When a pane dropped out of a glass bridge and crashed to the ground last week during a test, project leader Dr Fred Veer wasn’t much impressed. “The test was 90 percent successful.”
“Our insights into how to design with glass as a structural material have developed considerably over the last ten years,” says Dr Fred Veer (Architecture) two days after the fateful test. “Most people think that normal annealed glass cannot take tension, that you cannot drill holes in it and cannot put pressure on it. We now know it can all be done and we’ve shown it.”
Indeed, the glass bridge that was tested last week carries all the mentioned hallmarks of construction. Constructed from glass panels, it spans 6 meters and weighs 600 kilograms. The tension at the lower half of the side panels is distributed evenly over the glass by steel bolts. The upper half of the panels experiences the pressure of the bridge’s weight, which small lead blocks coupling the panels convey. The glass consists of two layers stuck together by a rubbery glue that holds the splinters together if the glass bursts.
Tension and pressure in the construction have previously been calculated using a finite elements computer programme. Dr Veer: “The compressive strength of annealed glass is 300 Newton per square millimetre. That puts it in the same category as concrete.” So why did the test bridge fail?
In fact, the bridge did not structurally fail. Rather, to be precise, one glass panel carrying the test load of 120 kilograms of bricks fell out and crashed to the ground. Dr Veer inspected the panel afterwards and saw that the glue between the glass panels had become unstuck. He blames that on the faulty workmanship of the students who built the bridge. “They’re intelligent, but they’re unskilled,” he comments. Before the glue is applied, the glass must be cleaned with alcohol, which must be subsequently dried and removed before applying the glue. Ignore that and the glue will not harden properly.
The issue of structural safety in glass structures is addressed in the PhD research of Christian Louter. He developed and investigated reinforced glass beams that take large loads even when the glass is broken, thanks to the integrated tendons of stainless steel or glass fibre. The multilayer glass beams are laminated with the innovative SentryGlas interlayer, which not only holds the splinters together after breaking but also stops cracks from spreading.
Photos of a beam prototype show a crew of five adults happily standing on a broken beam hanging in chains. No worry because the beam’s internal tendons maintain the strength. Experts believe that the introduction of reinforced glass will have the same effect as the invention of reinforced concrete: much more daring and challenging structural designs.
Christian Louter, ‘Fragile though Ductile, Structural Aspects of Reinforced Glass Beams’, 18 April 2011
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