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So, part of my thinking on "how to build a house" includes the use of a structural floor (slab) with a crawlspace framed atop it to provide space for, and access to, service runs. (Air handling equipment may also be located in there, or it can be put in a mechanical closet instead.)

However, I'd much rather the structural floor support the crawlspace framing (instead of having a second set of seats on the load-bearing walls); this way, a failure in the secondary framing won't impact the primary load-bearing paths in the house, and it also allows the finish floor and crawlspace to be omitted intentionally in unfinished spaces using a steel stud cripple wall under the end of the finish floor structure. However, while NFPA 13D 8.6.5 permits this space to be left unsprinklered even if constructed from combustible materials (i.e. wood), the full NFPA 13 standard (see 8.15.1.1 and 8.15.1.2.2) requires such a space to be of noncombustible or limited-combustible construction in order to be exempt from sprinkler protection. Flame-retardant-treated (using a pressure treatment process) wood could be used as per NFPA 13 8.15.1.2.11; however, that doesn't seem to gain me much over simply using cold-formed light-frame steel joists, or some other noncombustible system, instead.

With this in mind, I did a whole bunch of research, and hatched two possible ideas for a floor system, both based on cold-formed steel joist framing (Marino/Ware JoistRite is what I've been looking at, but I'd be open to other systems that provide easy-to-use blocking). However, neither system quite falls within the available prescriptive design guidance for steel joist floors, so I'm not sure if my proposals are something that won't give the (obviously mandatory for final planning) structural engineer nightmares. Or in other words: "are these concepts for a floor system within the realm of what a structural engineer could turn into a finished plan without weeping or gnashing of teeth?" The loads I've been figuring on are a 25psf dead load (worst case, with a tiled floor finish accounting for 16psf of that) and a 65psf live load (50psf by default with 15psf for partition walls as per 2018 IBC 1607.5), by the way.

The first idea: steel frame, cement board deck

The first idea I came up with for this would be to use a steel joist frame, but with structural cement board decking (StructoCrete fiber-reinforced cement board or a cement-bonded particle board) instead of plywood or OSB. This does impose the design constraint of requiring a support every 24" though, so we use 2x8 steel joists on 24" centers, with blocking every 48" in all bays, aligned to create a neat grid of joists and blocking, and a maximum cantilever of 24" at all edges, with the edge track floating (i.e. not fastened to structure). Supporting this is a 48" by 48" grid of Schedule 40 pipe posts, maximum 30" tall, attached to the structural floor and the finish floor structure with pipe flanges and appropriate mechanical fasteners, and set so that the pipes coincide with joist/blocking junctions. Finally, a 3/4" structural cement board subfloor is attached to the top of this, providing further composite action to stiffen the structure. This idea raises the primary question, though, of "is the 'floating' joist between the two supported joists adequately supported by the blocking?" as going to 24" spacing for the support posts would make the crawlspace rather tough to navigate.

The alternative: steel frame, steel deck

As an alternative solution, I decided to research a system using shallow corrugated steel form deck and 2x8 steel joists. This would allow me to use joists on 48" spacing due to the longer spans the deck permits, with the same 48" spacing for the blocking and 48" by 48" post grid, since steel deck is allowed to span longer distances than structural cement board panels are. However, the SDI Floor Deck Design Manual isn't exactly something one can readily get their paws on (it's $85 for a PDF), and the available manufacturer guidance for steel floor deck only seems to cover its use as a form deck for a structural slab or in a composite concrete slab system, not as a stand-alone subfloor or with a non-structural leveling topping. This raises the question of "is it practical to span 48" with steel deck, given the loads involved?"

Also, is there something I'm drastically missing here, with either plan, or some out-of-left-field idea that'd work for this situation?

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  • To me, a “structural floor slab” is designed to “span” a certain distance using rebar. These slabs are usually elevated. A “slab on grade” rests directly on the ground and relies on the soil bearing to support loads from above. A structural slab could cost 5-10 times that of a slab on grade. Why use a structural slab? What are your soil bearing values? – Lee Sam Jun 4 '20 at 5:45
  • Residential fire sprinkler systems are different than commercial fire sprinkler systems and are generally NOT required unless in high density residential areas, large condo complexes, forested areas, zoning requirements, etc. What most people don’t consider is that residents that have sprinklers need additional home owners insurance for “water damage”. Why fire sprinklers and can they be the residential type? – Lee Sam Jun 4 '20 at 5:53
  • Comfort of walking on wood floors vrs. walking on concrete floors is significant. We usually use wood floor systems in custom homes due to the “feel” of wood frame floors. Why concrete on steel floor system? – Lee Sam Jun 4 '20 at 5:59
  • Seldom do we design the perfect house that never needs remodeling. Wood framing is easier to remodel than concrete on steel floor systems. Why not wood floor system? – Lee Sam Jun 4 '20 at 6:03
  • In my area, there are many excellent carpenters to build a wood frame house, but very few workers that could build a concrete and steel frame house. The cost is proportionately higher for these limited type workers, thus the cost of construction is higher and often can’t be started at certain times. Is cost important? – Lee Sam Jun 4 '20 at 6:14
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I think the main reasons for using wood floor framing in lieu of concrete and steel is 1) ease of construction and cost, 2) soil bearing values, 3) fire sprinkler costs vrs. smoke detectors, 4) higher cost of insurance, 5) higher taxes, 6) ease of future remodeling, 7) appearance

1) Traditional wood framing is significantly less cost than elevated concrete and steel floor systems. Not only is the labor more expensive for concrete and steel, but the size of the members to support such a heavy load is bigger (deeper). This depth is transferred to the minimum overall height of the structure requiring more exterior cladding, more stairs (larger area to ascend to a higher level), etc. all which adds to the cost.

2) Code requires a design “Live Load” of 40 lbs. per square foot plus any “Dead Load”. Wood framing will total about 10-15 psf and concrete and steel will total about 80-90 psf. Therefore, the total design load for wood is about 50-55 psf and concrete and steel will be about 120-130 psf.

In addition, concrete cannot be supported by wood, therefore concrete walls, special steel posts, etc. are required.

All this additional weight requires a larger footing...regardless the soil conditions.

3) Fire sprinklers are used to “save property” while smoke detectors are used for “early detection”. Strictly from a safety standpoint, smoke detectors saves lives more than sprinklers.

4) A more expensive building will cost more to insure. Any savings because of non-combustible construction will be eaten up by higher replacement costs, water damage coverage, etc.

5) Buildings with higher values are assessed with higher taxes.

6) Future remodeling is more difficult.

7) Concrete floors cannot be poured without expansion joints. These joints are continuous and located throughout the building. These joints require special treatment so that they do not “Telegraph” through finish flooring that is installed on top of these joints.

Summary:

I see no advantage of using concrete on steel joists over wood framing, unless it’s required for zoning, codes, etc.

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