Wind and seismic stresses - increase in allowable buckling load
Wind and seismic stresses - increase in allowable buckling load
(OP)
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.
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.





RE: Wind and seismic stresses - increase in allowable buckling load
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.
RE: Wind and seismic stresses - increase in allowable buckling load
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?
RE: Wind and seismic stresses - increase in allowable buckling load
RE: Wind and seismic stresses - increase in allowable buckling load
RE: Wind and seismic stresses - increase in allowable buckling load
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?
RE: Wind and seismic stresses - increase in allowable buckling load
RE: Wind and seismic stresses - increase in allowable buckling load
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?
RE: Wind and seismic stresses - increase in allowable buckling load