The warehouse requires an open floor area and a large elevated porch to facilitate moving inventory in and out of storage racks and trucks at the loading dock. Today, robots are replacing forklifts to move inventory in large e-commerce fulfillment centers. Floor areas without load-bearing walls and a minimum number of columns require a roof supported by some type of truss.

Fire recruits must grasp the huge difference between "fire-resistant" and "non-combustible" classifications early in their fire academy training. Some warehouse truss roofs are non-combustible, but no common warehouse roof can be considered fire-resistant. The floors, walls, and roofs of buildings classified as "fire-resistant" or "type 1" by building codes can withstand collapse and fire penetration for up to two to four hours.

Refractory structural components are tested in large furnaces in test facilities such as Underwriters Laboratories. Although the test performance does not guarantee that the refractory structural components will perform similarly under actual fire conditions, they are unlikely to collapse.

Refractory buildings generally use reinforced concrete structural members. When steel is used in fire-resistant construction, it must be protected by wrapping it in concrete or enclosing it with a fire barrier. Later, this article will study the prefabricated "double T" warehouse roof, which is a perfect example of a concrete roof structure that is far from fire resistance and prone to premature collapse.

Almost every American city and town’s oldest warehouse has roofs supported by trusses, including bowstring trusses; many of these buildings still stand today. Bowstring trusses can be made entirely of wood or steel, but most are a mixture of wooden strings and webs tied together with metal fasteners. The bowstring truss roof has a typical arched profile, but this may be obscured by a large railing. You can determine the height of the railing by the position of the roof drain hole on the roof line.

The bowstring truss roof is the killer of firefighters. Due to the collapse of the bowstring truss roof, multiple firefighter deaths (LODD) have occurred. Students of the fire department must learn the lessons of tragic fires, such as Waldbaum's grocery store (1978, Brooklyn, New York) and Hackensack Ford dealership (1988, Hackensack, New Jersey).

The height between the upper and lower chords of the bowstring truss can reach several feet; this space, a "truss attic", is an attractive storage space. When the floor is constructed in a truss attic for storage, the risk of collapse of the bowstring truss roof increases significantly. This formed a huge attic space, which was difficult for firefighters to enter, direct the flow of water, and determine the location and extent of the fire burning above them. In addition, heavy storage places dangerous, undesigned loads on the trusses, which can accelerate their collapse.

When a fire occurs on the bowstring roof assembly, firefighters must use the main stream running outside the collapsed area to conduct a defensive attack, and the distance from the building must be at least 1.5 times the height of the railing. The bowstring truss project requires this large collapse area. Like any truss, a bowstring truss has an upper chord in compression and a lower chord in tension.

Consider archery bows. Cut the bowstring under considerable tension and the bow will be forcibly straightened. Similarly, when the lower chord of the bowstring truss fails, the upper chord tends to straighten and exert considerable lateral thrust on the outer wall, throwing the bricks a considerable distance greater than its vertical height.

When bowstring truss roofs are inclined at the front and rear of a building (such as a hip roof), they present an additional risk of collapse. The hip rafters support the sloped part of the bowstring roof, which spans the front and rear trusses and front and rear guardrails. The collapse of the front and rear trusses can exert considerable leverage on the front and rear walls, launching bricks at a distance of up to twice the height.

When establishing a collapse zone, consider the wires and utility poles that will collapse when the wall collapses. If the width of the street or alley does not allow firefighters and equipment to maintain a sufficient safe distance from the fire-fighting building, they must take a flank position outside the brick falling range. Therefore, aerial equipment should be placed in a "corner safe" area.

Since the late 1940s, flat gypsum deck warehouse roofs have become common in warehouses and supermarkets. What firefighters might call "lightweight concrete" is probably gypsum deck. The gypsum roof is composed of gypsum cement, which can be cast into a panel or poured on a gypsum board or corrugated metal deck. For cast gypsum roof decks, steel wire mesh provides tensile strength.

