Perka: Fixed-Base vs Pinned-Based Columns

Return to Perka Library Fixed-Base vs Pinned-Based Columns Most experienced structural engineers will agree that it is rather difficult to achieve a full column-base fixity in metal building systems. Outside designers routinely assume the column bases to be pinned, that is, not transmitting any bending moments to the foundations. Unfortunately, not everybody remembers to put this assumption in writing with the contract documents. Without being “nailed down”, the pin-base assumption exists only in the minds of the specifying engineers. Manufacturers may feel free to propose a flagpole-type system with fixed-base design, which is a much cheaper solution for them. Since manufacturers do not see the added foundation costs, they may sincerely believe that the fixed-base solution is truly economical. By this time, however, the foundations may have already been designed with the pin-base assumption in mind and could be partially constructed by, or at least awarded to, a concrete contractor. Since the fixed-base column design results in an unanticipated bending moment applied to the foundations, a redesign is almost always required. At this point, the owner’s choice is between accepting a large change order from the foundation contractor or a protracted battle with the metal building manufacturer. Both could have been easily avoided with one sentence inserted in the contract drawings and specifications: “The pre-engineered building columns shall have pinned bases and transfer no moments to the foundations.” (Source: Metal Building Systems by Alexander Newman)