Theres a new residential housing complex under construction near me; a series of buildings containing about 250 units. They are five storeys tall, and they’re framed with freaking 2 x 4s! I can’t get close enough to check with a tape measure, but they appear to be on 12" centers, with prefab trusses for joists, and trusses in place of traditional cripple studs over the larger window openings on the exterior walls. The stairwells and elevator housings are built with concrete and CMUs, but everything else is wood.
Is five storeys a realistic limit, or can buildings designed to carry these types of loads be built even taller using only 2 x 4s? What would be needed to make a wood sky scraper?
Are you sure they’re 2X4’s? Most jurisdictions have modified their code to require 2X6’s, for increased insulation and strength. I doubt they would use 12" centers throughout construction, it’s impractical for several reasons. As to a 5 story wood framed structure I’m not an expert, but it seems a bit much if all the elevations are independent, as you say.
I must say that I’m astonished that they would build a five-story building using wood frame construction, especially out of 2x4s. Also, putting the 2x4s at 12-inch O.C. seems pretty impractical. There would be hardly any room for utilities in the wall cavities.
Building codes for residential construction (up to three stories, IIRC) usually require 2x4s at 16-inch O.C., or 2x6s at 24-inch O.C. (I built my two-story house using 2x6s at 16-inch O.C. )
I’m particularly surprised because any multi-story building built to commercial standards (which generally includes apartment complexes) is usually built using steel columns (i.e. I-beams) along with non-load bearing light steel framing. The exterior sheathing is usually Dens-Glas instead of plywood, and the floors are usually concrete. Often you see concrete masonry units (CMUs) used for walls as well, especially on the first floor.
The general guideline that I’ve heard for wood-frame construction is a maximum of three stories, and again, this is generally only used for single-family houses and townhouses.
That being said, I don’t know what the building code requirements are in your area. Presumably the builder received a building permit, and the local building inspector is conducting inspections?
P.S. While I am an engineer, I’m not a structural engineer, so take this all with a grain of salt.
Is it possible the concrete walls are being prefabbed off site and the 2x4’s are for aligning the walls when thay are later dropped into place? The 2x4s could also be part of the frame for molds used to pour concrete onsite.
Its about 5 miles from there. This is being built on land surrounding the PG Plaza Metro station.
Hyattsville is known for occasional microbursts though. During a storm a year or two back, the roughly one square mile around my neighborhood accounted for almost 1/2 of all of the power outages due to falling tree limbs and the like that PEPCO reported for the whole county.
From a purely load standpoint, I am not surprised; a stick-built structure is very light. The vertical stress on 2x6 framing for five storys doesn’t seem high, assuming appropriate interior load-bearing walls and columns (even if the columns are made by built-up 2x6s).
There is some reason to be intuitively anxious, though: Stick-built framing is extremely dependent on the quality of all of the connections between the individual framing pieces. The whole thing at the end needs to end up as a single integral unit to be maximally strong. Every nailed joint counts; what is glued; how carefully the siding is applied; how accurately every piece is sawn so that all the load is not on one member; use of hurricane ties; and so on. In commercial construction where guys are using power nailers in cold weather, no inspection can really substitute for workmanship, and workmanship can be hard to come by when you are getting paid by the job.
It won’t fall down but it wouldn’t be my first choice in a stiff breeze. Specially a tall skinny sucker like the one in the picture.