ASCE 12.4.3.3 Allowable Stress Increase Factor

[JoshPlumSE]

This section of ASCE troubles me and I'd like to get some other opinions about its applicability. It's in section 12.4.3 which deals with the overstrength load combinations. So, it clearly only applies when your load combinations include the Omega overstrength factor. So far, so good.

But, then the section (12.4.3.3) says the following:

Where allowable stress design methodologies are used,... allowable stresses are permitted to be determined using an allowable stress increase of 1.2

What is considered an allowable STRESS methodology? Does AISC's new 13th edition Allowable STRENGTH design qualify? Or, are we really just talking about Masonry and wood design? The only codes that I know of which are still stress based. Or, maybe it applies to allowable stress for soils or piles or such.

I believe this provision dates back to the old 1.7 Allowable Stress Increase Factor from the 2000 IBC or so. And, that seemed to be used mostly for steel detailing when using the 9th edition Green book. However, it's not clear whether it applies to today's ASD steel design.

Also, why the heck can't we get rid of this stupid provision and just move this over to our load factors like we've done for most other allowable stress increase factors!

 

[sandman21]

Do you use the load combinations it lists? If yes then you can use it. ASD, strength or stress are the same thing.

P.S. ACI 530, Green book are allowable strength design.

 

[ATSE]

Josh,
I don't believe it is intended to be used for the soil component of foundations, which is still ASD in the code and practice. I just ran some numbers, and using AISC 360-05 with this 1.2 factor. The demand/capacity ratio using LRFD or ASD could be better.
As an example:
Narrow braced frame
D=40 kips
E=100 kips
SDS = 1.0g
Ru = (0.9-0.2)D-Omega*E = 172 kips for LRFD
R = (0.6-0.14)D-Omega*0.7*E = 122 kips for ASD

172/0.75 = 229 kips LRFD
2.0*122/1.2 = 203 kips ASD
About 13%. However, you can get much more variance with different D and L and E values.
All things considered, not terrible agreement for my made-up, and this allows all the old-timers to keep ASD.

 

[ATSE]

Sandman,
ACI 530 is the worst written code. Ever.

 

[RFreund]

My first thought was that stress is stress and strength is strength so no the 1.2 factor would not be applicable for allowable strength design.
However the reason (I believe) they got rid of the 1/3 stress increase is because it was moved to the "load side" of the combinations in the form of 0.75*Load. So if your using the ASD (strength) load combo's then it would make sense (to me) that you can increase the allowable strength by 1.2 because this is not accounted for in the load combos.

EIT

 

[JoshPlumSE]

I think I'm beginning to understand it better.

In the 2000 IBC, the ASIF was 1.7. But, it was applied only when using ASD design for strength level overstrength LC's which did not include the 0.7 factor on the earthquake loads. Therefore, the new codes (which include the 0.7 factor on EQ loads) now use a factor of 1.2 (because 1.2 = 0.7 * 1.7).

Now I understand where the number comes from historically and when it applies. The only part that still confuses me is how it related to ASD vs LRFD steel.

Here's my thinking:

1) LRFD loads are roughly 1.4 times ASD loads. This is based on the EQ load factors only. Which should be reasonable because we're only talking about earthquake load combinations.

2) The LRFD capacities are 1.5 times what ASD capacities. This is based on the 0.9 phi factor compared to the 1.67 safety factor. Or, the 0.75 phi factor compared to the 2.0 safety factor.
0.9*1.67 = 1.5
0.75*2.0 = 1.5

3) By this rationale, an ASD increase factor is needed to set these two design methods to be equivalent. But, we should set this to 1.5 / 1.4 = approximately 1.1 rather than 1.2. So, why do we end up using 1.2 instead?

 

[smwa]

Josh,

I have the same question myself. It seems to me that one should be allowed to use phi = 1.0 when applying the overstrength load combination, i.e. comparing "probable" maximum seismic load (omega) to the "probable" strength (phi=1.0). However, I can't find anything in code to back this up.

If we run your numbers again using phi=1.0:

1.0*1.67=1.67, and 1.67/1.4 = 1.2

The fact that the code allows the 1.2 increase in ASD, seems to imply that phi=1.0 when checking for overstrength combination in LRFD. Otherwise it is more advantageous to use ASD when checking for overstrength load combinations.

Again, there is nothing I can find in the codes that allows the use of phi=1.0 for overstrength combination, so I'm interested to hear other people's thought on this subject.