When the Houston Astrodome, baseball's first indoor stadium, opened in 1965 it had natural grass. The grass required a translucent roof to let in sunlight. Unfortunately, it produced such glare during day games that fielders routinely lost track of fly balls. So the Astros repainted the roof and replaced the natural grass with an artificial surface now known universally as "Astroturf."
Fast forward 35 years to Houston's new baseball stadium, Enron Field. Thanks to a retractable roof, the new stadium is able to feature a natural grass field. A unique suspension system enables the roof system to open and close flawlessly. This is in sharp contrast to retractable roofs at other ballparks, where operational glitches are common.
Bart Riberich, VP of engineering at Uni-Systems, the Minneapolis-based firm that designed the roof travel mechanism for Enron Field, traces roof operating problems to the bogie suspension systems on which they ride. Bogies are similar to the undercarriage of railroad cars. "Bogies tend to concentrate the load at a single point when the wheels hit a high spot on the rail," Riberich explains. Instead of bogies, Enron Field uses independent suspensions for each of the 142 wheels that carry the three huge roof sections.
Uni-Systems envisioned a suspension spring that would compress by a small amount when the wheel meets a high spot on the rail. Almost from the start, Uni-Systems eliminated conventional metal springs because they could not achieve the targeted 25-year lifespan. That left elastomers -- not just any elastomer, but a highly resilient, durable, system with exceptional load capacity. Uni-Systems was already working with Kastalon Polyurethane Products (Alsip, IL) on a bumper-stop assembly for the roof system when the two companies began to explore the use of urethane in the suspension springs.
The team selected Adiprene urethane prepolymer from Uniroyal Chemical, a Crompton business. As Kastalon president Bruce DeMent points out, "Polyurethane was the only material that could provide consistent performance and compactness of size." Adiprene urethane elastomers combine the outstanding extensibility and elasticity of rubber, distinctive load-bearing capacity, toughness, durability, and abrasion resistance. They also stand up well to oils, chemicals, ozone, and radiation.
The patented cylindrical springs installed at Enron Field consist of alternating layers of steel and urethane around a central shaft. Only 8 in. dia and 9 in. tall, the assemblies support a normal load of 125,000 lb and peak loads of up to 325,000 lb.
"The springs had to be compact to make the design work," explains DeMent. "The versatility of Adiprene urethanes allowed us to compound an elastomer that would give us the required performance."
Innovative design, engineering, and materials halved the cost of the Enron Field suspension system when compared to a conventional suspension used at a similar stadium. In addition to initial cost reductions, Uni-Systems projects additional savings through reduced maintenance expenses.
Uni-Systems also selected Adiprene for another Enron Field application. The stadium's roof trusses rest on bearings that move when the trusses expand and contract in response to temperature. When originally specified traditional bridge suspensions proved inadequate, Uni-Systems and Kastalon collaborated to develop a spring to suspend the roof trusses.
"It had to bear a million pound load and fit into an existing space that was 18 in. square and 5.5 in. high. Once again, the Adiprene urethanes were the key to making it possible," says DeMent.
Adiprene urethanes are also used in the stadium's roof tie-down system. Despite its great weight, the roof of Enron Field must be tied down during high wind and hurricane conditions. Urethane enables the tie-down springs to maintain constant tension during the lift/drop cycles that are often experienced during high winds. In addition, the strength of the elastomer allowed engineers to specify fewer straps than they would have needed if they installed steel coil springs. Although Adiprene applications at Enron Field may appear unique, Kastalon's Bruce DeMent says the material has very broad potential. "It is a high-quality engineering elastomer that is useful in a variety of mechanical and environmental conditions, including heat, humidity, and cold. We've found that there are many applications where urethane is the best and only solution," he says.
