One-way slab or steel on metal deck?

[PowersPE80]

I am designing an underground corridor which connects two buildings. The walls are reinforced concrete basement-type retaining walls. The floor is a concrete slab on grade. There is about 2 feet of soil above the roof of the tunnel. I am trying to decide if the roof should be a one-way concrete slab or structural steel with slab on metal deck. Does anyone have any recommendations/suggestions for the roof of this underground structure? Is it better to provide a one-way slab or steel with slab on metal deck? Thanks in advance.

 

[hokie66]

I would prefer the solid one-way slab. Steel framing with steel decking and slab just introduces more trades than necessary, and a system which is more prone to damage by corrosion if there is water intrusion.

Whichever system you use, make sure of the waterproofing, and protection for the waterproofing, before the backfilling is done.

 

[grantstructure]

Cast in place. You may also want to consider the possibility of losing soil support on one side, in which case the box will have to work as a frame to resist lateral loads, and your corner zones would then be detailed for both opening and closing moments. Also crown the slab slightly to promote drainage and pay attention to waterproofing (and waterstops) as hokie says. Your construction and contraction joints details will matter.

 

[mark519]

Thanks for the responses. There is already steel on the job (the tunnel is a small portion of the entire project), so the steel trade is already involved. I was leaning towards the cast in place option as you both mentioned. Out of curiousity, are there any disadvantages to the steel/deck option?

 

[JedClampett]

One disadvantnge is that the steel deck is prone to corrosion. Even if it's hot dip galvanized, it gets scratched or the spot welding burns off the zinc and you get corrosion.

 

[shobroco]

CIP from me too: rusting of the deck, poor detail where the deck sits on the walls, aesthetic finish of the tunnel, tendency to reduce concrete thickness when using decking, more reasons I can't come up with instantly

 

[chicopee]

Tunnels with arch roofs have endured the test of time.

 

[ajk1]

Cast in place is the way to go, for the reasons listed below. I would never consider steel deck for this situation.

a) I assume that there will need to be a waterproofing membrane on the top of the concrete to prevent leakage into the space below, but it is not good practice to apply a waterproofing membrane to concrete that cannot breathe on the opposite side; most waterproofing manufacturers / contractors will not give any warranty for this condition (see CSA S413);

b) eventually all waterproofing leaks and this will lead to corrosion of the steel deck.

 

[RareBugTX]

Hi:

Am with Chicopee and AJK1, Structure is one consideration, waterproofing a very important as well. I have seen large concrete garages with serious water problems. If posible, design it with at least some slope for drainage and a very good waterproofing detail. Arched would be best...I think.

 

[hokie66]

Other, non-structural, issues could also favor the one depth cast in place system. Steel framing would mean the height of the tunnel, thus the excavation depth, is greater. Also, a non-flat soffit could make the lighting and/or ventilation more complicated.