Uplift on concrete roof slab

[ntpe]

I am looking at the design for a small storm shelter. For the roof system, I would like to use a 3 1/2" concrete slab over a metal deck. The design uplift exceeds the slab DL so I need to anchor the slab to the deck. I don't want to use studs on the joist because I would like to avoid composite action. Does anyone have a suggestion for attachment of the slab to the metal deck? Can I thru bolt the slab to the deck? Can anyone think of any potential problem with that?

 

[UcfSE]

I would use the puddle welds attaching the deck for shear and uplift. You can use the 2001 NASPEC to calculate a puddle weld strength and then check the interaction of tension and shear. If you are concerned about the bond between the deck and the concrete, you can probably try using a composite metal deck. With uplift on the concrete slab you'll need rebar to resist the flexural tension unless you design the concrete to work unreinforced. With headed studs, you don't necessarily have to put in so many that you will have composite action with the joists. You only need enough to hold down your slab. After looking at all these costs, consider increasing the slab thickness to eliminate the uplift. The difference in concrete might not be that much, and the added dead load will help you with uplift on your footings. I've not been able to go that route yet however.

The website below has tables for the effect of uplift on the shear strength of metal deck diaphragms but I don't think they have one listed as of yet for form deck with concrete. They are near the bottom of the page.

http://www.njb-united.com/usd/ddds.htm

 

[ntpe]

Thanks UcfSE. Maybe I need to be a little more specific. I have an uplift of about 120psf. I would like to avoid the use of an 8" slab just to resist uplift. I don't think it will be a problem to hold the metal deck down. I just want to be sure to keep the concrete slab in place. I know that studs attached to the joist will work, but then I need to consider composite action at the joist, and I would rather avoid that.

 

[Lutfi]

To what code/standard are you designing the shelter (State, FEMA)? For example FEMA has strict requirements for the wall and room construction. I like to refer you to FEMA Guidance 362, chapter five. Here are the wind load criteria, (to be used with ASCE 7-98), FEMA's requirements:

Wind speed = 200 MPH (3-second Gust).
Importance factor = 1.0
Exposure = C
Directionality factor = 1.0
Internal pressure coefficient = +/- 0.55
Height of shelter is not restricted.

They also have the following requirements:

A) 6 to 12 inch CMU walls with #4 vertical bar fully grouted in each cell with horizontal reinforcement as required.
B) Reinforced concrete roof and wall sections at least 6 inches thick with at least #4 reinforcing steel at 12 inches on center each way.

I will let you extract the rest from the chapter. Here is a link to this manual:

http://www.fema.gov/fima/fema361.shtm

Regards,
Lutfi

 

[Lutfi]

Two more points I failed to bring up:

1. try to add reinforcing steel in the slab so that it takes negative bending. You cannot simply rely on weight of slab to resist uplift! If you are going to do that, I would make sure that I have at least safety facto of 1.5 against uplift so I can go to sleep at night. Things are not always that simple.

Why are you opposed to composite action? It is a great system.


Regards,
Lutfi