Tacked to his bulletin board was a postcard with pictures of stone bridges in Scotland. Boothby returned with a cup of coffee and apologized for the clutter. He stirred his coffee, and began talking about masonry arch bridges. These bridges typically fail in two ways, by hinging and by sliding. A wedge-shaped gap between two blocks he called hinging.
Problems happen when a heavy load hits in one place.
To demonstrate how sliding occurs in masonry arch bridges, he pulled out a little arch made of wooden blocks. Boothby talked as he pushed, not paying attention to the block. The arch fell. Wooden blocks clattered across the desk. The walls between the arch and the sides of a bridge, called spandrel walls, might also fail, tumbling outward and allowing the arch to collapse.