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RE: RE: Special Moment-Resisting Truss Frames[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: RE: Special Moment-Resisting Truss Frames
- From: hsprague(--nospam--at)aspen.klaalov.com (Harold Sprague)
- Date: Fri, 14 Aug 1998 15:14:41 -0600
The encumbrances were more with the concept of a "seismic-fuse", and before 1997 there were no provisions like 2213.11. What was intended with the truss frames I was familiar with was to allow ductility to occur in the end connections. The gravity connections were predicated on the amplified plastic moment of the connections plus full gravity loads. The trusses spanned in excess of 100 ft which is a violation of UBC 1997 sect 2213.11. The pre 1988 UBC was a lot more dependant on the engineer understanding the intended performance of a structural system in a seismic event. But this is unfortunately difficult to administer. And as EQ damage reports indicate, some engineers did not understand how to design for intended performance. It is much easier to design and administer prescriptive code requirements. I have designed industrial structures in many foreign countries where specific code compliance was ambiguous. But with some prompting, the owner could tell you what he intended regarding performance. I guess I am of the opinion that I would like to have the latitude of performance driven codes. Some of our most important structures are driven by performance requirements. Various aspects of nuclear structural design, petro-chemical plant designs, dams, hazard material containment structures, etc. are designed based on performance. A liquefied natural gas impoundment structure is required to not leak following an earthquake. Granted, some manufacturers take liberties with performance specs. But in that case, let's talk about peer reviews as opposed to prescriptive codes. It can be observed by the examples cited that life safety and collapse prevention are well short of intended performance. There is some research being conducted on seismic fuse / semi rigid connections, but I do not think that it is in a truss application. Harold Sprague, PE Krawinkler, Luth & Assoc. -----Original Message----- From: Eddie Gonzalez [SMTP:Eagonzal(--nospam--at)ENG.CI.LA.CA.US] Sent: Thursday, August 13, 1998 6:04 PM To: seaint(--nospam--at)seaint.org Subject: Re: RE: Special Moment-Resisting Truss Frames The issue was ENR June 1, 1998. The project was an office building in Sacramento, California. Althought the article doesn't address it specifically, the picture seems to indicate that the ductility is provided by a sacrificial x-brace bay which is smaller than the rest of the chord elements. Can you specify what UBC restrictions you ran into when using it/ if your were to use it in the States, particularly, western states? Is it in reference to yielding of members or something else? The article notes that "Codes changed appreciably after the 1994 Northridge Earthquake, which exposed a tendency for brittle fractures in SMRF connections. Such changes included precursors to performance-base design, to allow owners to specify expected degrees of quake damage. Such a change in the 1996 supplement to the 1994 Uniform Building Code effectively permits the use of moment-resisting trusses with members that yield." Of course, as noted on the thread discussion regarding performance-base design, the owner's performance objectives must meet some be justified, particularly, to local building dept. The article indicates that the analysis had to be done by hand because of a "dearth of relevant software." I am assuming the analysis is iterative/nonlinear. If you know of any Research/Testing on this system, please let me know. Thanks again for your comments. Ed Gonzalez >>> Harold Sprague <hsprague(--nospam--at)aspen.klaalov.com> 08/13/98 02:55PM >>> What issue was it in? I have designed trusses to resist seismic and wind in frames all over the world where UBC does not dictate or limit designs. You basically must think in terms of nonlinear performance. Trusses in high seismic applications offer special challenges that were addressed by designing the truss for a controlled mode of failure. This was generally accomplished by using the "seismic fuse" top ductile connector plate and designing the truss for the applied ultimate strength / moment induced into the truss with an appropriate overstrength factor like the 1.25 used in concrete moment frame design. Harold Sprague, PE Krawinkler, Luth & Assoc.
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