CONCRETE PARKING DECKS: CONSTRUCTION AND HAZARDS

BY BRIAN BENNETT, Ph.D.

Concrete parking decks pose some unique challenges to firefighters. These decks are becoming more common as our towns are developed and builders are forced to find more room for vehicle parking. The parking deck is the obvious solution for providing large vehicle capacity in a small footprint. These structures are built using concrete that usually is precast at a remote facility and assembled at the construction site. To understand these challenges, it is helpful to understand how concrete parking decks are built.

CONCRETE AS A BUILDING MATERIAL

Concrete is a cementitious material produced by a chemical reaction of Portland cement and water, to which inert materials (such as stone), called aggregates, are added. Concrete is favored for its low cost and ease to produce, high strength, durability, sound control, fire safety, and ability to carry large loads.

CONCRETE PROPERTIES

Good concrete results from proper handling of carefully controlled materials. In general, concrete has weak tensile strength (it cannot be stretched out), has poor shear strength (it will fail quickly if only one end is supported and a load is placed on the opposite end), and is very strong in compression (heavy loads can push down on properly supported sections). Concrete also provides good fire resis-tance. Since the structural members must resist loads through a combination of compression and tension forces, concrete is often reinforced with steel, which provides tension and shear strength.


(1) A beam being placed on columns. The beams are notched to accept the double Ts. (Photos by author.)

null


(2) A double T being lowered into place on the beams.

null


(3) The double Ts are notched to fit onto the beams and are held in place by their weight.

null

TYPES OF CONCRETE CONSTRUCTION

The two basic techniques used to build concrete structures are precast and cast in place.

Precast concrete construction involves pouring the concrete at a location remote from the construction site, moving it to the job site, and lifting it into place. Precast concrete includes plain concrete, reinforced concrete, and pretensioned concrete.

Cast-in-place concrete construction involves pouring the concrete into forms that are set up at the desired location. Cast-in-place concrete includes plain concrete, reinforced concrete, and post-tensioned concrete.

TYPES OF CONCRETE

Plain concrete, also known as unreinforced concrete, is simply poured into a form that will ultimately provide the shape desired upon hardening.

Reinforced concrete uses steel beams, mesh, or rebar to provide additional strength or load-bearing capacity and reduce sagging. Metal reinforcement is positioned in the forms in a spiral or square tie system. The concrete is then poured over the metal support and hardens. Reinforced concrete may also be tensioned.

Pretensioning of concrete is done at the point of manufacture. Steel strands are stretched in the forms to a predetermined tension. Concrete is then poured over the tensioned cables; as the concrete sets, it bonds to the tensioned steel. When the concrete reaches a specified strength, the tension is released from the strands. As the strands attempt to contract back to their original size, they compress the concrete, which provides additional strength and load-bearing capacity.

Post-tensioning of concrete is done at the job site. With this type of concrete system, high-strength steel tendons, in combination with steel reinforcing bars, are encased in plastic or paper tubing (to prevent bonding to the concrete when it is poured), embedded, and anchored in the concrete. When the concrete has acquired adequate strength, usually three or four days after pouring, the tendons are tensioned (stretched like rubber bands) and anchored at the ends, imposing a compression force on the concrete. The tendons remain stressed throughout the life of the structure. Post-tensioned concrete seeks to mitigate concrete’s natural weakness in tension by imposing a permanent compression load on the structural members. Post-tensioning allows for thinner structural members, increased load capacities, and less deflection.

  • Columns. Columns are vertical structures that hold up a load (the beams, girders, and double Ts). Concrete is used for columns because of its superior compression strength. Reinforced concrete can also carry a tensile load. Precast concrete columns combined with beams or load-bearing steel stud walls create the framing system. Columns can be manufactured round, square, or multisided in heights up to 60 feet.
  • Beams and girders. Beams and girders are horizontal structures that run between the columns and carry a load to the columns. The bottoms of beams have reinforcing rods for tensile strength (precast beams are usually marked at the top for proper positioning). Cantilever beams (those supported on only one end) have the tensioning at the top to support the load. Beams are available in square, rectangular, inverted T, L, I, and spandrel styles and are designed for both their heavy load-bearing capacity and long-span capability (up to 80 feet in length).
  • Double Ts. Double Ts, placed on top of columns and beams, make up the horizontal parking deck surface. Standard double Ts are available in 10- and 12-foot widths and 24- to 36-inch depths. The standard length is 60 feet, but custom designs can exceed that.
  • Spandrels. They are used as closures at the ends of the parking deck or within the deck to provide for traffic control. Spandrels may or may not be load bearing. Standard lengths up to 60 feet are available.

