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Re: UBC97-- Redundancy Factor[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org, flynn(--nospam--at)aiscmail.com
- Subject: Re: UBC97-- Redundancy Factor
- From: Mlcse(--nospam--at)aol.com
- Date: Thu, 25 Feb 1999 23:37:42 EST
In a message dated 2/25/99 1:30:10 PM EST, 1548(--nospam--at)idc-ibg.com writes: << Re: UBC97, Reliability/Redundancy Factor 1. What is your interpretation of the UBC97, section1630.1.1, eq. 30-3, if you have a chevron brace? 2. Do you still consider chevron brace as two brace elements when calculating r(max.) or one? 3. Is there any difference between X-brace & chevron brace when calculating r(max.)? 4. Maybe X-brace counts for two brace elements since each system (diagonal plus column) could be an "independent", statically stable system? 5. However with chevron is a different story -- buckling, let's say, of one diagonal make the whole system unstable ( assuming that there are no special provisions for beam). 6. What is your opinion? regards Lee Konczak SE Industrial Design Corp. Portland, Oregon >> I numbered the responses relative to above and will try to address them individually. 1. I personally think the redundancy issue speaks for itself, the more frames you have the lower the forces which you have to design for, especially the connections on a SCBF. You can rearrange the redundancy equation to determine the preliminary minimum number of braces required so you won't be penalized by the redundancy factor. The analysis should be done on a floor by floor basis. The UBC allows you to start at the two thirds level of the building height and check from that floor level down. 2. A chevron braced bay would be considered as being two brace elements (the steel section 2220.127.116.11 talks about limiting the horizontal force component of all members acting in compression to more than 70% of the total force acting at that level). 3. I see no real difference if you are using an "X" brace (two story x brace) or a chevron. I would consider the x brace as two seperate chevrons (one inverted and the other normal). The bottom part of a two story x will have higher axial loads in the brace. For a single story x brace (I might try a single diagonal instead), I would consider this as two seperate brace elements. 4. The x brace is two seperate elements, therefore you can use it as two elements. I would not consider the column as a bracing element. 5. The two story x brace is a unique animal. You do not need to do the special beam provisions since if one brace buckles, you have bracing above or below the beam to resist the UBC unbalanced load combination acting on the beam. This can save about 180#/foot in beam weight (example: SCBF two story x brace could use a W18x50 where a multistory chevron would require a beam in the range of W24x220 for TS8x8x5/8 tubes). 6. To be cost effect for steel weight, the two story x brace should be encouraged. Not necessarily cost effective for the designer though when using the SCBF, getting the geometry correct is time consumming regarding out-of- plane buckling of the brace gusset plates. Performance wise, the SCBF is better than the OCBF, and the final brace sizes may be very similar due to b/t thickness ratio requirements. My two cents Michael Cochran S.E.
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