Different Rigid Frames May Mean Different Deflections

It is common practice to add interior modular columns to reduce overall frame weight, reduce frame deflection, or for architectural or functional requirements. It is a given fact that rigid frame rafters deflect under any environmental load. The magnitude of deflection is limited by the design code or contractual requirements (e.g., L/180, L/240, or L/360).

Sometimes, the deflection of frame rafters becomes a concern that needs attention. One case is when adjacent rigid frames have modular columns at different offsets from sidewall columns. The maximum rafter deflection location changes with the frame profile.

Differential deflection between frames along purlin runs can significantly affect roof drainage and the final appearance of roof panels. If the deflection between frame rafters is substantial, American Buildings will need to use heavier frame rafters, resulting in additional costs for the customer. In the worst-case scenario, fitting issues may arise during erection, leading to extra time and expenses.

This engineering tip is to give builders and estimators a heads-up. When locating the interior modular columns, be sure to account for the impact of the differential deflection of the rigid frame rafters. As a rule of thumb, differential deflection between adjacent rigid frames should be considered if there is a clear span frame rafter adjacent to a frame with interior modular column, or if the unsupported frame rafter length between two frames with interior modular columns exceeds 150 ft. (Building Floor Layout: Frame 3: C-D verse Frame 4: B-D).