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RE: Advice on a 230 ft long bldb[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: RE: Advice on a 230 ft long bldb
- From: "G M" <newabhaju(--nospam--at)hotmail.com>
- Date: Fri, 11 Jul 2003 17:36:38 +0000
Dennis: Thank you for your input. I do appreciate it.Regarding conventional construction, I do not feel comfortable with it - I do not even consider it in my design unless it is a small single story addition to an existing building where analysis would not make sense because the rest of the building does not meet code anyway.
You raised a vaild point about the validity of the analysis especailly if the building is modelled or changed at a later date. Howeve, as an EOR, I would presume my responsiblity ends when changes to design is made without my approval.
Regarding the conservativeness of the envelope solution, I agree with you - it is consverative. However, I do not believe it adds a whole lot to the cost of the building (I have not done the acutal cost breakdown; this statement may be premature). Moreover, a recent CUREE seminar I attended at San Francisco seems to support my use of the envelope solution. From what I understood, CUREE tests results show that a complete rigid diaphragm analysis did not fully support the test result - in fact, the speaker recommeneded the envelope solution.
I have not started the design yet - but, if you are interested , I can send you a copy summarzing the shear values.
Your statement "By using an envelope solution, you must maintain a copy of your engineering ( analysis and drawings) and be available ...." in your last email caught my attention. I presume, based on this statement, that you do not keep a copy of your calculations. I have heard that engineers submit the original calcualtions to the city with permit applicaiton so that in case there is a lawsuit, they can claim that they do not have the calcs hoping the city would have tossed the submitted copies. Is this the concept here. I would definately appreciate your thoughts on this.
I have heard about your program but have not used it yet. I had develped my own spreadsheet for analysis purpose using the envelope solution ( with option to use flexible diap. method only). Wall rigidity is based on wall length. (I found that nail slip and shear deformation has a much larger contribution to walll deflection than flexure and holddown slip). If you are interested, I can send you a copy - I have not finished the manual yet and can include simple examples that explains the input. Please e-mail in private at grm(--nospam--at)engineer.com
From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net> Reply-To: <seaint(--nospam--at)seaint.org> To: <seaint(--nospam--at)seaint.org> Subject: RE: Advice on a 230 ft long bldb Date: Tue, 1 Jul 2003 14:22:34 -0700 Gautam, Wonderful responses and I think you see my point. The weak direction is longitudinal as there is a garage below each two story unit. I raised the issue of conventional construction, because I wanted to raise the issue of that possibility that the developer of a similar project would have, at his disposal, a means to design the structure for the greatest profit and the lease structural materials. A project like this might promote conventional construction. This does not imply that I worry an engineer might lose work - but that the code would create a potential for a hazardous structure designed to a lower standard that would be provided by an engineer. The issuance of a building permit and compliance with this section of the code by non-engineers would be enough justification to clear a developers conscience should anything happen and the structure collapse in a strong motion earthquake. If the units are condo's, the cost to repair would virtually break the homeowners association and the developer walks away free of any responsibility. If you were a developer and were ignorant of the structural issues that we raised (or aware of them but not caring or believing) which choice would you make. Unfortunately, in my area, I am confident that the developer would choose the least costly means to get the project through the permit stage and free his/her conscience. This is food for thought when our profession is promoting conservative solutions that result in high construction costs. Now on to the next issue. Envelope Solutions verses simplified Static approach. First off, I would be very interested in seeing a comparison of solutions for a building this long and this narrow regardless of where your choice of shearwalls will be placed. In other words, does the horizontal shear transfer through the diaphragm using rotational analysis contribute any significant shear to exceed the capacity of the shearwalls (or frames) that exist from the static design methods (flexible)? In other words: when adding in the effect of rotation, will the new or revised demand exceed the capacity of the shear elements designed for static design only? By increasing the shear using the Simplified Static principle, will the revised shear wall capacities have enough reserve to compensate for the envelope solution. Now let's get into some interesting areas. I propose that