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Re: P-Delta
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- Subject: Re: P-Delta
- From: "Kris Sathia" <kris(--nospam--at)ca.reiusa.com>
- Date: Wed, 7 Apr 2004 15:56:44 -0700
Computer programs based on the stiffness method typically handle PDELTA ANALYSIS in the following manner : 1) For the load case being considered, first determine the nodal displacements. As Charlie has mentioned, the load case should consist of the lateral and vertical loads in order to be able to correctly account for the PDELTA effect in the steps to follow. Next, for each member, do the following. 2) Using the nodal displacements, determine the axial force in the member. 3) The relative displacement of the 2 ends of the member in the 2 orthogonal global directions is then measured. 4) The axial force is multiplied by the relative displacements to arrive at the bending moment on the member in each of the two orthogonal directions. 5) Each of these 2 moments is divided by the member length to arrive at coupling forces (equal and opposite forces) at each end in the 2 orthogonal directions. 6) These forces are added to the global load vector and the nodal displacements are recalculated. Steps 2 thru 6 are repeated if more cycles of PDELTA effects are required. Because the forces which are obtained in step 5 form a couple, and hence contribute no net force, the net additional horizontal force or vertical force on the structure is zero. The only effect that PDelta analysis should produce - when compared with a situation where PDelta effects are not evaluated - is additional moments on the structure which should be reflected in the support reactions. The software should not produce any extra horizontal or vertical reaction forces. Kris, STAAD Support Group ******* ****** ******* ******** ******* ******* ******* *** * 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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