Mehrdad Sasani, PhD
Professor of civil and environmental engineering at Northeastern University in Boston. His research focus is the collapse of structures and structural integrity. Northeastern.edu
The tragic collapse of Champlain Towers South, a 12-story condo in Surfside, Florida, this past June triggered sympathy… and concern. Are other US high-rise buildings at risk as well? Short answer: US high-rise buildings are at extremely low risk of collapse—but there always is some small probability of failure.
As of early August, it was not known exactly why the Surfside collapse occurred—it likely will turn out to be a combination of factors, not a single flaw.
What is known is that certain signs may appear when a concrete high-rise is in danger, and some high-rises are at greater risk than others.
Here’s what to watch for and what to do if you live or work in a concrete highrise or if you’re considering moving into a condo or an apartment building…
If any of the following factors apply to a high-rise where you live or work, the danger is somewhat elevated and vigilance is especially warranted. Both applied to Champlain Towers South…
It was built before 1989. New building code requirements for the integrity of structural concrete took effect in 1989. Concrete high-rises constructed in the US after that year are much less likely to collapse following an initial failure than those that were built before.
It is located near the ocean. Champlain Towers South was right on the beach, and the ocean may have played a role in its collapse. When concrete undergoes alternate wetting and drying, ocean water is much more dangerous to high-rises than fresh water, because salt can corrode the rebar—the reinforcing bar—inside the concrete. Even just the salt in the air near the ocean can be corrosive—that’s why metal bridge components often are visibly corroded even though a bridge is far above the water.
Some high-rises fall more abruptly. In many high-rises, the concrete slabs of each floor rest directly upon concrete columns, with no horizontal beams in between. This was the case for Champlain Towers South. Called “flat-plate construction,” this lets builders squeeze more floors into a building of a given height, and it’s generally as safe as any other floor system. But: When flatplate construction fails, it sometimes gives people inside little warning—the columns quickly “punch through” the floors, then the floors can, in some cases, pancake down on the ones below. When high-rises have supporting beams beneath floors, this type of floor punching often is avoided.
You don’t have to be a structural engineer to identify the following potentially serious high-rise safety concerns. It’s especially important to watch for these issues if any of the risk factors above apply to your building, but everyone who lives or works in a high-rise should be on the lookout.
Cracks in the concrete. If you look for cracks in structural concrete, you’re going to find them—they’re almost inevitable. While some concrete cracks are perfectly normal and not at all dangerous, others point to possible problems.
Rule of thumb: In general, sloping cracks on walls or beams—from around 30 to 60 degrees—are potentially more troubling in structural concrete than cracks that are largely horizontal or vertical. These types of cracks can point to shear or compressive failure, which are more dangerous in concrete structures. Also, cracks that are wide and/or widening, are more troubling than those that are stable and extremely thin.
What to do: Every so often, look for cracks, particularly sloping cracks, in concrete columns, beams, floors and walls. In some high-rises, you will have to visit the garage or underground parking area to find exposed concrete. If you spot cracks, purchase a concrete crack gauge online—they cost less than $20—and use it to measure the width of the cracks. A crack wider than about 0.016 inches is a concern, particularly if it is active or widening. Alert the building manager or condo board, and/or call in a structural engineer for an assessment if you have the power to do so. Monitor the width and length of cracks to determine if they’re growing.
Small sections of peeling or flaking concrete. This is called “spalling,” and it could be a serious problem or no problem at all, depending on why it’s happening. Sometimes sections of concrete flake off because the rebar inside the concrete is corroding—corrosion causes rebar to expand, pushing away the surrounding concrete. But sometimes spalling occurs without corrosion and is superficial.
What to do: If spalling exposes rebar previously hidden inside the concrete, examine that rebar. If it’s visually corroding, alert the building manager or contact an engineer—more corroding rebar could be hidden inside the concrete, putting the entire building at risk. If there’s no visible rebar or the exposed rebar shows no sign of corrosion, there’s nothing significant to worry about. Just have the cover concrete repaired.
Crushing or crumbling concrete. This often is a sign that concrete is failing under compression. It occurs most commonly in vertical columns but also can occur at the top or bottom of beams. Crumbling was spotted by residents of a six-story Singapore building before it collapsed in 1986.
What to do: As above, notify the building manager or a structural engineer.
Sloping floors. This can be a warning sign that the floor is failing. Fortunately, it’s probably failing slowly, giving everyone sufficient time to respond.
What to do: If the slope is slight—only noticeable because round items roll across the floor, for example—and not rapidly increasing, notify the building manager or an engineer. But if the sagging is obvious and growing worse at a noticeable rate, consider moving out until a structural engineer confirms that the building is safe.
Loud sounds. If a high-rise’s structure is the source of these sounds, that could mean there’s a very serious problem and could point to imminent failure. But it can be difficult to identify the source of sounds in a large building— was that noise rebar fracturing…a nearby car backfiring…or something dropped by an upstairs neighbor?
What to do: Ask your neighbors if they know the source of the sounds before jumping to conclusions. If you suspect that the building structure is the source, consider vacating until an engineer declares it safe and/or a nonstructural source of the sounds is identified.
Significant weight recently added to the building. While some collapses occur because the building’s capacity to support weight has decreased, others occur because additional weight has been added. Example: A 16-story Boston apartment building under construction collapsed in 1971 in part because of the construction weight on its roof.
What to do: If a new penthouse, heavy air-conditioning unit or other substantial weight has been added to your high-rise, that’s a good time to monitor concrete for the problems described above—and make sure that the added weight was approved by an engineer.
