Wind Analysis

[hemal1214]

Hi,

Does anyone know a good reference material/book to distribute wind loads? I am not using any software which generates wind loads automatically and even if I will be using any software I want to understand how the loading is applied to MWFRS.

The information given in ASCE7-05 regarding wind load is very vague to my knowledge. So the question is how do we apply wind load on a structure? My approach is:

1) Calculate wind pressure on the structure using ASCE7-05.
2) Find out the tributary area (width x height/2 for roof) and (width x avg. height (between floors) for floors). Multiply wind pressure with the tributary area and apply to respective node.
3) Design a MWFRS system for the loading and then for load combinations.

Questions:
1) If the tributary area is same between floors, does the wind pressure changes with height of the structure?
2) How do we account for shape of the structure, Center of rigidity of structure and other structural irregularities?

Looking for a good guidance. A reference material would be great.

 

[JoshPlumSE]

Answer to your questions:

1) Yes, wind pressure changes with the height of the structure.

2) Center if rigidity is irrelevant when we're talking about loads (unless the structure is very flexible). The centroid of applied wind forces can certainly change from level to level.


If you are looking for a reference for wind (and it sounds like you need one) then I recomment the Structural Engineers of Washington's commentary on the wind code... It's published as ATC 60. The one I have was published in 2004, I think. Therefore, it's pretty up to date. Though there are supposedly some big code changes in the 2010 ASCE-7 which will not be included in the one I have.

 

[boo1]

1. yes, pressures change with height (h). For residential many use 1/2 height of roof for h.
2. It is common break up into rectangle sections for analysis.

wind speed can be found at:

http://www.windspeedbyzip.com/

training link is:

http://timber.ce.wsu.edu/Supplements/WindDesign/intro.html

 

[frv]

It looks like you are a new engineer looking for some guidance. Although this website is a great resource, you really should be asking your supervisor all of your questions for the first few months.

 

[pmblair]

It depends on the structure. If the height is over or under 60 feet.

Is the diaphram flexible or rigid. For a square 1 story building, with masonry walls your on the right path.

As the aspect ratio, heights, change it all changes. Then you could have moment frames, braced frames, shear walls,......

 

[hemal1214]

Thank you for your replies. I understand with the change in height, wind speed and exposure category, the wind pressure changes. Let me describe as an example:

Design Parameters:

1) Building(steel structure) is located in New York City i.e. wind speed = 120 mph
2) Building is 3bay@40'= 120' in x-direction and 3bay@30'= 90' and height is 100'-0" with the first story is 16' and 12' height for rest of 7 story.

Based on ASCE7-05, 6.5.3
1) Wind Pressure = 120 mph and Kd = 0.85 (Table 6-4)
2) For Importance Category (II) I = 1.00
3) Velocity Pressure Co-efficients, Kh and Kz = 0.57 to 0.99 for 0 ft to 100 ft (For exposure category B and case 2, Refer Table 6-3)
4) Topographic Factor Kzt = (1+K1*K2*K3)^2 (This one has always confused me as all the data are not available) say Kzt = 1.2
5) Gust effect Factor G or Gf = 0.85 (Assuming the structure is rigid with fundamental frequency is greater than or equal to 1hz, is there any simplified method to calculate this? Or do I have to do a dynamic analysis using software to calculate this?)
6) Assuming "Enclosed Building"
7) Internal Pressure Coefficient GCpi = +0.18 or -0.18 (Fig. 6-5)
8) External Pressure Coefficient Cp or GCpf, Wall Pressure Coefficients, Cp, Windward Wall = 0.8, Leeward Wall (L/B=1.33)= -0.43, I am not discussing about Roof pressure for now.
9) qz (for 100ft,kz=0.99) = 0.00256*kz*kzt*kd*V^2*I = 0.00256*0.99*1.2*0.85*(120)^2*1.0 = 37.2psf (seems to low)

This qz pressure I need to multiply by tributary area to find out point load at node. Let's say if I am using moment frame in the center bay in front and rear frames. For side walls, I am using a braced frames in the center bay.

My wind point load on roof moment frame node = 37.2psf*120ft*6ft = 26.8kips
Similarly, my moment frame node for the floor below roof is 88ft, the pressure qz = 36.1 psf, so my wind load at node = 36.1psf*120ft*12ft = 52 kips. Same way the wind point load on node will reduce with the reduction in qz value with height.

I would like to know, if this approach is correct or am I missing something here?

Thanks again,