Deck collapse: Swaying people can be worse than a hurricane

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By Rebecca E. Phillips, University of WSU – Communications

Don-Bender-in-the-Composite-Materials-and-Engineering-Center-lab

Don Bender in the Composite Materials and Engineering Center lab at WSU. (Photo by Rebecca E. Phillips, WSU)

(PULLMAN, Wash.) Sultry summer barbeques on the deck don’t usually include third-degree burns and concussions. But for dozens of people each year, happy gatherings are cut short when the floor below their feet suddenly gives way, resulting in serious injuries and death.

“For every deck collapse you hear about there are many more collapses that never make the news. It’s a serious national problem,” said Donald Bender, director of the Composite Materials and Engineering Center at Washington State University.

Public safety paramount

 

Bender has discovered a deck design that resists the mysterious lateral motion that can lead to catastrophic collapses. His findings were the first to show that people can be the cause of these “lateral loads.”

The study was published online earlier this year in the American Society of Civil Engineers Practice Periodical on Structural Design and Construction.

Bender is collaborating with engineers to write a national deck design manual and make appropriate changes to building codes.

“This work with decks is probably the most significant research I’ve ever done in my career concerning its impact on public safety,” he said. “And in my area of research, that’s what matters.”

DIY not always best

Bender said decks tend to fail along the railing or at the “ledger”- the interface where the deck attaches to the house.

The problem is amplified when do-it-yourself homeowners build decks without professional input. He estimates only about 10 percent follow the minimum safety standards: getting a building permit, using proper design and having a final inspection.

Contrary to popular belief, overloading does not trigger most collapses.

“Many decks fail long before they reach the building code weight limit, which is basically wall-to-wall people,” he said.

Lateral motion

But sideways – or lateral – movement of decks is something different. Lateral motion, which pulls the deck out and away from the house, is most often seen during hurricanes or earthquakes. Bender said safety specs addressing it have been vague. And, compared to weight loading, lateral loading is difficult to measure.

With good intentions, provisions to help limit lateral motion were added to building codes in 2009.

The provisions required installation of tension tie-downs to reinforce the connection between the deck and the ledger. Despite the extra level of safety, Bender said the builders despised the tie-downs.

Installing them requires tearing up a home’s flooring and ceiling, which increases costs, he said. Not only that, there was no clear evidence that the tie-downs were necessary or effective in preventing lateral motion. Not surprisingly, many builders ignored them.

A eureka moment

As a structural engineer, Bender decided to tackle the question of lateral loads himself. Working with a team of graduate students, he calculated wind and seismic forces on decks across the U.S. and meticulously diagrammed the findings on color-coded maps.

The results, reported in 2013, were eye opening.

“We found lateral loads were insignificant for wind except in the tip of Florida where they get the worst hurricanes,” he said.

Likewise, lateral loads from earthquakes were minor in most areas of the nation. Larger forces did show up along the San Andreas Fault and in Yellowstone National Park. But they were much smaller than predicted by the building code.

“We thought we were on to something and maybe we didn’t need such strict tie-down requirements,” said Bender.

Then he had a eureka moment.

Dangerous dance

“I got to thinking about being here in a college town where there are lots of young people out on decks, moving around and feeling festive. I realized that the lateral loading could also be caused by people,” he said.

Using a deck simulation in his lab, Bender’s team discovered that a group of people moving back and forth in rhythm, as when dancing, displaced the deck a dangerous 16 inches.

“If you or I felt a deck moving, we’d probably get off,” said Bender, “but college kids call more friends out to share the fun. The swaying back and forth generates forces that far exceed the worst-case hurricane or earthquake.”

Strength in triangles

To offset the lateral motion, Bender tried a time-honored alternate design – nailing boards to floor joists in diagonal patterns to form triangles. Triangular braces can add four times the stiffness and six times the strength to a structure.

Even when fully loaded with swaying people, the reinforced deck moved only one inch.

Encouraged, Bender tested decks with and without tension tie-downs to see how lateral loads were actually being transferred into a house when a deck was thrust sideways.

Using “smart bolts” equipped with sensors, the team measured forces on every screw in the deck and discovered that tie-downs were only necessary in specific cases. It came down to the type of joist hanger – the hardware that attaches the deck to the ledger on the house.

Bender said that angled joist hangers, attached with nails, ripped out easily under lateral loads and could lead to deck collapse. But straight joist hangers used with approved structural screws were much more stable and lessened the need for the invasive tie-down system.

Source: https://news.wsu.edu

Contacts:
Donald Bender, director, WSU Composite Materials & Engineering Center, bender@wsu.edu, 509-335-2829

Rebecca E. Phillips, WSU University Communications, beccap@wsu.edu, 509-335-2346