using an envelop solution, albeit conservative, creates solutions that have no practical future value should the building be remodeled or changed. I will admit that the likelihood that this building will be modified is of much less chance than had this project been a 230 foot long single family residence. By using an envelope solution, you must maintain a copy of the engineering (analysis and drawings) and be available to provide your work to another engineer who will need to recreate what you have designed or at least know where your load-path exists. It is highly unlikely that the building department will keep these records or the that building owner will properly maintain the records. I haven't seen it except in buildings that are managed and in these cases, there are generally some drawings that are maintained. The reason I raised this issue is two-fold: Having created a spreadsheet called Multi-Lat(r) with David Merrick,SE (Multi-Lat) [which is available at http://www.structuralist.net/yabbse/index.php?board=3 ] I was able to compare the results of each method on building I designed and found that in most cases the additional shear contributed by rotation does not add significantly to the capacity of the shearwalls designed to the higher base shear. The example posed in the ICBO Seismic Design Manual II had two such problems designed to full compliance where the shear wall demand would have contributed very little to upgrading the walls since there was sufficient reserve in the panels for the additional shear caused by rotation. I'm not trying to argue that we should omit rotational analysis because it is difficult or unfamiliar to those who specialized in light-framing. I suppose that I could argue that we have no "intuition" as to how the materials will behave when rotational design is considered. Another problem with full compliance is a two-fold issue. If you contend that you will choose the worst case shear caused by seismic (flexible vs. Rigid) and wind and apply this load to each line of resistance it will, as noted above, be difficult to recreate. However, this is fairly straight forward. The second part of the problem comes if you rely upon rotational design, and try to balance the walls which thee demand from wind or flexible design replace those of the straight rigid analysis. Almost every other wall will need to be tweaked in order to balance the distribution of shear. The resulting "Envelope" solution would be highly conservative. The bottom line of each of these issues is that we might be doing something wrong here. We might be trying to compensate for what we can not change in the field - improvement in quality and performance of construction. We end up designing ourselves out of the business when there are less conservative buy reasonable performing (if this has ever been described) buildings. I think as a professional community we are indirectly creating incentives for poorer performing buildings. Had we stuck with a rational method of design for low-rise structures, but became more involved with the detailing and construction of the buildings we might solve some of these problem. Sorry to use this as an example as I know this is not what you wanted, but these are the issues that need to be resolved. Thanks Dennis -----Original Message----- From: G M [mailto:newabhaju(--nospam--at)hotmail.com] Sent: Tuesday, July 01, 2003 12:15 AM To: seaint(--nospam--at)seaint.org Subject: RE: Advice on a 230 ft long bldb Dennis: Thank you for your input. The project consists of 10 units each 23' by 37. The walls on the back is about 50 percent open with at least 4'0" wide shear wall at each unit. The second story wall above the open front is also about 50% soild. There is garage under each of the unit at the first floor - i.e. every unit has an open front. The shear walls are generally stack up. You raised several issues: 1. conventional framing: The building meets the conventional framing requirement; however, I do not feel comfortable using the conventional method and try to stay away from the prescriotive method. 2. Simplifed base shear 3 Ca W/1.4R versus 2.5 Ca W/1.4R: Several San Francisco Bay area cities waive the rigid diaphragm analysis requirement if one uses the simplified base shear. However, I do not use the simplified base shear method because I do not find it conservative. It is true that the total base shear is increased; however, because the code does not require a vertical triangular distribution of base shear (first mode of vibrartion), I find that the use of simplified base shear can grossly underestimate the overturning, and therefore the holddown, requirements. I use the 2.5 Ca W/1.4R base shear and use the envelope method (flexible and rigid diaphragm analysis) in my design. Once a spreadsheet is set up, the rigid diaphragm analysis and shear wall deflection calc is only a click away. 3. Open front: Seeing the damage from Northridge, I neglect the fact that the code allows shear trasfer by rotation (section 2315 of the code). I have not started the design yet - I am still in the contract phase. Gautam ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. 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