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RE: P-Delta Analysis(clarification)
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: P-Delta Analysis(clarification)
- From: "Haan, Scott M." <HaanSM(--nospam--at)ci.anchorage.ak.us>
- Date: Sat, 2 Feb 2002 14:49:32 -0900
"They look at each degree of freedom as not depending on the other degrees of freedom." Sorry, I was baffling with BS. A diaphragm works by making stuff deflect together. I was thinking of mass, damping and stiffness for dynamics or something. My head hurts, ask a college professor. 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. Sent: Saturday, February 02, 2002 1:41 PM To: 'seaint(--nospam--at)seaint.org' Subject: RE: P-Delta Analysis(clarification) "My question why we don't consider the axial load in the calculation of stiffness which is more correct ." So are you asking why the rotational and translational stiffnesses in American finite element modeling programs do not consider axial load affects or are you asking why a textbook moment frame drift approximation formula does not include axial effects. Most moment frame buildings have more drift from translation then from building curvature unless they are super tall. Curvature (effects of columns squashing and stretching) adds up as you go up a tall building, so the higher you go the more drift is affected by "flexural" effects then by racking. Approximate formulas for moment frame drift are pretty accurate for short buildings. Ommitting axial stiffness from the analysis of a short moment frame building actually does not affect the results very much. American building codes assume people are using software that does structural analysis in the elastic range. Most programs used for design are doing stuff in the elastic range and consider rotational stiffness and translational stiffness to be independent on axial effects. They look at each degree of freedom as not depending on the other degrees of freedom. I can't remember what that is called. If you don't use these assumptions there is not a closed form solution for your stiffness matrix and you need to iterate or do something more difficult. Most modeling programs [anything that is not free] will include axial stiffness in the calculations. They calculate how much columns stretch on the upwind building face and squash on the downwind face and in this way the effects of translational displacement from building curvature are included in diaphragm displacements. It is more accurate to let a program automatically include P-Delta effects then to use more approximate methods. 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: maaz siddig ibrahim [mailto:maaz_siddig(--nospam--at)hotmail.com] Sent: Saturday, February 02, 2002 6:42 AM To: seaint(--nospam--at)seaint.org Subject: P-Delta Analysis(clarification) Dear Sherman/William : I mean by the beam stiffness and the (beam/column) stiffness that: I have checked some software uses the stiffness of the beam considering only the bending deformations or the bending and the shear deformations without considering the effect of the axial load in the rotational and translation stiffness ,this what I means by the beam stiffness ,The other one (beam/column) stiffness is used when we consider the effect of the axial load in the stiffness calculations. The P-Delta analysis is equal to : when we use the transltional stiffness ( Stt=(S1+S2+2*T)/h^2-P/h ) [where : P is the axial load ] but the rotational stiffnesses S1 & S2 should calculated considering the effect of the axial load to give the correct critical load ,when the load increased . My question why we don't consider the axial load in the calculation of stiffness which is more correct . Thanks Engineer: Ma'az Siddig Ibrahim. U.A.E Abu Dhabi maaz_siddig(--nospam--at)hotmail.com _________________________________________________________________ Send and receive Hotmail on your mobile device: http://mobile.msn.com ******* ****** ******* ******** ******* ******* ******* *** * 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. 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