ASSESSING STRUCTURAL COLLAPSE FROM ACTS OF TERROR

BY LARRY COLLINS

Incident commanders on the scene of major terrorist attacks and other emergencies resulting in structural damage or collapse are often faced with a “go” or “no-go” decision: Can they commit first responders to immediately enter the structure to begin life-saving operations with a reasonable assurance that what remains of the structure (or the collapse pile itself) won’t come down on their heads? Do they have the ability to evaluate the structural integrity with a reasonable degree of accuracy? Is the structure reasonably safe to enter without major stabilization, or will it require hours-long shoring operations?

A truly accurate assessment of the stability of damaged structures often requires the skill, experience, training, and knowledge of a certified structural engineer, who is prepared to perform a risk analysis and to make certain calculations about the weight of material, the status of key structural members, how the loads have been redistributed after the event, and the need for stabilization or evacuation.

Unfortunately, first responders typically don’t have those capabilities, and when lives are hanging in the balance, they don’t have the luxury of time to await the arrival of a structural engineer. Someone needs to make immediate decisions about entering to conduct firefighting, search and rescue, and other emergency operations.

WHY STRUCTURES FAIL: A PRIMER

All structures are erected with components and systems intended to resist the force of gravity. Significant damage to one or more of those components can result in a loss of stability. This may be due to horizontal offset or decreased vertical support (complicated by lateral loads from earthquakes or wind, vertical/lateral loads from explosions and impacts), which creates a moment of opportunity for gravity to overcome the structure’s resistance. Down comes the building.


(1) LAFD and LACoFD firefighters rescued a victim trapped for nine hours when a three-story reinforced concrete parking structure suffered multiple pancake collapsed during the Northridge Earthquake in 1994. (Photos by author.)

In the simplest terms, structure collapse is caused by the loss of stability that was originally designed into the building. At the instant of explosion, impact, ground shaking, or other conditions, the building changes shape. If the new shape of the structure was not part of the design (and thus is unable to carry the loads or resist the force of gravity), the building will then continue to change shape (collapse) under the force of gravity until a state of stability is reached.


(2) Damage to a multistory medical office building in the Northridge Earthquake.

The big question for firefighters and rescuers dispatched to the scene is this: Did the structure reach its final state, or is it likely to collapse further as gravity continues to pull at it? This is where well-trained and highly experienced rescue companies, Urban Search and Rescue (USAR) units, USAR Task Forces, and other rescue assets (augmented in some cases by structural engineers and other experts) “earn their keep,” because it is their job to help answer these questions and to guide the incident commander regarding what should be done next to protect the lives of those trapped or missing in the collapse and the first responders who will be called to get them.

HOW EXPLOSIVE FORCES AFFECT STRUCTURES

Few buildings are designed to withstand the internal and external pressures (or the initial uplift and overpressure) created by explosions, even if the duration is measured in milliseconds. When large structures are exposed to explosive forces, they move in reaction to the passing shock wave, but the vector of the net force is determined by the configuration of structures and terrain, the origin of the blast in relation to the structures, and other factors.


(3) In October 2001, 26 workers were pouring the reinforced concrete roof of this water tank in LA County when it suddenly collapsed, dropping 13 of them into this tangle of 3⁄4-inch rebar. Three men were impaled on rebar, requiring rescuers to use circular saws, reciprocating saws, and an electric rebar cutter to extricate them.

The initial blast wave may in one instant destroy key structural components, including columns, transfer beams, and other main supports. It may also destroy floor-wall connections, roof-wall connections, and steel I-beam welds. In the next instant, gravity takes over. The building will either redistribute the load or collapse.

We know that some terrorists place explosives to maximize the destruction of columns, each of which supports a significant portion of the structure. Very few structures are capable of redistributing the load when a column suddenly disappears. It is interesting to note that construction of some new government structures requires that the structural systems be capable of accommodating the loss of columns from explosions or sudden impact.


(4) Access to the 2001 roof collapse site was limited, and all rescuers and victims had to be moved with a personnel basket hoisted aloft beneath a crane.

