SAN FRANCISCO—With the potential for more people driving into cities and workplaces rather than taking public transit once staggered workforces get the green light, parking structures will be more in demand. When considering parking structure construction, how can owners determine which type is best for any given project?
Rodney Riddle, vice president of parking structures with McCarthy Building Companies Inc., recently shared insights into the benefits of using precast versus cast-in-place deliveries.
GlobeSt.com: What is the difference between precast and cast-in-place concrete parking structures?
Riddle: When it comes to parking structures, there are three types to be considered.
Cast-in-place concrete: These structures are constructed by using ready-mix concrete poured into removable forms on-site. High-strength tendons in the concrete are anchored at the outer edges and tensioned after the concrete has gained sufficient strength (48 to 72 hours). Post-tensioned concrete is a very common type of concrete structure for parking garages today, especially for those above ground.
Double-tee precast: As the name implies, a precast, pre-stressed double tee resembles two side-by-side capital letter Ts. The panels, ledger beams, columns and double-tee members are fabricated off-site and assembled onsite. High-strength tendons in the concrete are anchored at the outer edges of the concrete forms and pre-tensioned before the concrete is placed and then released after the concrete has gained sufficient strength. In California, this method requires a cast-in-place topping slab be placed over the double-tee members in order to create a structural diaphragm and meet seismic requirements.
Concrete/precast hybrid: This is a composite structural system that uses the precast process for columns and beams and formed, cast-in-place decks. Precast beams are tied into the cast-in-place decks by lacing deck reinforcing through exposed reinforcing cast in the precast beams.
GlobeSt.com: What are the advantages/disadvantages of each delivery?
Riddle: Cast-in-place concrete delivery offers the best seismic performance and the lowest cost solution. Its lighter structure equates to less footings and fewer joints, which minimizes the amount of ongoing maintenance for items such as sealant replacements, and reduces overall lifecycle costs of the project. This option also allows for greater flexibility for mechanical, electrical and plumbing/MEP design. On the other end, cast-in-place concrete involves higher field labor risk and lengthier construction time with more concrete pour days involved.
Double-tee precast structures require a significant investment up front for engineering and prefabrication, but the time of construction can be faster than other structure types. These structures require less onsite construction time by utilizing the bulk of the manpower in a controlled, prefabrication plant. Structurally, these buildings involve more long-term maintenance and appear darker from a lighting standpoint after completion.
The hybrid option merges the pros and cons of cast-in-place and precast deliveries. These structures also require a significant investment upfront for engineering and prefabrication, but the time of construction can be slower than the other structure types. For contractors that do not have available manpower, these structure types reduce the amount of manpower required on site and instead utilize the manpower in the prefabrication plant.
GlobeSt.com: How can owners determine which method is best for any given project?
Riddle: It really comes down to evaluating what is deemed to be the best value to the owner based on having a complete understanding of the advantages and disadvantages of each structure type. If lower costs and long-term durability are important, then cast-in-place should be considered higher value. If speed of construction is more important then, a double-tee precast option should be considered higher value.
Regardless of what structure type is utilized, selecting a contractor who performs the structural work with their own forces can provide advantages to owners as far as cost, schedule and control of the quality on the project.