The new parking structure at the Mystic Transportation Center in Medford, Mass., is the first of two planned facilities for the space. The completed portion features 1,389 spaces on eight levels. A later, nearly identical structure will offer space for 1,200 cars. The spandrel design was considered interesting enough to be repeated many times over the eventual 500-foot length of the garage. Two blue vehicles, used to transport commuters to the Wellington Orange Line station 800 feet distant, stand ready on the guideway connected to the parking garage.

Parking structure spandrel units that serve as exterior bumper guards and life-safety guardrails serve double duty and save construction costs. But by turning it into a load-bearing unit, the spandrel goes the extra mile to really become a money-saver.

Such was the case with the Mystic Transportation Center in Medford, Mass., a Boston suburb. The garage stands at the center of the Mystic Center Office Park, which was master-planned in 1986 by Thomson Design Associates of Boston. Thomson Design also designed the new parking facility, which is complementary but not identical to the office buildings. The parking structure provides 1,389 spaces for workers in the nearby office park and for Boston commuters with easy access to the commuter station via an 800-foot-long people mover.

The garage has a footprint of 256 by 180 feet and provides 410,000 square feet of parking space in its eight stories. It is considered phase one in a two-phase construction plan. A nearly identical structure, slightly smaller at 226 by 180 feet and with 1,200 parking spaces, is to be added.

The eight-story building was planned so that standard 11-foot-wide double tees could be used throughout. Even the two elevator-stairway towers are placed outside the main structure, simplifying the construction process. The basic layout of the garage features an uninterrupted array of three 60-foot-wide bays in a combination of 33-foot and 43-foot lengths totaling 253 feet long. The center module is supported by a shear wall system that also provides lateral bracing.

Other Systems Considered
"Both precast and cast-in-place, post-tensioned concrete were considered," says Bill Wieners, director of marketing for Northeast Concrete Products LLC, the precast supplier. Precast's initial cost was less than other systems, he adds, partly because of the standardization in the construction of the precast components. "With the quality control that came with factory casting and the use of steel forms to construct precast components, including the spandrels, precast clearly was the optimum way to build."

The advantages to using precast were fourfold, says architect Brian Thomson, president of Thomson Design. These were "fast erection time, economy, low maintenance and integration of a high-quality architectural finish in structural components that were used as the building's façades."

The structure cost $12 million, with another $4 million allocated for the people mover, consisting of an 800-foot guideway and two vehicles, each with a capacity of 50 people. The vehicles are pulled by cables linked to a motor, "like an elevator, except it's horizontal," according to Thomson. It links the garage with the Wellington Orange Line Rapid Transit station of the Massachusetts Bay Transportation Authority (MBTA).

The spandrels were constructed in lengths of 33, 43 and 60 feet and are patterned "to provide sufficient visual interest to withstand being repeated for 500 feet of total garage length" when phase two is completed, says Thomson. "The garage was designed to serve as a buffer between the office buildings and the rail yard," he adds. "One of our main challenges was to find a panel design that was expressive, that could be created in three sizes and that could be replicated over and over while still creating visual interest."

Exposed red aggregate used at the inner arch in the middle of each spandrel increases the apparent depth of relief, achieving a visual lightness. Changing light patterns across the surface "render its curving planes in subtle to bold relief," Thomson explains.

To achieve maximum construction cost efficiency, the spandrel beam at the exterior column line of the garage was designed as a load-bearing structure, bumper guard and architectural façade together, Thomson explains. Using six-inch projecting bullnose shapes at the top and bottom of the spandrel, in concert with curving arches of one-inch relief, the spandrel was conceived to grow almost treelike from columns with arching haunches.

"A reverse scalloped curve was formed out of the top of spandrel connections at the columns to lighten the mass of the spandrel, to articulate the column haunch and to modulate the rhythm of the façade," he explains.

"The pattern of columns and spandrel beams achieves a visual lightness and movement that belies the strength and weight of these structural members."

The two elevator-stairway towers are built of glass and brightly painted steel. These were located outside the main structure to avoid cutting the double tees, simplifying construction and reducing costs further. The towers offer "light, transparent counterpoints to the mass of the precast structure. Their dramatically curved roof forms are complementary to the precast panel design," Thomson adds. A byproduct, he says, "is the drama of evening lighting of these elements." The towers are considered safer than if they were built inside the garage, points out Wieners, "because there's no place for people to lurk."

Once the spandrels' design had been selected, Northeast Concrete ordered steel forms for the precasting operation. "Steel gives a sharp definition to the spandrels even after the operation is repeated many times," says Wieners. The forms, manufactured by Helser Industries in Tualatin, Ore., were shipped by flatbed trailer truck to Northeast's Plainville, Mass., facility. The first of the completed spandrels were ready to be placed only 25 weeks after the contract for construction was awarded. Installation of all precast members was finished 12 weeks later.

The double tees, cast with 5,000-psi strength, are welded flange-to-flange with stainless steel connections and provide both the floor diaphragm and drive surface. Joints are filled with a high-performance pourable sealant. Hot-dipped galvanized end connections of double tees-to-spandrels are common, and shear wall-to-tee connections are covered with washes of four inches of latex-modified concrete to further protect from corrosion.

Double Tees Less Costly
"Pound for pound, double tees are the least expensive method of support," says Brent Goldstein, project engineer from McNamara/Salvi Inc. of Boston. "We looked at the project and did two sets of drawings, precast vs. poured-in-place. When we priced them out, precast was less expensive."

Other considerations, Goldstein adds, were "winter weather and speed of erection, plus the economics - it's less expensive to build a garage this way. The use of spandrels as both architectural panels and supports is one of the great benefits of the precast system. You get a spandrel and a support beam at the same time."

In all, 861 pieces of precast were needed. This included 541 double tees, each 11 feet in width and weighing between 24.8 and 29.1 tons each. It also comprised 81 load-bearing spandrels weighing between 13.9 and 18.5 tons, plus 22 non-load-bearing spandrels with weights ranging from 13.7 tons to 27.9 tons. In addition, there were 67 double and single ledge beams, weighing between 4 and 20.9 tons; 40 columns, each 24 by 24 inches and weighing between 13.7 and 27.4 tons; 50 12-inch shear walls weighing 13 to 30.5 tons, and 60 12-inch lite walls weighing 23 to 27.2 tons. The component design detailing was performed by The Consulting Engineers Group in San Antonio, in accordance with McNamara/Salvia's performance design documents.

Goldstein says there are many precast garages being built in the Boston area, some with steel frameworks and precast concrete façades, and others almost entirely of precast. In the case of the Mystic Transportation Center, Goldstein says, "The contractor had concerns about the weather." As a result, the contractor preferred the precast option, he adds, from the standpoint of speed and the ability to fabricate the panels in a controlled environment.

"The advantage of treating the structural spandrel panel as an architectural feature cannot be overstated," Thomson concludes. "We were convinced we could make an architectural-grade structural member and we did. Whatever the viewer's interpretation, the architect has achieved his goal of visual interest by the most economical of means."