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Re: Site specific response spectra[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaoc(--nospam--at)seaoc.org
- Subject: Re: Site specific response spectra
- From: John Nichols <cejn(--nospam--at)engmail.newcastle.edu.au>
- Date: Mon, 18 May 1998 10:27:24 +1000
Dear Chris, I have been analysising two storey to seven storey masonry builidngs as part of my research work. Its purpose is to try and get a handle on what a shear wall sees in a real building. The building model I used was based on Benedettis work at Milan. It is interesting to note that one collapsed when they were tuning the table. I have been using a number of earthquake traces, but the one that is most interesting is Nahanni. It is a horrendous event. What I have found is a very strong selection of the dominant mode in the two principal directions. That's waht one would expect from the mathematics so I am not trying to teach about egg sucking. The problem is in an event like Nahanni there is so much energy around right across the spectrum that most buildings will find energy at their natural modes. It may be less than the peak ground acceelration, but 0.1g is enough to stuff brickwork and their are very few frequencies at Nahanni that are less than that level. The Fast Fourier transform of acceleration trace from nodes in the shear wall shows a very strong peak at the dominant frequancy of the direction of the shear wall. So depending on where you live and how long since your area has had a major event I would be loath to scale back any loads that can get into the building, unless you have a basement. Rutherford and Chekenes recent report for Abrams on the LAMB project comments on this point. By the way the UBC loads are implicitly minimum design loads. For Californai the method is probably ok, but for Eastern North America, I would be extreemly loath to design a building for less than a good Intraplate event say Nahanni. Yes I am conservative, make no apologies for that. Your problem is picking the buildings in America that will last a millenium and be hit by a New Madrid 7+ or series. The problem is of course cost in terms of human life and buildings. This is of course always teh problem with codes. it would be interesting to known teh retrun period for teh design of teh white house. Then again the old british saying probably applies The King is dead long live the King. (Dennis I have no idea who coined this phrase so please if you know dinna tell me.) The other thing is it is much more imporatnt to get a good bond on the masonry. If you are using calcite white bricks Sugo's recent work in Australai definately shows that that brick needs lime in the moratr to achieve bond. A 1:1:6 mix is best. Good luck john nichols Chris Palmateer wrote: >Due to the occupancy of the structure, a site specific analysis was >performed by the geotechnical engineer...for short period structures, >the accelerations are greater than the normalized response spectra >given in Fig. 16-3 of the UBC...we are now designing the "structure" >for this increased base shear...Do "elements" of the structure also >need to be scaled up by the same factor? I'll give you two answers: 1. Since the UBC allows you to "scale down" the results to the code required base shear, as far as I can see there is no code requirement to "scale up" the element forces. The sole purpose of the site specific response spectrum in this case appears to be to determine a more accurate distribution of the base shear within the structure. 2. However, if the intent of obtaining a site specific response spectrum is to design for the potential of higher than code prescribed forces, amplification of code values for elements should be considered. Since you state that you are designing for increased base shear, it appears that it is your intent to design for forces beyond code prescribed forces. (This is not uncommon for critical structures.) In this case, I would consider the code element forces as minimum forces, and I would calculate the acceleration at each level of the structure from a dynamic analysis using the site specific response spectrum. Then I would base the acceleration of each element on these results. (I would divide the calculated acceleration by the R-factor for design.) Hope this helps.
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