have one 3 storey building project, ground floor, 1st, long span Steel roof truss. the steel roof is light weight compate to the rest floors. And the 1st to roof is 20m high. My Steel roof column is sitting on the 20m high RC column ( RC　column from　1st to roof)

Based on code, I run a dynamic analysis in etabs, my mass participation is 95% and Base shear for response spectrum (Rs)/ Static(E) is 85 percent.

I want to clarify the load combination 1.2DL+1.0LL+RSx+0.3RSy.....1.2DL+1.0LL+RSx-0.3RSy .... SO on for dynamic analysis load combination.

the analysis shows the steel roof column connect to RC column moment reaction is 3 times for the dynamic load combination and equvalent load combination. (equvalent method 3 times more than dynamic).

My question is， is this correct??? if so, can I use the dynamic analysis load combination reaction force to design my steel columns.

Thanks

### RE: Etabs dynamic analysis (more understanding)

Well Not sure if it is "correct" but depending of irregularities definitely could be expected, equivalent static analysis and dynamic Response spectrum analysis can not be scaled with one factor for all dynamic quantities, for RSA each demand is combined using modal load combination rules, so there is never correspondence between them, also static equilibrium does not apply to RSA.

### RE: Etabs dynamic analysis (more understanding)

(OP)
Thanks, my thinking it might be due to:

1) the roof structure is too light and siffness is too low so the Seismic load is lesser till the roof
2) the column grids is 14m from gound to 1st, and 28 m from 1st to roof
3) the column is too high 20m from 1st to roof

All these will cause the roof T period is high and acceleration is too low, so the shear force is low at roof.

It means the roof stiffness is low and too slender. So the below floor columns need to be further strengthen.

Is this correct?

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