Designing manufacturing facility layouts to mitigate congestion

When workflow congestion is prevalent, minimizing total expected material handling time is more appropriate than minimizing a distance-based objective in manufacturing facility layout design. This article presents a model labeled the Full Assignment Problem with Congestion (FAPC), which simultaneously optimizes the layout and flow routing. FAPC is a generalization of the Quadratic Assignment Problem (QAP), a classic problem for the location of a set of indivisible economic activities. A branch-and-price algorithm is proposed and a computational study is performed to verify its effectiveness as a solution methodology for the FAPC. A numerical study confirms the benefits of simultaneous consideration of layout and routing when confronted with workflow congestion. A detailed simulation for a case problem is presented to verify the overall benefits of incorporating congestion in layout/routing. A critique of FAPC with two alternative models is also provided. Three conclusions are offered from this work. First, a combination of re-layout and re-routing is a more powerful way to mitigate the impact of workflow congestion rather than using just the re-layout or just the re-routing options. Second, it is important to model workflow congestion in a manufacturing facility-that is, ignoring it can result in a significantly poor design. Third, the QAP layout is dominated by the FAPC layout for situations of medium workflow intensity.