How to account for joists in stiffness analysis in STAAD.Pro ?

The joists modeled in STAAD.Pro can be used for transferring gravity loads to the supporting members only and these are not accounted for in the stiffness analysis. If you would like to consider these members in the stiffness analysis, the only way would be to come up with the analysis properties. This is not possible without knowing the actual configuration of the truss used by the manufacturer.

So here are some options. You can refer to the deck manufacturer’s catalog and find out moment of inertia. You would also need to come up with an equivalent cross sectional area based on the weight per unit length and the material density. You can then define these as a general section with cross sectional area and moment of inertia as inputs. Alternately after figuring out the moment of inertia and cross sectional area, you can use the Section Wizard program which has an equivalent section calculator that can come up with approximate section dimensions based on properties ( like say A , I ) and cross section type. See picture below