Merits of steel frame construction vs. poured concrete
With all of the recent talk about shoddy construction, I wanted to post a question that reflects me total ignorance about new building construction. Observing all of the next construction, especially in Williamsburg, it seems that approximately half of new buildings are steel frame girder construction and the other half poured concrete. Why does a…
With all of the recent talk about shoddy construction, I wanted to post a question that reflects me total ignorance about new building construction. Observing all of the next construction, especially in Williamsburg, it seems that approximately half of new buildings are steel frame girder construction and the other half poured concrete. Why does a developer (or is it the architect?) choose one method over the other? Is one method typically cheaper? Lower quality? Given the propensity for cheap construction nowadays, one would think that which ever method was cheaper will be the clearly preferred one.
cm’s answer is mostly correct (though I would strenuously disagree with his definition of building height as being the overriding factor in decision making) but leaves out some subtleties that are worth mentioning.
Up until about 3 to 4 years ago it was a no-brainer to go with cast-in-place concrete for residential buildings. Other than load-bearing masonry walls and concrete plank floors (a system that has a practical height limitation of an absolute maximum of 8 floors or so and which puts severe limitations in interior layout), a flat plate concrete structure is the only one that keeps the ratio between structural floor-to-floor height and ceiling height to a minimum. Because there are no beams and the ceiling finish can be applied in most areas (typically excepting kitchens, bathrooms and corridors, where ductwork and pipes run) directly to the bottom of the concrete slab, you can get a 9′ 0″ ceiling height out of a 9’8″ structural height. With steel construction each floor has additional structural height by virtue of the presence of beams and the necessity to conceal the metaldeck (upon which the concrete floors are poured) and the steel fireproofing with a hung drywall ceiling. The concealment problem may not be an issue if wide concrete planks are used for the floors but obviating the joints between the planks is pretty damned difficult at anyone who has stared at the ceiling from a cheap motel bed will testify. Thus, with steel construction you’re building an extra 1′ 0″ to 2′ 0″ of every vertical building element than you would be doing with a concrete job: exterior walls, interior partitions, plumbing risers and waste pipes, etc., etc.. Plus the cost of putting in a drywall ceiling is $10/SF versus the $2/SF or so for the direct-on-concrete Kadex finish.
All that logic changed, however, when about 3 or 4 years ago the cost per SF for concrete frames started rising from the $30 – $35 they had been coming in at for years to $40, then $50 and ultimately, today, $75 to $80/SF. Of course this increase is out of all proportion the escalation in the cost of labor and material and can be explained in two ways. One is the increase in residential construction volume during that period and the consequent ability of the limited number of concrete subcontractors to pick and choose jobs and increase profit margins. The other possible explanation is rather less savory and I’d probably end up with concrete boots in the East River if I expounded on it.
So, now it’s suddenly a horse race cost-wise between the two systems. Concrete still has two distinct advantages not directly related to construction cost, however. One is that, because of the lesser structural floor-to-floor height you can fit more floors in tall building that is limited by zoning regulations to a maximum height. If there’s a 300 ft height limitation you can fit in 31 floors with concrete but only 26 or so with steel. The other of these two advantages is that a concrete frame can be started much sooner; you don’t have to deal with the delay in getting a mill order in and with the time it takes for shop drawing preparation and approval and for steel fabrication. As we all know, time is money….and I’ve spent much to much time on this lengthy explanation.
I think the shoddy construction that people refer to does not included this aspect. I say that because people don’t see this part of the project and if it were to fail the result would be the building falling down.
Most of the time when people talk about shoddy construction it is either finishing work such as moldings that don’t line up or walls/floors not being plumb or level, or issues such as thin walls, leaks in plumbing or the roof, etc.
The selection of one framing method or the other is usually decided upon jointly by the developer, architect and construction manager and usually dicteated by the size of the project and market forces.
Poured in place concrete will not be cost effective until you have a minimum number of typical floors over which you can spread the expense of building forms. Usually it’s not even considered for buildings of less than 12 floors or greater after which it becomes more economical than steel framing. In this market however their is a shortage of carpenters experienced in building formwork.
Steel framing for smaller buildings is more common, either with concrete plank or poured concrete and metal deck. Remember however that all that steel framing and decking needs to be fireproofed.