The pouring gypsum deck structure is conceptually similar to a suspended ceiling. Just as the ceiling panels are placed on the flanges of the inverted T-shaped members, the gypsum formwork and gypsum panels for the cast gypsum roof are placed on the flanges of the bulb T-shaped purlins, usually supported at right angles by unprotected steel joists. The top of the column of the bulb T is wider than the bottom, just like the outline of the bulb. This bulbous shape helps limit the rising force of the roof deck against wind. Photo 1 shows the underside of the gypsum roof; unprotected steel beams support the beam joists, which appear to be spaced four feet apart in the center. The strip joists support the bulbous T-shaped purlins, separated by two feet in between, to support the plaster formwork. Photograph 2 shows a cross-section of the gypsum roof; visible is the bulb T, orange steel joists, gypsum board formwork, approximately 2.5 inches of cast plaster, and 1.5 inches of the top layer of the combined roof covering. Photo 3 is another cross section, but instead of using a plaster template, the plaster is poured three inches deep on a corrugated metal plate. The gypsum is covered by foam insulation, rigid "recycled boards" to protect the insulation, and a "cover board" roof with a granular surface.

(1) Unless otherwise stated, photographs of Eric Goodman.

To understand the extreme risks faced by firefighters working on or under a gypsum deck roof, consider what happens to a drywall with gypsum board in a fire. When gypsum is heated, it loses moisture and becomes brittle. Any firefighter who pulls through a water-saturated gypsum board ceiling knows that it takes very little effort to lower the entire 4 x 8 foot section.

The gypsum deck roof behaves similarly when exposed to fire and water. The heat from the underside of the gypsum deck roof absorbs moisture, making it brittle and unable to support the weight of firefighters. In addition, steel beams, beam joists and spherical T-shaped purlins will begin to lose strength, warp and deform at approximately 800°F.

Most professional roofing contractors are proud of their excellent craftsmanship using high-quality materials. This is for roofers who "fly at night" who started a roof fire and then lied about how the fire started. Firefighters reacted when a fire broke out in the attic or attic and learned that workers were repairing holes in the roof a few hours ago; this is not a coincidence-it is a torch!

Low-density fiberboard roof insulation is prone to smoldering; therefore, a fire on the roof or attic may not be detected until a few hours after the roofer has left for a day. Any time there is a fire and the roofer is or is working there, he must be very suspicious! If they are using asphalt or asphalt-based patches and there is no tar pot on the job site, how can they use the patch to mop the roof without molten asphalt? Most likely, they used a flashlight.

Don’t expect to find a flashlight on the job site, which is usually fueled by a 20-pound liquefied propane cylinder, and don’t expect workers to tell the truth when asked if they use a flashlight. Before the first fire brigade arrived at the scene, the torch had already disappeared.

Leaking gypsum roofs are insidious, and they are not water-saturated from the outside and cannot support the weight of firefighters. As water leaks from the roof covering, over time, there will be a lot of water stains on the lower side of the roof, but the upper side looks completely normal.

Don't assume that the steel mesh can support the weight of the firefighter. Moisture will corrode the steel mesh, bringing it close to disintegration (photo 4).

Firefighters should not work on or under the gypsum deck roof exposed to fire conditions. The Fire Department of New York (FDNY) is very clear in its directives. If the firefighters of the trapezoidal company performing vertical ventilation notice that their rotating saw blades throw out cement powder, they must send an emergency message informing them that there may be a plaster deck and leave the roof quickly.

The warehouse roof terrace is insulated to prevent the passage of solar heat in hot weather and the loss of heat in cold weather. Old roofs are usually insulated with low-density particleboard, which consists of wood chips or bagasse (residue from milling sugarcane). Low-density fiberboard is notorious for smoldering, so it is easy to re-ignite. Modern roofs are usually insulated with thermoplastic foam panels (such as polyisocyanurate). The covering is installed to make the roof waterproof, resistant to ultraviolet (UV) light, and prevent damage when walking.

The following inspections of roof coverings are just a general overview; the composition of roof coverings varies from region to region, depending on the climate. This is based on my experience and may not be applicable everywhere. Therefore, firefighters must be familiar with the roof structure, insulation and covering composition of their area. Generally, roof covering systems are either constructed or membranes.

When the roofing contractor towed the "tar" kettle trailer to the job site, he planned to install a modular or "tar and gravel" roof. In a modular roof system, a layer of molten bitumen (incorrectly called tar) is "hot dragged" on and in the middle of the bitumen-impregnated felt layer. The roof surface is usually covered with gravel ballast. Roofing contractors can use thick felt cover slabs impregnated with granular materials such as slate or perlite instead of gravel for protection.

