Effective Wind Area 2

[Lina]

There was a great confusion in ASCE 88 about what is called a tributary area when calculating pressure coefficients. That was wrong they should have never called it tributary area because we all know what is a tributary area. however, ASCE 98 is very clear now in calling it effective wind area. I am reviewing a design by other Engineer and I believe he did not use the right value for the effective wind area. Let see if you can agree with me.
A Tower 109 feet heigh 90' long by x 81' wide. The engineer used an effective area that is less than 10 sf to calculate GCp (external pressure coefficient for the roof). I beleive this is wrong. Way over design. We all know the smaller the effective area the higher is the pressure. I believe the effective area of this building is based on whether the roof slab is one way slab or two way slab. If it is one way slab the effective area should be 81'x 81'/3=2187sf; if it is two way slab the effective area is 190'x81'= 15390 sf.

Let me hear from you before I redesign. Thanks in advance.

 

[JAE]

Lina:

The effective area used in the ASCE 7 wind coefficients is meant to apply to the element you are designing. For instance:

Suppose your roof plan is made up of a one-way beam and slab system where the beams are spaced at 8 feet o.c. and are supported by concrete girders spanning 28 feet on columns.

For your slab design, each slab is taking a very small area of wind in that you are really designing a 1 foot wide strip, spanning from beam to beam. Therefore, the 1' x 8' distance is what I'd use for SLAB design.

Now, as you move to the beams, suppose they are spanning 32'. Their effective area would be 32' x 8'.

The girders supporting the beams have an effective area of 28' x 32'.

The effect that is being modeled by the GCp coefficients is that, for a component wind load, there are wide variations in the wind pressure over a roof. These variations (high peak areas in some locations and low pressures in others) have a large effect on a small "piece" of your structure in that an element with a small effective area has a higher probability of experiencing one of those peaks.

As your element gets larger, the peaks and valleys average out and statistically, the wind pressure comes down lower.

So for a roof, there isn't ONE effective area to use, but numerous areas, depending upon which element you are designing.

 

[Ron]

JAE...nice answer

 

[Lina]

Jae
Thanks for your detailed answer. Your comment on the uplift pressure was excellent. However, I am still not convinced of these effective areas and may be you can help me further.
ASCE 1989 defined the Effective wind area as follows:
The effective wind area used to determine GCp is the span lenth multiplied by an effective width that need not be less than one-third the span length.
For the slab design in your example the effective area should be 8' x (8'/3) = 21.33'sf keeping in mind that the 1' strip you are refering to is less that what the code is refering to. Therefore a value less than the minimum should not be used. In this case you have used smaller effective and you are over-designing for the pressures.

The same with the beams. The effective area is 32' x (32'/3)= 341 sf. the effective width you have used (8') is less than (32'/3). Therefore a value less than the minimum should not be used. In this case you have used smaller effective ara and you are over-designing for the pressures.

The same with the girder design the effective area should be 28' x (28'/3)= 261 feet. In this case you have used larger effective area and you are under-designing for the pressures.

Again I am trying to translate the code and I hope I am doing it right. Tell me what you think.
Thanks.

 

[tw]

If we're talking about the main wind-force resisting system then I don't see where the magnitude of the effective wind area makes a difference. That only applies to components and cladding, which I equate to windows, curtain walls, etc.

The structural system (beams, slabs, columns, x-bracing, etc.) would be subject to the force pressure coefficents found in ASCE 7 for the main wind-force resisting system. (In ASCE 7-98 the design procedure is summarized in 6.5.3)

Tom

 

[JAE]

Tom:
The main wind-force resisting system winds should be used ONLY with your design of the main wind force resisting system (the beams, columns, footings, shearwalls, etc. that perform along your lateral load path).

For other beams, joists, slabs, girts, etc., you still have to check and design for component wind. Some members occurring on the outside of the building shell must be checked for both component and primary wind.

Lina:
Yes, you are correct that the 1/3 factor on span can apply under the ASCE 7 and my example indicated a conservative approach. The main point I was making was that there area multiple effective areas used on elements a building, not just one.

 

[tw]

JAE:

The commentary in ASCE 7-98 under section C6.0 addresses this subject directly.

Thanks for your reply.

Tom