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Re: bracing conections[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaoc(--nospam--at)seaoc.org
- Subject: Re: bracing conections
- From: Evan.Jorgensen(--nospam--at)fluordaniel.com
- Date: Tue, 28 Apr 1998 9:18:00 -0400
Original message: >I'm designing a tall, flexible, steel-braced structure to the 1994 >UBC, in seismic Zone 2A. This structure is slender with large >equipment loads high in the structure, therefore, a Rw of 4 was >chosen from Table 16-P. The resulting C/Rw is on the order of 0.24. >The period was calculated using RISA-3D, since the building is being >analyzed using dynamic lateral force procedures. Per Section 1632.5 >(Other Nonbuilding Structures), the provisions of Section 1628 >(Minimum Design Lateral Forces and Related Effects) shall be met with >the exception of having a minimum C/Rw of 0.40 vs the usual 0.075 for >buildings, among other details. >Therefore, when designing the bracing connections per Section >2212.6.3, do I need to design the connection for (3/8)*Rw times the >minimum load ( i.e., using C/Rw = 0.40) or times the calculated load >( i.e., using C/Rw = 0.24, in this case)? If the calculated C/Rw is >used, the connection would then be designed for the brace load, since >(3/8)*4*0.24 = 0.36 < 0.40. Basically, the question is, does the >relaxation of the use of the (3/8)Rw scale factor in Section 1628.2.1 >apply to the design of bracing connections? >As written, I think it does. Bill Sherman wrote: >Since Section 2184.108.40.206(2) refers to "the force in the brace due to >prescribed seismic forces", I would say that the "prescribed seismic >forces" are the forces calculated using C/Rw=0.40. This is >consistent with the intent to have the connections stronger than the >member. Aren't the "prescribed seismic forces" those forces calculated in Section 1628? Section 1628.2.1 specifically states, "Except for those provisions where code-prescribed forces are scaled up by 3(Rw/8) the minimum value of the ratio C/Rw shall be 0.075." The exception stated in the original message for nonbuilding structures changes the minimum value to 0.40. Therefore when the code in Section 2220.127.116.11(2) scales up the "prescribed seismic forces" for the connection design, the code is referring to the actual calculated base shear, not the minimum base shear using a C/Rw = 0.40. Evan Jorgensen, S.E.
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