Install membrane roof coverings during the construction or renovation of old warehouses. The membrane can be made of synthetic rubber—for example, ethylene-propylene-diene-monomer (EPDM); plastic-based [thermoplastic olefin (TPO)]; or asphalt-based, such as modified asphalt. Granular materials are embedded in the surface, similar to a composite roof used for protection.

The diaphragm comes in the form of a large roll. EPDM and TPO membranes consist of a single layer; modified asphalt is usually spread on a substrate reinforced with glass fibers. The single-layer film is fixed by the weight of gravel, mechanical fasteners or "chemical welding" with glue or adhesive. A common method of using modified asphalt is to glue it to the roof and torch the joints (see the sidebar "This is a torch!") The flammability of roof coverings varies depending on the chemical composition; some are easy to melt and burn , While others will extinguish by themselves when the heat source is removed.

Metal deck roof fire and collapse

The roofs of today's modern large shops and warehouses consist of corrugated metal decks (photo 5). In addition, when the gypsum roof cannot be repaired, it is usually replaced with a metal roof. Similar to gypsum roofs, metal deck roofs are usually supported by unprotected steel beams and beam joists. Steel structural elements are non-combustible, but far from fire-resistant, and will fail after being exposed to fire for "a few minutes". How many minutes is "what"? Some firefighters apply the “20-minute rule” to unprotected steel structural components—that is, if exposed to fire for more than 20 minutes, the roof assembly is in danger of collapsing.

The 20-minute rule is a decision guide, but it is definitely not absolute. Factors affecting the collapse time include the volume and intensity of the fire and the static load on the roof, such as HVAC equipment. Perhaps the most important factor in determining the time to collapse or preventing collapse is the ability of the firefighter to cool the underside of the roof assembly.

Acre of land fire

Some of the biggest dollar-loss fires in history were caused by self-propagating metal deck roof fires. Huge industrial and storage buildings, measured in acres under one roof, have been destroyed by the metal deck roof fire. The fire spread rapidly and exceeded the firefighters' ability to extinguish fire.

Operating in a public warehouse

If the metal deck roof is not combustible, how does it spread the fire? The fire heats the underside of the metal deck roof as if it were a frying pan on a stove. The heat liquefies and evaporates the combustible mopping asphalt layer and roof covering. The expanding steam passes through the partially overlapping seams of the metal deck, ignites, and can spread like wildfire under the roof. Just like wildfire, when the flame rolls over the underside of the roof, the fire will preheat the roof before the flame front (photo 6). The antidote to a metal deck roof fire is the same as the method to prevent collapse: cooling the underside of the roof.

Don't be fooled by the appearance of the warehouse roof built with precast concrete double Ts (photo 7). They may look like heavy, strong concrete members, but they are far from fire-resistant. Similar to trusses, concrete Ts has elements of tension and compression. In the double T, the stranded cables that extend along the length of the member are tensioned to thousands of pounds before the concrete is poured into the formwork in the precast factory. Therefore, the precast Ts is considered "pre-stretched" (photo 8). The tension cable exerts a compressive force on the concrete, enabling the component to span without the support of a column or load-bearing wall in its center.

Don't confuse pre-stressed and post-tensioned concrete; both gain strength from cables under tension. However, the cables in the precast Ts are stretched before the concrete is formed and cured. The cables in post-tensioned concrete are tensioned after the concrete is poured and cured for a specified time. Another difference between precast concrete and post-tensioned concrete is that the cables in the pre-stressed Ts are in close contact with the concrete, while the cables in the post-tensioned concrete are in the plastic sheath.

The steel cables in the prefabricated double T are susceptible to heat, which will corrode and peel off the concrete on the underside of the component. It does not require large and intense fires or long burning fires to peel off concrete and expose cables. Once the cables are heated, they lose tension, causing the double T to lose the ability to span between its supporting members, and the double T starts to sag.

Double T can be pulled out of their load-bearing surface or wall socket without much sagging, because building codes usually only need four inches from the end of the double T to rest on a beam, pilaster or socket on a load-bearing wall . Experience has shown that the steel embedded in the double T-end is welded to the steel embedded in the supporting member, and it is usually not sufficient to restrain the member to prevent it from collapsing.

