Thursday, April 25, 2013

Stabilizing the Unstable Stable

11th Street Entrance to the hotel

On the National Register of Historic Places since 1990

Yesterday Forrester Construction Company took down the wall that was lurching into the alley behind the Morrison Clark Hotel, thereby immediately stabilizing the stable.

One can easily appreciate the opening for the original ridge beam just below the apex of the roof line in this photograph of the East wall. 
Analysis of structural building failures like this is a little like an architectural autopsy. One takes many small clues and ultimately puts all the pieces together and reflects until it makes sense. An experienced and highly placed authority on historic preservation and Washington architecture shared the following fascinating and elegant insights into why this stable roof and wall collapsed.
“There was a ridge beam. The original brick west wall had been removed by the 1980s addition. Then the only thing holding up the ridge beam was a furred wall of nothing more than drywall on metal studs. There were no ceiling joists or collar beams tying the roof rafters together like a letter “A” so the only thing keeping the bottomless triangle of roof rafters together was friction with the 1980s addition and the nails at the rafter/ridge beam connections."
 (photo courtesy of Kathy McEnany)

 (photo courtesy of Kathy McEnany)
"When the builders began to remove the drywall looking for the beam’s support (which they presumed had to be somewhere under the drywall) they realized there was no support and the beam fell down. The beam fell down, but not straight because it was still nailed to all the rafters. The rafters being rigid, they don’t bend or break. If the north and south walls had been equal, the collapse would have been symmetrical and both walls would have pushed out as the diagonal rafters became horizontal (horizontal being laterally longer than diagonal).
But the South wall was buttressed by the modern exterior brick stair well. That acted like a buttress laterally supporting the south wall. That was an anchor point that didn’t move. So all the lateral movement was concentrated on the North wall pushing it way out.
The collapse was a little bit like a rubber band too, meaning that the alley wall went out then back in a little. Midway through the rafters’ fall, they reached a point of being perfectly horizontal. At that point they reached their maximum lateral dimension. But as the beam continued to fall, the rafters became diagonal again (just inverted), reducing their lateral dimension and as much as the wall/rafter connections survived the shock, cinched the North wall back in a bit. That kind of explains why the gutter on the South wall is pulled up and in too.”
(photo courtesy of Kathy McEnany)

In the author's opinion, Forrester, DCRA and HPO deserve commendation for reacting so quickly and in professional synergy to ameliorate a potentially serious problem. Once rebuilt to the original configuration, this stable will regain its integrity and smile with its new facelift. 
These small alley buildings may not seem like very much today to most people, but on reflection 20 years from now, they will be recognized as the crucial little ingredients that make the difference between whether an alley is interesting or not. They speak of human scale and activity - something that is hard to capture in the massive block structures that surround and dwarf them.