Composite Deck and Uplift

[slickdeals]

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?

 

[MiketheEngineer]

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

 

[msquared48]

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

Mike McCann
MMC Engineering

 

[slickdeals]

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.

 

[ron9876]

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

 

[hokie66]

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.

 

[Ron]

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.

 

[JAE]

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.

 

[a2mfk]

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?

 

[hokie66]

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.

 

[slickdeals]

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

 

[ron9876]

a2mfk that would be 1.67' of concrete.

 

[hokie66]

Why, ron? Nobody mentioned 250 psf uplift.

 

[slickdeals]

He is using a 0.6 factor as required by code.

 

[hokie66]

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.

 

[ron9876]

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

 

[epitome1170]

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).

 

[JAE]

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.

 

[hokie66]

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?

 

[JAE]

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.