By Mike LaBella
---- — HAVERHILL — Cheering for the collapse of a bridge isn’t something you’d normally encounter. But at the Haverhill Alternative School, teams of students competed to see whose bridge made of pasta would fail and collapse first after hanging heavy loads from them.
It was a competition to see whose bridge was the strongest by design and just how much weight they could bear before crashing to the floor.
“The project emphasizes tension, compression, torsion and shear, but it’s really all about breaking stuff,” science teacher Neil Wilkins said with a smile.
Wilkins teaches a unit on engineering and technology to his high school students at the Alternative School, located on the second floor of the St. James school building on Primrose Street.
During a recent hands-on project, he asked his students to plan, draw and build a bridge prototypes made from one pound boxes of spaghetti strands, using hot tape and glue so their bridges would support the greatest possible load, over a minimum one-foot span.
“Spaghetti bridge building exercises are widely used in national engineering programs to demonstrate the structural possibilities of enhancing seemingly ‘weak’ materials through engineering design,’” Wilkins said.
His students based their bridges on Warren truss and Pratt truss designs, which incorporate triangular structural components, and the arch bridge, an ancient design with abutments at each end. A few years ago he challenged his students to use plastic drinking straws to build the longest bridges they could before collapsing, and prior to that they built tall towers.
Students and staff gathered in the school’s science and art room last week and eagerly watched as Wilkins began testing the strength of six bridges built by teams of students. Using classroom desks as abutments, Wilkins placed a wooden stick across the width of Wilgen Montero’s bridge and carefully suspended the bucket beneath with string.
Slowly, Wilkins began to fill the bucket with cups of rock salt, adding more and more.
“Any guesses?” Wilkins asks his students.
“Ten pounds, 15 pounds!” they shouted.
Then came the first creaking sounds marking an impending failure. Suddenly, it was a spaghetti catastrophe.
Wilkins weighed the bucket of rock salt at 10 3/4 pounds.
Students Tyler Dube Perez, grade 12, and Shaine Reynolds, grade 9, hoped their design would withstand a much heavier load.
Wilkins set up their bridge but after adding just a few pounds of rock salt the wooden stick he’d used to suspend the bucket cracked and the bucket fell to the floor. He rigged things up again and continued adding rock salt until the bridge finally collapsed, falling into the bucket in two sections.
“I’m sorry about your bridge,” Wilkins told Perez and Reynolds, who were in first place with a load of 23 3/4 pounds.
“I’m kind of surprised it did so well as we had the smallest bridge and I didn’t think it would hold that much weight,” Perez said.
“I think the triangles were a big key in our bridge design,” Reynolds said.
Next up was a bridge built by Lathan Nagle, grade 11, and Chris Smith, grade 10.
Wilkins began pouring rock salt into the bucket he’d suspended from their bridge and everyone watched and listened.
“Look closely,” Wilkins told the boys. “Your cross beams are buckling.”
He added more rock salt and remarked, “That’s not a happy bridge.”
Suddenly, the little bridge that managed to hold 21 1/2 pounds collapsed.
“Is it heavier than Tyler’s?” Chris Smith shouted.
Jose Cabrera, grade 9, built a sturdy looking bridge that other teams were in fear of.
“This is the one they worried about the most,” Wilkins told the group.
He poured and poured cups of rock salt into the bucket suspended from Cabrera’s bridge, yet it would not budge.
“It’s all about how the load is distributed,” Wilkins said aloud. “That’s why triangles are so good. They spread the load in different directions.”
He continued adding rock salt yet the bridge stood strong.
“He’s going to beat the record!” one student shouted.
“I hear it cracking!” another yelled.
“I should have had music,” Wilkins tells them. “Where is my showmanship?”
Wilkins, nearly out of salt, tells the group to expect a “spectacular and explosive failure.”
The bridge finally succumbs to a weight of 34 3/4 pounds, setting a new record.
“I knew it was going to win,” Cabrera said.
Wilkins said the lesson builds on his students’ knowledge of the Engineering Design Process, orthographic drawing, and the application of scale modeling. He said the focus on Construction Technologies applies to their understanding of structural forces of tension and compression, as well as material properties and the calculation of live and dead loads.
He said his students were introduced to large scale local projects such as the construction of Boston’s Central Artery Tunnels and Zakim Bridge, taking advantage of their awareness of the Bates and Rocks Village bridge rebuilding efforts. Future field trips are planned to visit the construction site of the I-95 Whittier Bridge in Newburyport.