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Wind Direction : Was Wind Load Topographic Effects
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- Subject: Wind Direction : Was Wind Load Topographic Effects
- From: "Conrad Harrison" <sch.tectonic(--nospam--at)bigpond.com>
- Date: Tue, 6 Jan 2009 12:17:03 +1030
Joe, Not sure I would approach it that way. As I mentioned the pressure coefficients given in the code (ASCE7-05) are based on the extreme, within the quadrant. To AS1170.2 we refer to direction measured by angle theta relative to building orthogonal axes, and angle beta are the compass cardinal directions. Theta=0 gives transverse, and theta=90 gives longitudinal. We are required to check minimum of 4 orthogonal directions, theta =0, 90, 180, 270. Referring to these angles then the implication is that the pressure coefficients ASCE7-05 Fig6-6, are based on extremes for theta=0 (transverse) taken with airflow between -45 and 45 degrees. Whilst those for theta=90, longitudinal are taken between 45 degrees and 135 degrees. Since pressure coefficients are not equal for both directions, the shared angle (45) only provides the extreme for one of the directions: I've not read anything which says which direction. Taking this into account. Then I would draw the building plan, and the orthogonal axes, and the 8 compass directions correct orientation to the building. Then qz would be determined as previously mentioned: the maximum in the quadrant under consideration. For any wind direction theta, I would then draw a line normal to the direction and project the building onto that viewing plane. That which is visible is the windward face, that which is hidden behind is the leeward face. Apply windward, leeward, and sidewall pressure coefficients accordingly. So for example one short and one long wall may experience Cp=+0.8, and the other short and long wall would experience Cp=-0.5 (max. mag.), that is no wall experiences the sidewall coefficients (Fig6-6). The roof planes would experience windward and leeward pressure coefficients. If both roof planes are largely visible from the viewing plane then adopt the longitudinal pressure coefficients. That is adopt pressure coefficients relative to the standard quadrants (transverse (0),longitudinal(90)) the wind direction angle theta lies within. I would guess that 45 degrees belongs to the longitudinal quadrant. The resultant distribution is likely to produce more of a twisting affect compared to the typical theta=0,90,180,270. Which is generally the reason why consider angles other than the orthogonal directions: if need be. Depending on pressure coefficients given they can be adjusted using trig: many of the drag coefficients for steel sections are typical of coefficients adjusted for direction using sine and cosine functions. Most pressure coefficients don't have this option. The projected dimension (or diagonal) of the building would be a simple approach, for a simple windward pressure on a projected area. But it doesn't take into account the change in distribution of pressure over the surface of the building. Also wind blowing against the long or short sides is not relevant once calculated the dimension projected on the viewing plane normal to the direction of the wind. Hence the tough question. For a non-orthogonal direction cannot have bracing in planes parallel to that direction because don't have any such walls: so how does the wind on that projected area distribute to the bracing in the walls? I don't believe need to answer that question because the air is a fluid and acts normal to the surfaces of the building. Our first task is to estimate the distribution of pressure over those surfaces. Which involves calculating qz, determining Cp[external] and Cp[internal]. Once have the distribution then can determine the effect. Hope that doesn't over complicate things. Regards Conrad Harrison B.Tech (mfg & mech), MIIE, gradTIEAust mailto:sch.tectonic(--nospam--at)bigpond.com Adelaide South Australia ******* ****** ******* ******** ******* ******* ******* *** * 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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