ICC 500 / ASCE 7 wind design for safe room

[conradlovejoy]

I am designing a small multi-purpose/safe room and ran across an issue. I obtained some calcs from an old job that were done by an engineer that no longer works for my company in order to check my work against hers. I noticed in the C&C section that when she calculated the areas for zone 1, 2, and 3 for the roof deck that she divided the areas by 2. When I looked in the ASCE 7-05 (this is for an IBC 2009 job in a small town) I saw that the figure I used (Figure 6-14A) only covers monoslope roofs with theta between 3 and 10 degrees and Note 5 states that for roofs with theta less than 3 degrees that values of GC_p should be taken from Figure 6-11B (Gable roofs with theta less than 7 degrees). Since the diagram in that figure is for a gable roof it has a ridge and zones 1 and 2 are effectively halved compared to a monoslope roof. I am unsure if the area should be calculated for a monoslope (i.e. A = B-2a * L-2a) and then taken to the table in Figure 6-11B or if I should calculate the area as shown in the zone map on Figure 6-11B (i.e. A = (B-2a * L-2a)/2. This is not significant for areas less than 100 sq. ft. as the value of GC_p flattens out past that value but for smaller areas like zones 2 and 3 on a small bldg like a storm shelter the result could be effected.

 

[mike20793]

You use the same process as a typical monoslope roof, only with the increased wind speed at ultimate level rather than service level. Monoslope roofs with theta less than 3 degrees have similar external pressure coefficients to gable roofs with theta less than 7 degrees, so your allowed to use the same table. To answer your question, you need to enter Figure 6-11B with the area of the monoslope roof to obtain the correct GCp value. Also, make sure your load combinations jive with the 2nd edition of FEMA 361.

 

[conradlovejoy]

Thanks for the response, Mike.