designing buildings for tornadic winds

huangyhg

designing buildings for tornadic winds
i am working on an steel-framed office building (5-6 stories) that has to be designed as an essential facility (requirement by client). i would really appreciate if anyone out here could through some light on designing such a building for tornadic winds. i did a bit of code review on this and i am totally confused. is it even practical to design such a building for such high wind velocities?? if yes, where can i find the design criteria. asce 7-05 commentary does have some information regarding tornadic wind velocities to be considered. but, i couldn't find any information on how to calculate pressures (both mwfrs and c&c) and load factors to be used in the final design. i would really appreciate your thoughts and comments on this. thank you.
it isn't very practical to design an entire building for tornadic winds.  the only thing that changes with the essential (cat. iv) building category for wind is the importance factor.  if there is a storm shelter you need to include in your building, then that would need to be designed for tornadic winds per icc-500 (i believe there is also a fema document that supplements this).  but for an entire mwfrs, the standard 90 mph wind is all that needs to be designed for (assuming you're in the 90 mph region).  of course, you can always overdesign the hell out of everything, but it may not be practical from a budget standpoint especially when the mean recurrence interval is 100,000 years.
the issue with tornadoes isn't so much of the wind speed either, but more with the openings and penetrations that are created in your building by debris.  these then cause pressure buildups as you approach a partially enclosed building envelope.  but since they are not predictable, they are nearly impossible to design to.  i'm not sure asce-7 made any load combinations for a volvo flying through your third story windows.
most practical solution may be to design the building as if it were in a hurricane zone. the building code requirements, construction techniques and materials to do this are available. here is a discussion of the probabilities (which bnickeson pointed out) some thoughts that are in line with this approach:
fema has some guidance on storm shelters.
this can be had at the link.
maybe only critical items need to be placed in the safe room rather than design the entire building for a torndao.
as others have stated, internal pressures can be come significant for a direct hit by a tornado.
i know that we bid on a job in our area to build a doppler radar facility where it had to stay operational during a tornado. we did not get the job but there was a special heavily reinforced concrete vault for the equipment and operators. i do not know what standard they built it.  
as stated above, when a facility is designated 'essential', the only thing that changes is the wind importance factor in calculating the design wind pressure. this does not imply that the facility has to be designed for tornados.
if the owner wants the facility, or parts of the facility, designed to withstand a tornado, that is another issue. and a question that should be asked of the owner. what is he/she intetending when they say they want their facility to be 'essential'.
fema 361 is guidance for the design of tornado shelters. fema 453 mentioned above if for shelters offering protection from man-made events.
we have been asked many times if it is practical to design multi-story buildings to meet fema 361 guidance (we are located in the midwest, and tornados are an issue around here). our response always is that yes, we can design a multi-story building to fema 361. the only problem is the owner likely cannot afford it.
the best solution is to locate the shelter underground. if that is not feasible, 1 story 'protected areas' on the ground floor are the only practical solution.
jmho.
portions of nuclear power plants are designed for tornadic winds.  there's the wind issue and the pressure drop.  the tornado moves at some speed (like 60 mph) and the circular wind at the edges is in the range of 240 mph as i re  
quote:
fema 361 is guidance for the design of tornado shelters
lkjh345, as stated by bnickeson above, check out icc 500.  this document is based upon fema 361 and essentially provides a "code-language" specification on shelter designs vs. 361's "guide-language".

the international code council (icc) was in the process of issuing a standard a couple of years ago that dealt with designing emergency shelters for tornados.  try their website at iccsafe.org.
uh....icc 500