Even if heavier columns remain intact, entire floors that provide loading and lateral bracing to keep the columns in place (and, because of their surface area, are more vulnerable to the blast wave) may be obliterated. Steel frames, beams, and columns may survive the initial blast wave, but the bracing and connections that hold them in place may be damaged or destroyed. Thus, they will fall apart when gravity takes over.

Wall and floor panels are particularly vulnerable to being blasted away, which further erodes the stability of structures and may lead to partial or total collapse. Lift pressures associated with explosions are especially dangerous to concrete slab buildings that rely on the constant downward pressure of gravity to hold them together. Concrete slabs in buildings of this design may “hinge up” because there is relatively little top reinforcing.

Engineers have identified critical blast points at columns where the slab-column joint is damaged by the uplift pressure, causing upward punching shear, followed by “a combination of gravity and positive overpressure that tends to drive the already damaged slab downward.” This is what brought down the Alfred P. Murrah Federal Building in the 1995 Oklahoma City Bombing. The same effect caused severe damage to the World Trade Center in the 1993 terrorist bombing.

Secondary explosions are a serious hazard to all first responders. Secondary explosions are common after dust explosions, gas explosions, and those involving stored explosive materials that may be set off by smaller detonations. Secondary terrorist attacks are becoming more common, and we should anticipate them whenever there is a hint that terrorism is involved.

IDENTIFY AND REPORT THE STRUCTURE COLLAPSE PATTERN

The 9/11 attacks demonstrated the insidious and unpredictable effect that fire can have on structural components. Nobody anticipated that the World Trade Center towers could fall in so short a time, and no one predicted that both towers could collapse with such totality and ferocity from the effects of fire (complicated by the impact of the hijacked airliners and the fire load from thousands of gallons of jet fuel).

Today, there is newfound recognition of fire as one of the most significant causes of structural failure. The new awareness should cause firefighters and other first responders to rethink old assumptions and to pay special attention to the potential for collapse in terrorist attacks that cause large fires.

The way in which a structure collapses is somewhat indicative of the stability of the remaining structure and the rubble pile (as well as survivability for victims trapped within). Structural collapse patterns are broadly classified according to what part of the structure has failed, how it fell, the configuration of the collapse, the characteristic void spaces that are left, and the hazards they present to firefighters and other rescuers.

Structural failures leave typical patterns of hidden void spaces that may be survivable. If we understand how these collapses occur and if we recognize where survivable void spaces are likely to be found, we can exploit the strengths and weaknesses of the collapsed buildings to find better, quicker, and safer ways to locate, reach, and extract trapped victims.

One school of thought says that structure collapses can be broadly characterized as Internal Collapse (interior walls or floors have failed, but the exterior walls and sometimes the roof remain intact), External Collapse (outside features such as fire escapes, scaffolding, roofing materials, chimneys, and outside walls have fallen), Total Collapse (the frame, walls, floors, and other components have failed), or combinations thereof. But there also are some specific collapse patterns about which rescuers should be familiar.

PANCAKE COLLAPSE

Pancaking of floors may result from several causes. Explosions can cause columns to disappear, resulting in a pancake collapse. An imposed load (i.e., a crane dropping a heavy beam or slab on the top floor during construction, a helicopter crashing on the roof, or floor or roof trusses failing during a major fire) may precipitate a pancake collapse. Earthquakes and other events can cause the failure of load-bearing walls or other supports on one or more floors, which can sometimes result in the pancaking of floors in the middle levels of a building (Mid-Story Collapse) or the pancaking of all the floors down to ground level.

Once a single floor falls onto the next floor, the imposed load causes a high potential for that next floor also to fail and for the remaining floors to fall in succession. This has occurred in earthquakes, explosions, construction accidents, and the 9/11 attacks on the Pentagon and the World Trade Center towers.


(5) Civilian USAR structural engineers monitoring a collapsed portion of the Pentagon for signs of impending secondary collapse. The engineers also advised rescuers about the safest way to stabilize the structure during Stage 3 (Void-Space Search) operations after the 9/11 attacks.

Pancake collapses tend to leave potentially survivable void spaces beneath strong structural and nonstructural members like furniture, desks, beds, cabinets, beams, and anything else that may prevent the floors from coming in direct contact with one another. Only a small space is required for a person to survive, and the smaller void spaces may provide safe harbor for children and infants.