EXPANSION JOINTS

Expansion joints, where two pieces come together, are often sealed with a rubber, silicon, or expanding-foam sealant material. The joint seal also provides waterproofing. Joint detailing determines whether or not the expansion joint deflects. In cast-in-place construction, the joint often occurs over a beam, permitting horizontal but not vertical movement. In precast parking decks built with double Ts, there may be a shear key to prevent deflection of one double T relative to the adjacent one, or beams may be close enough to prohibit excessive movement. Some joint-sealing systems permit vertical and horizontal movements, and if there is no other reason (such as snow plowing) for not permitting vertical movement, some vertical movement may be permitted.


(4) Note the alignment plate on this column; the plate will be bolted onto the footer.

null


(5) A column in place with the alignment bolts cemented.

null


(6) Columns notched to accept a spandrel. The holes are for rods that will secure the spandrel to the column so it does not fall over.

null


(7) Spandrels in place at the edge of the parking deck.

null

BUILDING A PARKING DECK

A parking deck is put together like a giant house of cards. It relies on the component weight to hold itself together. Very few fasteners are found in concrete parking decks.

Construction of the parking deck starts with the footer. The footer is dug down into the ground to a firm surface. It is a poured concrete block that provides a foundation on which the structural elements (columns) will be set. Normally, several alignment rods are placed in the footer block, to provide proper orientation of the column.

Once the footer has set, the column is placed over the alignment rods and bolted down. Mortar is placed over the alignment rod/bolt assembly, and the hole is filled with soil.

The columns are poured with notches that will accept the beams, spandrels, or deck assemblies. Typically, the assemblies are matched with alignment rods similar to those found on the column footers. When the deck structure is joined up alongside a column, a stainless-steel connector plate is welded between the deck and the column.

Walls can be prepoured or poured in place. They may or may not be reinforced with rebar or mesh. Walls may be load bearing or non-load bearing. They are typically used as retaining walls to hold back soil from running onto the deck structure. Retaining walls may be fitted with tiebacks to provide vertical strength.

The decks themselves are the horizontal structures that make up the floors of the parking deck. They are used to transfer the loads to the columns or load-bearing walls (if so designed). The most common precast deck size is 12 feet by 60 feet, with a weight of 32 tons. The most common shape for the deck structure is a double T. The deck structures are lifted by crane and placed atop the columns and beams.

Spandrels are the enclosures found at the ends of the decks. They may be load bearing, in which case, they support part of the weight of the deck, usually at the ends. Nonload-bearing spandrels are used as closures or as barriers to direct the flow of traffic on the deck.

The decks are connected by small, stainless-steel slugs (typically 3 inches 2 3 inches or 4 inches 2 4 inches), which are welded together. The slugs are connected to the reinforcing material encased in the concrete and help to transmit the load over wide areas of the deck surface.

Stairs are usually precast or metal pans with concrete fill and simply dropped into place. Elevators on lower decks are usually hydraulically powered; elevators on higher decks can be electrically powered. Hydraulic elevators will have a piston pit; electric elevators will have a penthouse machinery room.

All joints and openings in the deck surface are usually sealed with mortar or caulking to prevent water from traveling from deck to deck. Some decks are provided with washouts, which direct storm water to floor drains. The floor drains pierce the deck and are piped through PVC or cast iron to a discharge area. These drains and piping systems provide a possible travel route between various levels of the deck for gasoline if a vehicle’s tank were to rupture.

