PRESTRESSED HOLLOW-CORE CONCRETE PLANKS
HOWARD J. HILL
Prestressed concrete (hollow core) planks are being used more extensively as flooring and roofs with masonry and steel as the bearing walls. The hollow-core design uses less concrete than a poured floor and thus is economical. The prestressed concrete planks, prevalent in motel construction, are used to span long distances with much thinner sections while carrying heavier loads.
The planks are available in various sizes, ranging from four to 15 inches in depth, 40 to 96 inches in width, and 16 to 64 feet in length. The plank`s length and the required load it must carry determine its depth.
Made of hollow-core concrete, the planks have prestressing strand wire running through them. They are manufactured at the plant and transported to the job site ready for installation. The builder submits to the manufacturer a design that includes all major openings, sections, and details showing connections, weld plates, and support conditions.
Weld plates are required when a plank is resting on and is supported by a steel girder. These plates are molded into the plank. When the plank is in position, the plate is then welded onto the steel girder. Also included on the premanufacturing design drawing is the location of all dead, live, and other applicable loads.
The planks are set close together in their final position, and grout is placed in the keyways. All required supports should be in place before the plank is erected. The planks are tied into the bearing wall by placing an “L” reinforcement bar into the keyway before the grout is applied. This bar is also fastened into the bearing wall.
A major concern arises with regard to cutting openings in the planks. The prestressed wire molded into the concrete plank creates tension in the concrete and holds the concrete in compression. When this wire is cut, the concrete loses its strength and can collapse.
FIRE OPERATIONS
Although this building technique offers protection against fire spread because of the continuous cement ceiling, the potential for collapse is present and officers must consider this factor during fire operations.
Firefighters operating in buildings constructed with prestressed hollow-core concrete slabs should keep in mind three major considerations:
Never cut into concrete that has stressed cables. Once the cable inside the concrete is severed, the tensile strength in that area of the concrete is lost, and the concrete could fail. This collapse area might be localized to the area being cut. However, if the cut is near its supports and the end section completely fails, the entire slab section could fall to the floor below. Also, if the cut is large enough so that the slab collapses into itself, the entire slab could collapse into a “V” pattern. Anyone caught beneath this slab would be crushed by the weight of the concrete floor.
It has been reported that the prestressed hollow-core slab can be heated enough to cause severe spalling. This spalling can weaken areas of the slab and produce the same catastrophic effects mentioned above. Constantly monitor any highly heated ceiling area. If the cables inside the concrete are visible as a result of spalling, consider the area under that entire slab a collapse zone until the concrete has been inspected.
As reported by Francis L. Brannigan (The Ol` Professor, Fire Engineering, March 1999), prestressed concrete can totally lose its tensile strength at 800 degrees F. As noted, many of the ceilings are left unprotected and can be directly exposed to a fire burning in the room below. The heat from the fire could cause the plank to collapse completely.
The slabs are generally supported with masonry, concrete block, or concrete walls. When there is little ventilation and heavy fire, the inside area will reach extreme temperatures that will be very punishing to firefighters. To counter this, attack hoselines should be capable of delivering large volumes of water, and members must don full personal protective gear. Officers must be aware of the heat stress to which firefighters are being subjected and must relieve them accordingly.
ADVANTAGES AND DISADVANTAGES
An advantage of this type of construction is that it provides compartmentation, which will limit fire extension. At the same time, however, it increases heat retention in the compartment. Carbon monoxide will be present at higher levels and will remain in the fire area longer because of this lack of ventilation. During overhauling operations, constantly monitor the area with meters until a tolerable level of carbon monoxide is reached.
If a fire should occur during the construction phase, the connection points of the panels might not be properly secured, and any movement of the walls could cause the entire length of a slab resting on it to fall. On the plus side, there is less combustible construction material.
All building projects require that holes be opened for plumbing, electrical, and HVAC equipment. The builder is permitted to make small cuts (10 inches or less) in the precast planks. All other openings must be shown on the approved shop drawings and cut at the factory. Any field cuts in the planks require experienced workers using proper equipment. If the general contractor makes any unauthorized field cuts, structural problems can occur. Consider the possibility that there may be weakened areas in the plank or other structural problems when sizing up these buildings.
ADDITIONAL CONSTRUCTION FEATURES AND HAZARDS
Various methods, depending on the weight of the objects to be attached, are used to anchor and fasten items to these planks. For suspended ceilings, expansion anchors and toggle bolts are used. For heavy loads, through bolts with large plate washers and weld plates are needed.
The plank`s bottom surface is flat and uniform, allowing the ceiling area to be exposed. The ceiling may be painted or have tile applied. The top surface needs only minimal preparation before padding and carpeting can be directly applied. If tile is to be installed, a “cleavage membrane” is required to isolate the tile from the structural movements of the prestressed concrete floor.
Structural toppings can be applied to concrete of a specified thickness. This topping must be continuous from support to support and can distribute loads, add strength, and improve the floor system`s fire rating.
The planks can be cut angled and cantilevered to accommodate structural requirements such as balconies.
The prestressing force creates camber, the upward deflection created by the prestressed strand located below the center of gravity in the hollow-core plank section, which compresses the bottom more than the top. Variations in the concrete strength caused by exposure to the elements may also distort the shape of the planks and create stresses that may weaken the structure. Camber and deflection can be kept within acceptable tolerance if the builder uses a greater span-to-depth ratio–for example, if an eight-inch-thick plank is needed to carry the load, a 10-inch-thick plank would be used to offset the camber.
Collapse can also occur at the point where the plank attaches to a steel girder. If the girder distorts because of heat, the welded plates might pull loose and the planks will fall off the girder, possibly causing a substantial collapse.
A fully constructed building gives no indication that hollow-core prestressed concrete planks are present. It is incumbent on fire department members to inspect all new building construction in their area to identify building techniques that will affect firefighting operations.
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(1) Hollow-core prestressed concrete slabs, cut to the builder`s specifications, are delivered to the job site. (Photos by Jerry Tracy.)
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(2) Slabs are often supported by concrete block walls. They are tied into the wall with steel reinforcing rods, which are then cemented in place.
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(3) Slabs can be cut to any shape and can be supported by steel girders.
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(4) When the slabs are supported by steel girders, they are constructed with steel plates embedded into the slab. These plates are welded onto the steel girders at the job site.
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(5) Sometimes a builder will cut holes into the hollow-core prestressed slabs at the job site. This can result in structural defects in the slabs.
HOWARD J. HILL is a battalion chief in the Fire Department of New York, where he has served for more than 27 years. He has written and lectured on a variety of operations and technical topics.
