Wind and seismic stresses - increase in allowable buckling load

[GraemeL]

In Section A5.2 of AISC - ASD, one is allowed to increase the allowable stresses by 1/3 for wind and seismic loads.

I was interested in finding out whether others increased the allowable Fa determined in Section E2 when using eqn E2-2. That is, when Euler buckling prevails and Cc > Kl/r.

I have always increased it based on what is shown in Section H for the definition of F'e. However, I have come across others who say a buckle is a buckle and don't give any allowance for the increased axial carrying capacity under short term loads when Cc > Kl/r.

So I would appreciate seeing what others think.

 

[JAE]

From a "pure" logical flow of the ASD, the 1/3 stress increase would apply to either E2-1 or E2-2. The concept of the 1/3 increase is to supposedly account for the fact that full gravity loading in addition to full code wind load will rarely occur simultaneously.

Thus the increase is dealing with the statistical behavior of the LOADS and is really independant of the CAPACITY.

However, there was once a great article written in the AISC Structural Journal entitled, I think, "The Mysterious 1/3 Stress Increase factor". The author spent a lot of time researching the source of the 1/3 increase and came up empty.....

he may be looking for the true source of the Nile now.

 

[GraemeL]

Thanks for your thoughts.

I have not heard before that the 1/3 increase was due to the load statistical behaviour not short term capacity.

This is interesting as API RP-2A "Planning, Designing and Constructing Fixed Offshore Platforms" Section 2.3.6.c4 allows the basic AISC allowable stresses to be increased by 70% for seismic loadings for offshore platforms. Thus permitting some minor yielding but not significant damage.

The allowing of minor yielding may be a different issue rather than accepting members loaded quickly by short term loads have a 33% or 70% increase in capacity over what they would have with the same load applied long term.

Anyone else into the fray?

 

[ajh1]

I'm not familar with the off-shore platform spec, but most building codes today define seismic in an "ultimate strength" or "member strength" format. The clearest example is in UBC where it adjusts the ASD allowable by a factor of 1.70 for those loads. Alternatively, there is the option to divide the seismic load by 1.4 and use ASD allowables. The 1.4 factor brings the 1.7 boost down to approximately the old 1/3 stress increase level.

 

[par060]

I believe if you go to the AISC web site you can down load the errata for the ASD manual. The 1/3 increase for these loads has been removed

 

[GraemeL]

I cannot find details of such a change to the ASD manual on the AISC website. Can you list a detailed website address that can take one direct to the errata?

The "Seismic Provisions for Structural Steel Buildings", that you can download, Part III, Section 4.2a changes the ASD manual Section A5.2 to allow the 1.7 factor in place of 4/3 for the earthquake load combinations given in Section 4.1. Is that what you mean?

 

[par060]

It is in the resource section of the AISC site(

http://www.AISC.org)....You

can down load "Supplement No. 1 to the Specification for Structural Steel Buildings" Allowable Stress and Plastic Design...the section I'm speaking of is on page 24.

 

[GraemeL]

Thanks for the location of the supplement. As I see it, this supplement is the way AISC are transitioning ASD into LRFD. The 1/3 increase goes away if you use the load combinations given in ASCE 7.

You may have picked by my second post that my main concern is in the area of offshore platforms. There, API RP-2A Working Stress Design governs and I assume that ASCE 7 would be used for structures in USA only for onshore structures.

Thus in the API RP-2A WSD world, a strict working stress regime applies and the 1/3 increase still is allowed. API have a separate code covering LRFD design of offshore structures that specifically excludes the use of the AISC LRFD code.

So I am now interested in how others handled E2-2 in the pre-LRFD days. The 1/3 increase for E2-2 reduces the safety factor of 12/23 to 48/69. Was everyone comfortable with this? Even if the majority of the axial load was the result of just the wind or earthquake forces?

 

[RobertoMiguel]

You can avoid all this speculation by just "normalizing" the applied loads. In the Air Pollution Equipment industry we use a factor of 3/4 on load combinations containing one transient load (be that wind, seismic, or some excursion operating load) and a factor of 2/3 for combinations containing two transient loads. This is compatible (or conservative, in comparison) with most building codes.