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Time, Money and Precast

Faster construction with precast concrete equals more sales for national retailers.

By Carrie Johnson, P.E., and Doug Tayrien, AIA, LEED AP

Photos courtesy of SGA Design Group

Precast concrete Walmart Supercenter in Chester, S.D. Main shell building walls are integrally colored precast panels.Why would a national retailer consider using a precast concrete wall panel system over another exterior wall system? From our experience, it boils down to construction schedule and construction cost. Precast wall panels can be a very attractive design option for exterior walls. Yes, the quality and durability of the panels are important to a retailer, but the key deciding factors for using precast are reduced construction time and competitive costs compared with other exterior building wall systems.

Retailer opens for business sooner

Time really is money in retail construction. The faster a retail building is built, the faster a merchant can start selling products. Even a few weeks of construction time savings can mean significant additional sales for a large retailer.

Our clients like to use structural insulated precast concrete panels, in part because of their speed of erection. The precast panels do not need structural steel columns at the perimeter or additional insulation to install, which means more exterior walls can be erected in less time. Construction time using precast structural panels can be months less than with CMUs (Concrete Masonry Units) and can also be significantly less than with site-cast concrete tilt-up walls if the precast panels are ordered early in the design/construction process. The goal is to have the panels installed immediately after the installation of the foundations. Having the building dried in (enclosed and protected from the elements) quickly means you can allow more trades on site installing more materials. This is particularly true in northern climates.

Frigid weather not a problem

Quickly erected precast structural panels are shored and waiting for structural steel for roof.A few years ago, we traveled with a client to look at some of their first precast projects in Minnesota and North Dakota. It was winter, 10 F, snowing, and a north wind was tearing across the northern plains. One of the projects was located just south of the Canadian border in North Dakota.

The project had started in the early fall, and by winter the precast panels were up and the building was dried in. Inside, four space heaters were warming 150,000 sq ft of space. The subcontractors were pouring concrete floor slabs, erecting interior walls, installing mechanical and electrical equipment – all while working without winter jackets. Fantastic! They were months ahead of schedule compared with a comparable CMU project. This example shows why precast in northern climates is a great solution for winter construction.

But does precast make sense for southern climates? Yes, there can still be a time savings using precast versus CMUs and versus site-cast, tilt-up panels. Again, the key is to have panels on site and ready to install when the foundations are complete.

Competitive cost and column elimination

Even in frigid weather, a subcontractor is comfortably working in short sleeves once the precast enclosure and roof are complete.So why isn’t every national retailer using precast? Cost can be a significant factor. Getting your retail store open earlier is desirable, but if the cost of the precast panels is more than CMU or tilt-up wall systems, it can eat into sales profits, and thus the benefits of a shortened construction schedule could be reduced or eliminated altogether.

While the initial cost of precast panels may be higher than tilt-up or CMU walls, a detailed cost comparison may reveal that precast construction compares favorably with the other methods.

Precast walls are most competitive on large one-story buildings with roof framing heights of 20 ft or more. The large footprints make precast panels more competitive due to the increased number of repetitive panel sizes. The tall spaces are ideal for precast walls due to the elimination of perimeter columns and girders required with other wall types. This also provides a second advantage in that it allows for a larger uninterrupted space within which to merchandise.

Precast walls are also more competitive on buildings with large cantilevered parapets (distance from the top of the roof framing to the top of the wall). If the cantilever is more than 6 ft, wall systems like CMUs require an extensive amount of additional grout and reinforcing steel, while light-gauge systems require extensive back-up structural steel. Precast panels can be used for these spans with little or no additional reinforcing. More and more jurisdictions are requiring taller parapets to screen rooftop equipment. At the same time, recent codes have more than doubled the required design wind load on parapets. Structural precast panels are better able to handle these cantilevered loads.

The time savings can also be considered a cost benefit. A shortened construction schedule means a reduction in job-site overhead (salaries, trailer and utilities) that results in significant savings.

Retail representative, structural engineer and architect are inspecting precast in 10 F weather. Precast wall systems mean early completion of building enclosure.A cost comparison should include both short-term and long-term cost savings. By using integrally colored concrete panels in lieu of painted CMUs, both the costs for initially painting and then repainting every few years will be reduced. The increased thermal mass of precast walls also reduces temperature swings, which can reduce heating and cooling costs. These maintenance cost reductions may make the life-cycle comparison of using precast walls competitive even if the initial costs are higher.

Conclusion

There are a number of advantages that make precast walls an attractive alternative for retailers. In order for precast to be selected, it really needs to meet two needs: reduction in construction time so that stores can open more quickly, and competitive pricing compared with other exterior wall systems. To get a retailer’s attention, you need to be able to demonstrate satisfaction of both of these criteria.

Carrie Johnson, Master of Architectural Engineering, P.E., is a principal of Wallace Engineering Structural Consultants Inc., Tulsa, Okla. She has 26 years of experience in structural engineering. Carrie has been on the board of directors for the National Council of Structural Engineers Association (NCSEA) for the past five years and is currently the vice president of the organization.

Douglas Tayrien, AIA, LEED AP, is a Principal with SGA Design Group. He has 29 years of experience in architecture and specializes in retail prototyping and design. Tayrien is a licensed architect in multiple states and a member of the American Institute of Architects. He is certified with the National Council of Architectural Registration Boards (NCARB) and is a LEED Accredited Professional. 


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