Fire protection on parking decks varies from jurisdiction to jurisdiction. Almost all modern parking decks are equipped with standpipe systems, which are supplied through a fire department connection (such systems may have no water supply other than the FDC or may be tied to a public water supply). The deck may be divided into multiple zones, depending on the deck’s size. Risers and connections are normally found in the stairwells, although it is common to find connections on the deck itself on larger parking decks. Some parking decks are built with a sprinkler suppression system. Automatic sprinkler systems are usually dry type, especially in the Northeast.


(8) Flammable liquids from ruptured fuel tanks can travel to lower levels through penetrations in the deck.

null


(9) A parking deck attached to an office building.

null

HAZARDS OF PRECAST CONCRETE

Slabs are transported to the site and then lifted into position with a crane. During the construction process, floors are temporarily connected to the columns. Particular attention must be given to ensure that the concrete has properly cured before being placed into position and loaded. Loads need to be constantly monitored during the construction process to prevent overstressing components. Local failure could trigger a progressive (pancake type) collapse of all or large parts of the structure.

HAZARDS OF POST-TENSIONING

The weight of the concrete is transferred to the columns after tensioning. This situation presents a potential for catastrophic collapse if a column fails. The tendons and anchors used for post-tensioning are left exposed. They may become heat collectors in a fire. Therefore, it is highly recommended that anchors be fireproofed immediately after tensioning.

CUTTING THROUGH TENSIONED CONCRETE

Cables embedded in the concrete are under tension. Cutting a tensioned cable can cause severe injury to the firefighters as the tension is relieved and damage the concrete with a loss of load-bearing capability. Cutting tensioned cables can lead to a collapse of the structure. Therefore, do not cut through a concrete parking deck structure. Construction workers often X-ray the floor before cutting to locate the tensioned cables.

FIRE PROBLEMS IN CONCRETE PARKING DECKS

Concrete is inherently noncombustible. However, it can be destroyed by fire given sufficient time and fire load. The contents of the concrete structure is the most significant problem—the extensive use of plastics and large quantities of flammable liquids or flammable gases found in newer vehicles present a significant fire load with elevated temperatures. Concrete can be made to deliver different levels of fire resistance. Fireproofing of steel structural elements in a concrete parking deck is critical, and it is quite common to find sprayed-on insulation on key components.

Because of a lack of ventilation and the low ceilings (typically seven feet or less), hot, combustible gases are trapped within the parking deck and are unable to escape. This leads to an increased probability of rapid horizontal flame spread and flames’ rolling along the ceiling. It is common to find multiple vehicles involved because of the rapid flame spread.

Vertical fire spread most commonly occurs through penetrations in the floor. Therefore, penetrations must have adequate fire protection. Expansion joints—because of the likelihood of large, improperly sealed gaps between deck sections—readily transmit fire between floors. Also, shrinking concrete can develop cracks that allow the passage of fire.

CONCRETE BEHAVIOR IN FIRE

Under fire conditions, concrete resists compressive stresses and yet protects the tensile strength of the encased steel. Spalling, the failure of the concrete because of entrapped water’s rapid expansion as it turns to steam, may expose the steel reinforcement. Once the unprotected steel is exposed to fire conditions, collapse may occur rapidly as the steel loses its ability to carry the load. Concrete absorbs a great amount of heat. In the short term this is not a problem, but long-term heat exposure complicates firefighting efforts.

TYPES OF PARKING DECKS

There are three basic types of parking decks:

  • Stand-alone structure—a parking deck not connected to other structures.
  • Connected to a structure—a parking deck connected to another building, such as a shopping mall or an office building.
  • Contained within a structure—a parking deck built underneath or above other occupied spaces.

Parking decks can be above or below grade.

FIREFIGHTING OPERATIONS

Fighting a well-involved car fire in a concrete parking deck requires techniques commonly used to battle high-rise and basement fires. When confronting a car fire in a parking deck, expect heavy smoke conditions and high heat on the fire floor. Visibility will be significantly reduced, and SCBA may be required to make the trip up the stairwell, as most stairwells are open and rapidly fill with smoke. It is also quite common to have multiple vehicles involved in fire, partly because of the time it takes to find the fire and set up hoselines. If the fire is on the ground floor, it is often possible to use a preconnected hoseline to attack the fire by simply humping the hose over the retaining wall. In most cases, a chain-link security fence that stretches from the top of the wall to the ceiling may have to be cut to provide unobstructed access.

