An engineer and the owner of Monterrey, Mexico-based OptiCretos, serving also the southwestern U.S., Jesus Eduardo Gonzalez is a SlenderWall licensee
An engineer and the owner of Monterrey, Mexico-based OptiCretos, serving also the southwestern U.S., Jesus Eduardo Gonzalez is a SlenderWall licensee who has observed growing recognition and adoption of studcast architectural precast concrete cladding. We’ve been involved with a number of projects that were changed to [Easi-Set’s] SlenderWall after originally being specified for conventional precast. One advantage the system offers is the ability to position panels with a tower crane, because of their light weight. Another is the fact that SlenderWall can provide better thermal insulation than conventional precast.
A HYBRID MODEL
The past two decades have seen development of a number of proprietary systems that maintain conventional precast’s widely recognized benefits Û durability, affordability, rapid erection, consistent quality due to plant fabrication, plus nearly unlimited aesthetic options in form, color, and texture Û while offering even more advantages: lighter weight, streamlined construction, and reduced environmental impact. Such hybrid systems of studcast architectural precast cladding comprise a thin facing of concrete integrally cast with panelized steel-stud frames. The light-gauge studs serve as the bones of the panel, resisting horizontal loads, while the relatively thin precast surfaces provide a weather-resistant skin. A modest amount of welded-wire reinforcement adds strength to precast spanning the steel studs. Moreover, since the panelized framing features headed-stud anchors integrally cast in the concrete, composite structural action of the two materials is effected.
Studcast panels typically incorporate a minimum 2-in.-thick concrete component, possibly full-density with compressive strength in the 5,000-psi range. Most aesthetic options, e.g., color and exposed aggregate, offered by conventional precast panels are available also with the hybrid system. Form liners can be used for cast-in textures, such as brick or stone; and, reveals, projections, and returns are achieved by applying materials to the form’s inside surface.
The panels generally are produced by licensed precasters, operating with the technical support of licensors for design and fabrication. Easy positioning of the frame further facilitates casting.
Despite obvious advantages of studcast cladding systems, a barrier to their more widespread market acceptance has been perceived cost premiums. Studcast panel costs per square foot of exterior wall may range higher than those for conventional precast, because the reduced material cost of thinner concrete is more than offset by inclusion of steel-stud framing. Accordingly, precasters who fabricate studcast panels gain the steel-stud share of a project’s overall material sales.
Yet, closer examination reveals that studcast panels often reduce overall construction outlay. Due to thinner concrete, the panels weigh less per square foot than conventional precast, enabling fabrication, transport, and installation of larger panels for more rapid building enclosure. And, depending on distance to the job site, additional panel cost may be offset by freight savings. Lighter-weight cladding also reduces the loads borne by a building’s structural frame, which allows a reduction in the size and cost of structural members. Costs associated with excavation and foundations similarly can be minimized.
Midland, Va.-based Easi-Set Industries Û developer of leading studcast system SlenderWall Û set out last fall to determine how such differences impact a project’s overall construction expense. To that end, the company engaged BKI Consulting principal William Kneeland, a professional engineer and construction-cost estimator, to develop a detailed cost comparison. The study was based on schematic plans for a hypothetical, five-story commercial building in Dallas. Sufficient design elaboration allowed comparison of different cladding methods, as plan specifications included building length, width, and height, floor plate dimensions, and exterior treatments, i.e., color, ornamentation, and fenestration.
To ensure the accuracy of estimates for a 113,375-sq.-ft. building Û 175 feet long and 125 feet wide, featuring 38,124 sq. ft. in exterior wall-panel surface area Û Kneeland contacted precasters, crane operators, and vendors for pricing, rather than relying on standard construction cost databases. Covered in the estimate are costs of excavation, structural foundation, perimeter structural steel, exterior metal-stud framing and batt insulation, plus exterior brick-patterned precast cladding.
The cost study was detailed in a 10-page Excel spreadsheet, available by request from Easi-Set Industries. Key findings can be noted among multiple project parameters.
Panel Costs Û While prices upon delivery of conventional precast and SlenderWall units are closely aligned in many markets, the estimated cost for the prototype project of studcast panels per se was 17.9 percent higher than that of 5-in. precast members, i.e., $1,000,760 [$26.25/sq. ft.] versus $848,263 [$22.25/sq. ft.]. Partially offsetting that premium was an estimated $4,100 savings in shipping costs: the lighter SlenderWall panels often are fabricated in larger sizes than corresponding conventional precast units and, thus, require fewer truckloads and round trips for transport to the building site.
Crane Use Û The prototype project required 178 conventional precast panels, weighing up to 13 tons, and deployment of a 100-ton crane. SlenderWall estimates indicated 120 panels Û none exceeding about six tons in weight, despite larger dimensions Û so, a 60-ton crane was determined sufficient for installation. By minimizing crane lifts and time-consuming connections, furthermore, SlenderWall cladding offered additional savings for an estimated erection cost about $51,000 less than that of conventional precast panels.
Dead Load Û SlenderWall fabricated with the same concrete mix used for a conventional precast panel can cut cladding weight by approximately 60 percent. That decrease has a small, but significant effect on gravity loads the building’s structural frame and foundation must support Û an especially important consideration for seismic zone structures. In the study project, increased structural costs associated with the heavier precast cladding totalled about $197,000, i.e., $4,600 for foundation excavation, $91,000 for foundation concrete work, and $102,000 for structural steel.
Contractor Overhead Û Estimator Kneeland found that the choice of cladding has a notable impact on the construction timetable. Since SlenderWall erection would take four fewer days than conventional precast panel placement, and eliminating construction of a separate stud wall should save 31 working days, studcast cladding could cut the project’s anticipated 12-month construction schedule by at least a month. Erection of a separate stud wall, moreover, delays installation of insulation, interior finishes, electrical runs, and glazing. Thus, cutting the contractor’s overhead by a month via SlenderWall installation promised savings of approximately $100,000.
Floor Space Û Except in the unlikely case that a site has zero lot lines, thinner panels add free usable floor space. Because steel studs are part of SlenderWall cladding, not resting on floor slab edges, four inches are gained around the building perimeter. Consequently, studcast cladding for the five-story, 113,375-sq.-ft. study building stood to expand floor space by 984 sq. ft.
With construction costs for similar buildings averaging about $24.18/sq. ft., this found space represents a value of $23,793.12. If Class A space in Dallas rents for $20/sq. ft./year, the bonus space potentially earns the owner about $20,000 annually Û plus, earlier project completion generates earlier income.
Successful studwall installation enabling customers to parlay its advantages requires that SlenderWall producers ensure the panel erector and general contractor are knowledgeable in the system’s use. Scheduling and trade coordination should be discussed during a pre-bid meeting. Collaboration with contractors new to the system will facilitate learning and accelerate construction.
Easi-Set Managing Director Moffette Tharpe emphasizes the licensor’s readily available support: We provide collateral materials that describe existing projects and outline proper installation techniques. Often, to ensure a project’s success, we will put new contractors in touch with experienced installers. We also direct licensees seeking help to approved, prequalified providers of engineering and drafting services Û firms that are familiar with the SlenderWall system.