Article and photos by Gregory Havel
This type of concrete is reinforced by steel wire tendons that are placed inside the forms and the tendons are tensioned before the forms are filled with concrete. The concrete bonds to the surface of the tendons as it sets and cures. After the concrete has cured sufficiently, the tension held on the tendons by the forms is released, which pulls the mass of concrete tightly together as the tendons attempt to shorten. The prestressed concrete structural member is then lifted from the form and moved to a storage area where its curing continues. These finished units are loaded onto large trucks and transported to the construction job site, where they are set in place with a crane.
Photo 1 shows 80-foot long, prestressed concrete “double-Ts” in place as the roof deck and beams of a new factory building. The precast concrete wall panels are insulated, and each contains two steel-reinforced concrete “wall columns” that carry the load on the shelves from the load-bearing points of the double-Ts. The connection plates between the wall and roof sections have already been welded in place. The steel plates on the exposed edge of the roof section will be connected to the next roof section by steel plates welded to both sections of the roof. The joints between the roof sections and the joints between roof and wall panels will be sealed with cement grout. The roof deck will be insulated with several layers of flame-retardant-treated extruded polystyrene foam in 4 x 8-foot sheets, and covered with a synthetic rubber roof membrane and gravel ballast.
Photo 2 shows one end of one of the two beams making up a double-T. Note the steel plate at the top of the T, which will be used to connect this roof panel to the wall panel by welding; and the steel plate near the bottom of the T, which will be used to connect this roof panel to the bearing plate on the wall panel by more welding.
Also note the exposed ends of the 13 steel wire tendons in the lower half that are used to reinforce this beam, and the marks from the abrasive saw blade that was used to cut them.
The ends of these tendons are the weak point of prestressed concrete. The high-tensile-strength wire used for tendons loses its temper and strength in a fire at about 800°F, and can cause the collapse of the prestressed member. The exposed ends of the tendons need to be protected from the potential heat of a structure fire to prevent early collapse. This protection can be achieved by completely sealing the joints with cement grout; or by using a UL-listed system of mineral-wool insulation and fire-rated caulk in joints where expansion and contraction is expected.
Buildings like this are often classified by the building and fire codes as Type II Noncombustible, even though two-hour and four-hour fire-resistance ratings can be achieved in ANSI/UL 263 testing. Visit the UL On-Line Certifications Directory at www.ul.com for details on the fire-resistance rating of this type of construction. (At www.ul.com, click on “Go directly to a UL service”; click on “Architectural Services”; click on “On-Line Certifications Directory”; and enter the file number as J944. Select and click on the result to view the design.) Look for BXUV.J944 regarding the ratings for this type of floor assembly. Look for BXUV.P905 and P906 regarding the ratings for this type of roof assembly.
Sometimes, protection of the ends of the tendons is omitted during construction of sprinklered buildings because the architect or engineer (or a state or local building code or authority) considers this level of protection to be redundant and an unnecessary expense. In these instances, the structural integrity of the building in a fire will depend on the proper functioning of the fire pump and automatic fire sprinkler system.
Firefighters are accustomed to redundancy in protective systems and equipment: two methods of closure on turn-out coats; suspenders plus waist adjustment or belt on turnout pants; visual plus audible low-air alarms on SCBA; city water supply plus fire department connection for automatic fire sprinkler systems. We must note on our prefire plans the absence of the expected redundant (thermal plus sprinklers) protection of these structural connections, to remind ourselves that this building may not behave as we expect during a fire.
Subjects: Prestressed concrete, building construction