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3_D analysis non-parallel walls
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: SEAonc seaint <seaint(--nospam--at)seaint.org>
- Subject: 3_D analysis non-parallel walls
- From: merrick group <merrickgroup(--nospam--at)compuserve.com>
- Date: Fri, 19 Nov 1999 15:31:39 -0500
I missed the begining of conversations for the subject on 3-D anaysis, But thought some conversation on non-parallel walls could help. The principle axis is the key to combining perpendicular base shears to create a needed resultant. Does anyone know of a 3-d program that will automatically adjust for a rotated principle axis? Diagonal walls will rotate the principle axis of a building. Recall that a force is in line with the principle axis when the resulting deflection vector is equal to that direction of the applied force. Don't make "local" wall vectors for the non-parallel wall and ignore the "global" principle axis affects on the structure's analysis. A code "minimum base shear" is assumed to be applied in-line with the global principle axis. The rigid diaphragm analysis that is modified for the attachment to Dennis Wish's wood shear wall program has a principle axis check. This can find a principle axis by trial and error, or the first set of deflection results can be used to calculate the principle axis angle. For a symmetrical and triangular plan shape, all angles that the horizontal force can be applied results in at least one of the walls receiving maximum possible resulting forces. Here's how to apply a force to a principle axis that is not parallel to a computer model reference axis, if one knows the principle axis. Apply two loads, sized to create a resulting vector that is parallel to the principle axis. The resulting vector needs not to be greater than code required. A second set is required for a perpendicular resulting vector. In the early 70's, I increased a 2-D base shear analysis by a factor of 1.41 to compensate for not knowing the exact principle axis alignment. It was a small job and the extra reinforcement was insignificant. This factor could be lowered, if one knew of an approximate range of the principle axis angle. It may not have covered all possilbe combinations. As mentioned by others, using the both, directional code base shears, together in a 3-D analysis and not reduced, is too much but works. Using 2 perpendicular base shears, equal and 70.7% of code is the 45-degree case and is too low. UBC: 1631.3 Mathematical Model. . "A three-dimensional model shall be used for the dynamic analysis of structures with highly irregular plan configurations such as those having a plan irregularity defined in Table 16-M and having a rigid or semirigid diaphragm." . For the rigid wood diaphragm users: Are we going to do a 3-d dynamic analysis for a system with a diagonal wall? UBC 1633.1 General. This section requires uplift considerations for irregularities defined by table 16-m type 5. That is the diagonal wall plan irregularity. These are ramblings from memory and hunches. If you need to know, check the statements for yourself. David B. Merrick, SE MERRICKGROUP(--nospam--at)compuserve.com
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