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RE: Progressive Collapse Design[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: RE: Progressive Collapse Design
- From: "Michael Valley" <mtv(--nospam--at)skilling.com>
- Date: Thu, 12 Aug 1999 13:18:48 -0700
- Comments: Authenticated sender is <mtv(--nospam--at)linux.skilling.com>
- Priority: normal
James: You wrote: > The pushover analysis gradually increases the load to a specified > maximum deflection or failure. If as a minimum columns must be able > to be removed how would a push over analysis help. Educate me. I use SAP2000 for both dynamic nonlinear and static nonlinear analysis. I'll outline the concept you might use as the basis for your analysis and another important consideration. First, remember that SAP2000 and XLinea are general purpose analysis programs. Don't be discouraged if the manual or tutorials ignore your class of problem. My most interesting and rewarding analyses have involved figuring out how to get the program to do what I want. The simplest approach is to simply design the system (statically) for each possible missing column condition. However, the resulting design might not be economical and certainly wouldn't indicate what happens when the element goes away. Saying this method is not an option... The basic approach to the removal of an element (using a program like SAP, that doesn't directly support element removal) is as follows: With the element in the model, apply the static loads that simulate the initial condition. Find out the internal loads in the element in question. Modify the model by removing the element and introducing loads that are equal and opposite of the internal loads previously noted. Now you should be able to run a static analysis with the applied loads and the new "internal" loads and get the same answer as before. The analysis proceeds by ramping the "internal" loads to zero, which simulates loss of the element. The idea is to remove the element and watch what fun things happen as a result. If you are performing a dynamic analysis, the rate of load removal (element death) can be significant; you will need to play with this parameter. Because the analysis is nonlinear dynamic, it is important that the mass, stiffness, damping, and strength characteristics of the system (and elements) are appropriate. Simply run time history analysis where the load ramps to zero and stays at zero for a suitable length of time (for instance, until the resulting motion damps out). If you are performing a nonlinear static analysis, the rate of load removal is not as significant since the analysis consists of a series of static analyses; inertia and damping do not come into play. The strength and stiffness of the system (and elements) are the important parameters. Using SAP2000, a pushover analysis can be either force or displacement controlled. The controlling parameter can be in the vertical direction (although that's certainly not how most pushover analyses are performed). The important consideration I alluded to involves the decision of whether to use dynamic or static analysis. If the response of the system is expected to be dynamic (dependent on inertia and damping), you really should not use nonlinear static analysis. I've had a lot of discussion with the folks at CSI and with the developer of XLinea on this topic. We all agree that one of the two biggest weaknesses of the pushover method is the fact that it misses dynamic response. Even when the final results aren't dependent on dynamic interaction, the presence of inertia and damping in the dynamic solution method significantly improves convergence. In the static method there are several key decisions that must be made by the software developer and user that affect load redistribution, etc. I can provide a little more detail offline if you're interested. > Correct me if I am wrong but SAP and Xlinea both do a pushover > analysis. SAP does 3-D and Xlinea only 2-D. SAP does not do shear > walls while Xlinea does. XLinea is still based on the DRAIN-2Dx elements, input file, and solution engine. The DRAIN panel element is limited to elastic behavior, just as the SAP2000 shell elements are. SAP2000 can do 3D pushover analysis. However, I would caution that it becomes much more difficult to understand what's happening as the structure becomes more complex. Under most circumstances 2D pushover analysis offers improved understanding without being so difficult to understand. (Of course, there are times when you may really need 3D interaction; make sure your fee is large.) -Mike - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Michael Valley E-mail: mtv(--nospam--at)skilling.com Skilling Ward Magnusson Barkshire Inc. Tel:(206)292-1200 1301 Fifth Ave, #3200, Seattle WA 98101-2699 Fax: -1201
- RE: Progressive Collapse Design
- From: James Lane, P.E.
- RE: Progressive Collapse Design
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