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Re: R-values in seismic design
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- Subject: Re: R-values in seismic design
- From: majid Sarraf <msarraf(--nospam--at)uottawa.ca>
- Date: Sat, 26 Sep 1998 18:56:17 -0400
Jon, IMHO, first, when we apply a redcuction factor to base shear it means that during a earthquake we expect members to develop inelastic deformation within their ductility capacity. So load readuction factor comes at the cost of predictable and controlled damge, or yielding or ductile deformation in strcutural members. Second, it is important to note what happend to the bracings during sever exciations. As lateral load due to earthquake increases, first, the compression member buckles and quickly losses its strength and stiffness, then to resist the same panel load, a greater share of load is resisted by tension member and it is overstressed untill it yields. Further, and to qulify for load reduction of R=2, tension member must deform inelastically, maybe, upto 3 or 4 times it inital yield displacement, during which end connections must not fail. In next half cycle the compression member acts in tension, it yields and must deform in a ductile manner while its connection must well resist the yield force of the member. So the idea of designing end connection of the member to resist the yield force developed in the member sounds very logical to me. In fact, I believe, we must try to design the end connection by considering a safety factor to its prevent premature damage before or exactly when the member yields in tension. (e.g design the end connection for 1.1 Ag Fy) I hope this helps, Majid Sarraf Civil Eng. Dept. University of Ottawa Http://www.uottawa.ca/~s056517 At 03:29 PM 9/26/1998 -0400, you wrote: > Excuse me for not explaining myself fully when I introduced the R-factors >without referencing their respective codes and location. The R-factors I >was discussing were with respect to the National Building Code of >Canada(1995). I will explain briefly how , we Canadians, obtain our >elastic base shear and design base shear. > >Elastic Base Shear >Ve= v*I*F*S*W >v is the zonal velocity ratio ranging from 0 to 0.4. In my city Ottawa >our zonal velocity ratio is 0.1 >I is the importance factor of the structure which varies between 1 to 1.5 >F is the foundation factor which varies from 1 to 2 >S is the seismic response factor which is dependent on the natural period >of vibration of the structure and the ratio of acceleration related to >velocity related seismic zones. This factor varies form 4.2 to 1.5/T*0.5 and >W is the weight of the building (includeing 25 percent of the design >unfactored snow load. > >Design Base Shear >V=0.6*Ve/R >0.6 is a factor representing level of protection based on experience. Our >R-factor is dependent on the type lateral force resisting system used. Our >R-values range from 1.5 to 4. To obtain a greater R-value more stringent >detailing requirements are required. Some of the corresponding lateral >force resisting systems include: > >STEEL >R=2 BRACED FRAME WITH NOMINAL DUCTILITY > MRF WITH NOMINAL DUCTILITY > >R=3 DUCTILE BRACED FRAME > >R=4 DUCTILE ECCENTRICALLY BRACED FRAME > >CONCRETE >R=2 WALL WITH NOMINAL DUCTILITY > >R=3.5 DUCTILE SHEAR WALL > >R=4 DUCTILE COUPLED SHEAR WALL > >A R value of 1.5, which corresponds to any system which has no ductile >detailing requirements, is permitted depending on location and the number >of stories of the structure. > > Our load combinations from the NBCC 1995 are the following > >1.0D + 1.0E >or >1.0D + 0.5L + 1.0E > > > My question was with regards to the detailing requirements for a steel >braced frame with nominal ductility(R=2.0). Our requirements are governed >by CAN/CSA-S16.1- >M which states (clause 27.5.3) the connection must be designed to > >"In velocity or acceleration related seismic zones of 2 or higher the brace >connections shall have a factored resistance at least equal to the axial >tensile yield strength of the brace (AgFy) unless the designer can show >that a lower resistance is adequate." > > I find this clause leads to excessive forces for connection detailing and >that the final clause...(unless the designer can show...) is too open >ended. Any suggestions? > >thanks > >Jon Turner > > > > ~ ~ ~ ___ [_________]-|__) __________OO______O__O_O___________________________ |==========================================| | | \ Majid Sarraf / \ Ph.D Candidate / \ Department of Civil Engineering / | University of Ottawa | | Ottawa, Ontario | | Canada K1N 6N5 | | Tel:(613) 562-5800 Ext. 6159 | | Fax:(613) 562-5173 | | | ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
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