Differential Deflections and Intermediate Diaphragms
I have a question about designing intermediate diaphragms when there are differential deflections of adjacent steel girders. Typically, intermedate diaphragms are designed to transfer wind loads and to meet all applicable slenderness and minimum material thickness requirements. This is appropriate for tangent bridges with small skew angles. However, as the skew angle increases, the differential deflections between girders increase. What procedure is generally used to determine the force effects from differential deflection? A computer model (STAAD, GTSTRUDL, etc.) could be used, but this is time consuming to generate and check. How large does the differential deflection between adjacent girders have to be before this type of analysis is required? NCDOT requires a more refined method of computing deflections due to the weight of the slab when the deflections between adjacent girders differ by more than 1 inch. The analysis method chosen must take into account the effect of the diaphragms on the stiffness and relative deflection of the girders. Other states require oversize or slotted holes for the diaphragm connections when the differential girder deflections are greater than 1/2 inch. Is this a common solution? Large differential deflections can generate large forces in the diaphragms.
How small does the differential deflection have to be to ignore the force effects on the diaphragms? Do these forces cause problems if the diaphragms are designed for wind forces only? What methods are typically used to calculate these force effects? Any suggestions would be greatly appreciated!