Q&A: How Do Explosions Cause Building Collapses Like Harlem's?

Older masonry buildings can collapse rapidly after room-size explosions.

The deadly collapse of two apartment buildings in New York on Wednesday has an engineering explanation—the "brittleness" of turn-of-the-century brick buildings. (See also: "Why Do Building Collapses Like Philadelphia's Happen?")

Triggered by what authorities said was a natural gas explosion, the collapse in East Harlem killed at least two people, with more than a dozen reportedly missing. Apartment buildings in that part of East Harlem date to the 1900s, prior to the advent of modern building codes.

"Buildings of that vintage are quite brittle," says structural engineer Donald Dusenberry of Simpson Gumpertz & Heger, editor of the Handbook for Blast Resistant Design of Structures. "Even a room-size natural gas explosion can trigger the collapse of an older masonry structure."

National Geographic asked Dusenberry to explain how the buildings, one six stories and one four stories tall, may have collapsed:

How does a natural gas explosion trigger the collapse of two large buildings?

Masonry buildings are able to support very heavy loads, compressive ones, from gravity. But older ones weren't built to take something extra pushing on them in the opposite direction [from gravity] or sideways, and they are quite brittle and can collapse. That's what happens in an explosion.

How does it happen?

When there is an explosion, assuming this was a gas explosion, which is usually the case, pressures build up very suddenly in all directions and expand in the direction of the walls and ceiling.

The ceiling, the floor of the apartment above, can blow out from the force acting in the opposite direction of gravity.

When you lose the floor, you lose the sideways web of support for the columns holding the building. [The columns] are suddenly twice their unsupported length and they buckle.

The columns are great at supporting the compressive forces holding the building up, but they are quite weak, relatively speaking, at resisting the bending forces acting sideways to them that put them in tension.

They are brittle. It's like a stick. You bend it and they snap.

How large an explosion does it take?

That is the tough question. It depends on the building. Normally [natural] gas explosions start in one room or place of origin. Along with the ceilings, they can blast out entire walls, which can leave the wall above not supported at all. So they collapse.

Because the [older] masonry is so brittle, it just breaks into small pieces, leaving piles of debris behind.

How are modern buildings different?

Today we add steel reinforcement, rebar, to masonry building walls. [The steel reinforcements] help to resist to sideways forces . . . Structures built to modern building codes are 10 to 20 times more resistant, normally, to these kind of forces.

Is there a building code change needed here for older buildings?

Well, typically, there aren't requirements for upgrades in older buildings, unless renovations are made of a sufficient magnitude to trip local ordinances.

Is it always a leaky stove that triggers these blasts?

No, I've seen natural gas explosions happen all kinds of ways. Most modern stoves actually have a cutoff switch if left on too long without the burner firing. But that has happened.

It also happens in repairs or with construction, where a leak is triggered by work that damages pipes. I've heard of digging in the street outside a home snagging a pipe and leading to a house filling up with gas to trigger an explosion.

It can happen accidentally in a multitude of ways, unfortunately. It isn't common, and people shouldn't spend all their time worrying. But natural gas leaks should be taken seriously.

This interview has been edited and condensed.

Follow Dan Vergano on Twitter.

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