By Andrew Schrader
When firefighters think about structural collapse from an urban search and rescue (US&R) perspective, it’s easy to consider incidents such as the 1994 Northridge earthquake or man-made explosions like the 1995 Oklahoma City Bombing. However, a basic knowledge of structural collapse does matter whether you’re attached to a big Federal Emergency Management Agency (FEMA) US&R team or a rookie firefighter riding a rescue or an engine all day. If a tornado hits a trailer park and your crew is the first on scene, pulling up to mobile homes strewn in a pile, you just became the world’s smallest US&R squad!
Of course, collapse knowledge also matters in fire situations. For example, the tendency of floor truss systems to deteriorate quickly under extreme heat can turn a routine single-family residential house fire into a potential collapse situation.
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Structures specialists are licensed structural engineers who have received additional US&R training from the U.S. Army Corps of Engineers and FEMA. They normally support heavy rescue teams and function as advisors when the teams are performing search and rescue inside of damaged buildings. However, the insights they provide to those teams can be useful to all firefighters regardless of in which seat or vehicle they ride.
Less “Nerd Speak”
Following is an overview from one structures specialist’s perspective of structural collapse knowledge pertaining to the buildings in which we work most often. It incorporates less theoretical information and more rule-of-thumb and hard-won knowledge not necessarily covered in the books. Instead of describing how to build another shoring tower, I focus on shoring locations, load amount considerations, and whether you need to shore at all.
Causes of Collapse
Building collapse can occur through natural (earthquakes, tornadoes, hurricanes, fires, and floods) or man-made (vehicular impact, construction accidents, and terrorist attacks) events. From a responder perspective, earthquakes and active fires necessitate the most active monitoring and constant vigilance. In an earthquake, aftershocks, sometimes as strong as the original quake, can and will occur while the would-be rescuers are working inside. Fires need continuous reevaluation of the structure since it is being affected in real time.
Besides those exceptions, the search and rescue team usually has one big advantage: The worst thing that was ever going to happen to that building has already occurred before you arrived. If a bomb went off inside of the building and it’s still standing, it’s unlikely that the additional weight of the rescuers and their gear will cause any significant movement or make much difference.
In one of its structural collapse courses, FEMA says it this way: “In evaluating, if a specific structure is at rest, one could state on the positive side that the structure that was moving had enough resistance to stop moving and achieve at least temporary stability. However, the damaged structure is difficult to assess, weaker, and more disorganized than the original.”
Once an object is moving, it takes more force to stop it from moving, right? Anyone who has caught a baseball with bare hands can attest to this. But, once that ball has been caught, it’s nothing to let it drop from one bare hand into the other. It’s the same ball, right? But the ball has no more movement (kinetic energy) and is no longer a threat to your hands or shins.
In the same way, if a partially collapsed building is still standing when you arrive on scene, something else has already arrested the collapse pattern. No one can guarantee it will remain stable; but as long as the conditions remain constant, experience has shown that the building will usually remain in place.
Construction Types
Just how concerned we should be about the likelihood of secondary collapse depends in part on what type of construction we’re looking at. Some types of construction, such as wood frame, will almost always have redundant load paths and ways to safely shed the load. If an elderly driver takes out six studs on an exterior wall, there are probably more studs on either side of them that are now pulling “double duty” and shouldering the additional load. On top of that, we know that conventional wood structures typically fail in a relatively slow and predictable manner. The structure, as a whole, bends before it breaks; some advanced warning is observable through sight or sound before it fails.
As a general rule, the more of the structure that is built on site (vs. being premanufactured, precast, or preengineered), the more resistance it will have to secondary collapse. These structures often have more redundancy or alternate means of support that can transfer load from damaged elements to intact elements and then down into the ground. This means that as a responder to the scene of a collapse, most search and rescue structural specialists would probably feel more comfortable in a wood or metal-frame building (assuming it is not on fire) or a cast-in-place concrete building with reinforced block [concrete masonry unit (CMU) masonry] walls than they would inside of a precast concrete parking garage or a big-box store with precast tilt-up walls.
(1) Photos by author.
From a structural collapse standpoint, the concern with most precast concrete components is that the connection points are highly vulnerable. If one connection fails, it is much more likely that nearby connections will fail as well, producing progressive and increasing instability. This is an example of secondary collapse and is more likely when there is less structural redundancy. These buildings are also more likely to fail in what is called a “brittle” collapse pattern—relatively quickly and with little warning.
