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RE: QUESTION ON USING ACI'S MOMENT MAGNIFIERS FOR COMP. MEMBER DE SIGN
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- Subject: RE: QUESTION ON USING ACI'S MOMENT MAGNIFIERS FOR COMP. MEMBER DE SIGN
- From: "Rogers, Robert" <rogersr(--nospam--at)amkinney.com>
- Date: Fri, 5 Oct 2001 08:36:28 -0400
Scott, Thanks for the feedback. Your comments did start me thinking again. A P-Delta can be done by I want to know what's going on here in the code. If doing a first order or second order frame analysis using factored loads the code recommends using the moments of inertia in 10.11.1 (unless a more exacting analysis is performed). The frame analysis would thus use load factors 1.4 for DL and 1.7 for LL. When you go to compute the Stability Index for the Story, it tells you to if you used service loads for the frame analysis you can still calculate the Stability Index by mulitplying the summation of the column axial service loads by 1.2, using the service load story shear, and mulitplying the relative deflection by 1.43. If one conservatively assumes that all column loads are due to live load (i.e., thus using the 1.7 load factor), and that the story shear was due to wind or earthquake thus dictating the following load factors: 1.7 x 0.75 = 1.275 1.7 x 1.1 x 0.75 = 1.40 1.3 x 1.1 = 1.43 and using the worst-case value from above in the denominator yields: 1.7 -------------- = 1.19 ~ approx 1.20 1.43 thus, I believe this explains the mulitiplication of the column loads by 1.2 and leaving the story shear at the service load level (i.e., effectively making the loading "factored"). However, I still am unclear about the multiplication of the relative deflection by 1.43. In the commentary to 10.11.1 it tells me to take the reduced moment of inertias given in 10.11.1 and mulitply them by 1.43 to do a service load frame analysis (thus making the structure stiffer). Then, in the commenentary over in 10.11.4.2, it tells me to take the deflection generated by a service load frame analysis (utilizing the reduced moments of inertia termed "service load moments of inertia" in 10.11.1) and multiply the deflection by 1.43. This I don't understand....... if I used service loads and moments of inertia meant for a "factored" analysis I have a mismatch. I used lower loads than I should have but I also used lower moments of inertia than I should have (as evidenced by them telling me to mulitply the moments of inertia in 10.11.1 by 1.43 if doing a service load analysis). When I go over to the stability index calculation, if I increase the service loads to factored loads (by using the 1.2 multiplier in the stability index equation) I have accounted somewhat for this mismatch. However, by rights, I should have used stiffer moments of inertia as well. Whether this increase in loading, but also increase in moment of inertia, leads to greater deflection (particularly 1.43) seems somewhat ambigious to me in origon. Where does the 1.43 x relative story deflection come from ? Perhaps I'm just missing the boat somewhere.... possible......I would like someone from an ACI code committee to respond...... Robert C. Rogers, PE Senior Structural Engineer A.M. Kinney, Inc. Consulting Engineers / Architects 150 East Fourth Street, 6th Floor Cincinnati, Ohio 45202 PH: 513-421-2265, ext. 125 FX: 513-345-1318 rogersr(--nospam--at)amkinney.com www.amkinney.com -----Original Message----- From: Haan, Scott M. [mailto:HaanSM(--nospam--at)ci.anchorage.ak.us] Sent: Thursday, October 04, 2001 1:36 PM To: 'seaint(--nospam--at)seaint.org' Subject: RE: QUESTION ON USING ACI'S MOMENT MAGNIFIERS FOR COMP. MEMBER DE SIGN One other thing on your question. It says Q can be [(Pservice*1.2)*(Dservice*1.43)]/[Vservice*H]. 1.7 gravity factor /1.4 seismic factor =1.2 and .7Ig/.7=Ig so Dservice is a function of unfactored loads and an increased-reduced moment of inertia. Where I don't see parity is Do is based on factored loads and is divided by a reduced moment of inertia. So Do is not exactly the 1.43*Dservice, but there is some spare change if the gravity load factor is less than 1.7. Scott M Haan P.E. Plan Review Engineer Building Safety Division Development Services Department Municipality of Anchorage http://www.muni.org/building phone:907-343-8183 fax:907-249-7399 mailto:haansm(--nospam--at)ci.anchorage.ak.us -----Original Message----- From: Haan, Scott M. [mailto:HaanSM(--nospam--at)ci.anchorage.ak.us] Sent: Thursday, October 04, 2001 9:09 AM To: 'seaint(--nospam--at)seaint.org' Subject: RE: QUESTION ON USING ACI'S MOMENT MAGNIFIERS FOR COMP. MEMBER DE SIGN ------------------------------------------------------------------------ -- Your following message has been delivered to the list seaint(--nospam--at)seaint.org at 10:16:25 on 4 Oct 2001. ------------------------------------------------------------------------ -- Robert: I would take the easy way and use factored load combinations and reduced moments of inertia for the strength check. Do a P-Delta analysis and you do not have do worry about the moment magnification due to drift. If you have to calculate the bulding period to figure seismic force use the 1.43 increased reduced moments of inertia [since the less flexible the building, the shorter the fundamental period, the higher the seismic force]. The drift checks in the IBC and 1997 UBC are based on strength level loads and would not require additional messing around if you use the factored loads. You are right. It appears there is not exactly parity. I think the point was to give people using working stress loads some tools. Respectfully, Scott M Haan P.E. Plan Review Engineer Building Safety Division Development Services Department Municipality of Anchorage http://www.muni.org/building phone:907-343-8183 fax:907-249-7399 mailto:haansm(--nospam--at)ci.anchorage.ak.us ******* ****** ******* ******** ******* ******* ******* *** * 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. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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