When there are signs that the fire in the warehouse is small or controlled by sprinklers, it is in the best interest of the building owner to allow firefighters to enter through the revolving door and open the elevated door from the inside. Commercial overhead doors usually do not have any locks or latches on the outside; they are lowered and fixed inside by the occupants who leave the building through the revolving door. In a severe warehouse fire, overhead doors have the following tactical advantages over swing doors:

The main flow running through the elevated doorway can cool the roof structure, possibly preventing collapse. By deflecting a powerful stream of water from under the roof, a fire can occur in an area too dangerous to enter by hand (photo 9).

There is a plan A, B, C...

Various forced entry techniques may not be effective every time, every door, or every area. If plan A does not work, the company officials who instructed the forced entry operation should have alternative technology and know when to move to plan B, plan C, and so on. When possible, company managers should not operate these tools; he must monitor and judge the effectiveness of actions.

Don't wake up the sleeping dragon

Opening the elevated door may endanger the firefighters, who enter the warehouse through the revolving door before opening the elevated door. Consider fires in industrial parks that are largely empty at night and on weekends. Without a fire detection system, the fire can burn for several hours, consume available oxygen and cause ventilation to be controlled or restricted. If a firefighter enters through a revolving door, it may take 100 to 200 seconds to allow enough air to flow into the building and exacerbate an oxygen-deficient fire. If the elevated door opens and a large amount of air flows in, awakening the "sleeping dragon", that is, a fire with restricted ventilation, please consider the impact on people a few feet away from the escape route.

Before opening the elevated door, carefully consider evacuation of people entering through the revolving door, or preferably by initially entering through the elevated door, to reduce their exposure to hostile fire conditions. If the air flows through the doorway overhead and causes the fire to come back to life, so be it, but personnel will not be caught in a hostile fire deep in the building.

Fire researchers recommend not delaying the fire attack while establishing a continuous water supply. This is an effective recommendation for residential fires, but it does not apply to fires in enclosed commercial buildings. Firefighters who forcibly enter a closed commercial building should expect that when they breathe oxygen into it, the fire will intensify, and it is best to prepare adequate, redundant water supply and ready-to-flow water source equipment.

On the elevated doors, firefighters perform primary and secondary cuts. The main incision is a small opening through which the nozzle can be directly aimed at the burning fire behind the door, and locks, latches and lifting chains can be contacted. They allow the door to be raised largely intact, minimizing property damage and maximizing the size of the opening.

In hurricane-prone areas, a small main opening in the elevated section door allows firefighters to enter the interior to determine the size of the horizontal wind support, which is a key factor in choosing the method of cutting the secondary opening. When the lock and latch cannot be operated or the door cannot be raised due to fire damage, the overhead door needs to be cut a second time.

Overhead scrolling. These doors consist of interlocking slats that move up and down on rails fixed on both sides of the doorway, and bottom rods of channel steel or angle iron. The slats are hinged to each other and rolled into an elevated drum. If the building code requires the doors to be wind resistant, they are equipped with wind locks/pull tabs riveted at the end of each or every other slat. The wind lock engages with a channel in the track, which resists the force of the wind that bends the middle of the door and pulls the slats out of the track. The wind lock also prevents firefighters from pulling the slats out of the track after cutting the triangular openings unless they cut the ends of the slats that hold the wind locks.

Large and heavy overhead rolling doors are usually mechanically operated by chain hoists or electric motors. Heavy-duty overhead doors are assisted by powerful torsion springs in overhead rollers, which can balance the weight of the door. When the door is operated by a chain hoist, a continuous chain loop extends from a gear on the overhead drum to a few feet above the ground, where it is usually secured with a padlock.

Firefighters should inspect overhead doors during pre-fire planning; they may find that the lifting chains in their area may be concentrated on one side. For example, in South Florida, the chain is usually on the right.

Plan A. The effective plan A for mandatory overhead rolling doors is to cut vertical slices along the track on one side of the door, where the lifting chain is believed to be located (photo 10).

