How can the capacity of an existing joist be determined?
The first information to look for is the manufacturer’s name, job number, and mark number from either the metal joist tag found on the joist or the erection placement drawings for the building. The manufacturer can usually determine the loading the joist was designed for with this information. If this information cannot be found, field measurements must be made. The top and bottom chord angle sizes must be measured along with the joist depth. An approximate capacity of the joist can be determined by calculating a section modulus with this information and using a reasonable value for allowable compression.
