wind: why is it a problem?

by William J. Hope, CSI, CDT, PE, RRC

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tornado winds create such low pressure that structures explode in an effort to equalize the pressures

wind: in the field of the roof 

The sun heats different parts of the earth with different rates of intensity. The heated air rises and creates a low pressure. The rising air cools as it rises and eventually the water vapor in the air condenses to form clouds. The low pressure has created a disturbance of unequal pressures. The surrounding air will flow to the low pressure to equalize the pressures and in a counterclockwise pattern due to the coriolis effect. When we introduce the rotation of the earth, we have an oversimplified global weather pattern.

The element we wish to address is the air flow to equalize the pressure systems: we call it “wind”. As wind travels parallel to the ground, it will impact vertical building components facing the wind with positive air pressure. The air pressure is directly proportional to the wind velocity. The “impact wind” is subsequently pushed around and over the top of the building. Since it is interrupted from its original path and forced to travel a greater distance than its “non impact wind” component, the impact wind is forced to accelerate to a higher velocity. Thanks to Bernoulli (1700-1782), we know that high velocity wind will cause a “lifting effect” or negative pressure on the adjacent surface.

This phenomenon can be seen on a tightly tarped trailer truck traveling at a high speed. The tarp will “balloon up” due to the wind passing over the tarp and creating a negative pressure. An extreme example of high wind causing a low (negative) pressure gradient is a tornado. The winds create such low pressure and are so pronounced that structures explode in an effort to equalize the pressures as the tornado passes near the structure.

The wind does eventually reform into a single air mass. Its speed will still cause uplift or negative pressure on the interior field of the roof. But, a greater uplift pressure is felt at the perimeters and the greatest uplift pressure is felt at the corners. These pressures can be two to three times the magnitude of the uplift pressure experienced by the interior portion of the roof. It is no surprise that almost all wind blowoffs of low slope roofs begin at the corners and perimeters. Future articles will examine the components of the roof and perimeters needed to maintain a secure roof system.

© 1999 William J. Hope, CSI, CDT, PE, RRC on line at www.extconsulting.com. This article appeared in the December 1999 issue of “Crystal Clear”, newsletter of the Toledo Chapter, CSI. Reprinted with permission. 
Want to know about light colored roofs? Read what Dick Fricklas has to say in his article “Reflections on a Cool Roof” 

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pconway@imiweb.org
 

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