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Composite Deck and Uplift

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?

RE: Composite Deck and Uplift

Probably - but a 125 psf of up lift seems a bit high??  Like over 300 mph?? approaching 400 mph??

RE: Composite Deck and Uplift

This canopy is a portion of the wing of an old 747...

Mike McCann
MMC Engineering

 

RE: Composite Deck and Uplift

(OP)
Its' been designed for 146 mph wind. The pressures are what they are per ASCE 7. I am sure the South Floridians in this forum will commiserate.  

RE: Composite Deck and Uplift

Yes they will. Seems reasonable to me. Be sure the puddle welds are good enough.

RE: Composite Deck and Uplift

You are going to pour concrete on the canopy...why not make the slab thick enough to resist much of the uplift as dead lead ballast?  Should get a tradeoff in your structure, as the up and down loading will be more equal.

RE: Composite Deck and Uplift

slickdeals...yup...that's what we have to deal with here!  Your approach is reasonable.  Keep in mind the NOA for LWIC is only applicable to the roof membrane attachment.  You could probably make a case that there is no need for an NOA for its attachment to the metal deck by adhesive bond. However, I assume you're dealing with Miami-Dade, so probably better to save your breath and go with what you're doing!!

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

The only warning I would offer is that composite decking works compositely due to shear-bond between the deck and concrete.  In an uplift condition, you will have some net tension between the deck and the concrete.

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

I'd use just the concrete for uplift resistance. See if I can do the math:
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

Using concrete for DL on a canopy may seem odd, but is often done in high wind areas if the outside of the canopy can't be tied down.  I have done it several times for airport terminal buildings.  It's especially applicable to places like Florida and Queensland, where snow is a minor or nil consideration.  Using enough concrete so that the combined uplift and the combined downward loads are equivalent allows the supporting structure to be optimised.  Worrying about whether the concrete is bonded to the deck is nitpicking...the uplift is not just suction on the top, for a canopy it is mostly buildup of pressure from underneath.

RE: Composite Deck and Uplift

(OP)
The only reason I have to use concrete is because there are no NOA's for LWIC on metal deck for those pressures.

RE: Composite Deck and Uplift

a2mfk that would be 1.67' of concrete.  

RE: Composite Deck and Uplift

Why, ron?  Nobody mentioned 250 psf uplift.

RE: Composite Deck and Uplift

(OP)
He is using a 0.6 factor as required by code.

RE: Composite Deck and Uplift

I'm certainly not familiar with the US wind loading code, but I thought the load case was 1.0W - 0.6D, where both are characteristic loads.  So 125 psf uplift could be counterracted by 125/.6 = 208 psf of concrete, about 1.4' or 17" of concrete.  But I didn't suggest using that much concrete, but only enough to more or less equalize the upward and downward loading conditions.  If 125 psf is already a factored load, then less concrete is required.  Maybe somebody can enlighten me.

RE: Composite Deck and Uplift

My comment was to a2mfk's calculation of 1.0' of concrete. 1/0.6=1.67.  

RE: Composite Deck and Uplift

Since we are on the subject, does anyone else find it ridiculous that you have to decrease the dead load of a KNOWN weight with the 0.6DL load case.  I understand when you have an assumed uniform load that the reduction is necessary, but in situations like this and, say, uplift at footings the 0.6 should be able to be omitted (not that I do, but I think it should be allowed).

RE: Composite Deck and Uplift

epitome1170.  Yes, they used to let you use 1.0D but then you still would want some level of safety against uplift or overturning right?  Perhaps 1.5?

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

I'm still confused with this US load case.  Maybe I will remain so.  What do you do when the wind and gravity load are in the same direction?

RE: Composite Deck and Uplift

The W in the combo can be in any direction.  The + sign is simply the addition (i.e. inclusion) of the wind effect, whichever way it can possibly be applied.

 

RE: Composite Deck and Uplift

(OP)
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 smile

RE: Composite Deck and Uplift

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:
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

Got it.  Thanks, JAE.

RE: Composite Deck and Uplift

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 :)

RE: Composite Deck and Uplift

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.

RE: Composite Deck and Uplift

(OP)
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.

RE: Composite Deck and Uplift

In that case, it's probably more conventional to put the required mass all in the footings.

RE: Composite Deck and Uplift

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.

RE: Composite Deck and Uplift

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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