BERKELEY (CBS SF) — It is still unknown what caused a Berkeley apartment balcony to collapse early Tuesday, killing six people, including five Irish citizens, and injuring seven others who were celebrating a 21st birthday.

Grace Kang, a structural engineer, said the balcony  appeared too small to hold the 13 party goers that officials said had gathered on it.

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According to Kang, California building codes during the past decade have required balconies to support 100-pounds per square foot. The previous codes called for 60 pounds of support per square foot.

Kang, who is also a spokeswoman for Pacific Earthquake Engineering Research Center at the University of California, Berkeley, says moving and dancing would have put additional strain on the balcony.

The building is managed by Greystar Management. A call to company officials was not immediately returned.

Ronald Bertone, an architecture, construction & premises safety expert with Robson Forensic, told KCBS Radio: “What happened this morning should not have happened.”

Bertone speculated on the strain of the “live load” the balcony endured during the gathering – that is, the number of people on the balcony participating in activity considered to be “normal.”

Precise numbers aren’t available yet, but the balcony had at least 13 people on it. One could estimate 2,000 pounds total on the balcony at once if everyone was roughly 150 pounds each, putting about 50 pounds of stress on the balcony per square foot. Given those numbers, Bertone said the balcony should not have collapsed.

“There are a number of factors that could have come into play. I don’t know how old the building was [or] whether there was any degradation of the structure,” he said. It turns out the building was less than 10 years old.

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Another factor that may have contributed to the structure’s capabilities could be water intrusion.

Bertone said in a building like this, the floor joints would have been tied back into the structure of the apartment building and that balconies are part of regular building inspection and are also inspected as the building is being constructed.

Another structural engineer, Enjin Yagmur, told KPIX-TV the balcony should be able to handle this kind of load. “Balconies are typically built for higher loads than the rest of the building, sometimes two to three times,” he said.

Yagmur only examined photos of the damaged structure but says to him, it’s pretty obvious. “I believe  there was damage after construction and from the photos I observed some water damage.

Water damage can cause dry rot, which may not have been visible before the accident. Yagmur said for multi-unit structures, inspections are recommended every five to 10 years, or when there’s an ownership change.

But balconies may not be on the checklist. “Balconies are part of individual units so typically they come and inspect the roofs but for the balconies there is not an inspection requirement,” he said.

Dry rot doesn’t happen overnight, and a permanent resident of the apartment might have noticed signs. In this case, students rent it for just a few months at a time and most likely weren’t looking for those signs.

Yagmur thinks that investigators are going to find the cause very quickly, because balconies are unique separate structures. Until then all the balconies on the building will remain red-tagged.

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