Readers Dislike the WTC/I-580 Comparison

A lot of readers have criticized an Underwire postput up yesterday morning regarding the collapse of the Bay Area's I-580 overpass.

This line seems to be fueling the most anger:

When the I-580 overpass buckled, it brought back memories of the World Trade Center. The towers collapsed due to the excessive heat caused by the exploding planes, not from the initial impact. Steel loses half of its structural strength when heated to 1,000 degrees -- a temperature easily hit when a gasoline tanker (or plane) explodes. It begins to melt at 2,750 degrees Fahrenheit.

I, in no way, am comparing the horrors of 9/11 with what happened on the morning of the 29th in Oakland, CA. But I am comparing the structural impact the two locations had when faced with fire bombs (an airplane in one scenario and a gas tanker in the other). The WTC was on a much larger scale, but the two seem to have had similar structural responses.

I apologize for oversimplifying what happened with the WTC towers when I wrote, "The towers collapsed due to the excessive heat caused by the exploding planes, not from the initial impact." Obviously this isn't the full story, as there were many factors that added up to the final collapse, but fire did play a major role in diminishing the structural integrity of the skyscrapers.

According to the NIST

“Like most building collapses, these events were the result of a combination of factors,” said Shyam Sunder, lead investigator for the agency’s building and fire safety investigation into the WTC disaster. “While the buildings were able to withstand the initial impact of the aircraft, the resulting fires that spread through the towers weakened support columns and floors that had fireproofing dislodged by the impacts. This eventually led to collapse as the perimeter columns were pulled inward by the sagging floors and buckled.”

The same paper states that, "The floors weakened and sagged from the fires, pulling inward on the perimeter columns. Floor sagging and exposure to high temperatures caused the perimeter columns to bow inward and buckle—a process that spread across the faces of the buildings. Collapse then ensued."

While what happened at the I-580 overpass is nothing in comparison to the tragedy of the WTC, I think we still have a lot to learn about how large fires affect structures, especially structures that use large amounts of steel. The beauty of the WTC design, was that it was built in such a way that it was able to withstand a jumbo jets crashing into each tower at 429 and 503 miles-per-hour. How long would it have stayed up if the fire hadn't weakened the rest of the structure?

What we can learn from both the WTC collapse and that of I-580 could lead to stronger buildings, freeways, and bridges, which are susceptible to these types of attacks and accidents.

More fodder regarding the steel in the WTC can be found in this infographic from the Sydney Morning Herald.