A block of Vermont marble broke off from the facade of the Supreme Court Building on Monday. The chunk of marble came from the molding above and to the right of the allegorical statue of "Liberty," who carries the scales of justice. What could make part of a building fall off?
Water, in most cases. Moisture can seep into cracks or joints in the building and then freeze. Since water expands as it turns to ice, this process can crack hard materials from the inside and—given enough cycles of freezing and thawing—loosen up chunks of concrete, stone, or marble. A weakened chunk might drop off a building spontaneously or in bad weather. Water can also cause problems if it reaches the metal attachments that connect decorative facades to the frame of a building. Metal expands as it rusts, which adds another source of internal pressure to the material; it can also weaken so much that it snaps in two.
The watertight seals over structural joints tend to deteriorate over time as the caulking becomes less sticky and dislodges. The first cracks in a building's exterior can form as it undergoes slight movements associated with thermal expansion and contraction. Rain poses greater risks if it's accompanied by high winds that drive it deeper into the cracks and joints. Acid rain and rain that mixes with corrosive bird droppings also tend to do more damage.
Earthquakes can also break off the heaviest, most brittle parts of a structure. (Chimneys are particularly vulnerable.) Termites or mold can weaken a wooden frame, which might serve as the attachment point for a veneer of brick or masonry.
Structural engineers often identify weakened parts of a building before they crumble. (The pediment of the Supreme Court building was found to be in good condition when it was checked two years ago.) Any cracks they find can be sealed or covered over with cement paste, or the entire area can be removed and replaced. The rustproof metal attachments in modern buildings are safer than some of the materials used in the old days, and structural components can be coated with epoxy to keep out moisture.
Bonus Explainer: What caused the marble to fall from the Supreme Court building? We don't yet know, but from the looks of it water might not have been the culprit. The molding that failed seems well protected from rain by an overhang. The surface of the break suggests that it may have resulted from a flaw in the marble itself rather than from a broken attachment to the building.
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Explainer thanks Robert Frosch of Purdue University, Sidney Guralnick of the Illinois Institute of Technology, and Josh Kardon of Joshua B. Kardon + Company.