TORNADO LOADS PRESSURES

[alh]

What would be the differential pressure on a tight structure when a tornado pass nearby. Internal pressure of the building did not have time to equalize with the external pressure, thus causing an outward pressure or a failure (explosion) of the weakest area of the building and then equalizing the pressure differential.

 

[JAE]

If you are in the U.S. there is a FEMA document (FEMA 361 - Design and Construction Guidance for Community Shelters) that has a lot of good information about tornado design using ASCE 7 provisions. They recommend using the following:

Wind speed 160mph to 250 mph depending on the F magnitude of the tornado you are designing for

Importance factor = 1.0
Site Exposure = C
Directionality Factor Kd = 1.0
Internal Pressure Coefficient (GCpi) = +/- 0.55

Use the ASCE 7 wind provisions with the above using the Method 2 - Analytical Procedure...not the Method 1 Simplified.

 

[alh]

FYI this was a E-Mail from NOAA
NOAA states that the presuure drop within a tornado is likly only 6-10% of atmospheric pressure. Modeling studies by Lewellan at the University of West Virginia show this and our own measurement from one third of the way across a violent (F4-F5) tornado vortex likewise showed a 60 mb pressure drop (about 6%). Weaker tornado will have lower pressure drops.

They also stated that studies of the tornados in OK and KS of the May 3, 1999 indicate that the outward damage is the results of the wind breaching the envelope of the structure and appling an additional outward pressure.