Wind loads on a small structure?

[SteelPE]

I have a structure that is square in shape and relatively slender (12’ square by 70’ tall). I would like to capture the wind loads applied to the structure in each orthogonal and along an axis skewed 45 degrees to the structure. In my computer model I ended up making two separate structures one oriented along the orthogonal axis and the other where I rotated the structure 45 degrees.

When it came time to apply the loads to the skewed structure I used the same loads I had used along the orthogonal axis. I am now questioning if this was the correct approach as the wind load will never actually hit the broad side of the structure.

The possible error was brought to my attention as I was calculating the reactions at the base by hand and realized that my moment at the base was 41% higher along the skewed axis. I thought this was find until I broke out the loads into the footing orthogonal axis and realized I had the full wind applied in both directions at the same time which didn't seem correct to me.

Hopefully my picture will be able to better describe my question.

 

[msquared48]

I would say not as 40% or so more volume of wind has to get by the structure on the orthogonal axis.

There was a famous case a few years ago regarding this analysis involving a high rise in New York designed by a very reputable structural engineer. He initially neglected this scenario and had to revise much of the reinforcing in the building after a student pointed out his mistake to him, if I remember correctly. As I recall, there was a rush to reinforce the structure locally as a major hurricane was approaching.

Mike McCann, PE, SE (WA)

 

[sponton]

Loading should be the same, 100psf as it's based on height not directions, however, I don't know how your software/analysis works, are you doing a quick check up? adding the loads to check the shear? does the software apply load directly to the projected area perpendicular to the wind direction and checks on that?

if you're doing it by hand the force is normal to the projected areas and then you'd apply the loads at the corners and such but the same load values as in the orthogonal example should be used. May i ask why are you using a skewed direction? the wind provisions are already cumbersome.

 

[KootK]

Canadian wind provisions deal with this explicitly for high aspect ratio structures. It ends up being an in between value. If you don't find what you need in ASCE let me know. I'll chase down the details for you.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SteelPE]

msquared, I am familiar with the LeMessurier Citicorp building as it was something that was introduced during my first year ethics class..... but I'm not sure if his problem was slightly different

sponton, I am telling the computer program what loads to use, it is not calculating them I am manually. The question is, if I have 41% more load at 45 degrees then I will end up with 100% of the load in each axis at the same time. This just seems excessive to me. I'm also using a skewed direction because..... I want to make sure I accurately account for all of the loads.... and the addition of the diagonal wind wasn't that difficult.

KootK, the is an AISC 7-05 structure so I don't think I am going to get into the Canadian Code.... but thanks for the offer.

In the end my structure works with these loads..... and I'm sure the footing will work to0 (as I'm pouring a large footing under the entire structure because it is so small).... so I'm not sure I am even going to change the size of anything.

 

[JAE]

Would this fall under Figure 6-9 of ASCE 7-05?
(assuming you are using method 2)

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[SAIL3]

I may be missing something here, but, I thought it was as simple as multiplying each orthagonal loading case by 0.707 and adding them together for the skewed case...

 

[JAE]

Which is sort of what Figure 6-9 does for you.

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[DaveAtkins]

I have always thought if the wind hits a surface at 45 degrees, one component (0.707 times the wind load as mentioned by SAIL3) is perpendicular to the structure, and the other component is ignored, since it is parallel the face of the structure.

DaveAtkins

 

[steellion]

ASCE 7 is pretty clear on this. Apply 75% of the wind load in each orthogonal direction simultaneously. You will also need to consider torsional loading cases (ASCE 7-10 Figure 27.4.8). The building you described is not a low-rise building, so the "all heights" method must be used. I would recommend doing a 3D analysis rather than two 2D analyses if you have that capability. Your P-delta effects of a slender frame element loaded in two directions could have a fairly substantial effect on your design. You may even have "vortex shedding" effects on a structure that slender.

 

[msquared48]

Dave I'd right. I think the overall S of the structural shape plays into the New York problem though.

Mike McCann, PE, SE (WA)

 

[SteelPE]

I am running a 3d analysis, unfortunately with the software I am using I will not be able to accurately analyze pdelta as the structure has tension only members and the program doesn't like to run pdelta when I have these types of members. P is very small anyway (the weight of the structure itself which is only about 15k and is hollow on the inside) so pdelta shouldn't be large.

I didn't think to apply figure 16-9. I was using 1.41x the orthogonal load.... using these numbers I would use .75*1.41=1.06x the orthogonal load. I think this will help.

 

[Tomfh]

You're using the same uniformly distributed load and have rotated your square 45 degrees, so it seems correct that it's now catching 1.414 times are much wind. You've simply opened up your window by a factor of root 2. Right?

In le Messuriers case, the columns are closer together under quartering winds, rather than further apart which is normally the case. That's why it didn't work. At least that's my understanding of it...

 

[SteelPE]

Tom, while that may sound right when you back everything off you end up applying the full wind in each direction at the same time.... .707*1.41=1. This would be done for each direction.

 

[WinelandV]

SteelPE,

When I designed bucket elevator towers, I always did my wind load the way you show in red - the 100psf across the whole tower and 70.7plf to the members. The 41% more force sounds right, because you just picked up that much more length across the diagonal.

One question, why will the wind never be able to hit the diagonal of the building?

Also, if this is an open tower, the diagonal force will need to be amplified, per figure 29.5-3 (ASCE 7-10) or Figure 6-23(ASCE 7-05).

 

[SteelPE]

wine,

I'm not saying that the wind will never be able to hit the diagonal of the building, I'm saying when the wind does hit the diagonal of the building it will be hitting the building at a 45 degree angle which would different than if it were to hit the building head on. It is just a little odd to me that the building technically ends up seeing full wind load in both x and y directions at the same time. Seems excessive. Now, my structure at this point can resist those loads..... I also suspect the foundation will have no problem with the loads either but I guess I am more interested in getting it right for future reference.

Yes, the structure is open and I have applied a Cf of 1.6 to my wind loads.

 

[msquared48]

SteelPE:

You may not be hitting the building at right angles, but you are hitting a 41% greater width than the normal direction, even though the forces are shedding.

Mike McCann, PE, SE (WA)

 

[SteelPE]

msquared,

I understand that I am hitting 41% more area so I agree that while the areas are larger the actual load wouldn't be as large as if the wind was hitting the structure head on.

If you look at the tables JAE provided, I would end up using 1.06x (1.41x.75)the perpendicular loading along the diagonal direction. I think this would capture both the fact that the area has increased and how the wind is not actually hitting the building head on.

If this building were 100'Lx100'Wx15'h you design the building to resist the load in each direction..... but not necessarily full wind both directions at the same time do you?

 

[msquared48]

Absolutely not.

Mike McCann, PE, SE (WA)

 

[manstrom]

Full wind in each direction is not required. This is addressed in ASCE 7 like JAE said.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

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