ASCE 7-16 Components and Cladding h > 60 Fig 30.5-1: No positive roof GCp values?

[gmoney731]

Is there a reason why Figure 30.5-1 does not provide positive GCp factors for h > 60'? They do for h < 60'...just wondering what the justification is? Do we typically not design for any "downward" C&C pressures then, only uplift?

 

[driftLimiter]

I'm not an expert of wind flow, or wind tunnel tests so take this with some salt.

Generally the wind flow is horizontal. When it reaches a building 60ft or taller it has to go around that building. Much of the flow would shed to the sides, some of the flow will be pushed up along the walls until it reaches the roof. When it reaches the roof the upward flow mixes with the horizontal free flow above the building. There is both a wind speedup, and a lag as the flow tries to go up and over the roof. The area of the roof is in a classic low pressure vortex zone.

As a token, the MWFRS all heights method offers some insight. If you Review Fig. 28.3-1 of ASCE7-16 you can see that as the roof angle increases the pressure on the windward face of the roof (zone 2 and 2e) goes from negative to positive. I suppose review of Ch 27. also would be interesting as there are MWFRS provisions for tall buildings in there as well.