(6) FEMA USAR Task Force personnel shoring and stabilizing sections of the Pentagon where the airplane blasted through, damaging and destroying many reinforced concrete columns. These stabilization operations continued around the clock for days, while in other areas rescuers probed void spaces looking for survivors and recovering victims.

The roof and upper floors are usually found collapsed in such a way as to give the impression that there is a complete structural failure (hence, pancake collapses are sometimes mistakenly referred to as total collapses). To the layman (and even to some first responders who have not been adequately trained or who are not experienced in such matters), it may appear to be hopeless for victims who are trapped inside. To the contrary, pancake collapses are notorious for harboring live victims for many days; these victims can be trapped in survivable void spaces, but hidden from the outside.


(7) Canine search team at the site of a major train derailment in the East Los Angeles area in 2003. Firefighters probed void spaces looking for survivors in a neighborhood buried beneath 30-foot piles of debris. Under the debris, homes were found in various patterns of collapse, including pancake, lean-to, and total collapse.

Firefighters and rescuers who see a pancake-style collapse should immediately think “live victims are likely to be found inside this collapse unless some other factor (like fire) has sealed their fates.” Survivable void spaces are more likely than not to be found inside pancake collapses. Therefore, round-the-clock, nonstop urban search and rescue efforts should be initiated until all potentially survivable void spaces have been searched by physical or technical means.

MID-STORY AND LEAN-TO COLLAPSES

Some structures have floors that are built to different specifications, and others have been renovated or added on in such a way that one or more mid-story floors have different levels of stiffness than those above and below. A typical example is a floor with short columns surrounded by floors with longer columns. The effect is similar to a pancake collapse, except the “pancaking” does not extend to the roof or to the ground; it is restricted to the middle levels of the building.

Explosions, impact disasters, fires, earthquakes, and other collapse events can cause a single bearing wall to collapse. These events can also cause the end of one or more beams to pull away from an outside wall and fall. In either case, the result is a “lean-to collapse,” a reference to the floor, roof, or beam finding itself “leaning” against the remaining bearing wall, debris, or even another building.

Imagine a “V” laid on its side. That’s what the void space below the floor or roof of a lean-to collapse looks like if you were to draw a cutaway diagram. The void spaces are characteristically found within the two lines that make the “V,” which places them below the sloped floor or roof. That’s where live victims are likely to be found. Even if the sloped floor is piled high in debris, there may be live victims in the survivable void space below. Victims may also be found in the debris that has piled up on top of the sloped floor, but their survivability often is reduced because of the direct contact with (and crushing by) the furniture and other debris.

OTHER TYPES OF COLLAPSES

Imagine a “cutaway” view of a floor, a roof, or a floor slab breaking in the middle (often after one or more interior support columns or walls have failed), with the center falling to the next lower level. The floor, the roof, or the floor slab should be left in a “V” shape, with all the furnishings and victims who were on that level piled in the middle of the “V.” Void spaces will be found on either side of the sloping walls of the newly-formed “V.”

V-Shaped Collapses: These are commonly caused when heavy dead loads or imposed loads cause the floor or roof to break in the middle. It is a common consequence of earthquakes, renovation mishaps, construction accidents, and severe fires (which cause steel columns to fail, dropping the floor or roof above).

Victims will typically be found in potentially survivable void spaces beneath the collapsed floor on either side of the sloping collapse. They also may be found trapped within and beneath the furniture and debris that has slid to the center on top of the collapse.

“Tent” or “A-Frame” Collapse: Imagine a tent with a sloped roof and a pole in the center to hold it off the ground, with vertical walls surrounding it. Now stack debris and victims on top of the tent, with victims trapped beneath the roof and between the walls. This is a “Tent” or “A-Frame” collapse. It is similar to the type of collapse that occurs when an earthquake, a fire, an explosion, or other force causes the floor beams to collapse near the outer walls, leaving an interior bearing wall to support the center of the floor. It is essentially the same situation you would find if two V-shaped collapses occurred side by side.

