ASCE 7-22 Roof Wind Load 2

[Althalus]

Why are there two sets of values for Cp in Figure 27.3-1 (Continued)?

I can understand if there is a positive or negative. But when they are both negative, I cannot conceive of a situation where the lower negative value (closer to zero) would govern.

 

[271828]

Crossing my fingers someone has an explanation for this! I've been wondering about it for years.

 

[phamENG]

It's meant to envelope the worst case scenarios for everything MWFRS is used for.

Say you have a building with a gable roof, a 10 degree roof slope, and h/L = 0.5.

-A Cp of -0.18 on the windward side and -0.5 on the leeward side will result in the greatest lateral load from the calculation when you do your vector sums.

-But what about the ridge beam? It is receiving wind load from more than one surface, so MWFRS loads are used to design it. The worst case uplift is going to come from a windward Cp of -0.9 and and a leeward Cp of -0.5.

 

[Althalus]

The worst case uplift is going to come from a windward Cp of -0.9 and and a leeward Cp of -0.5.

I'm looking at the part of the table you cited (Windward side for h/L = 0.5 @ 10 degrees)

Cp = -0.9 and -0.18.​

Both of those numbers are for the windward side. Both indicate uplift.

When would the -0.18 ever control? This is only a 10 deg pitch. I can't come up with a scenario where the -0.18 would ever control. Why is it even there?

 

[JAE]

phamENG just answered your question.

-0.9 WW combined with -0.5 LW results in a worst case uplift on the roof (and potential ridge beam).

-0.18 WW combined with -0.5 LW results in a worst case for overall lateral wind on the building.

 

[Althalus]

Sorry, I missed that detail.

One is max vertical. The other is max lateral. Interesting.

 

[Althalus]

OK, I did some calculations and I don't see those numbers generating the max vertical and horizontal as described. It must be some odd combination I'm not seeing.

So, I spoke with a real wind expert on this matter.

He said
[ul]
[li]There is only one direction of load. It is not vertical or horizontal (per se). It is normal to the roof surface regardless of the slope. So, we would ideally perform some vector math to determine horizontal and vertical forces for the overall frame. For fairly flat roofs (like 10 deg or 15 deg, it is close enough that you can just use the normal force as the vertical load. But the roof sheathing and framing would always be designed per the normal load directly.

[/li]
[li]Why 2 numbers? They are simply a continuation of the chart from the steeper sloped roofs where both positive and negative values must be checked. And who knows? Maybe there will be some weird load condition where a lighter uplift would actually control? We can't think of everything So, why not provide the numbers? But the vast majority of the cases, the second load will not require checking.[/li]
[/ul]