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Linear Time history Analysis against Response spectrum Aalysis
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- Subject: Linear Time history Analysis against Response spectrum Aalysis
- From: "A.M.Zaki" <amzaki(--nospam--at)soficom.com.eg>
- Date: Sun, 01 Nov 1998 08:51:22 +0200
here is a discussion under the titel 'LONG SPAN GIRDER'. Now I think our subject is the comparison between the results and capabilities of the Linear Time History Analysis and Response Spectrum Analysis. I want to put this in a new discussion so more engineers can share us. I would appreciate any comment, so please read the following e-mails. on Sat, 24 Oct 1998 osama ree wrote, "Hi Every body I'm trying to design 20meter span frame, which will carry a tower above it consist of 12 floor, the depth of the girder to be 3.5 meter, column dim. are 1.5X1.5 meter. 1. What should i take care of rather than the usual design criteria? 2. is some one knows about good references dealing with this subject? thanks" On Tue, 27 Oct 1998, my reply was, "I think you should take care of earthquake load, because your structure is not regular so you should analyse it against earthquakes using time history analysis (equivilant static load and response spectrum analysis can not be used) You need a design earthquake to excite the structure. An expert can simulate such earthquake according to the seismicity of the place, where the structure will be built, and the dynamic characteristics of the structure. This will cost too much. In general you should be careful of seismic loads and hence the ductility of the structure." On Wed, 28 Oct 1998 Mark Gilligan, reply to me, "Mr. Zaki I would be interested in the rational why only a time history analysis is appropriate for an irregular structure. Is the time history analysis linear or non-linear? If the analysis is linear what information is reflected in the results that is not reflected in a response spectrum analysis? The answer is none since a linear time history analysis can be defined in terms of the mode shapes just as a response spectrum analysis. In both cases the usefullness is dependent on the validity of the input motion and both types of input have their limitations. If you are doing a non-linear analysis what non-linear aspects are you considering and which are you ignoring? >From my point of view the most critical issue with transfer structures and earthquakes is that the transfer structure not fail during the big one. To deal with this you should consider the maximum forces from the supported structure in a major earthquake. Given that the transfer structure won't fail the next point to consider is whether the structure is torsionally balanced. The point being that with a little common sence you can safely design an irregular structure with rather simple analysis tools. It amazes me that people design a URM upgrade, which can asume very non-linear behavior, using manual calculations, while a base isolated structure which is typically a very regular structure requires time history analyses. " On Fri, 30 Oct 1998, my reply to Mr. Mark was, "Mr. Mark There is a difference between the results of the linear response spectrum analysis and time history analysis. 1- Finally in a response spectrum analysis you solve a static problem with a lateral story shear forces that correspond to the instant where the base shear is maximum. Some elements may have maximum response at different instant, so its response may be under-estimated. 2- In a response spectrum analysis, the response is restricted to the number of modes specified by the engineer. 3- In a response spectrum analysis you can not take the effect of soil structure interaction, which may cause significant change to the structure response. 4- Some Codes do not allow the use of the response spectrum analysis for irregular structures (e.g. The 1995 Egyptian Code). " A.M.Zaki Structural Engineer Cairo-Egypt
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