Where to Start Searching
At the scene of a building collapse, get the utilities turned off as soon as possible. Stretch and charge a line even if no fire is visible. Once the undamaged portions of the building have been cleared, it usually makes sense to start searching for victims next to the parts of the building that are still standing because voids, or spaces between debris, are most often found on one side or another of a freestanding wall.1
Once these parts have been searched, the search teams should investigate the strong or sheltered parts of the building, which would be more likely to resist total collapse. This would include stairwells and foundation wall areas, which are more heavily reinforced. Victims may also have been able to shelter underneath or adjacent to large furniture or equipment.
Responders need to consider structural triage on the site of a large collapse or in a case where multiple buildings are down. The team must consider how hazardous it will be for rescuers to enter the area and how likely it is that live victims will be found. If the collapse occurred at a school at 0300 hours on a weekend, it’s very unlikely that students will be inside, and it wouldn’t make sense to put rescuers into a high-hazard situation. But, if a collapse occurred at an office building at 1400 hours on a weekday, it’s very likely that workers will be somewhere inside the wreckage.
Although the risk/reward analysis is relatively simple regarding when and where the collapse took place, it becomes more complicated when evaluating the relative hazards present in the building itself. In another section of its structural collapse course, FEMA states the problem this way:
“The problem of identifying, let alone properly evaluating, structural hazards is staggering. A well-trained engineer may, at best, be able to rate the risk of various hazards on some arbitrary scale like bad, very bad, and deadly.”
For firefighters, this means that even for specialists with a strong background in engineering and search and rescue, quantifying how risky it is to be working inside or adjacent to a partially damaged building is far from an exact science. Sometimes, the best you can do is figure out whether you’ll have minimal notice or zero notice before the building collapses.
A building with minimal structural damage is relatively easy to assess; the hazard is likely to be very minor (and is unlikely to have victims). But a fully collapsed building that was three stories tall but is now in scattered pieces on the ground also poses (relatively) little threat. There’s nowhere else for the building to go; everything that could come down has already come down. Hence, a total “pancake” collapse, in which all of the concrete slab floors have compressed down onto each other, is often considered a low-risk situation for rescuers even though it may look hazardous to the layperson.
Other collapse types often considered low risk would be a “soft” first-story collapse caused by an earthquake and one- and two-story wood structures (photos 1-2).
(2)
Medium-risk structures could include partially collapsed cast-in-place concrete buildings, a racked wood building three or more stories high, and freestanding (not fully supported) reinforced masonry or concrete walls. High-risk situations will usually include buildings with brittle failure modes such as tilt-up wall construction, unreinforced masonry, and precast concrete structures (photo 3).
(3)
You may run into trouble when trying to assess a partially collapsed building—one that falls between the two extremes of lightly damaged and completely destroyed. It becomes difficult for anyone to say with certainty exactly what’s going on and how close the building is to collapsing. To help reduce the risks, you can do the following: identify the structural hazards, avoid those hazards, or install shoring or shields to help mitigate the hazards.
Hazard Identification and Assessment
The types of hazards present will depend on the type of construction and the reasons it collapsed. Hazard and damage identification aids are contained in the U.S. Army Corps of Engineers Urban Search & Rescue Field Operations Guide (FOG),2 which you can download free, and in the rapid inspection guidelines from the Applied Technology Council (ATC) ATC 20-13 and ATC 45.4
Some experience and practical construction knowledge are needed when looking for hazards. For example, just because reinforced concrete is cracked doesn’t mean that it’s going to fall down. Cracks in sidewalks appear every day, yet pedestrians still dare to walk over them. If someone points out a crack, look inside and see if there’s mold and dirt. If so, that means that water has been getting in there for a long time and the crack is very old.
In addition, there are often hazards related to a structural collapse besides the threat of the building’s collapsing on you. Use a six-sided approach, and consider what’s above as well as below you. Small, nonstructural elements can be the greatest hazard; wind or aftershocks can easily knock them off the building. A pane of glass falling from a height may end your life just as effectively as an entire multistory building collapsing on you. Hazards don’t always come with a note and a sign saying, “I’m going to reach out and touch you.”
Also, remember that hazard assessment is not a one-time event. If you’re working in a building for an extended period, you will need to reassess the conditions on an ongoing basis.
Concepts of Shoring
In some cases where rescuers will be working in a damaged building for an extended time, it might make sense to install shoring inside or outside of the building. Sometimes, a technical rescue team may train so much on shoring that it seems as if installing shoring on a building is a foregone conclusion, but this is not always the case.
