Precast Prestressed Adjacent Slab Beams - Charter School Entrance Bridge, Naples, FL
If cost is the primary concern, the best approach is to give the contractor the option to use precast or cast-in-place concrete. Some contractors prefer precast and some prefer cast-in-place depending on their experience, equipment and personnel. The market will always provide the true lowest cost. When we follow this project approach, we provide drawings for the cast-in-place structure in the contract plans and provide design criteria to change the design to precast. During the bidding phase the precaster’s specialty engineer performs a conceptual design for the precast elements which allows the manufacturer to quote the cost of the precast to the bidders.
If time is the primary concern, precast is usually the way to go. There is typically a premium to pay for the precast structure. However, the time gained by having the structural components fabricated off site and out of the critical path could pay high dividends that would lead to a lower overall project cost. In addition, if dewatering the excavation is difficult and therefore costly, precast reduces the time required for dewatering which could make precast more cost effective. Cold weather construction also gives precast a cost advantage (not an issue in Naples).
We are aware that some civil engineers or their clients, get their advice on structural engineering from a precast product supplier’s sales person (we assume) because the service is free. As Kurt Vonnegut stated, “In this world, you get what you pay for.” While these precast sales persons are very knowledgeable about the products they sell, it is highly unlikely that they will suggest considering a cast-in-place structure option or a competitor’s product. Only a qualified independent structural engineer will provide a thorough evaluation of multiple precast and cast-in-place alternatives that are in the best interests of the owner. Also be aware, if a civil engineer reviews and stamps shop drawings and calculations prepared by a vendor’s specialty structural engineer, that civil engineer is accepting the responsibility as the structural engineer of record. Most civil engineer’s don’t have professional liability coverage for structural engineering, so if something goes sideways with the structure, they may be on the hook for the cost of claims out of pocket.
