Composite steel Deck

[Sweever]

I have a unusual question regarding the minimum thickness of concrete above the profile of a steel composite deck within a localized area (see attached sketch). I have a exterior balcony that spans approximately 6'-8" (simply supported). We are using 3" composite steel deck and 2" concrete (above the deck) for a total balcony floor of 5" (note, the thickness of concrete at the end of the balcony is more like 1.5" with the desired slope. The contractor wants to install a continuous low profile drain in the balcony floor near the support where the concrete is more like 1.5" deep. The continuous drain will running perpendicular to the deck flutes. The drain pan is 13/16" deep and 2.5" wide (the flanges of the drain are 1" each side for a total width of 4.5", see sketch). The thickness of concrete directly under the drain pan will only be 3/4" thick above the top flutes of the composite steel deck. Not sure this is a good idea but the client is pushing hard for this drain. The unfactored dead and live load on the balcony is minimal (less than 100 psf) and if the deck was 5" thick would be adequate for a load of 375 psf. (3" deck was utilized for architectural reasons, that why it is so over designed). The drain is located very close to the support as shown (3") and I am sure I can determine that the shear strength of the steel deck will take the load without any concrete. Just worried about the vibration deflection and weak spot this drain may cause. Comments?

 

[hokie66]

I am "sure it is NOT a good idea". Steel deck for exterior balconies is never appropriate. Corrosion will soon be an issue. Form it conventionally.

 

[JAE]

I would agree with hokie66. No deck - will look like a mess eventually unless the top surface is adequately waterproofed and maintained...which it never is.

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[mfrad]

Definitely corrosion issues. Also the trench needs a cross slope to pitch to drain. The trench will interfere with any reinforcement from the railing curb into the minimal concrete deck. The railing may not be able carry 50 plf. also need to consider the drain pipe; where is it being plumbed to. Is it an interference to the balcony below?

 

[Sweever]

I understand where you are coming from on the corrosion issue. The slab will be waterproofed and maintained. This is a high end beach house where the owner is highly particular about upkeep and maintenance. Regarding the railing, it is welded directly to the outside steel beam and is adequate structurally. Also there are no issues with the drain pipe. I am really just needing input on the issue stated. We have thought about the other potential issues already.

Thanks

 

[XR250]

Beach house? Yea, it ain't gonna hold up. Form it up and add the metal deck later as an accent.

 

[KootK]

Very important corrosion issues aside:

1) Probably not an issue for bending so close to the support.

2) I'd base shear on a depth reduced by the presence of the drain. Again probably not an issue.

3) I'd be concerned about the quality of the concrete between the drain and the end of the member. Kinda tough to get in there to pour and consolidate etc. You thinking wet set?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Sweever]

Thanks for the response KootK.

I am concerned about cracking at this drain/concrete interface given its reduced concrete thickness. Also, potential vibration issues with the deck, and absolutely the installation/ concrete consolidation issues. I would definitely be using a small pea stone for aggregate. Have not got to thinking about the wet set feasibility yet.

 

[KootK]

I don't see much potential for vibration issues. Short span, fair bit of mass relative to probably a pretty small forcing function.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[bones206]

I think the most probable crack location would be directly beneath the drain where the slab cross section is thinnest. In that case it wouldn't necessarily be visible, but you never know which way a crack will propagate. Maybe the drain manufacturer could provide some photos or references of similar existing installations.