ASCE 7-10 Wind loads on rooftop platform

[ENGINEER92]

I am designing a platform to hold equipment on top of a building. I have already calculated my wind pressure. I am a bit confused on the force coefficient to use. I know to use Figure 29.5-1 for the force coefficient on the equipment, but unsure on what figure to use for the force coefficient on the platform itself. I am thinking I use the force coefficients in figure 29.5-2. The platform has four pipe columns, the beams are made out of wide flanges and channels and i have knee braces to stabilize the structure.

It would greatly appreciated any help on the correct way to calculate the wind force on the platform.

 

[JStephen]

Which edition of ASCE 7?
It's possible that an uncommon geometry simply may not be covered by the standard. I suspect a platform above a building could have large uplift forces that aren't covered by the projected-area methods. Also note the section dealing with increased loading for rooftop equipment.

 

[msquared48]

I will check tomorrow, but above a certain height, you have to increase thenpressyer, I believe as much as 30 percent.

Mike McCann, PE, SE (WA)

 

[dik]

In these environs... the bigger problem is usually snow accumulation.

Dik

 

[ENGINEER92]

I am using the ASCE 7-10 third printing. And my rooftop is about 100' in the air. And our live load governed over our snow load.

 

[ENGINEER92]

Also, when I am applying my ice weight to my structure. Would the equipment on the platform have ice weight?

 

[WARose]

I am a bit confused on the force coefficient to use. I know to use Figure 29.5-1 for the force coefficient on the equipment, but unsure on what figure to use for the force coefficient on the platform itself. I am thinking I use the force coefficients in figure 29.5-2. The platform has four pipe columns, the beams are made out of wide flanges and channels and i have knee braces to stabilize the structure.

Yes, getting Cf from 29.5-2 (and loading each individual member for wind) would be one acceptable approach.

Also keep in mind that there is a uplift case to consider with rooftop equipment/framing.

Also, when I am applying my ice weight to my structure. Would the equipment on the platform have ice weight?

Unquestionably. (Unless it runs hot/warm.....and even then I'm not sure you should ignore the ice load because you can always lose power in a ice storm.)

 

[ENGINEER92]

Thank you WARose for responding. Section 29.5.1 is for "Rooftop structures and equipment for buildings h<60ft" if my building is greater then 60ft does this section still apply?

 

[WARose]

Section 29.5.1 is for "Rooftop structures and equipment for buildings h<60ft" if my building is greater then 60ft does this section still apply?

I didn't notice your height until after I posted. I think at that point, you can use Part 6 of Chapter 30 (C&C).

 

[ENGINEER92]

Thank you that is helpful, I should of posted my height in my original post. When I am calculating my ice load of the equipment, do I use equation 10.4-1 to get my ice weight?

 

[WARose]

When I am calculating my ice load of the equipment, do I use equation 10.4-1 to get my ice weight?

I think we are talking (then) more of a structure composed of flat surfaces. (I.e. then you would figure it by volume, ergo Eq. 10.4-2.)

To go back to the wind load for a second (while it is on my mind), I wanted to say: I went to a seminar on wind loading (based on ASCE 7-10) some years back and the guy giving it said that the wind load on rooftop structures with h ("reasonably") greater than 60' is just about what you get if you did it by that section anyway. At the time, I don't think they had the research to justify it in excess of that. Not sure if that has changed since. (I know that is not much of a source.....but I thought I'd mention it anyway.)

 

[ENGINEER92]

Yeah, equation 10.4-2 makes a lot more sense.

So basically there just isn't enough research on rooftop equipment above 60ft get mentioned in asce 7-10. Just to double check, the uplift acts simultaneously with lateral wind force on the equipment?

 

[ENGINEER92]

Also, since my platform deck is metal grating, does that mean I have to consider a ice weight on it?

 

[WARose]

Just to double check, the uplift acts simultaneously with lateral wind force on the equipment?

That's what I've typically done. (In the proper combination with minimal Dead Load.)

Also, since my platform deck is metal grating, does that mean I have to consider a ice weight on it?

Most definitely.