ASCE 7-16 Chap 28

[SE2607]

I guess I'm still having difficulty applying wind loads on buildings based on ASCE 7. I got over it on C&C with help from members here by pointing out I don't have to use a trib area less than L^2/3.

Now, I'm stuck on the MWFRS, chapter 28. Here, I'm only interested in the net horizontal force. If I have a straight gable roof with a slope of 20 degrees, I get gust factors of -0.69 for the windward side (zone 2 and 2E) and -0.48 for the leeward side (zones 3 and 3E). To me, that would mean the net wind load on the roof element is in the opposite direction of the wind. What sm I missing?

Also, wouldn't I multiply the net by sin(20)?

All this would say is it's a waste of time to go through the motions and just design for the minimum of 8 PSF (LRFD, 4.8 PSF ASD) unless I'm working with high wind velocities. Am I wrong here?

TIA

 

[DanKile]

You didn't get the right edge coefficients.
2E is -1.07 for both case A & B
3E is -0.69 for case A and -0.53 for case B

The roof is like a wing. You'll see uplift while the wind flows over it. The zone 2 & 2E stops getting uplift at about 28 degrees.

Multiply the net by sin(20) if you only want the lateral load from the wind. The uplift may affect your lateral bracing size depending on the load path.

I find it better to go through the motions and end up with the minimums than to use the minimums and later find I should have gone through the motions. It's fairly easy to make a spreadsheet to do the calculations needed.

 

[DaveAtkins]

To me, that would mean the net wind load on the roof element is in the opposite direction of the wind.

That is correct. Depending on the roof angle, sometimes the wind on the roof pulls the building "backwards. This is why the Code also requires design for a minimum wind force of 16 psf times the projected area.

DaveAtkins

 

[P1ENG]

Multiply the net by sin(20) if you only want the lateral load from the wind. The uplift may affect your lateral bracing size depending on the load path.

Use caution with this so that you don't double dip. If you multiply your pressure by sin(20), then don't use projected areas. You'd have to apply that wind load on the full length of the roof member. If you don't multiply your pressure by sin(20), then you can use projected areas on your roof elements (elevation height and plan width) rather than element lengths. Usually in MWF, you are working with projected areas, so that is why I'm chiming in.

Juston Fluckey, SE, PE
Engineering Consultant

 

[P1ENG]

SE2607, attached is an example of my wind calculation using Chapter 28 of the ASCE 7-16. I convert the Chapter 28 pressures into equivalent net pressure zones (Wall Zones A & C and Roof Zones B, D, E, F, G, H), so I don't actually give you the GC values of the Chapter 28 zones. But I did confirm that as Dan said, Zone 2E is -1.07 for case A. In my case, Zone 2E is synonymous with Chapter 27 Zone E (uplift) and Chapter 27 Zone B is the net pressure of windward 2E and leeward 3E. I do this because almost all of my building are simple rectangles with symmetric roof slopes, so I can switch between Chapter 27 and 28 quickly.

In this example, I made some assumption on the building dimensions. You will see that Zone B pressure (lateral pressure on sloped roof) is negative, which means that the roof is actually pulling the building backwards as Dave stated. So either that or the minimum lateral 16 psf (walls) and 8 psf (roof)is considered when I check the wind base shear, overturning moments, and diaphragm loading (see page 4 of attached).

Juston Fluckey, SE, PE
Engineering Consultant

 

[SE2607]

I get it in that the roof acts like a wing. I'm only interested in the horizontal component over the projected height, hence the sin(20). Regardless, just looking at the coefficients, if both coefficients are negative, that means the windward roof is in the opposite direction of the wind. Forget wind speed, etc., I'm just looking at the horizontal component for the MWFRS.

 

[P1ENG]

... if both coefficients are negative, that means the windward roof is in the opposite direction of the wind.

That's not true. If the leeward is more negative than the windward, then the net pressure is positive. Remember, the wind pressures are normal to the surface. So on the windward side, a negative pressure has a horizontal component in the opposite direction of the wind. On the leeward side, a negative pressure has a horizontal component in the direction of the wind. So, the net affect is determined by which side's magnitude is bigger.

Juston Fluckey, SE, PE
Engineering Consultant

 

[SE2607]

That's not true. If the leeward is more negative than the windward, then the net pressure is positive. Remember, the wind pressures are normal to the surface. So on the windward side, a negative pressure has a horizontal component in the opposite direction of the wind. On the leeward side, a negative pressure has a horizontal component in the direction of the wind. So, the net affect is determined by which side's magnitude is bigger.

Mathematically, you are correct. However, in my case, the roof angle being 20 degrees, the coefficients for Zones 2 and 2E, the windward side, are greater than for zones 3 and 3E, the leeward side.

 

[P1ENG]

However, in my case, the roof angle being 20 degrees, the coefficients for Zones 2 and 2E, the windward side, are greater than for zones 3 and 3E, the leeward side.

Ahh, ok. Yes, you are correct. When you said "both coefficients" you were referring to 2 and 2E not windward (2E) and leeward (3E) like I had assumed.

Juston Fluckey, SE, PE
Engineering Consultant

 

[SE2607]

Ahh, ok. Yes, you are correct. When you said "both coefficients" you were referring to 2 and 2E not windward (2E) and leeward (3E) like I had assumed.

2 AND 3 are additive as are 2E and 3E.

This phenomena (the horizontal component of the wind on the roof acting in the opposite direction of the applied wind load) occurs up to a roof slope of about 5:12.