90-Degree Collapse: A 90-degree wall collapse is an obvious danger to passersby and rescuers alike. Although it is possible for external walls to collapse inward, it is probably more likely that they will fall outward, away from the building. An earthquake, an explosion, a sudden impact, or some other precipitating event may cause one or more walls to fall outward onto streets and sidewalks.

If the wall remains intact and falls over like a tree, this is known as a 90-degree collapse. The wall may fall its full height away from the building, or it may fall even farther (we should anticipate that walls will fall up to 11/2 times their height). Some walls, like those of tilt-up concrete buildings, may create a cushion of air just before impact with the ground, carrying them even farther from the building. Debris thrown from the collapsing wall may travel even farther.

Curtain-Fall Collapse: Imagine a masonry, brick, or stone wall one or more stories high. Where is the wall going to go if the event destroys the consolidation of the mortar or other supports holding them in place? Naturally, the result is often a huge pile of brick, masonry blocks, or stones, beneath which may be buried a number of victims. Even some curtain-wall-construction buildings can be “defaced” in an explosion or earthquake in such a way that the facing material buries victims and automobiles beneath huge piles of metal, glass, and other materials.

A related problem is glass falling from high-rise buildings during explosions and earthquakes. One estimate of potential damage from a major earthquake in Los Angeles County includes scenarios where victims would be buried on streets and sidewalks beneath 13-foot-high mounds of broken glass from high-rise buildings.

Cantilever Collapse: When the outer wall is destroyed, leaving the roof and/or upper floors dangling in thin air as unsupported members, the result is a cantilever collapse. Cantilever collapses have also been described as a pancake collapse with floors extending as unsupported planes. They are among the most dangerous and unpredictable collapse situations because a serious amount of weight may be suspended in midair, with overloaded and unsupported floors or the roof ready to snap and fall without warning or debris on the upper floors ready to cascade down if the floors begin to sag.

Cantilever collapses are notoriously difficult to assess and stabilize, and search and rescue operations below a cantilever collapse involve a great deal of risk for anyone near the potential fall zone. Trapped victims are in imminent danger of being further buried by cascading material or by the entire floor or roof coming down (potentially pulling other parts of the building down with it). Likewise, firefighters and rescuers are placed in harm’s way just trying to assess these collapses, search for victims, install shoring, treat trapped victims, and extract them from the rubble.

Inward/Outward Collapse: In this type of collapse, the outside walls fracture horizontally along the middle during a collapse event, and the wall collapses in large chunks. Sometimes the top half falls inward while the lower half falls outward, or vice versa. In either case, the result is a pile of debris at the base of the wall, sometimes projecting more than half the height of the wall away from the base.

TOTAL AND COMBINATION COLLAPSES

When shearwalls or foundations fail during a collapse event like an earthquake, heavy-floor buildings may fall over sideways by their full height, sometimes remaining intact but lying on their sides.

In some cases, damage or loss of a single column, transfer beam, or system of structural components can precipitate progressive failure of an entire building (or major portions of a building). The Oklahoma City bomb caused destruction of a major horizontal transfer beam near the ground floor and damage to several columns, which in turn contributed to the collapse of all nine floors on nearly half the structure’s footprint.

A Total Collapse indicates complete failure of the structural systems, with the building crumbling into an unconsolidated pile that’s difficult to characterize. As of this writing, the most lethal example of total structural collapse in history was the fire and impact-induced failure of the World Trade Center towers in New York City on September 11, 2001.

No single collapse has been as devastating to the occupants and those attempting to rescue them. In addition to the direct human toll to the victims and their families, there was the loss of so many of the world’s most experienced USAR practitioners and officers, some of whom helped develop, test, and establish the concepts on which this instruction is based. These practitioners were instrumental in conveying the message that rescue is an integral function of the fire service and that lives can be saved if fire departments are better prepared for disasters resulting from terrorism and other causes.

Some buildings fail in a way that leave multiple types of collapse, each with its own characteristic pattern of void spaces. The Pentagon collapse following the 9/11 attack is an example of a Combination Collapse. Because the areas surrounding the actual point where the airliner impacted the Pentagon were so sturdy, the collapse area fell in pancake style, but part of the pancake ended up leaning on the uncollapsed portions of this massive structure. Consequently, firefighters and federal USAR Task Forces were faced with victims trapped within a combination “lean-to/pancake” collapse.

