Two Dimensional Strip Packing with Unloading Constraints

In this paper we present approximation algorithms for the two dimensional strip packing problem with unloading constraints. In this problem, we are given a strip S of width 1 and unbounded height, and n items of C different classes, each item ai of height h(ai), width w(ai) and class c(ai). As in the strip packing problem with rotations, we have to pack all items minimizing the used height, but now we have the additional constraint that items of higher classes cannot block the way out of lower classes items. We design a bin packing based algorithm with asymptotic approximation ratio of 5.745. These problems have practical applications on routing problems with loading/unloading constraints.