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Using Data, Not Assumptions, To Make Safe Structures For Less Money

Sami Rizkalla standing next to a full-scale spandrel that is poised for testing in the CFL.

Each level of a parking garage is held up by a structure called an L-shaped spandrel beam. For years, manufacturers have been making these beams using inefficient assumptions, which meant they were using too much steel and putting it in the wrong places. Researchers have now collected actual data on these structures, allowing them to create new beam designs that are just as strong – but can be made more efficiently, using less steel.

Specifically, previous spandrel beam designs were created using inaccurate data on how the beams deform under pressure. These flawed assumptions led to beams that contained complex internal steel structures. To be clear, the beams were (and are) safe – they’re just inefficient and difficult to manufacture.

Researchers at NC State’s Constructed Facilities Laboratory (CFL) ran tests to determine exactly how these beams behave when overloaded, using full-scale spandrels. They then used this (accurate) information to craft a new design approach.

“This new approach allows us to use from 30-50 percent less steel, without affecting the integrity of the beam. There is no reduction in strength,” says Greg Lucier, the CFL lab manager and lead author of a paper describing the work. The paper is forthcoming from PCI Journal.

The new design method also makes the beams much easier to produce. Traditionally, it took a five person crew three to four hours to prepare the steel cage used in a spandrel beam. Using the new method, the steel reinforcing structure can be prepared in half that time.

“This approach allows manufacturers to nearly double the output of their production line,” says Dr. Sami Rizkalla, director of the CFL and a co-author of the paper.

The decrease in steel content, and improved manufacturing efficiency, should make the beams significantly less expensive.

Some companies are already adopting the design approach, and researchers expect the entire industry to move in this direction in the coming months.