- How can a snail crawl upside-down on the underside of the surface of a pond?
- Why is mercury liquid at room temperature?
- How does glass change over time?
- What makes wood rot so slowly?
- Why doesn’t a plain, white piece of paper reflect light, but a mirror does?
- What’s the difference between premium-grade and regular gasoline?
- Is fire a solid, a liquid, or a gas?
- Why do plastics get brittle when they get cold?
- Does a golf ball really change its shape when struck by the club?
- What are the basic forces behind tape and glue?
Is there a way to check a building for structural damage without knocking down walls?
Signs of deep structural distress are often visible on the surface…By Sarah Jensen
When buildings survive an extreme event, the people who live and work inside them naturally fear there might be hidden damage that could send the walls tumbling down around them. But taking a close look is generally enough to allay their fears — or suggest that something’s seriously amiss.
“You can’t beat visual inspection for discovering indicators of bigger problems,” says Professor Jerome Connor, a structural engineering specialist in MIT’s Department of Civil and Environmental Engineering. “If you see large cracks in the walls, you know the building’s frame has experienced some significant motion and the structural plane has a problem.” Rust seeping from cracks in concrete, he adds, can be a clue that the rebar inside is corroding. And of course if a building is listing to one side, it’s a pretty safe bet its underlying structure has problems.
Following a calamity, local building inspectors can assess the initial damage. Their background in carpentry or construction can generate an initial assessment. Depending on what a building inspector sees, professional engineering consultants with a high-level understanding of how buildings are put together, and how they fall apart, may be required. They perform a global assessment, a qualitative observation and testing of the structure to identify specific damage and the building’s potential weak points. This is not a quick process.
“You can’t go in after a hurricane and tell exactly what’s happened unless you know the condition of the building before the storm,” says Connor. Just as a doctor runs an EKG and compares it with last year’s results, engineers might use a building’s original specs and blueprint as a baseline. In regions where seismic activity occurs regularly, sophisticated vibration equipment is used to collect data on changes caused by the temblors. “Buildings have natural periods of vibration,” says Connor. “These engineers purposely vibrate the structure and come up with an acceleration measurement. If a building is hit by an earthquake, they vibrate it again and compare the results against past data. If they’ve changed, they can tell something’s going on.”
Identifying local damage — the breaks and cracks and deterioration hidden deep within a structure — can involve dismantling the building to reach the innards of its frame. Examining the condition of a skyscraper’s steel beams after an event entails removing thick coats of fire prevention material before performing a visual assessment, no small or inexpensive task. Most of the time, such invasive exams are unnecessary. If a building still stands upright and the walls aren’t cracked, it’s a safe bet that all is well. “Generally, if the structure is damaged, you’ll see signs of it right away,” says Connor.
Apocalyptic storms and the earth’s shifting plates may cause plenty of damage, but another concern has given rise to structural health monitoring, an emerging engineering discipline that is focused on predicting the life expectancy of built structures. “Sixty years ago, we built a beautiful infrastructure of highways and bridges but never thought about deterioration,” says Connor. “It has now become old, and structural health monitoring is a wide open field for engineers. It’s our obligation to examine and maintain this aging infrastructure.”
Thanks to Suzanne of Lakewood, N.J., for this question.
Posted: April 09, 2013