Structural engineers are most commonly involved in nonbuilding structures, as well as building structures, but they can also be involved in the design of machinery, medical equipment, vehicles or any item where structural integrity affects the item's function or safety. Structural engineers must ensure their designs satisfy given design criteria, predicated on safety (i.e. structures must not collapse without due warning) or serviceability and performance (i.e. building sway must not cause discomfort to the occupants).