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RE: EQ resistant Design
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- Subject: RE: EQ resistant Design
- From: Mark Gilligan <MarkKGilligan(--nospam--at)compuserve.com>
- Date: Thu, 21 Mar 2002 02:50:08 -0500
This whole topic has been made much more complex than it really is. The basic provision is that you must consider the effect of the earthquake acting in the most critical direction for each member. For many structures this requirement is satisfied by analyzing the structure in the principle directions. This assumes that the lateral resisting elements are parallel to the principle directions. When dealing with non perpendicular systems or when the same element participates in non-parallel walls, braced frames, or moment frames then you need to consider the effects of most critical direction. In order to simplify this process the code provides several methods by which you can approximate the expected forces using the results of analyses in two perpendicular directions. The extreme situation is when you have 4 columns in a square with braced frames on each of the sides. In this situation when the direction of seismic forces is at 45 degrees with respect to any of the sides two of the columns will see a reaction 1.414 times that that you would obtain when the forces are parallel to one of the sides. The critical direction for the braces would probably be one of the principle directions. Each of the approximations is based on this relationship. If the force in each direction is the same (fx=fy) the SRSS method gives you (fx^2+fy^2)^0.5=1.414*fx The 100% and 30% gives you fx*1.0+fy*0.3= 1.3*fx. The difference between 1.414 and 1.3 results from the fact that some SEAOC Seismology Committee members believed that 1.3 was conservative for most situations and that there was little reason for concern in those situations where it wasn't. Some others will use 0.707*fx+0.707*fy= 1.414**fx. This is as simple as it gets. Forget about fancy dynamic effects. Their is no difference between the concept of orthogonal effects and most critical direction. Mark Gilligan ******* ****** ******* ******** ******* ******* ******* *** * 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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