Victims were sandwiched in void spaces that one would expect in a pancake collapse; however, in this case, the sloped angle of the collapse affected their final resting places. Some other victims were found where one would expect them in a “lean-to” collapse, but the void spaces were overlaid by multiple layers of pancaked slabs. Tragically, the intense, fuel-fed fires that accompanied the airliner impact also tranformed potentially survivable void spaces into nonsurvivable tombs.

The attack on the World Trade Center caused a total collapse of the twin towers, but the falling debris struck other buildings on the way down, causing a combination of collapse types over the entire disaster site. To make the situation worse, some of the most experienced collapse search and rescue experts, the people best able to assess the collapse after the towers fell, were themselves buried in the debris.

ASSESSMENT BY FIRST RESPONDERS

The structural assessment of a collapse should begin with an “eight-sided” size-up of the involved building(s) and the surrounding area. This includes

  • The roof.
  • The bottom/basement.
  • Side A.
  • Side B.
  • Side C.
  • Side D.
  • Rotary sweep of the aerial space around the building (falling hazards from adjacent structures and other aerial hazards).
  • Rotary sweep of the ground around the structure (looking for hazards like ruptured gas mains, broken water mains, railroad tracks, and other potential ground-level problems).

It’s important to determine what caused the collapse: Is there any evidence of a terrorist attack (bomb in vehicle, suicide bomber, improvised explosive device, large vehicle bomb/large vehicle improvised explosive device? Is there any evidence of NBC attack (e.g., victims down outside the collapse area)?

Rescuers need to be prepared to identify the type of construction. Is the structure reinforced concrete, pretensioned or post-tensioned slabs, wood frame, steel frame, tilt-up, or some other style?

Here are other assessment steps that must be taken:

  • Quickly identify the basic collapse pattern.
  • Determine if there are any signs of impending secondary collapse.
  • Determine the structure’s occupancy, which, in correlation with the time of day and the day of the week, indicates the likely number of victims in the structure at the time of collapse.
  • Estimate the number of potential occupants at the time of collapse.
  • Look for the presence of hazards like gas leaks and secondary devices.
  • Remember that structural stability of adjoining buildings might have been affected.
  • Assign at least one aerial truck (preferably an aerial platform) at the front of the affected building(s), to be raised for use as a Lookout platform. Consider designating a member to observe the entire collapse zone and surrounding areas for shifting walls, smoke or water appearing from cracks in walls, sagging roofs, and other signs of secondary collapse. The Lookout should have means to immediately notify everyone on the scene when signs of impending collapse or explosion are noted. This may include a hand-held radio, an air horn, a whistle, or some other signaling device. Consider aerials at different sides of the collapse zone to maximize the observation capabilities for Lookouts.
  • Using the standard USAR search markings (Figures 1, 2 and 3), mark the building as the process of size-up, recon, and search proceeds. Every building that has been searched should have the search marking spray-painted on the front to signify that the search has been completed.


Figure 1.

 


Figure 2.

 


Figure 3.

 

APPLYING LCES TO STRUCTURE ASSESSMENT OPERATIONS

Assessing structural integrity can place first responders in life-threatening positions due to proximity to (or entry into) damaged buildings. It is, therefore, necessary to adopt the same basic precautions that are taken before search and extrication operations are undertaken in earnest. This includes the use of LCES (Lookout, Communication, Escape Plan, Safe Zone).

The concept of LCES was developed for wildland firefighting operations and has been successfully used during structure collapse operations since it was implemented by several USAR Task Forces during urban search and rescue operations at the Oklahoma City Bombing in 1995. LCES was successfully applied during the Pentagon collapse rescue operations. Before committing personnel to the danger zone of a partially collapsed structure, the incident commander should always ensure that LCES has somehow been addressed and that all members are aware of it. The following is a brief review of LCES as it applies to structure collapse operations.

