ASCE 7-02 Seismic Load effects 3

[tvar]

In ASCE 7-02 Section 9.5.2.7, there are 2 equations that account for the combination of vertical and horizontal effects. My question is are the effects from the vertical and horizontal combined into a single horizontal force and then into a single vertical force, thus having to use the equations twice or does the equation include one horizontal force componenet and one vertical component that have to be combined algabraiclly to get a single force ie. a shear foce or an axial force? Thanks

 

[JAE]

Be careful not to confuse "effect" with "load".

The equations you speak of - 9.5.2.7-1 and 2 are combinations of effects, not loads. There is no vector stuff going on here.

For example: say you have a three member bent - two columns and a beam connecting them at the top.

=========
| |
| |
| |
| |
^ ^

Say you have a dead and a live distributed load on the beam and a lateral (horizontal) seismic point load on the left end of the beam.

After you analyze this you have at the left end of the beam:

Shear force from D, L and E
Moment from D, L and E
Axial from D, L and E

The equation you speak about, 9.5.2.7-1 and 2 are combining these EFFECTS (V, M and A) not the loads.
The load combination D+L+E would be used in the formula by combining the following:

The design axial load in the beam at the left end would be:

rho x (A(dead) + A(live) + A(seismic)) + )0.2 x S(DS) x A(dead))

Likewise for shear and moment. So keep in mind you are adding the effects (shear moment axial) in the member being designed due to the loads applied as they are (horizontal or vertical).

 

[tvar]

Thanks JAE, that's what I thought but couldn't convince a co-worker but he should get it now with your answer. Thanks again.

 

[haynewp]

JAE, why would the normal live and dead load effects be multiplied by rho?

 

[JAE]

Wooooooppppsss.

I think it should read:

(A(dead) + A(live))
+ rho x [((A(seismic) + (0.2)(SDS) x A(dead)]

Does that look right?

 

[JAE]

Then again - I don't think that the rho factor applies to the 0.2 x SDS x D.

 

[haynewp]

Yes it's just rho times the horiz. seismic.