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Bracing Example 2

The empty lot along West Broadway is being prepared for new construction, meanwhile the loft building to the east on Warren Street has been temporarily braced. The loft building most likely has bearing walls on the lot lines with either wood joists or steel beams spanning side to side. The bracing looks good – well-designed and well-built – but there’s an interesting question here. What is the bracing for? I can think of at least three very different reasons.

The first possibility is the entire loft building is unstable and has a tendency to tilt to the west, and this became clear when the adjoining building was demolished. You can see the ghost of the old building as the off-white stucco at the steel bracing. The yellow stucco above was above the roof of the demolished building. (If you look closely, you can make out the ghost of the stair bulkhead on the demolished building.) The first problem with the whole building being unstable in that manner is the question of why it would be. The soil in this area is generally pretty good sand, and the water table is not high. So the kind of soil conditions that cause differential settlement aren’t really present. The second problem is statics: the building is visibly pretty close to vertical if not vertical, which means it’s nowhere near tilted enough to be unstable. The third problem is the old bracing: if the demolished building was holding the entire building in place, it would have to be quite strong and it was clearly not very big east-west and so would likely have tilted along with the loft more than it would have restrained it.

The second possibility is that the demolition has flooded the west side of the foundations of the loft, or something similar, and the bracing was put in place preemptively. The generally good soil (mentioned above) argues against this. But the most convincing argument against this is that people are consistent. An owner and contractor sloppy enough to allow their site to damage their neighbor’s foundation in that manner are not the kind of people I associate with well-made bracing like that.

The last possibility is a bit more subtle. The loft building as a whole may not be moving west, but its west wall might be. Joists and beams were supposed to be tied to the bearing walls but often were not, and any bulge in that wall will have a tendency to grow. And since stability in an unreinforced masonry wall is generally based on the load remaining with the middle third of the wall thickness, an out-of-plane bulge as small as 2 inches (for a 12-inch wall) is enough to threaten stability. We’ve dealt with these situations and you can fasten the wall to the interior floor structure from the outside, with some difficulty.

Finally, Woolworth in the background on the left and 3 WTC in the background on the right.