ASCE 41-13: Seismic Evaluation and Retrofit of Existing Buildings 2

[ChipB]

Hey all:
I've been away from the threads for a while. Sorry...

I'm evaluating a building using this standard. In Chapter 7 of ASCE 41-13, it defines the Gravity Loads, Qg as 1.1(Dl+0.25LL+SL). It does get a little bit further in defining what to use for torsional effects, etc, however, it really doesn’t discuss the use in evaluation that I can see. It doesn’t define a method (ASD or LRFD).

The way I am interpreting it is:
The 1.1QD+L+S Value is to be used in place of D in 1.2D + 1.0E + L + 0.2S of the Strength Design Formula
Effectively creating the formula: 1.2(1.1QD+L+S) + 1.0E + L + 0.2S
Which once you go to Chapter 12 of ASCE 7-10 creates the formula:
(1.2+0.2Sds)(1.1QD+L+S ) + 1.0E + L + 0.2S
And
The 0.9QD+L+S Value is to be used in place of D in 0.9D + 1.0E
Effectively creating the formula: 0.9(0.9QD+L+S) + 1.0E
Which once you go to Chapter 12 of ASCE 7-10 creates the formula:
(0.9-0.2Sds)(0.9QD+L+S) + 1.0E

Am I interpreting this correctly? ASCE 41-13 specifically states, it is a “Standard”. So do I take this “Standard” and evaluate the building using the load combinations above?

To be honest, I will be using ASD combinations, not strength, but the same principle applies.

Thanks in advance,
Chip

 

[structSU10]

I have been digging into ASCE 41 a bit myself, but do not have extensive knowledge (only used it for one project so far). The seismic forces in ASCE 41 are "amplified" to produce the expected drift on an elastic analysis, it uses no R factor, but it related to using a R factor less than unity. In that sense, the load combinations and acceptance criteria within ASCE 41 should be followed and not mixed with ASCE 7 provisions. the use of 'm' factors, deformation controlled actions, and force controlled actions are unique to that code, so ASD vs LRFD doesn't exactly apply. You do use 'LRFD' capacities for force controlled issues however.

 

[MrHershey]

I wouldn't mix and match. ASCE 41 is a standalone standard. Where it references ASCE 7 you can look at the sections it references specifically, but don't try and mesh the two otherwise.

Your load combinations in ASCE 41 are essentially Gravity Loads (QG) + Seismic (QE). QG can be either 1.1(D+L+S) or 0.9D, depending on if gravity load helps or hurts. 7.5.2 gives combos of essentially QG+QE for linear procedures. So it's 1.1(D+L+S)+1.0E or 0.9D+1.0E for load combinations for linear procedures.

 

[ChipB]

Thanks guys!
Ugh.... Now my head hurts worse. I have members failing under DL only, which, since the building has been there since the mid '50s, we know is not possible.