The jigsaw is a natural gyroscope, which is relatively easy to operate during vertical cutting; in addition, the gravity is on the operator's side. Continue cutting the slats until the cut part of the door can be pushed in sufficiently so that the firefighter can use bolt cutters to cut through the padlock holding the lifting chain, not the chain itself, and operate it to lift the door (photo 11).

The door can also be secured by L-shaped sliding latches on the bottom rod, which engage with holes in the rail. If no chain is found, enlarge the cut and push in the cut part so that the firefighter can enter and lift the door from the inside if conditions permit, or cut vertically at the other end of the door again.

Option B. You may not be able to lift the overhead rolling door through its chain hoist mechanism due to the following reasons:

When the manual lift door does not work, plan B will depend on the condition of the door. See "Rotary Saw: Lessons Learned from Cutting Overhead Doors" (Training Notebook, Fire Engineering, May 2020). The author Stephen F. Shaw Jr. is correct: pulling out slats from elevated rolling doors is not always easy, and it may be too labor-intensive and time-consuming to be practical.

If the door is in good condition, make a vertical cut at the end of the door, as high as possible to its bottom crossbar, which will effectively cut each slat from its wind lock. Large overhead doors or doors that are deeply embedded in the doorway will require a third vertical cut in the center of the door to reduce the overall length of the slats and make it easier to slide.

Now use the locking pliers to clamp the slats while hitting the other end with a flat axe (photo 12). Another way to use locking pliers is to insert the spiked end of Halligan with an axe, mallet, or baseball swing, and then hit Halligan. This is an effective way to slide difficult slats. This method is explained in "Forced Entry Technology of Rolling Safety Doors" (Fire Engineering, March 2012). Captain Daniel M. Troxell of the Washington (DC) Fire Department suggests that you must push the spikes in before making any cuts on the door.

Caution: When pulling out the slats from the large and heavy overhead roller shutter door, remember that the strong spring in the overhead roller can balance the weight of the door. Pulling out the slats from the overhead rolling door effectively reduces its weight and changes the balance between tension and door weight, which is beneficial to the spring. A door separated from its bottom rail will suddenly rush into its drum with great force, just like an old curtain, rotating in the drum until the spring tension disappears. Then, just like suddenly, the door can fall again. When cutting and pulling the slats from the overhead roller shutter door, it is expected that the door may suddenly rise at any time and prepare to go back a few seconds, waiting for it to fall.

The heat can cause the balance spring to lose tension. In this case, the tension/weight balance will move in a direction that favors the door, causing the door to close unexpectedly behind the firefighters, trapping them in the building. If there is not enough spring tension, the goalkeeper is too heavy to be manually lifted to rescue trapped firefighters. When any type of overhead doors may be heated, use a spear rod (preferably long) to fix them open. This will reduce the amount of free fall of the door before the bottom of the door comes into contact with the spear rod. As an extra measure of protection, a Halligan was vertically wedged on the track at the bottom of the doorway. It is hoped that this will give firefighters enough room to crawl under the door if the spear rod cannot restrain it.

Plan C. The proven method of cutting a large triangular opening in an overhead rolling door is still a viable alternative to the method described above, and may be Plan A, when the fire is behind the door and needs to be opened quickly when the nozzle is inserted or when the heat, Corrosion or damage (for example, from a forklift) prevents the slats from sliding freely out of the door. Triangular cut or inverted V cut basically requires two cuts-one vertical and one diagonal-to overlap on top. However, it is easier to cut the triangular opening by making three cuts:

Top curtain. Their operation is the same as that of overhead rolling doors, but without slats. The curtain door is composed of metal corrugated plates pressed together to form a continuous plate that is strong and flexible and can be rolled up in a roller overhead. Scheme A for forced rolling doors is the same as for forced overhead rolling doors: a vertical slice is cut along its track at the end of the door, where it is believed that the hoisting chain will enter and operate the chain crane to lift the door. If the door cannot be raised, plan B is to try to cut its bottom rail. There are two techniques for cutting the bottom bars of top curtains and rolling doors: (1) Cut a triangle at the bottom of the door, enough to insert saw blades and guards (photo 14). (2) Place the Halligan lamp parallel 3 to 4 inches from the door. A halligan with adz and spikes touching the ground is a good fulcrum, allowing the steel roof hook to exert considerable leverage to raise the bottom pole; when the bottom rebar is 1 to 2 inches in front of the floor, this is usually necessary. If the bottom railing of the door can be cut off, make a second cut on the door as high as possible horizontally to complete the "barn door cut" so that the cut part of the door can be hinged (photo 15).

