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Composite Deck and Uplift

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slickdeals

Structural
Apr 8, 2006
2,268
Folks,

I have a situation where canopy uplift pressures are of the order of -125 psf. This is for a canopy on the entrance of a big box retail.

The canopy pressures have been calculated based on ASCE 7-05 (using a open building with obstructed wind flow) Figure 6-19A.

There are no available NOA's (Miami-Dade County) for light weight insulating concrete on metal deck for these pressures. As a result, the contractor wants to replace the LWIC with 2" of structural concrete (for which NOA's exist).

My design approach is the following:

1. Design the concrete slab on metal deck for uplift, by providing top reinforcing in the 2" concrete slab. I am assuming a "d" = (distance from top of concrete to CG of metal deck). Follow ACI procedure for calculating As.

2. Since the slab will be subject to uplift, there will be positive moment at the supports. I intend to use the deck as reinforcing for this case (1.5 VL deck). There are no studs at the support. Only puddle welds, which have been calculated for the net uplift.

3. Use the SDI Method (for no studs) and calculate the phi*Mno = phi * Sc * Fy. Compare the capacity against the positive moment.

Does the above sound reasonable?

 
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Good point hokie.
There is a D+W load combination, which will control when the wind and gravity are in the same direction. No FS then :)

 
hokie. I screwed up with the load combination above. It should have showed 0.9D (not 0.6D) for the LRFD combo that I typed.

The correct combinations (with wind involved) are:

LRFD:
[blue]1.2D + 1.6(Lr or S or R) + ((0.5 or 1.0)*L or 0.8W)
1.2D + 1.6W + (0.5 or 1.0)*L + 0.5(Lr or S or R) [/blue]
[red]0.9D + 1.6W + 1.6H [/red]

ASD:
[blue]D + H + F + (W or 0.7E)
D + H + F + 0.75(W or 0.7E) + 0.75L + 0.75(Lr or S or R) [/blue]
[red]0.6D + W + H [/red]

The blue ones are where dead and wind work together.
The red ones are for the overturning/uplift checks.

But technically all have to be checked.
 
Slick- did you have to do wind analysis on the new Marlins stadium roof?

Interesting Hokie, not arguing that you haven't seen and used it, just amazed that is used... FWIW in 13 years of mostly Florida design, slick's post was the first I have heard of concrete being used on a canopy or even a roof, though other forms of ballasted roofs are nothing new. I would not want to be Slick if he goes with the 17" slab though, I don't think it would have a warm reception. Solves your wind problem, then you turn around to do your D + L calc and DOHHHHH!!!! These loads are like light storage design loads.

The other problem with canopies is they are often cantilevers and adding more dead lead solves one problem and raises another, unless you have some type of back span and/or counterbalance built into the structural system. Like a cable-stayed bridge that has a central mast with counterbalancing dead loads to each side. Calatrava uses this concept not just in his bridges but in his structures. Like the Milwaukee Art Museum that I went to last summer, he used this concept to support the walkway one one side of the mast and the roof of the main building on the other, along with angling the mast for effect and to help balance forces I assume.... Not many day to day canopy designs allow for such creativity however :)
 
 http://www.factmonster.com/us/history/milwaukee-art-museum.html
a2mfk,
We don't know much about Slick's canopy, but I assumed it was cantilevered and that some ballast to balance the up/down loads would be advantageous. I didn't suggest 17" of concrete.

The awnings/canopies I have designed with concrete ballast have indeed been cable stayed. When you only have tension members and can't take them to ground, mass is the answer.
 
a2:
Marlins was rigorously tested in the RWDI wind tunnel up in Ontario.

My canopy is not cantilevered. It is 20' long with the building walls (facade) on one side and steel columns on the edge.

 
In that case, it's probably more conventional to put the required mass all in the footings.
 
hokie66 I agree. Weight in the footings doesn't have to be resisted in D + L case for the canopy framing and I have always been told by contractors that concrete in the ground is as cheap as it gets.
 
That is true, but it is still worthwhile to consider the assistance that roof supported dead load provides for wind uplift. The roof members have to be designed for both upward and downward cases, so a balancing act can achieve lighter member sizes. The OP was going to use insulating concrete on the roof, so the operation was part of the construction anyway, regardless of the thickness.
 
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