Seismic load, horizontal and vertical 2

[BulbTheBuilder]

thread507-437683

I'm a young engineer in the industry and would like some help on these. I have read through "Seismic load, horizontal and vertical" and I would like to clarify these.
1. Both vertical and horizontal seismic loads are imperative in seismic design as seen in ASCE 7-16 (12.4.2).

2. cl (12.4.2.1) Eh=pQe,where Qe is the effect of horizontal seismic forces V or Fp required by section 12.5.3 or 12.5.4. If you read 12.5.3.1(a), it says, "...The structure shall be analyzed using the equivalent lateral force analysis procedure of Section 12.8.."

Now, equivalent lateral force (ELF) procedure (12.8) can be said to be used in determining seismic base shear and vertical distribution. Hence Eh = p x (results from ELF)? However, I have seen so many examples where they use the horizontal forces from ELF only without any Ev, is it to do that and assume Ev will be a resultant vertical reaction from ELF? or you'd need to compute the Ev afterwards.

 

[ProgrammingPE]

When the horizontally applied seismic load Eh results in vertical forces in your members (like what occurs with columns in braced frames), it is still Eh.

Ev is 0.2*Sds*D, and it needs to be included in addition to Eh. (The only exceptions to this are when Sds <= 0.125 or when checking soil-structure interface of foundations. See 12.4.2.2.)

Structural Engineering Software:

http://www.structuralcentral.com

Structural Engineering Videos:

http://www.youtube.com/channel/UCOj2mXXf3ZbZtgxTc6BtkGg

 

[AaronMcD]

When you say "p" do you mean rho?

Ev is not a resultant vertical reaction, it is a separate force in addition to any vertical reactions. In software, it is common to include load combinations with (1.2+0.2Sds)D and (0.9-0.2Sds)D with seismic combinations with ELF to account for vertical seismic force.

I don't know why it isn't included in many examples. Do these examples use full load combinations including dead, live, wind, etc, or are they examples focusing on the results of ELF analysis? Are the examples first order analyses? With first order, you just add the results from different loads, so perhaps the examples are just showing how to use the equivalent lateral force procedure, not a full analysis.

In some real world situations with hand checks, vertical seismic force may not be explicitly included, but rather the designer may keep it in mind and apply it when it may control.

http://www.hellodwell.design

 

[BulbTheBuilder]

@ProgrammingPE-Thank you for comment. That was/is thinking. I thought my designs were over-killed because I was using both Ev and Eh whilst I have seen Eh only being used often.

@AaronMcD- Yes "p" as in "rho".
These examples focuses solely on computation of seismic forces (as it's something I am reading on). Almost all that I have seen uses the ELF for the vertical distribution of shear and neglects the vertical seismic force. Maybe, I will assume they're just focusing on showing how to use the equivalent lateral force even though that it is the impression from their presentations.

Thank you very much for clarifying these for me

 

[AaronMcD]

Ev is included in load combinations. It is completely separate from ELF, which is a method to compute and distribute horizontal loads. If the examples are for ELF loads, they would not be focusing on other loads. Other loads would be vertical seismic, dead, live, earth pressure, wind, etc, and would be included in an analysis with load combinations including these loads. Yes, Eh and Ev are both seismic, but they are applied separately in load combinations, such as live and dead.

http://www.hellodwell.design