Text on Direct Analsyis for Stability
Text on Direct Analsyis for Stability
(OP)
Hello,
I'm supposed to do a small presentation to my department about how to use the Direct Analysis Method for Stability. I've downloaded some papers and articles from AISC, but I was curious if anyone knows of some good texts that explain it. As I haven't delved to deeply into myself, I'm not sure where to begin.
I'm supposed to do a small presentation to my department about how to use the Direct Analysis Method for Stability. I've downloaded some papers and articles from AISC, but I was curious if anyone knows of some good texts that explain it. As I haven't delved to deeply into myself, I'm not sure where to begin.






RE: Text on Direct Analsyis for Stability
Here's one.
RE: Text on Direct Analsyis for Stability
RE: Text on Direct Analsyis for Stability
RE: Text on Direct Analsyis for Stability
RE: Text on Direct Analsyis for Stability
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FWIW: One of my main points was that the method was actually a good bit simpler than it appeared when you first read the code. You've got three basic requirements:
1) Account for Member out-of-plumbness.
2) Account for Second Order Effects (P-Delta).
3) Account for the inelasticity of the structure as you approach buckling loads.
RE: Text on Direct Analsyis for Stability
- geometric imperfections and out-of-plumbness (through notional loads or direct moving of the joints in the more critical direction),
- both P-Δ and P-δ effects (probably using software), and
- residual stresses and inelasticity (through reduced stiffnesses of 0.8*τ_b*E*I and 0.8*E*A *only in the analysis not the spec equations* if your software doesn't already automatically do it).
I took a couple of classes in college with a professor who sits on the spec committee and helped with pushing DM to chapter C from the appendix, so if you have any specific questions about the method I'm sure I and a lot of other people here will probably be able to help.
Structural Design Engineer
New York, NY
RE: Text on Direct Analsyis for Stability
2. Also, P-δ effects is automatically accounted in the code checks UP TO the tolerances permitted for steel shapes being sold in the market. Hence use of K=1 and dividing the -initially surmised notionally straight- compressed members in a number of segments and using segment length for the checks should work to capture P-δ effects.
3. By AISC 360-05 it is clear that initial imperfections are to be considered and that out of plumbness should be taken 1/500 or equivalent notional floor level loads.
4. But it is not as clearly stated what would be the out of straightness to be considered -and need to be considered per 7.2 and 7.3- for compressed members, neither are the notional loads to be considered substitutively for the analysis. In my view it would be inconsistent in the frame of the regulation of the industry to enforce for the checks out of straightness above of the tolerance permitted for members to be placed at the works; if exceeding the tolerance should be rejected. So my view is then resumed respect the thing as stated in point 2.
5. If by constitutive geometry, like in bent or curved members (arch elements) or by protection against some degree of accidental out of straightness it is wanted a bigger sagitta than the 1/1500 permitted in the USA (Note the 1/1000 in the EU wouldn't be covered by the AISC P-δ member checks, these things happen when mixing international practices) by the tolerance for straight members, the appropriate corrective measures in whatever the way need to be taken.
6. For members theoretically straight initially, in my view this would encompass representing the expected imperfection directly, better than including a load more difficult to derive to mimick it; that is, except one is wanting to believe elephants fly.
7. And all this would be fine if 360-05 was the end of the matter; but it is not; 360-10 has appeared, I have to read it well but it becomes apparent it has reduced the applicability of the the direct analysis method to buildings with tiered levels; portal frames, gable type, have then dissappeared from the scope of direct design. In my view this reduction on the scope of the method may not to be warranted as a proper description of the same be included in the code; in my view, either you are confident that direct analysis is able to produce a sound and safe design, and AISC has retracted in 10 what in 05 accepted us to believe.
8. This more so because even if developing later as a consequence of theoretical and computing development, the alternative method of first order design with amplified moments should be thought to have appeared due to the means of the era not being able to produce what direct analysis can deliver. It has a sound track of practice, but you are closer to the behaviour in direct analysis than in first order with moment amplification.
RE: Text on Direct Analsyis for Stability
RE: Text on Direct Analsyis for Stability
RE: Text on Direct Analsyis for Stability
RE: Text on Direct Analsyis for Stability
RE: Text on Direct Analsyis for Stability
Your comments are interesting.... but, start another thread if you want a discussion on the subject. This was actually a pretty simple topic. Just looking for some references, not looking for a lengthy discussion on the background theory's strengths and weaknesses and how it relates to other codes.
RE: Text on Direct Analsyis for Stability
RE: Text on Direct Analsyis for Stability
RE: Text on Direct Analsyis for Stability
All this comes from that P-Delta methods are already in maybe their 3d decade of existence -with much things said in between- and are coming out to some kind of adulthood with the complex 360-10 statement of them. And so it is perhaps no bad then to remember that as much as they precise to be complex they get far from the conceptual former intent of simplifying design.
But take for example this recantantion for this case ... if the notional member needs to show a reduced stiffness to properly evaluate the solicitations it always will seem rare that then miraculously its stiffness gets revamped to full value when finding a critical value of its strength that must meet such kind of loadcase condition.
In the english speaking world you still retain the title or "Doctor in Natural Philosophy" for so much things and it is very proper because there is a philosophy for everything. The philosophy of division of problems may lead to practical solutions; but even if necessary, good and useful, it must be held inferior to the holistic omnicomprensive thorough and, hopefully, concise understanding showing the uttermost insight on the thing to which the efforts of those that want to understand comply.
RE: Text on Direct Analsyis for Stability
RE: Text on Direct Analsyis for Stability
"The design of members and connections shall be consistent with the intended behavior of the framing system and the assumptions made in the structural analysis"
To use different E in solicitations and checks IS inconsistent.
RE: Text on Direct Analsyis for Stability
RE: Text on Direct Analsyis for Stability
In short, mixing stiffnesses from the frame analysis and the spec equations is inconsistent with the way they were derived, and this approach would give you erroneous results.
Structural Design Engineer
New York, NY
RE: Text on Direct Analsyis for Stability