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Re: Section 1633.2.6 & 1612.4
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: Re: Section 1633.2.6 & 1612.4
- From: "Swingle, Mark" <Mark.Swingle(--nospam--at)dgs.ca.gov>
- Date: Fri, 10 Sep 1999 14:33:02 -0700
- Cc: "'mswingle(--nospam--at)earthlink.net'" <mswingle(--nospam--at)earthlink.net>
I am surprised that this question has not yet received a response (at least I didn't see one on the web site), so I will offer my comments. I believe your interpretation is correct. In the 1997 UBC, the demand Em to be calculated for a collector is omega times Eh. The capacity to be calculated is the allowable stress design (ASD) capacity times 1.7, which may ALSO be multiplied by an ADDITIONAL 1.33 for wood members, but only for stresses to which the LOAD DURATION FACTOR (C sub D) applies, such as tension, compression, and bolt bearing on wood (but not for compression perp to grain). Now let me add some editorial comments regarding the following items: 1) Em, 2) combined stresses, 3) rho, 4) C sub D, and 5) collectors in buildings with wood structural panels. ----ITEM 1)---- Do not forget to "put the 1.4 factor back in" before calculating Eh, if allowable stress design (ASD) is used. Let me explain what I mean. Most designers, after calculating the governing base shear equation, will IMMEDIATELY reduce the base shear coefficient by the 1.4 factor. Following is a lengthy example, so bear with me. Suppose one has the following building: steel ordinary braced frames (R=5.6, omega=2.2), Zone 4, I=1, soil type S sub D, rho=1, Na=1; and one is using ASD for steel. The governing base shear equation for this short period building is V=2.5(Ca)(I)(W)/R, which yields V=0.196W, which is a factored or ultimate strength design force. If allowable stress design is to be used for the design of the braces, most engineers would immediately divide 0.196 by 1.4 to get a base shear of V=0.14W, which is the same as the 94 code. This base shear will give you ASD demands on the braced frames. Now, suppose you start analyzing the building with the 0.14 base shear coefficient (ASD), and you have a collector force, F, for which you need to provide a steel member, let's say it is a TS section. The tension or compression demand will be omega times Eh which is 2.2(1.4)(F), where 1.4(F)=Eh. DON'T FORGET THE 1.4 FACTOR!! The allowable capacity from the AISC ASD book may be multiplied by 1.7, but may NOT be multiplied by an additional 1.33 allowable stress increase, since the 1.7 is INSTEAD OF 1.33 for steel members. Let's compare the 94 UBC with the 97 UBC for THIS CASE ONLY. In the 94 UBC, the analysis would yield the same collector force of F, also at an allowable stress design level (actually slightly less, since the base shear coefficient was 0.1375 vs 0.14, in insignificant difference of less than 2%). However, the 94 UBC had no requirement to increase the force on collectors, although many engineers did this anyway (the old 3Rw/8) for all collectors, recognizing their importance in the LFRS. The 94 code did allow a one-third increase in allowable stresses for this condition. Therefore, using the notation Cap for the allowable capacity we have (for this case only): demand < capacity 94 UBC: (F) < 1.33(Cap) 97 UBC: 2.2(1.4)(F) < 1.70(Cap) which is an EFFECTIVE increase in the required capacity (97 UBC vs 94 UBC) of: (2.2)x(1.4)x(1.33)/(1.7) = 2.4 If the collector is WOOD (with steel braced frames) then omega is 2.8 and the load duration factor of 1.33 MAY be combined with the 1.7 allowable stress increase, so the above equations yield the following: demand < capacity 94 UBC: (F) < 1.33(Cap) 97 UBC: 2.8(1.4)(F) < 1.33(1.70)(Cap) which is an EFFECTIVE increase in the required capacity (97 UBC vs 94 UBC) of: (2.8)x(1.4)/(1.7) = 2.3 ----ITEM 2)---- If the collector resists gravity loads as well, don't forget to check the combined bending and compression due to 1.2D + fL + 1.0Em and 0.9 +/- 1.0Em. ----ITEM 3)---- If rho is greater than 1, don't forget that you can "take it back out" before checking collectors, if your collector force is based on E, which already includes the rho factor. ----ITEM 4)---- If your steel braced frame building has wood collectors such as straps and blocking, don't forget that the load duration factor does not apply for compression perpendicular to grain stresses, according to the NDS. So, do NOT multiply the capcity by 1.33(1.70), but only by 1.70 ----ITEM 5)---- Finally, don't forget that this ENTIRE DISCUSSION does not apply to buildings braced by wood structural panel shear walls, whether with wood studs or steel studs (Exception to paragraph 1633.2.6). My guess is that there is virtually no known failure of a collector in a properly-designed wood building. Can you imagine the outcry if all double top plates (and their connecitons), as well as all straps in residences had to resist 2.3 times the 94 force? I welcome any and all comments. Mark Swingle, SE Oakland, CA Disclaimer: These are my own opinions. They are subject to change due to the ravages of time and after being subjected to reasoned criticism. ----------------------------------------------------------------------- On 9/3/99, ECVAl3(--nospam--at)aol.com wrote as follows: <I don't know if this has been discussed in this forum but if it hasn't <can anyone tell me how the requirements for collectors, etc. in <section 1633.2.6 and the reference to the "special seismic load" <of section 1612.4 translate to Allowable Stress Design? < <Does the last paragraph mean that after you factor the load Eh by <omega to get Em, you can than increase the allowable stresses <(or decrease the load Em) by 1.7 ? < <Also when the section states < "This increase shall not be combined with the one-third stress < increase permitted by Section 1612.3, but may be combined < with the duration of load increase permitted in Division < III of chapter 23" <does that mean you can increase the stress by one-third for <seismic loads as per Table 2.3.2 pg 2-291 as long as the collector <is made of wood? < <ECVAL3 <
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