question about wind load analysis of a open top screened structure

[gazibu]

I was wondering if some one can provide guidance form the following wind analysis condition:

a metal screen that is supported by vertical and horizontal steel tubes is supported on top of a 18 story bldg.the screen has 3 sides and the fourth side is the side of the existing bldg.The screen is perforated , but the design architect is not allowing for any reduction in forces due to the perforations. considering that the screen is open on the top, how do you analyze the wind force ? is this a partially open structure? or an open structure? secondly can the stresses due to wind be decreased by 1/3? using the ASCE7-02 wind tables I came up with a wind force of 61.5 PSf (35.6 psf pos + -25.9psf neg = 61.5 psf)would the positive and negative wind forces add in this situation?

 

[kslee1000]

No reductions. Design use the worst case possible, since the screens either could be plugged, or create turbulence affecting the structures.

 

[gazibu]

I understand that we cannot take any reductions, but it seems to me it is probably more realistic to analyze this as a other-structure system (as outlined in Fig. 6-20 ASCE-02 for solid free standing walls and signs).

 

[kslee1000]

I am not familiar with the newer code. But depends on its function and operation, the structures can be closed, open, or partially enclosed. List the possibilities and corresponding wind pressures, then you can clearily see and chose the design load. Remember that the loads are to be resisted by the support tube beams/columns.

 

[BAretired]

Is the 25.9 psf an internal pressure? If so, I don't think it should be included if the walls are open to the top.

BA

 

[CFSEng]

I would be cautious of using the other-structure (free standing walls and signs) as that neglects any possibilities of wind pressure increase due to corner (edge) zones.

 

[Ron]

I would design this as an open structure, using normal methods, not as an "other structure". Consider the three sides as A,B, and C. Sides A and C are opposite. When wind blows on A you get positive pressure and negative pressure on C. You will also get turbulent flow at the top of wall at, creating a negative pressure on the back side of a, much like a parapet. The problem is you don't know how far down to take that effect. You'll have to use your judgment, but conservatively you might want to use 1/3 of the height. If the separation between wall A and C is greater than the height of A and C, you will also have some positive pressure on the inside of C, much like the negative on the inside of A.

Assuming the "screen" is substantial and can develop tension, your corner column at A-B will now pick up both the positive and negative pressure of walls A and C. It becomes a shear wall. If it will handle compression as well (diaphragm), then corner column B-C will carry the same load, otherwise it will only carry the pressures on wall C....but when the wind blows from the other direction, the reverse will apply so essentially all columns carry both loads. Then you have to check the wind from all other directions...that's just an example of one.

No, you cannot take a 1.33 overstress on this.

 

[gazibu]

Thanks for your detailed response. I checked the wind pressure using the gcpi= + -0.55 (partially enclosed structure), which gives the highest wind pressure.Additionally I suppose to be conservative I can add the positive and negative internal pressures. If the existing members work for this load condition I suppose I can let it go.If they do not work then I may have to reduce the internal negative pressure or as you stated apply it only for a certain height.