Probably the quickest and most effective way to attack a car fire would be similar to that of attacking a high-rise fire. The engine crew should use the interior stairs in the parking deck to travel to the fire floor. The engine company should ensure all of the standpipe hose outlet valves are closed as they walk up the stairs to ensure adequate water pressure is provided to the fire floor. Typical equipment brought in by the first-due engine includes a high-rise pack with a minimum of 200 feet of hoseline, forcible entry tools, hand lights, and radios. A thermal imaging camera (TIC) would be helpful in locating the seat of the fire in smoky conditions.

Once the engine company arrives at the landing of what it perceives to be the fire floor, the officer should proceed onto the deck with the TIC to ascertain the exact location of the fire. Once the fire has been located, the engine crew should proceed to hook up to the nearest standpipe and begin attacking the fire. The hoseline should be connected to the outlet on the fire floor (not the floor below as in high-rise fires). Hoselines may also be stretched vertically up the exterior of the parking deck structure to the fire floor. If a ladder company is responding to the fire, the aerial or tower can be elevated to the fire floor to provide a secondary means of ingress or egress if needed. Also, additional handlines can be stretched off the aerial device waterway, if it is so equipped. Parking deck fires require a minimum of two handlines.

APPARATUS PLACEMENT

Consider reverse laying a supply line from the standpipe siamese connections to the hydrant. It will take several minutes to fill a completely dry standpipe system with water, so it is important to charge the standpipe system as soon as possible so the engine crew is not waiting for water. One supply line should be charged to start filling the standpipe system; the second supply line should then be charged.


(10) Parking decks can be found on top of or below occupied spaces. In this building, the first several floors of the structure are parking deck topped with offices.

null


(11) A stand-alone parking deck.

null


(12) Stairways in parking decks are often open. Expect heavy smoke conditions.

null

PARKING DECK TACTICS

The engine crew should not connect to the standpipe until the exact location of the fire has been ascertained. A minimum of two handlines is required to extinguish the fire (one attack line, one backup line); larger streams (21/2-inch) will likely be required if multiple cars are involved.

PARKING DECK HAZARDS

Expect heavy smoke conditions and high heat on the fire floor. Because of the delay in getting to the seat of the fire, multiple cars may be involved. Flowing gasoline fires, maybe extending to different levels through floor penetrations, may be encountered if gasoline tanks rupture. Be aware of spalling concrete caused by high heat conditions. It is very easy to become disoriented and lost in the large open areas of parking decks. Finally, expect a lot of intense physical activity when encountering car fires in parking decks.

PARKING DECK COLLAPSE

If the concrete parking deck structure has been rattled, such as in an explosion or other major impact, it is possible that components may have shifted and may be prone to collapse. Always evaluate the need to enter parking decks in these situations. Parking decks may also be in danger of collapse because of improperly cured concrete or overloading caused by vehicle or ice/snow/rain water accumulations. Personnel should also be observant for concrete that has been removed from columns, beams, or the decks themselves, exposing the reinforcing steel. Concrete may have been removed over time, before the fire, as a result of having been struck by vehicles. Exposed steel will begin to lose its strength when the temperature reaches 1,0007F and may be in danger of collapse, depending on the load carried. As mentioned previously, do not cut through the concrete structures because of the likelihood of compromising steel reinforcement and possibly causing injury or collapse.

Parking decks are becoming more and more common. Preplan these types of structures in your jurisdiction, and develop the appropriate operating plans and guidelines.

BRIAN BENNETT, Ph.D., is a 21-year veteran of the fire service and is currently serving as deputy chief of the Iselin (NJ) Volunteer Fire Company #1. He has three engineering degrees and is a NJ-certified instructor level 2, fire inspector, and fire officer I.

No posts to display