ASCE7 - 05 or 10 Parapet Wind Load

[RFreund]

Referencing ASCE 7-10 (same in 05 but different section):
In Figure 30.4-2A (components and cladding h<=60ft) the GCp coefficient for zone 1,2,3 is in the range of 0.2 to 0.3 however per note 5 if the parapet height is greater than 3ft the GCp from Figure 30.4-1 (C&C walls) for zones 4,5 should be used which ranges from 0.7-1.0. Should positive pressure on the parapet really be increased this much?
To add to this confusion the 'user note' in figure 30.9-1 says to see Note 5 (referenced above) for "reductions in component and cladding roof pressures when parapets 3ft or higher are present".
Maybe i'm missing something...

Thanks in advance

EIT

http://www.HowToEngineer.com

 

[RFreund]

Second question regarding parapet wind loading for flat roofs -
For case A the pressure on the parapet is the combination of windward wall pressure and leeward/negative roof pressure used as a leeward/negative wall pressure.
For case B the pressure on the parapet is the combination of the windward and leeward wall pressure.

The GCp values for negative roof pressures are greater than for negative wall pressures so it seems as if the roof case almost always controls it also seems that I get much higher pressures than Case B.
Am I missing something here?

H<60ft say Zone 4 and Zone 2;
Smallest GCp Zone 2: -1.1
Largest GCp Zone 4: -1.2
H>60ft say Zone 4 and Zone 2;
Smallest GCp Zone 2: -1.6
Largest GCp Zone 4: -0.9 (ouch big jump!)

Thanks in advance.


EIT

http://www.HowToEngineer.com

 

[TXStructural]

The short answer is Yes.
The greater pressure (+/-) will always be at the corners. As the air stream is forced across an edge, maximum negative pressure occurs on the windward edge of the roof and top edge of leeward wall or corner. It is frequently strong enough in hurricanes to rip whole bricks out of a veneer and lift the edges of a roof.

Adding a low height parapet may accentuate this effect. Taller parapets will disrupt the effect, eventually acting like a freestanding wall or open structure.

 

[RFreund]

Thanks TX, wanted to make sure I wasn't missing something.

I really need to proof read my posts better.

EIT

http://www.HowToEngineer.com