Effective Wind Area for Parapets: ASCE7-16

[ChiEngr]

Hello,

Say I have a 5 ft parapet for a building with an effective wind area of 8.33 sf. Per section 30.8 of ASCE7-16, there are two load cases for calculating the C&C wind loads on the parapet. If we take load case A, for example, would I essentially enter into the applicable figures for wall pressure and roof pressure using the effective wind area of the parapet? For such a small area, you get a ridiculously high wind pressure. This is how I read the Code, but I am not sure if others have a different interpretation.

 

[phamENG]

The area is the tributary area for what you're considering. So if you have steel channels cantilevered up from a frame below every 4 or 5 feet, your effective wind area is 20-25sf. If you are using stiff clips or something similar on light gauge studs at 16" o/c, there's an allowance for repetitive members (I think they have to be 24" o/c or closer), the effective area need not be less than (2/3)L² (I think - find it in the code and verify it before you use it.) If you have a CMU wall, I usually go with reinforcement spacing as the grouted cell is doing most of the work to resist wind loads.

You are correct, though, the smaller the effective area the higher the C&C loading. Do you understand why that's the case?

 

[lexpatrie]

Parapets receive high wind pressures. You can delve into the research if you like, but even woodworks (doing a seismic design) has a calc for the parapet wind pressure. The taller the parapet (and I've had some hotels with 10') the larger the loading gets to the point it dominates the shearwall design and loads the holdowns most viciously. Like the parapet is 40% of the holdown requirement. (Speaking of a three - four story wood framed hotel). Two story building had a 14' parapet once, made everything extra.