Due to the existing problems in cast iron manholes including stealing potential, heavy weight, high noise, and high rate of decay, it is suggested to redesign and manufacture manholes using composite materials. Due to high production repeatability of composite sheet molded compound (SMC), this paper focuses on designing composite manholes made of this type of materials. Despite the various advantages of SMC, some significant weaknesses in the production of these parts can be seen. For example, in the production of two types of manhole with similar diameter but different classes, two different metal molds are needed to provide thicker manholes for the one with higher grade. In this paper, to optimize manholes for higher load bearing capacity without changing the mold, a combination of woven fiber sheets and SMCs is studied. To reach the desired combination, a full factorial test of 486 manholes is needed. The well stablished Taguchi method is considered to reduce the number of tests from 486 to 18. It is proved that the optimum sample suggested by Taguchi method increases the load bearing capacity up to 50% compared to pure SMC sample. Finally, a finite element model for simulation purposes is provided and good agreement with experimental result is achieved.

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