Historic Structural Detail: Not Very New York-y

by Don Friedman on January 20, 2016

American construction in the last third of the nineteenth century, particularly after the Chicago and Boston fires, was a search for better passive fire protection. One method that was developed and is effective, but is rarely seen in NYC, is heavy-timber construction. This is not simply the use of large-dimension timbers in a building’s framing, but rather the elimination of any small-dimension exposed wood.

IMG_0626

Fireproofing is all about insulating load-bearing structure from temperature increases large enough o cause material degradation. As it happens, wood char is a pretty good insulator. So if we allow timber framing to burn, it will self-insulate by developing char that will then slow down further burning. (An alternate name for heavy-timber construction is “slow burning construction.”) For this system to work, all of the exposed timbers have to be big enough so that the loss of surface material to char does;t represent a significant percentage loss in strength. So 2xs and 3xs are excluded, because they would lose most of their thickness to surface char. The “joists” are usually 8x8s or bigger and the deck spanning between them is not just 3 or 4 inches plank, but is multiple layers or tongue-and-groove to close off any gaps where hot air could penetrate and facilitate fire spread.

It’s an effective technique and can achieve quite good fire ratings, but the trend of development was towards non-flammable construction. Yet another name for this system is “mill construction” because it was most commonly used in factory construction, particularly in New England. The picture above is an old industrial building, now residential, in lower Manhattan.

Previous post:

Next post: