Santiago-Montaño, Stephanie

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    Shared material handling strategies to minimize unloaded travel in LTL cross-docks
    (2019-05-15) Santiago-Montaño, Stephanie; Carlo-Colón, Héctor J.; College of Engineering; Bartolomei Suárez, Sonia M.; Torres García, Wandaliz; Department of Industrial Engineering; Vásquez Urbano, Pedro
    The current operational paradigm in less-than-truckload (LTL) cross-docks that use non-automated material handling systems to unload trailers is that each material handler (MH) is responsible for unloading a different inbound trailer (IT) (i.e., Dedicated Strategy). This study challenges this operational paradigm by proposing a Shared Strategy, where material handlers are allowed to collaborate in unloading all inbound trailers. The main objective of this study is to propose and evaluate collaborative material handling strategies and quantify the benefits of implementing them in terms of total distance traveled and makespan. This study addresses the internal cross-dock workforce scheduling problem, concerned with determining the routing for MHs to unload ITs under a Shared Strategy. A mathematical formulation for the problem is presented assuming the dock door assignment is given and that loads may be retrieved from the trailer in any sequence. It is found that the Optimal Shared Strategy outperforms on average the Dedicated Strategy by 24.12% and 48.49% in terms of total distance traveled and makespan, respectively. A second formulation presented studies the best inbound door assignment (IDA) under this strategy. It is concluded that the optimal IDA for the Dedicated Strategy is also good for the Shared Strategy. A third formulation assumes that the retrieval sequence follows a last-in-first-out (LIFO) policy within trailers. It is found that for the total distance traveled, this model yields the same result as the first one that allows any retrieval sequence. This suggests that for total distance traveled the load sequence in the trailer may be disregarded when evaluating the problem. Although the Optimal Shared Strategy yields very impressive results, its implementation is deemed impractical given the dynamic and low-tech environment of cross-docks. In order to develop an alternative shared strategy that does not require additional technology, a practical rule-based material handling collaborative strategy, termed the Monomaniacal Strategy, is proposed. Under this strategy, after delivering a load to an outbound trailer (OT), material handlers travel counter-clockwise around the perimeter of the cross-dock to find the next available IT to serve. The mathematical formulations for the Optimal Shared Strategy are modified to obtain the optimal routing of material handlers under the Monomaniacal Strategy. It is found that the Monomaniacal Strategy outperforms the Dedicated Strategy by 7.48% in terms of total distance traveled in small sized cross-docks. Given the long runtimes required to solve the formulations, a polynomial-time algorithm is developed to find the total distance traveled under the Monomaniacal Strategy. It was empirically observed that the Monomaniacal Strategy outperformed the Dedicated Strategy in terms of total distance traveled by on average 14.70%, 37.99%, and 43.02% for pure, mixed, and highly-mixed flow compositions, respectively, in medium sized cross-docks. Lastly, to gain a deeper understanding of the performance of the Monomaniacal Strategy, a discrete-event simulation model was developed in SIMIO. This model was used to compare the Dedicated Strategy with six (6) versions of the Monomaniacal Strategy under different experiment conditions and to fine-tune the latter. Experimental results suggest that the Monomaniacal Strategy with up to one queue position at ITs has a better overall performance in comparison with the other versions. The main managerial insight from the simulation study is that the Dedicated Strategy outperforms the Monomaniacal Strategy for pure flows, but as ITs are required to visit more OTs the Monomaniacal Strategy outperforms the Dedicated Strategy. For pure flows, the Dedicated Strategy outperformed on average the Monomaniacal Strategy by 13.13% and 19.41% in terms of makespan time and makespan distance, respectively. Nevertheless, the Monomaniacal Strategy outperformed the Dedicated Strategy by an average of 12.48% in terms of total distance traveled. For mixed flows, the Monomaniacal Strategy outperformed the Dedicated Strategy by 2.34%, 0.62%, and 16.85% in terms of makespan time, makespan distance, and total distance traveled, respectively. For highly-mixed flows the Monomaniacal Strategy outperforms the Dedicated Strategy by 14.38%, 20.26%, and 14.56% on average in terms of makespan time, total distance traveled, and makespan distance, respectively. Therefore, LTL cross-docks with mixed and highly-mixed flows can dramatically increase their performance in terms of makespan time, total distance traveled, and makespan distance by simply adopting the Monomaniacal Strategy, without requiring any investments.