Wind at 45°, two questions 2

[mte12]

Questions relate to an elevated structure, but also apply to structure on ground.


Question 1
Is my understanding correct that for wind at 45°, the pressure coefficients on a building, for design purposes, are the same as that for the wind at 0° or 90°.

BS EN 1991-1-4 says the following in Section 7.2.1 (2):
The values [c[sub]pe[/sub]] in Tables 7.1 to 7.5 should be used for the orthogonal wind directions 0°, 90°, 180°. These values represent the most unfavourable values obtained in a range of wind direction θ = ± 45° either side of the relevant orthogonal direction.


Question 2
If you have a building type structure which is much longer than wider/taller, if the above method is used, the load in the longitudinal direction would be underestimated I believe.
For Option 1 (long side windward), the pressure is normal only, windward and leeward pressure are normal, side wall pressures cancel out. No net load in longitudinal direction.
For Option 2 (short side windward), the windward and leeward walls are relatively small and net pressure in the longitudinal directions would seem low to me.

If the structure was solid, I presume the pressure is applied to the projected area, so for wind at 45°, load in the longitudinal direction would be 0.5 times the load for transverse wind (1/√2 pressure x 1/√2 area = 0.5 Force).
Similarly if the structure consisted of individual members, such as for an open truss.

Question is asked because there will be compression induced in the structure, and the support needs to take longitudinal load back to ground.

Has anyone seen any prescription in codes or technical papers.

 

[zackngineer]

I can't speak for Eurocode but we do a lot of wind design in FL using ASCE 7-22. I do residential and commercial mostly. We use the worst case wind pressures to design the whole structure. For our practical purposes, there is no point in splitting up the orthogonal directions to justify a "possibly" more economical design. If you're designing a large commercial or industrial project, then there may be merit to it. That being said, we did a +/- 300,000 SF manufacturing plant last year and we did the same procedure for designing the whole structure. We didn't worry about orthogonal directions and reduced pressures.

Ultimately, when it comes to wind design, the way it works is that the smaller your effective area of analysis the higher your pressures. Obviously, large wall faces experience more load but only because the effective area scales faster than the wind pressures. As an example: a 100 SF wall may have a zone 5 load of 45 PSF. A 1000 SF wall may have a zone 5 load of 30 PSF. Obviously the 30 PSF is lower but when applied to the larger face it is much greater than the smaller wall.

I don't really understand your comment about side wall pressures cancelling out.

Wind design bible in US is ASCE 7-22 chapters 26-32. The commentaries are very helpful too for answering a lot of questions you may have.

 

[HTURKAK]

DEAR @ mte12 (Structural)(OP),

I think you need to think for peak velocity pressure calculation procedure. You may look to NA specific for Table NA.1 Directional factor ( cdir).
Calculation of peak velocity pressure could be with four approaches . The simplest procedure is the use of the same peak velocity pressure for both orthogonal directions and using conservatively, cdir = 1,0 for all directions.
I will suggest you to perform a sample wind load calculation and post to get better responds. I am not sure if your approach is correct and got the clause Section 7.2.1 (2) .


According to the grace of God which is given
unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. . . .
I Corinthians 3:10

 

[HTURKAK]

@mte12 (Structural),

When you look Table 7.10 ( Frictional coefficients cfr for walls, parapets and roof surfaces ), The Friction coefficient cfr
btw 0.01-0.04.
You can calculate the friction forces using the expression ( 5.7) Ffr = cfr * qp (ze ) * Afr . In this case ,if the wind dir . 45 degree , you may use the vectorial component of ( qp (ze ) ) that is multiply with cos 45 or sin 45.
When you check , the most critical dir. would be normal to the faces.

I would remind the clause 5.3 Wind forces;

(4) The effects of wind friction on the surface can be disregarded when the total area of all surfaces
parallel with (or at a small angle to) the wind is equal to or less than 4 times the total area of all
external surfaces perpendicular to the wind (windward and leeward).



According to the grace of God which is given
unto me, as a wise masterbuilder, I have laid the foundation, and another buildeth thereon. . . .
I Corinthians 3:10

 

[mte12]

Thanks again HTURKAK.

It looks like I haven't interpreted correctly.