Segmental retaining wall (SRW) systems usually are built to hold up the earth. But at two Lawrence, Kan., high schools, they’re being used to hold up people as the primary structural component in four sets of massive outdoor athletic-stadium bleachers.
The bleachers were built in place of more traditional aluminum bleacher systems and saved the Lawrence School District tens of thousands of dollars and months of construction time. In addition, bleachers built from SRWs are quieter, warmer, and more attractive than aluminum bleachers — a fact that has other school districts green with envy, says Jeff Martin, C.E., principal with Landplan Engineering in Lawrence, which designed the stadiums.
Three of the four bleacher units were built on large artificial earthen berms topped with 2 feet of clean drainage rock, sloped on one side with amphitheatre-type seating on the other side. The fourth bleacher unit features SRWs on all sides, including a 16-foot near-vertical wall on the rear.
Landplan Engineering’s Lawrence office began investigating traditional bleacher systems after it was chosen to develop designs for separate stadiums at the district’s two secondary schools: Lawrence High School and Free State High School. Then, Capitol Concrete Products, a Lawrence manufacturer, came to their office to give a presentation on using Versa-Lok SRWs for the retaining walls in the project.
Aluminum bleachers could not be completed for more than a year — well past the schools’ football season. So, Landplan began looking at using Versa-Lok for the bleachers as well as the retaining walls.
The school district preferred enclosed bleacher seats rather than an open design. Cost estimates showed that enclosed aluminum bleachers would run about $400 per seat, while bleachers built with Versa-Lok would average about $300 per seat.
Another advantage of using a concrete SRW system built on a solid berm or base was noise reduction. The back doors of some neighbors were less than 100 feet from the back of the bleachers.
“On a given night, you walk into the stadium and it’s really loud. But with this system, when you walk around to the backside of the bleachers, you can’t hear a thing,” explains C.L. Maurer, landscape architect with Landplan. “With a mountain of concrete and soil, the sound’s gone.” Noise from fans jumping up and down on aluminum bleachers also has been eliminated.
And the concrete bleachers, which use pavers and wall-cap units for the seating surfaces, are warmer on the derrieres of fans, especially on chilly autumn football nights. Absorbing heat from the sun during the day, says Maurer, “They act like giant heat sinks and radiate warmth at night. The public loves them.”
The projects presented both construction and design challenges. As additional rows were added to the terraced bleachers, more soil was placed and compacted on the berms. Geogrid runs under each seat row.
Bleacher walls were built up before pavers were installed on the seat rows to avoid damage to the pavers during wall construction. Standard Versa-Lok wall-cap units were used for seating. The center of the bleachers is the high point, so water drains in either direction.
The non-terraced bleacher section features walls on all sides, filled with crushed limestone base material, much like a giant raised patio. A 50-foot conveyor was used to deliver the limestone gravel over the top of the 16-foot wall on the back of the structure. Geogrid is installed between every other course of the rear wall. Elsewhere, geogrid under each seat wall row overlaps with geogrid from the side walls.
The grid system was one of several design challenges on the project, says Martin. “Nobody on the design team agreed with what was required for geogrid reinforcement,” he notes. “We used the maximum anyone recommended, so we probably are over-designed.”
Portions of the walls are glued for extra strength at the corners. And, says Maurer, “The way the Versa-Lok system is pinned together is huge for this.” More than 65,000 square feet of retaining wall units and more than 35,000 square feet of pavers were used to build all four bleacher units. The guard railings at the top of the four-wall bleacher structure were particularly daunting, especially with a 16-foot drop over the back of the bleachers.
The entire bleacher system is “a flexible structure with people on top of it,” Martin explains, so a means had to be developed to keep the railings stable under the most extreme conditions. That’s when the design team came up with the “dancing tuba player problem.”
As they were brainstorming what could go wrong with the bleachers’ design, Martin says the worst scenario they could envision would be to place a marching band on the structure while someone tried to climb up the railing from the back side. Since the tuba players are always at the rear of the band formation, they determined they would need sufficient stability to withstand a heavy tuba player bouncing against the railing while someone pulled on the railing from behind, thus the “dancing tuba player problem.”
A 3-foot-square reinforced concrete beam runs the length of the structure behind the last bleacher row and the top 3 feet of the rear wall. Each of the columns that serve as posts for the railings are anchored by the beam. This design should be sufficient to keep any tuba players from going over the edge, says Martin. Railings dividing the stairs between seating sections were installed using Sleeve-It supports reinforced with rebar and concrete. “There aren’t codes for what we’re doing,” he points out. “We had to decide what our design standards were. I had it peer reviewed by three different engineers.”
Drainage was another issue that presented problems not normally associated with retaining walls. “Usually, people aren’t sitting around eating hot dogs and spilling sugary liquids on your retaining wall,” says Martin. “We needed to make sure we had a flushable system. So we have a massive drainage system buried behind all the short walls underneath the seats that will pick up most of the drainage before it gets to the core of the wall. In several places, we have the equivalent of clean-outs.” The system can then be flushed of food and other contaminants periodically, rather than allowing it to rot inside or clog up the drainage system.
The bleachers have exceeded their expectations, both structurally and aesthetically. After their first year of use, the structures remain impressively stable, says Maurer. “We went out there in May and took shots along the top of the bleachers, and it was within two-one-hundredths of an inch.”
Reaction from school officials and the public has been extremely positive as well. “I’ve done a lot of nice looking projects, but this is one of the neatest,” concludes Martin. “This is one of the best high-school stadiums around. There are a lot of jealous opposing-team coaches.”
Martin says he has a number of design variations of SRW-built stadium bleachers, and Landplan now is talking with several other interested clients about similar projects.
Karl Bremer is editor at Versa-Lok Retaining Wall Systems. He can be reached at 800-770-4525 or at firstname.lastname@example.org. For more information on Versa-Lok Retaining Wall Systems, visit www.versa-lok.com