Roof Parapet Pressure Calc

[conradlovejoy]

ASCE 7-10 - 27.4.5 states that p[sub]p[/sub] is the combined net pressure on the parapet due to the combination of the net pressure from the front and back parapet surfaces. The coefficient GC[sub]pn[/sub] is depicted at the net pressure coefficient and is equal to:
= +1.5 for windward parapet
=-1.0 for leeward parapet

Is this to say that a) for the parapet on the windward face on the building, 1.5 times the velocity pressure at the top of the parapet is yields the design pressure acting on the outside face of the windward parapet, and -1.0 times the velocity pressure yields the design pressure acting on the inside face of the leeward side parapet, OR b) all parapets shall be designed with +1.5 - (-1.0) = 2.5 times the velocity pressure to yield the total net pressure from force acting on windward and leeward faces of the same parapet?

Option a makes more sense to me, however, a coworker thinks the answer is b and I can't really argue that he is obviously wrong. His option is obviously much more conservative, but I want to know if it is unnecessarily so because I have always used 1.5 as the coefficient in this instance.

 

[JedClampett]

You are both right. If you had some element making up the parapet, like plywood sheathing, that only can see pressures from one side, you design that for he individual windward or leeward pressures. But for the overall parapet acting as a cantilever, presumably, you would add together the leeward and windward pressures.
Realistically, I would design everything for the biggest pressure. Who mess around with all that bookkeeping?

 

[Hokie93]

I would say neither approach is correct, if the issue at hand is main wind-force resisting system (MWFRS) parapet pressures. The net pressure coefficient on the windward parapet (considering BOTH surfaces/faces of the windward parapet) is +1.5. Similarly, the net pressure coefficient for the leeward parapet on the opposite side of the building (considering BOTH surfaces/faces of the leeward parapet) is -1.0. So a pressure equal to 1.5 times the velocity pressure at the top of the windward parapet plus 1.0 times the velocity pressure at the top of the leeward parapet is applied to the MWFRS. If I am understanding your approach correctly, it would apply twice the parapet wind pressure as required by ASCE 7-10 Section 27.4.5. Your co-worker's approach would apply four times the required parapet wind pressure.

The design of the parapet itself (for C&C wind loads) is a different story and is covered in Section 30.9.