Steel Porch/Deck Framing System

[T_Bat]

Hey everyone,

I have a project where an exterior porch is being added to an existing building. The porch wil be framed with conventional steel beams and columns. I originally designed it as a compsite slab - the spans are not large but I like the studs for getting my lateral loads into the LFRS and for vibration issues. I'm going to spec a heavier galvinization on the metal deck - and also consider it sacrificial (belt+suspenders?).

The client has requested we use metel deck with ridid insulation and a paver system on pedeastals instead of the composite system. I see how this would be cheaper but I'm concerned about redundancy. Once the deck deteriorates there is no "back-up" graity system with metal deck only.

Anyone think I'm being overly conservative here or are these reasonable concerns? Just seeing if there may some alternative I haven't thought of.

Thanks!

 

[Mjkkb2]

I personally would not be ok with a metal deck serving as the "floor" for a porch. I don't even see how you can do that. Isn't this basically a roof that you end up with instead of a floor? I would maybe compromise and forgo the composite slab system, you could just do form deck on beams with maybe a thinner slab to cut cost. IF the spans are not long then vibrations shouldn't be a problem anyway. The porch should not have lateral loads so high that one would need to rely on studs for diaphragm connection. Composite seems like an overkill for a porch but there should be some kind of slab, not just bare deck.

 

[XR250]

What about PT lumber on top of your beams instead of concrete and/or decking?

 

[T_Bat]

Mjkkb2 - The metal deck would not be the floor. It would be similar to roof construction with build up/tapered rigid insulation to provide drainage and the walking surface would be paves raised off the insulation with pedestals. You're probably right about the vibration stuff. I'm not sure how to get diaphragm loads into my lateral frames unless I provide some kind of connection between the slab and the beams. The deck is sacrifial so I don't feel like I can count on the deck attachment for this.

XR250 - That's another option. I've thought about proposing a wood system on steel framing. I'd have PT nailer plates on top of the beam and decking on top of that - or really I could just use steel for the main frames and hang wood joists off of the PT nailers. My concern is the drainage issue. As of now we have to collect and treat all runoff on the the site. But maybe we can get a pass since it would be a small area on the site.

 

[PaulG93]

I'm not sure how to get diaphragm loads into my lateral frames unless I provide some kind of connection between the slab and the beams. The deck is sacrifial so I don't feel like I can count on the deck attachment for this.

If you don't want to consider the deck attachment, have you thought of plan bracing?

We work in industrial/manufacturing facilities where 90% of the time we use grating (definitely not a diaphragm) for floors. We use angles or WTs (depending on forces) as plan bracing, typically 3" below T/STL to transfer the forces.

 

[JAE]

Steel beams supporting metal deck (galvanized) with roofing system over that. Then add the insulation and pavers.

Why doesn't that work for you? The metal deck provides some level of diaphragm rigidity. The insulation-paver system has been used extensively for years.



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[T_Bat]

Paul - horizontal bracing could be an option.

JAE - I have no issues with the insulation paver system per se. it's just that the structure (including the deck) will be exposed to waether. I've never used a pedestal system where the deck would be exposed. MY concern is what happens over time as the deck deteriorates? This would be a long term issues but worth considering on the front end.

 

[Mjkkb2]

I understand what you have, but in my experience a slab was always required if whatever is over it is an occupied area (other than just roof). It may be a locality thing but I have never seen a built up floor (pedestals or otherwise) over just bare deck without concrete. Based on other's comments it looks like it's not that uncommon, but a new thing to me here in VA.

Considering you're concerned about deck rusting out and loosing the diaphragm, shouldn't you be more concerned abut not having the concrete for gravity loading if deck is to deteriorate? What is to transfer the gravity load from pedestals to the framing, surely rigid insulation won't cut it....
Maybe a 'belts and suspenders' method is in order like slab on deck with some horiz bracing to substitute for diaphragm if it rusts away....

 

[T_Bat]

Mjkkb - that's exactly my point - once the deck deteriorates there is no gravity system, let alone lateral system... Hence the compsoite slab. Maybe a few studs on the beams in the lateral frame only so there is a load path? Otherzies I guess I could add some horizntal bracing.

 

[JAE]

Galvanize the deck - there's no significant deterioration if you have a water-tight "roofing" over the deck. I've seen deck exposed (below) many times. It isn't an issue unless you are next to a body of saltwater.



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