Truss Design and Partially Enclosed Buildings, by Steve Kastner, P.E.
When truss designers are designing a roof system in a high wind area of the country there are several things that they must address if the loading is to be applied accurately. Although truss designs rarely get rejected by building departments or the project professionals of record, they could be and should be. I have several times in the past. When a building is partially enclosed the uplift forces at the wall can be significantly higher requiring a much stronger connection.
1) The first thing to find out: Is the structure in an “Airborne Debris Region” such as in the coastal areas of Florida? Many coastal regions get hit with hurricanes and the design of the structure mandates that there will be protection on the openings. If so then the Partially Enclosed (PE) setting is not needed and selecting this option is incorrect.
2) Other areas the PE setting may be required. Then the amount of openings in the building envelope is used in determining the enclosure classification.
The definition of a Partially Enclosed building according to ASCE 7:
BUILDING, PARTIALLY ENCLOSED
A building that complies with both of the following conditions:
1. The total area of openings in a wall that receives positive external pressure exceeds the sum of the areas of openings in the balance of the building envelope (walls and roof) by more than 10 percent.
2. The total area of openings in a wall that receives positive external pressure exceeds 4 ft2 (0.37 m2) or 1 percent of the area of that wall, whichever is smaller, and the percentage of openings in the balance of the building envelope does not exceed 20 percent.
Some don’t even think about this condition. But, as you can see a large percentage of buildings should be analyzed as partially enclosed. The Florida Building Code mandates it.
Are you designing your exposures properly?
SE Kastner – Professional Engineer
Gould Design, Inc.