Increasing the capacity of non-composite metal deck 1

[RFreund]

I am looking at a non-composite metal deck floor that was previously used for storage. They want to change use to vehicle storage/parking. The live load reduces from 125psf to 40psf. However, we also need to meet the 3,000lb concentrated load. The existing deck is a 2.5" (total) thick LW concrete on a 9/16" metal deck. I'm looking to see how we can strength the deck. My first idea is to pour additional LW concrete in order to thicken the slab and increase capacity. Any other ideas?
Thanks!

EIT

http://www.HowToEngineer.com

 

[hokie66]

The additional thickness would be my first idea as well. Clean the surface, and provide some reinforcement in the topping.

 

[KootK]

1) How close to the 3000lb can you get with the deck as is?

2) I've seen some clever/desperate checks for punching on the deck using arching action. One has to consider the "tie" however and, perhaps, the lack of a tie at perimeter conditions.

3) If reinforcing is required, I struggle to think of anything other than adding a bonded topping that would be a practical solution.

4) Do you expect that the excess weight of the topping will be a problem anywhere? I'm guessing not if the original system was designed for 125 psf.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[JAE]

Most decks I've seen used this way ended in disaster.
Specifically a similar deck on steel joists with heavy forklift traffic on them. There were no direct punching shear failures but the entire deck-slab system was beat to hell and fractured everywhere due to flexibility and concrete fatigue cracking with wheel loads going all over and in all directions.

On another building - forklift traffic beat up a 5 1/2" cast reinforced concrete slab on wide flange beams. All the deflections were within L/360 but the repetition of the wheel loads created a maze of cracking all over the slab.

In both, eventually, you'd get some kind of wheel punch-through.

Even adding the concrete thickness may be a fruitless attempt. The two slabs would either be adequately bonded together to serve as "one" in flexure or they would each work as separate slabs and get cracked all up eventually.

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[KootK]

Well...it's hard not to read though JAE's post and not be a concerned. And I have had one similar problem myself. That was a forklift / hard wheel thing though. I'd imagined that this was going to be a pretty light duty application. Store a few cars, in doors, long term etc. If this thing is going to see serious traffic, or be exposed to salts etc, I'd definitely share JAE's concerns and, possibly, add corrosion potential to the list.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[RFreund]

Thanks for the responses.

JAE - Noted. and Thank you. I haven't really seen this happen, so it's easy to think "it probably doesn't happen". Good to know that it does.

Well...it's hard not to read though JAE's post and not be a concerned

Well said

1) How close to the 3000lb can you get with the deck as is?

Punching shear is about 50% over, but bending is even worse.

I'd imagined that this was going to be a pretty light duty application. Store a few cars, in doors, long term etc.

Correct.

Thanks again for the input. I'll see what we need either ignoring the old slab or trying to bond them. It seems odd to think, but if the old live load was 125psf, we could add about 9" of LW concrete.

EIT

http://www.HowToEngineer.com

 

[hokie66]

A well bonded topping, using shot blasting to clean and slightly profile the old surface, but not using a so called "bonding agent', would be my choice. That assumes automobiles, not fork lifts.

 

[TehMightyEngineer]

Oof, if you're also failing in bending that definitely changes the calculus here.

I'd acid etch or shot blast the surface and then use powder-actuated fasteners and pepper the exist in a calculated pattern for the shear-flow. Pour a hardened topping of something 5,000 psi (or more) of a high-density mix, low w/c ratio but high slump or SCC if you can get it, and properly groove and joint the slab to prevent it from cracking with such a thin pour.

9" seems like quite a lot; are you sure you're running the calcs correctly or is it really just that bad in flexure?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL, CO) Structural Engineer (IL, HI)

 

[RFreund]

Thanks again for the recommendations.

9" seems like quite a lot; are you sure you're running the calcs correctly or is it really just that bad in flexure?

I was just saying that we could get up to that thick for a LW slab "if" needed. I would only need about a 5-1/2" total thickness slab to get it to work.
125psf (designed Live Load) - 40psf (new LL) = 85psf. 85psf/110pcf = 9.27"

I am going to propose the bonded slab and see if they want to move forward. Thanks again!

EIT

http://www.HowToEngineer.com

 

[txeng91]

Here’s a nice spreadsheet for slab on metal deck:

http://files.engineering.com/downlo...f3ff-4fff-bcaa-9cda77806c94&file=DECKSLAB.xls

Personally, I would try to keep the topping to a minimum. Since it is a “real” full area load. Make sure to consider joist deflections and concrete ponding.