MWFRS vs C&C

[JoePrecast]

I've got a project with very tall parapets(approximately 20') using ASCE 7-16. It appears that the wind load from MWFRS is higher than the wind load from C&C(Cp of . Is this normal?

It's not a small difference either. C&C I have 56.4 psf, MWFRS I have 78.3 psf...

 

[JAE]

What effective area are you using to get your C&C value?
I'd typically use (20/3 x 20 = 133 sf)

 

[JoePrecast]

I used 300 ft^2 for the C&C loads. I’m just trying to understand why MWFRS is higher.

 

[andrews2]

Looking at C&C coefficients in ASCE 7-16 I don't think MWFRS should ever be higher than C&C.

First of all, why are you using 300 ft^2 as your area? If you are designing a vertical member spaced at 15' this makes sense, otherwise I would do what JAE proposed and use L^2/3.

Secondly, what are you using for GCp for your MWFRS parapet pressures? Is it possible you're using 2.5? If so, this is too high. You only need to use 1.5 for a windward parapet. The 1.0 applies to leeward parapets meaning parapets on the opposite edge of the roof, not the leeward side of a windward parapet.

 

[SE2607]

I'd typically use (20/3 x 20 = 133 sf)

I'm having difficulty visualizing how you are getting this effective area.

Thank you

 

[Aesur]

I'm having difficulty visualizing how you are getting this effective area.

ASCE7 has provisions for trib area = 1/3length x length, which is often used for walls and roof joists.

 

[Eng16080]

I suspect that you're doing something wrong in your calculations, but it is possible for MWFRS loads to be greater if you have a large effective wind area.

 

[BulbTheBuilder]

ASCE7 has provisions for trib area = 1/3length x length, which is often used for walls and roof joists.

I thought it had to be the larger of the [ 1/3length x length , length x Height]?

Edit; Is it possible OP is using a larger trib area hence a smaller wind load?

 

[JoePrecast]

“Secondly, what are you using for GCp for your MWFRS parapet pressures? Is it possible you're using 2.5? If so, this is too high. You only need to use 1.5 for a windward parapet. The 1.0 applies to leeward parapets meaning parapets on the opposite edge of the roof, not the leeward side of a windward parapet.“

I’m trying to find a reference for this as I’ve been told that you do need to add them. Can you please provide the code reference?

 

[SE2607]

ASCE7 has provisions for trib area = 1/3length x length, which is often used for walls and roof joists.

Thanks! Found it under section 26.2. That's going to help me a lot because, heretofore, I was using span x spacing.

 

[lexpatrie]

Parapet loads are very different from normal wind loads, read the code carefully. Done correctly for moderate height buildings they can be a strong contributor to the overall lateral loads on the building, or a major influence. Woodworks has a lateral design multi-story wood example that goes through it.

 

[andrews2]

"I’m trying to find a reference for this as I’ve been told that you do need to add them. Can you please provide the code reference?"

27.3.4 says the coefficient is 1.5 for windward parapets and 1.0 for leeward parapets. When calculating MWFRS loads the values are combined for a total coefficient of 2.5 (As long as you have a parapet on both sides of your building). However, you are wanting to compare these values to a C&C wind pressure. C&C pressures are for the design of individual elements, not the whole building, so you should be comparing them to the wind pressure on a single parapet, not the combination of the windward and leeward parapets.

It sounds like the reason your MWFRS pressure is higher than the C&C pressure is because the C&C pressure is looking at a single parapet, but your MWFRS pressure is adding the contribution of two parapets into a single value.

 

[Once20036]

Without seeing any of your calculations, we're all just guessing about how your results came to be.
Have you considered ASCE 7-16 section 30.6.1.2? It says that for components and cladding loads, you need to apply the design wind force to both faces of the parapet simultaneously.

 

[Rsd789]

It's also worth mentioning that ASCE 7-16 Section 30.2.3 indicates that any C&C element with a trib area greater than 700ft^2 shall be permitted to use MWFRS pressure