Consider that even the installation of shoring brings additional risk. Constructing and installing it necessitates that team members be inside a damaged building for a longer time, which may delay rescuers from locating and treating the wounded victims inside.
When thinking about how much shoring you need and whether you even want to install it, you can gain insights by looking at the FEMA definition of shoring: “Provides temporary support of the part of the structure that is required in order to conduct operations at a reduced risk.”
The three key words in that definition are temporary, required, and reduced. Shoring is a temporary solution. It helps reduce (not eliminate) the risk inside of a place where we don’t want to stay very long. We also are installing shoring only where we really think it’s required—it’s not just something we do to keep the team busy. If we can do our job effectively and remain outside of the most hazardous areas, that’s an even better option.
Shoring provides additional redundancy to a building in the form of alternate load paths where weight can be transferred into the ground. With few exceptions, rescue shoring is really never “holding up” a building—it’s already doing that on its own. Instead, the shoring is there so that if something shifts or moves while you’re inside, the shifting weight will be supported when it meets the shoring.
Think of it as an elderly grandfather having trouble walking on a sidewalk next to you. Would you hoist the guy up onto your shoulders and carry him? Not necessarily—he’s already carrying himself. He just needs a little steadying. Wouldn’t it make more sense to simply walk up next to him and offer him your arm to steady himself? Better yet, give him a cane so that even when he’s walking with one foot up in the air, he’ll still have two points of contact with the ground. You just gave Grandpa more stability and redundant points to transfer his weight down to the ground. That’s the right way to think about shoring.
Usually, start your shoring from the good areas and shore toward the bad areas from the outside in and from the bottom up for a multistory building. Don’t start at the bad areas on the inside and work your way out, and don’t start at the top and work your way down. Some teams will start with vertical spot shores or T-shores (Class 1 shores) and then expand to double Ts (Class 2 shores) or 3-D laced post shores (Class 3 shores). If you’re removing debris above shoring, remember to check and resnug or tighten the shoring so that it remains in constant contact with the structural elements it is supporting. Otherwise, the shoring may as well not be there at all.
Shoring should usually be placed adjacent to or underneath damaged floors, walls, beams, columns, and headers. The idea is that if the load-transferring element is damaged, place the shoring next to or under that element to provide another path for the building load (weight) to get to the ground. If possible, build the shoring outside of the collapse zone (1.5 × the height of the building) and walk it into place with as much prefabrication as is feasible.
Before you enter a building where structural collapse is a possibility, have your escape routes planned and reevaluate them while you are working. Remember that the safest route out may not be the most direct way.
If escape is not an option and if 3-D shoring such as laced post shores is present, consider attempting to squeeze inside one of them—sometimes you can leave out one of the low diagonals on the post shore to allow easier access to the inside. The load capacity of laced post shores is staggering; in fact, the laced post shores installed by rescue teams at the Alfred P. Murrah Federal Building collapse site survived the building’s eventual demolition.
If you arrive at a building where half of the structure is standing and the other half is on the ground, ask yourself, What caused the first half to come down? Why isn’t the second half also on the ground? Most importantly, what is stopping the second half from collapsing on top of me during our rescue operations? Hopefully, this article will help you to more carefully consider these questions the next time you’re faced with this situation.
Endnotes
1. O’Connell, John. (2011) Collapse Rescue for First Responders. Fire Engineering.
2. U.S. Army Corps of Engineers US&R Field Operations Guide, Seventh Edition. (November 2017). https://bit.ly/2v5MuAH.
3. Applied Technology Council. ATC-20, Building Safety Evaluation Forms and Placards. https://www.atcouncil.org/atc-20.
4. Applied Technology Council. ATC-45: Field Manual: Safety Evaluation of Buildings after Windstorms and Floods. https://www.atcouncil.org/atc-45.
ANDREW SCHRADER, P.E. is a licensed structural professional engineer, a certified general contractor, and a certified U.S. Army Corps of Engineers urban search and rescue structures specialist. He is attached to FL-TF8 and was deployed following Hurricane Hermine (2016), Hurricane Irma (2017), and Hurricane Michael (2018). He was a subject matter expert on structural collapse for the U.S. Department of State, assisting in the development of its post-earthquake search and rescue protocol for the Italian consulate. He is a guest lecturer on structural collapse. He has a bachelor’s and a master’s degree in civil structural engineering from the University of South Florida, Tampa. He is a certified continuing education instructor for the Florida State Fire Marshal’s Office.
