Incompatibility: Floor Finishes

It’s been a while since I did one of these.*

That beautifully polished marble is the floor finish in the new World Trade Center concourse, including the PATH station, the oculus, and the north, south, and west concourses. In some areas it is cracking very badly, and it is cracking across multiple panels of stone, which means it’s not failing because of a local flaw in the stone.

The stone is laid on top of a concrete slab (the structural floor) which is supported on steel beams. It is not spanning structurally at all, but rather is continuously supported. Poorly placed grout or adhesive between the stone and the concrete could lead to a failure, but again, that type of construction defect wouldn’t explain the concentration of cracks in specific areas covering multiple panels of stone.

The tip-off is the almost straight line of a crack running through three consecutive panels. The panels are independent of one another and cracks are not like rot in wood: they don’t spread by themselves. We’re seeing structural movement of the structure below “telegraphing” through the stone. It’s possible but extremely unlikely that there is an expansion joint in the structural slab at this location; it’s very likely that there’s a floor beam directly below that crack and the slab in the triangular area between that beam and the edge of the stair (the diagonal line at the bottom of the photo) is not as well supported as the slab in more geometrically-ordinary areas. The slab moves a bit under load, the line of the beam moves less than the slab adjacent to it, and voilà! there’s a crack.

Marble is brittle (like all stone) and weak, and needs to be supported with a very small tolerance for movement to not crack under load. The late-twentieth and twenty-first century fashion of using large panels of stone flooring does not work well with structure unless the deflection criteria used in design are far tighter than normal. And, as this example shows, that restriction on movement has to apply to every square inch of the floor structure.

* The most recent of incompatibility of rooftops, veneers, curtain wall decoration, floor loading, and curtain walls and frames was almost a year ago.