Cutting horizontally above the head with a rotary saw is hard work and requires the strength of the upper body. Even the most powerful saw operator will start to get tired. Often the first sign of fatigue is that the saw operator binds the saw blade and is unable to maintain the cutting height. Now, the company officials in charge of operations must step up their shift leadership. Don't expect a tired saw operator to stop cutting and hand the saw to another operator willingly. The firefighter who operates the saw is not only physically involved in the operation, but also has his ego in the game.

Overhead section. These warehouse doors have hinged panels, and these slats are mounted on rollers fixed on rails on both sides of the doorway. Sectional doors are reinforced by vertical members called lintels and horizontal wind bracing. The composite doors built to withstand strong winds are very heavy, so they are balanced by a torsion spring on the shaft above the door. The cables wound on the pulleys at both ends of the shaft extend down to the bottom of the door, where they are usually secured with aluminum cable clamps. Therefore, when the torsion spring is heated, not only will the overhead section door close, but the aluminum cable clamp will also fail, causing the heavy door to fall freely like a guillotine blade. In addition, as with overhead roller shutters, cutting and removing parts of segmented doors can reduce the weight of the doors; as a result, they can suddenly rise with considerable force.

Commercial sectional doors are usually secured with L-shaped sliding latches that engage holes in the track. The latch assembly is fixed on the frame of the door with metal plate screws, usually located at one or both ends of the second one at the bottom. Therefore, Option A of the forced sectional door is to cut a triangular opening in the second part from the bottom. The opening should be large enough to reach in and throw out the latch (photo 16). If you use cheap padlocks to secure the latches to the rail, widen the opening and cut them with bolt cutters. In photo 17, the firefighters were unable to lift the combination door or cut the opening of the barn door. By default, they use box cuts-two vertical cuts at each end and one horizontal cut across the door.

(17) The photo was taken by Robert Hernandez and provided by Miami-Dade (FL) Fire and Rescue.

"Skinned" wind-resistant elevated sectional doors. In Photo 18, the firefighter reached into the triangular primary cut and found that the horizontal wind support was large, which meant that the overall thickness of the door exceeded 5 inches, which is the maximum depth of a 14-inch blade rotary saw. Reaching into this opening also allows the firefighter to find the door frame so that he can avoid cutting them while cutting the door skin. In this case, immediately notify the incident commander of the delay in cutting the large opening; the door must be "skinned" first.

Two parallel vertical cuts are needed to skin the door so that the space for the saw blade is completely supported by the wind (photo 19). The first vertical cut is no more than 6 inches from the edge of the door to avoid cutting thick steel hinges. Tip: Whenever the saw operator encounters difficulties in cutting the skin of the elevated combination door, the blade is likely to cut the hinge or lintel. If the saw operator feels resistance and it is difficult to maintain the saw revolutions per minute, he should move a few inches and try another cut. In Photo 20, the roof hook and Halligan are used as levers and fulcrums to lift the bottom of the door to be cut.

The fireman opened the "barn door". If the firefighter cannot completely cut through the bottom of the combined door, then box cutting will be an effective alternative.

Fire companies must go to their area and be familiar with the local door components to determine which method is suitable for them from the many available methods. In addition, please consult with overhead door contractors, they are a valuable source of information on door design construction and forced entry techniques.

Author's note: Special thanks to Stephen F. Shaw Jr., Assistant Director of Fire and Rescue, Fort Lauderdale (Florida) for technical advice.

BILL GUSTIN is a veteran with 48 years of firefighting experience and a captain of the Miami-Dade (Florida) fire/rescue department. He started his firefighting career in the Chicago area and was the chief lecturer of the official development program for his department. He teaches tactics and corporate officer training programs throughout North America. He is a member of the advisory board of Fire Engineering and FDIC International.

Bill Gustin will present the "Operation of Newly Promoted Officials" on the FDIC from 8:00 am to 12:00 pm on Tuesday, August 3, and from 10:30 am to 12:15 pm on Thursday, August 5 "2021 Indianapolis International Championships.