ASCE 7-05 Seismic Provision

[rgerk]

I don't understand the thought process of splitting the seismic load effect "E" into two components (Eh and Ev). I have calculated the Eh and Ev components but am trying to grasp how to use them in the load combinations.

In my structure, I have assumed the Ev to be negative (uplift). No particular reason, just the way I chose to think about it. The Eh is also negative due to customer sign conventions. With all that said....

Section 12.4.2 states:
E = Eh + Ev in ASD load combinations 5 and 6
E = Eh - Ev in ASD load combination 8

In my case, looking at E = Eh + Ev. Do I add the two loads and then substitute the E into the load combination or should I use Eh and Ev in the load combination so that the result is additive? If I add the two loads, which direction is E applied.

Any help would be great.

Ryan

 

[dik]

It might be the primary and secondary wave of a typical earthquake where the primary one is essentially a compression wave and the secondary one is one of vertical translation.

Dik

 

[JAE]

rgerk,

Check out these threads that discuss the vertical component effect from seismic:

thread176-98527

thread487-121459

 

[JAE]

Ryan - per the two referenced threads above - the key to remember here is that E is not a load....its a load effect. E can be vertical, horizontal, or in any direction or directions.... its an effect DUE to seismic loads and in the case of E = Ev + Eh - its simply saying that in any combination that E occurs, you need to include ALL effects that are due to seismic...these effects can be due to horizontal forces and can be due to vertical forces.

You don't add the forces since they may have different vectors, you add the results of those forces.

Hope this clears it up.

 

[rgerk]

JAE,

Thanks for the information. I believe I understand it. Let me recap my thinking now.

Considering E = Eh + Ev:

load comb 5 - D + H + F + 0.7Eh + 0.7Ev
load comb 6 - D + H + F + 0.75(0.7Eh + 0.7Ev) + 0.75L + 0.75(Lr or S or R)

Considering E = Eh - Ev:

load comb 8 - 0.6D + 0.7Eh + H

In load combination 8, I wouldn't considered the load effects from the vertical seismic Ev.

Does this make sense?

Ryan

 

[JAE]

I think the negative for the Ev is intended to suggest that the vertical component of the seismic effect E should be taken upward to make the overturning check more conservative.