Cantilevered Roof / Grandstand

[slickdeals]

I am designing a building that has a cantilevered roof over bleachers. The cantilever is about 25'. There are solid walls at the fixed end of the cantilever running perpendicular. There are 15' long return walls at the end of the perpendicular walls.

The cantilevered roof framing consists of tapered steel members with metal deck + topping spanning between them.

See attached sketch.
I am in Hurricane country (South Florida) with V=146 mph (HVHZ zones)

My questions are as follows:

1. Can Figure 6-18A be used with the assumption of obstructed wind flow (more than 50% blocked)? If so, does this C_N value assume that there will be a stagnation pressure below the roof in addition to the uplift? If not, does the pressure calculated per 6.5.11.4.1 have to be added to this value?

2. As a secondary check, I am calculating the wind pressures (uplift) for this cantilevered roof considering it to be a partially enclosed building. This is for the case where the wind is blowing from left to right (in the cross section view).

3. Section 6.5.11.4.1 requires that roof overhangs be designed for a positive pressure on the bottom surface using a Cp = 0.80
Does this have to be in addition to any uplift calculated using Figures 6-6 through 6-10.

Any thoughts/suggestions on what sections of ASCE can be modified to apply to my situation?

As always, thanks.

We are Virginia Tech
Go HOKIES

 

[slickdeals]

A secondary question I have is the design of the 160' long perpendicular walls. For wind blowing from left to right (in cross section), there will be a positive pressure on the wall (causing stagnation below the roof intersection) and a negative pressure from the wind blowing over the roof.

Will this wall be then designed as a parapet wall, subject to both positive and negative pressures?

Does ASCE 7 address this issue?

We are Virginia Tech
Go HOKIES

 

[paddingtongreen]

Slick, I'm not up on ASCE7 these days, but when I designed a canopy front at an international airport arrivals building. It amounted to full pressure inside and full suction on top.

Have you looked for "Canopy"?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

slickdeals,

Be extremely conservative with a structure such as that. Do not try to interpret the wind pressure in the code too closely. Make sure you have plenty of additional capacity. And make sure you brace the bottom chord conservatively. The fact is that nobody really knows what the wind pressures are in a situation such as yours.

BA

 

[ToadJones]

I was going to replay along the lines that BA did, but I have never designed such a beast.
I have had to survey one that under construction loads was sagging severely. In modeling the structure in order to determine the problems, I realized how complicated they can be.

 

[ron9876]

That is one that certainly doesn't fall in any category in ASCE7. The obvious easy answer is a wind tunnel study.

I would at the least have stagnation wall pressure on the bottom of the roof and uplift on the top from the worst of the open building conditions. I would compare to an overhang pressure and look at partially enclosed pressures. That is a tough one to be comfortable with a judgement call and in the HVHZ you have to verify that it doesn't flutter (somehow). If the cantilevers were a little longer I would definitely push for a wind tunnel test but as it is I would look for the largest wind load that I could calculate and be sure that a reasonable solution for the structre will give you plenty of room to sleep at night.

Good luck.