Lookout: Some member of the team (or another reliable responder like a firefighter, police officer, structural engineer, construction worker, or public works member) should be assigned to observe the structure for signs of impending secondary collapse, secondary explosion, fire, or other immediate life hazards. It may be necessary to place the Lookout in the basket of an aerial platform, on an aerial ladder, or on an adjacent building, to ensure he can view the entire collapse zone. It may be necessary to designate multiple Lookouts. It may be necessary to use theodolite plumb bobs and other tools that can indicate movement of a building toward secondary collapse.

Communication: Each collapse SAR operation should have a communication plan that includes designated radio channels for certain functions and teams but also other forms of communication like voice, whistles, air horns, and hand signals. All personnel operating in and around the collapse zone should be familiar with the communication plan, and each officer should ensure his charges are using the components of the plan appropriately.

Clear position designations are also critical to communications. The use of identification vests, properly marked helmets, armbands, or other identifiers should be mandatory. If none of these are available, the use of marker pens to hand-print designations on shirts, helmets, or even on arms is preferable to the chaos that occurs when everyone looks the same and no one can identify who’s in charge of what. For disaster operations, predesignated caches of armbands, helmets, and vests can assist in the process of communication. Communication also includes the use of clear and concise Incident Action Plans (IAP) that coincide with what’s actually happening in the collapsed structure.

Escape Plan: Every team and team member should have a clear idea of the primary and alternate escape routes (preferably before entering the collapse zone and certainly once they’re in it). Each officer should brief his team on escape routes during each entry and as conditions change. Escape routes should be the fastest, safest way out of the collapse zone (or to a safe refuge) in the event of a secondary collapse, a fire, a secondary explosion, flooding, or other unexpected event. If necessary, the escape route should be identified by fluorescent spray-paint markings, signs, fireline tape, lumber crayons, or other clearly identifiable methods.

During search and rescue operations following an earthquake that shook the Philippines in 1992, urban search and rescue specialists from Dade County (FL) and Fairfax County (VA) found that rapid escape through the corridors of an overturned hotel was unfeasible during aftershocks. To expedite egress from the collapsed building, they stacked mattresses outside windows. The agreed-on escape route was this: Rescuers would simply scramble to the designated windows and dive out, one at a time, each rescuer rolling off the mattresses just in time for the next team member to land safely.

Stacking mattresses as part of the escape route might seem comical to some who have never operated inside a collapsed building with aftershocks continuing to strike, but it was clearly a simple and workable plan-one that was successfully used to evacuate rescuers from the collapse zone numerous times over a period of several days. When faced with unusual conditions, it’s important for team leaders and officers to “think outside the box” when addressing the safety needs of their fellow rescuers.

Safe Zone: Team leaders and officers should identify at least one safe zone, an area safe from secondary collapse and other hazards, into which rescuers can retreat in the event of an aftershock, an explosion, a secondary collapse, or other unplanned event. The safe zone may be outside the building (and beyond the collapse zone, usually the same distance as the height of the building).

If escape to the outside will take too much time or is otherwise unfeasible, the safe zone may be within a stairwell or other fortified area within a building. Or, in some cases, a safe zone may be “constructed” within a damaged building, fortified by shoring or other methods. Everyone entering the collapse zone should be clearly aware of the safe zone(s). In the case of an unplanned event, the team leaders or officers should conduct “head counts” at the safe zones to ensure that all rescuers made it to safety (and to determine if some are in need of assistance).

LARRY COLLINS is a 26-year member of the Los Angeles County Fire Department (LACoFD) and a captain, USAR Specialist and paramedic for the past 15 years, assigned to Rescue Task Force 103 (including on the day of the JPL Incident), which responds to all technical rescues and multialarm fires across Los Angeles County (including rescues in train tunnels, caves, abandoned mines, and other underground environments). Collins is a Search Team Manager for the LACoFD’s FEMA USAR Task Force (California USAR Task Force 2) and is assigned as a USAR Specialist on the “Red” FEMA USAR Incident Support Team, with responses to Oklahoma City Bombing; the 9-11 Pentagon collapse; and Hurricanes Frances, Ivan, Dennis, Katrina, Rita, and Wilma. He sits on local, state, and national rescue and disaster planning committees and is author of the book series Rescue: A Guide to Urban Search and Technical Rescue (www.technicalrescueops.com), published by Pennwell.

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