Wind forces on pyramidal structure

[atrizzy]

Does anyone have a reference for gust/pressure coefficients on a pyramidal shaped structure? Specifically a 4 sided regular pyramid, height matching the length of one of the sides.

Preferably the reference would be applicable to Canada, but at this point I'll take anything I can get.

Thanks in advance!

 

[msquared48]

Why would this shape be any different than a sloping roof?

Mike McCann, PE, SE (WA, HI)

 

[atrizzy]

msquared, because the slope tapers with height, which undoubtedly has an impact on the wind pressures. It's an abnormal shape for most of the relevant diagrams in the National Building Code. If possible I'd prefer to avoid treating it as a billboard... but if I can't find a reliable source for this then that's what I'll do.

Perhaps you could explain your position for me briefly?

 

[msquared48]

And that height variation is taken into account with the Lambda factor in the wind equations.

Mike McCann, PE, SE (WA, HI)

 

[skeletron]

I would compare a few different approximations and then make a judgement call from there. You could look at it like a monoslope roof to get the wind acting on the front face. You could split it into thirds and treat it like a free-standing wall and then determine if the variation in pressure is affected considerably. You could look at it like a standard building and make some modifications to the pressures you use.

 

[wannabeSE]

atrizzy,
Perhaps, I don't understand the description of the building's shape. From a wind load perspective, wouldn't it be a typical hip roof with no walls.

 

[Ron]

I've done a few of these in the past and I used the coefficients for a domed roof once and I used the hip roof approach as well. Not a lot of difference.
Keep in mind that depending on the orientation of the wind, you can get uplift on 3 sides at once.

One approach you can take is apply typical roof coefficient for two sides perpendicular to the "ridge" and two sides parallel to the "ridge" applied simultaneously.

 

[bridgebuster]

I wonder if Imhotep considered wind?

 

[atrizzy]

bridgebuster, unfortunately my structure isn't made of solid rock...

 

[bridgebuster]

I figured that; I couldn't resist being a wise guy.

 

[Ron]

@BB...yes. He considered it. Being a good engineer he used engineering judgment and concluded...Ain't no big deal

 

[msquared48]

Didn’t Imhotep write the song “Windy”?

Mike McCann, PE, SE (WA, HI)

 

[JStephen]

There's a couple of sizable pyramids at Moody Gardens on Galveston Island, presumably designed for hurricane loadings, so someone's bound to have looked into this a good bit. Morris Architects (now a Hewitt Zollars company) shows up on a search of them.

 

[Ron]

Also look at the Hard Rock Cafe in Myrtle Beach, SC.

 

[r13]

If you can settle for a 4 sides hip roof, this paper may help. Link

 

[bridgebuster]

You're right Mike, it was "Windy", I thought he wrote "Blowing in the wind". Ron has the hieroglyphics version of the lyrics.

 

[phamENG]

I agree with just about everyone else. A 4 sided, regular pyramid is just a hip roof on a square building - sans walls. A hip roof on a rectangular building creates a ridge, but as the length reduces to equal the width, the ridge shortens to a single point and creates a right pyramid.

Is your problem that the sides are too steep to fit into the rules provided by the code you're using? Or does your code somehow not provide rules for a hipped roof?

 

[atrizzy]

pham, I just haven't seen any justification to use the coefficients for a hip roof sans walls, as every diagram suggests that the walls are there and they may in fact influence the forces on the roof. I suppose I don't have enough background on the derivation of these coefficients to be able to definitively say that it's that easy.

Folks here seem to have their minds made up, though, so that gives me some confidence.

 

[phamENG]

So long as wind isn't going under the pyramid, it will behave in a similar fashion. Two of the main concerns are 1) projected area to windward/leeward and 2) the manner in which the flow of air separates as it goes around and past the structure. Neither of these really change by lowering it to the ground.

 

[atrizzy]

Well put, thank you pham.