By Mike Dunn
PHILADELPHIA (CBS) — Civil engineers not directly involved in yesterday’s horrific demolition accident in center city (see related stories) believe two key changes could have prevented the tragedy: the wall should have been temporarily supported, and the neighboring thrift shop should have been evacuated.
Drexel University professor Robert Brehm, who teaches civil engineering, says evacuations of neighboring buildings are not always needed during demolitions.
But in this sort of circumstance, he says, in which a taller building is being demolished next to a single-story site, evacuation would have been prudent.
“In that situation, where you have a four-story over a one-story, it would have been prudent to have evacuated for a short period of time any inhabitants that were in the one-story building,” Brehm tells KYW Newsradio.
In addition, Brehm says, it’s now clear that the masonry wall which collapsed should been given temporary support once the floors of the building were demolished.
“A proper plan of demolition would have had temporary bracing to keep that wall in a stable situation, so that you can bring it down in a controlled manner, so that it will fall in the direction that you want it to fall,” he says.
Without bracing, Prof. Brehm says, it wouldn’t take much to bring a four-story wall down:
“A little breeze… it can almost be a natural force can cause it to collapse. That’s why a good demolition plan provides temporary bracing.”
In agreement is Joe Graci of the Bala Cynwyd-based firm Franklin Engineering.
Graci has no direct knowledge of the work done by the demolition company working at 2136 Market. But based on media reports, he says, it appears this firm left more of the wall that adjoined the Salvation Army Thrift Shop freestanding than they should have.
“If you take too many floors and floor joists off, now you have these walls unsupported. Where they used to be supported every 10 or 12 feet, now they’re unsupported for maybe 30 feet. And now you have a wall that’s very flimsy and flexible, and the slighted movement, jarring wind, whatever would happen to it, could cause it to go over,” Graci tells KYW Newsradio.
Normally, he says, a wall would be demolished floor by floor as the building itself comes down.
“You want to do it floor by floor, so you don’t have these long heights of unsupported parts of the wall,” he explained.
Or, he adds, support columns would be added if the wall remains.
“It’s really important that, if you’re going to take the floor joists out, that you put temporary bracing into the wall, to take the place of the floor joists once they are removed.”