wind loads on parapet walls

huangyhg

wind loads on parapet walls
if there is a nonpourous (concrete or masonry) parapet wall, separated from the building below by a concrete roof, why would the gcpi be added or subtracted to gcp?  the commentary in asce-7 reads "using the value of gcpi for an enclosed building may be appropriate".  they hedge with "may", but the gcpi is in equation 6-24.
just not seeing it. maybe someone could enlighten us.

it looks like the decision is taken out of our hands in asce 7-05.  the windward pressure coefficient is 1.5 and the leeward pressure coefficient is -1.0.  
basically you multiply your wind load at the top of the parapet by 2.5 and design for that.
yes for main force, my question is for design of the parapet itself, using c and c loads.
because the parapet actually sees the positive and negative pressures.  it is similar to the building itself.  for the building, you account for positive pressure on the windward side and negative pressure on the leeward side for the overall pressures on the building.  the parapet is relatively thin, and provided there are few to no obstructions on the roof, the windward parapet will see positive pressure on the windward side (outside face of building) and negative pressure on the leeward side (on the roof side of the parapet) as the air flows over the parapet.  similarly, the leeward parapet will see negative pressure on the outside face and positive pressures on the inside face.
read the commentary c6.5.11.5 in the 7-05 standard.  for masonry parapets there is no internal cavity so there would be no internal pressure.  parapets of other materials may have internal pressure, thus the use of gcpi.  for mwfrs gcpi cancels out but for c&c the internal pressure on one side of the parapet can affect its net design pressure.
it is true that a parapet will see positive and negative pressures, but i am yet to find any reason why such high coefficients are used. a positive + negative for the building is typically 1.3 (0.8 + 0.5). however, for a parapet it is 2.5 (1 + 1.5). the commentary clearly states that research on parapets is not done, and this just seems like values that the experts decided to use. i will take it with a grain of salt till i see actual research.
the parapet values in asce 7-05 were somewhat reduced from asce 7-02.  slickdeals you are correct that the values in asce 7-02 where based on essentially best guess by the committee, but the revised 7-05 values are based on actual wind tunnel testing.  
i agree that these values are still high, and above about 7' in height i would be inclined to use solid sign type loads instead as i do not think the wind tunnel testing included parapets above that height.