An AS/R system is to be designed to store approximately 12,000 loads. Due to space limitations in the existing facility, it is known that the system will have eight tiers of storage. The choice has been reduced to having either

(a) 10 aisles, 75 openings long,

(b) 9 aisles, 84 openings long, or

(c) 8 aisles, 94 openings long. The S/R will travel horizontally at an average speed of 450 fpm; simultaneously, it will travel vertically at an average speed of 80 fpm. 48  40 unit loads, 44 high and weighing 2,500 lb, are to be stored. The I/O point is located at the end of the aisle, at floor level. The time required to pick up (P) or to deposit (D) a load is 0.30 minute. The throughput requirement on the system is to perform a total of 180 storages per hour and 180 retrievals per hour.

Randomized storage is used. Forty percent single-command operations are anticipated.

A maximum utilization of 95% can be planned for an individual S/R. A central computer console is to be used to control the S/Rs. Determine the least-cost alternative that satisfies the throughput constraint. Consider only rack cost and S/R machine cost; do not consider building cost. (Assume sprinklers and no transfer cars are to be used.)