Minimum Design Wind Loads 4

[Ayham Shahoud]

As mentioned in ASCE 7-10 / 27.4.7, "The wind load to be used in the design of the MWFRS for an enclosed or partially enclosed building shall not be less than 16 lb/ft2 (0.77 kN/m2) for walls and 8 lb/ft2(0.38 kN/m2) for roofs". As my project is classified as enclosed, the majority of the wall's design pressure values are smaller than(0.77 kN/m2). So, Should I replace them with the limit value 0.77 kN/m2 ??

 

[JStructsteel]

Thats how I comprehend it.

 

[phamENG]

I've always taken it as an additional load case, not an outright replacement.

If you have a windward pressure of 12psf and a leeward pressure of 5 psf, you don't replace it with 16 and 16...it's already a net 17psf. So you use the calculated design pressures for all of the load cases in chapter 27 (see Figure 27.4-8), and then apply the 16psf and 8psf to the vertical projection in each direction to be sure the demand on your LFRS components doesn't increase.

 

[Ayham Shahoud]

Dear phamENG,
Do you think it's economic to double the pressure value in certain walls where the current pressure there is almost 0.70 kN/m2??
In other words, is this practice that you suggest mentioned in the standard?? .. I actually cannot see it.

 

[phamENG]

It's not about economics, it's about maintaining a minimum standard for reliability. And if the windward pressure is already about 14.5psf, I seriously doubt that the minimum wind load load case will cause a change in your design since you'll have some leeward pressure as well...and it's probably more than 1.5psf.

As for where it is in the book...

This section specifies a minimum wind load to be applied horizontally on the entire vertical projection of the building as shown in Fig. C27.4-1. This load case is to be applied as a separate load case in addition to the normal load cases specified in other portions of this chapter.
(Emphasis Mine)

 

[Celt83]

phamENG:
For my own clarification you are saying apply the analytical wind, then apply the minimum wind ie as separate load cases and compare the results to verify the minimum wind case did not increase design forces.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[Aesur]

@Celt83 - the above statement and image is how I interpret the code. Most of the time I find that this only comes into play with gable roofs as flat roofs tend to have parapet loading which is significantly higher than wall loading. I am not sure I have actually encountered a case where the wall loading was less than 16 psf, however I have encountered cases where the diaphragm loading ,sum of horizontal components of the gable roof + wall loading is less than 16 psf + 8 psf on a horizontal projected plane and therefore the minimum loading controlled.

 

[phamENG]

Celt83 - that's exactly how I see it. Of course your analytical wind will also have the fun bi-directional/quartering and torsional load cases too if they apply. Minimum wind doesn't get used in those.

 

[Celt83]

got it, that was my understanding as well. Thanks

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[Ayham Shahoud]

Thank you for all you guys .. Your replies were really helpful.