Composite Deck and Uplift
Composite Deck and Uplift
(OP)
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?
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?






RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
Mike McCann
MMC Engineering
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
Hokie66 makes a good point...use the increased DL to help you out. Keeping in mind that the difference in unit weights between LWIC and lightweight structural concrete is about 80 pcf.
RE: Composite Deck and Uplift
I think that many of the SDI composite deck span tables are based upon a full composite action between the deck and slab and a simple span condition. With a tension applied, I would at least be a little bit concerned with the shear-bond. The net tension 125 psf equates to only a fraction of a psi, but I'd still think a bit about it.
RE: Composite Deck and Uplift
146psf/145pcf= @ 1' of concrete, yep, nobody will question that :)
Yessir, those S FL wind pressures can scare you if you aren't used to the magnitude. It almost just doesn't even seem right when you start crunching numbers... Didn't you say you worked on the Marlins stadium? Wasn't that a nightmare on that retractable roof?
Why are you putting concrete on a canopy in the first place? Strikes me as odd in my experience.
I am with JAE, even though its less than 1 PSI of gross uplift, what is the tensile bond of concrete on deck?
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
With that, the code dudes decided to just put the safety factor INSIDE the load combo and thus we have 0.6D + 1.6W. So in reality, the safety factor (load factor) against overturning/uplift is applied to the DEAD load instead of the WIND load.
If you read ALL the literature on statistical/risk based LRFD methodology (See Bruce Ellingwood for example) they suggest the load factor should be developed based on the variability of the load it is adjusting. In this case they did a counter-intuitive thing and down-graded the dead load when in reality, the overturning/uplift safety factor is meant to account for an accidental overload of wind.
I would have preferred a 1.0D + 1.6(1.5)W load case but they just didn't do it that way.
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
There is a D+W load combination, which will control when the wind and gravity are in the same direction. No FS then
RE: Composite Deck and Uplift
The correct combinations (with wind involved) are:
LRFD:
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)
0.9D + 1.6W + 1.6H
ASD:
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)
0.6D + W + H
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.
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
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 :)
RE: Composite Deck and Uplift
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.
RE: Composite Deck and Uplift
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.
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift
RE: Composite Deck and Uplift