This specification covers metal ceiling suspension systems used primarily to support acoustical tile or acoustical lay-in panels. Some suspension systems incorporate locking assembly details that enhance performance by providing some continuity or load transfer capability between adjacent sections of the ceiling grid. The structural classification or grade of ceiling suspension systems shall be determined by the capability of main runners or nailing bars to support a uniformly distributed load. These classifications shall be: light-duty systems; intermediate-duty systems; and heavy-duty systems. The structural classification of ceiling suspension systems shall be based on the load-carrying capacity of the main runners of the structural network. Suspension system structural members shall conform to the following tolerance requirements: metal thickness; straightness; length; overall cross-section dimensions; and section squareness.