Arch on spreading supports

An arch on spreading supports has an internal state of stress represented by the line of minimum thrust. Cracks due to spreading supports create hinges around which the arch can rotate to compensate for the increase in span. Hinges develop where possible thrust lines, in this case the line of minimum thrust, touch the boundary of the arch geometry.
Use the handle to move the right support. Note that the arch will collapse earlier than geometrically anticipated.