Q.: Do expansion joints between parking-deck slabs allow for differential vertical slab movement when one slab carries much greater loads--due to more vehicles--than the other slab?

A.: Joint detailing determines whether or not a parking-deck expansion joint deflects. In cast-in-place construction, the joint often occurs over a beam, permitting horizontal but not vertical movement. Differential vertical movement is undesirable for some joint-sealing systems--such as ones with a cover plate--because the deflection would interfere with this kind of joint seal. Also, if the deck will be snowplowed, the plow blade would hit a raised portion of the deck if the other side were allowed to deflect.

In precast/prestressed parking decks built with double tees, there may be a shear key to prevent deflection of one double tee relative to the adjacent one. Or beams may be close enough to the joint to prohibit excessive movement.

Some joint-sealing systems permit both vertical and horizontal movements, and if there is no other reason (such as snowplowing) for not permitting vertical movement, some vertical movement may be permitted.