Freshmen design, build Houston Arboretum bridge

Team Tree Amigos had a bridge to build at the Houston Arboretum and Nature Center, and not one of its four members knew the first thing about structural engineering.

“Nobody involved is planning to go into civil engineering. Some of us had never used power tools. We had our dark periods,” said Kathryn Hokamp, a freshman at Rice University who plans to major in ecology and evolutionary biology, and who served as construction foreman for the engineering design team.

The students first met last fall in their Introduction to Engineering Design class. The other team members and their tentative majors are Michael Donatti, mechanical engineering; Stephen Phillips, bioengineering; and Katherine Stiles, chemical engineering.

“We didn’t know each other. We didn’t start out as friends but we learned how to work in a team setting. The chemistry was pretty good. Basically, we were learning how to learn,” Donatti said.

The staff at the arboretum presented Tree Amigos with a straightforward problem: The low spot between the playground and a nearby pavilion routinely floods after heavy rains. More than a foot of water can accumulate and remain in place for several days. The span had to be at least 13 feet long, not counting the platforms at either end, to bypass the water and mud.

In their early brainstorming sessions, after three or four visits to the arboretum, the team came up with about 80 potential solutions, some extravagant or ridiculous. Additional meetings with arboretum staff and a review of the U.S. Consumer Product Safety Commission’s regulations for public playground safety reduced the workable solutions to one. By October they had a final plan for a suspended wooden bridge – one frequently tweaked, however, by contingency.

The arboretum staff poured the foundation – eight cylinders of concrete, each eight inches in diameter and four feet deep. The team used more than 250 board-feet of lumber, more than 85 percent of which was “repurposed” – that is, recycled from use in previous structures. Thus, their choice of team name: Tree Amigos. They intended the project to be ecologically benign, relying as much as possible on recycled material.

They also used 400 pounds of concrete, 45 feet of high-test steel chain (with a load limit of 5,400 pounds), more than 400 bolts and other pieces of hardware, eight steel posts and 80 feet of rope (for hand-holds across the span).

“All of the other teams had, physically speaking, smaller projects. We couldn’t really prototype anything. It was just too big,” Stiles said.

With the foundation in place, the rest of the work was completed in a single day, Feb. 16, between 8 a.m. and 5:30 p.m. “We had to make adjustments,” said Hokamp, who took wood shop in high school. “Some of the pieces in the foundation were uneven, off a little. It was like a trapezoid.”

“A slight rhombus, really,” Stiles added.

The arboretum has posted a sign limiting the number of children crossing the bridge at one time to three. Donatti knows better: “This thing is built to hold six adults, each weighing 200 pounds. It’s solid.”

undefinedPatrick Kurp, Engineering Communications