Temperature Can Affect Upper Floors of Precast Garages
BY MATT SHANE AND STEPHEN JOHNSON P
RECASTCONCRETEDOUBLE-TEES and joists are a common feature on many parking decks.While the use of these materials is proven for parking deck construction, several owners have
noticed that the pre-stressed framing members on the top level would begin to camber.While the tempera- ture remains fairly constant for the lower portions of the structure, the joists on the top level are directly exposed to the sun and therefore experience large tem- perature swings.
This process can create stress on the structure, causing
cracking and spalling. In response to this scenario, two common repair strategies have emerged to combat this challenge: the use of carbon fiber reinforced polymers (CFRP) to strengthen the structure and the installation of supplemental joist hangers.
Understanding the Issue In parking garage construction, pre-stressed framing mem-
bers are set in place and cast into concrete beams before place- ment of the concrete deck. Generally, pre-stressedmembers have an upward camber when installed, so that the member will not deflect below the level point when loads are added to the struc- ture. When exposed to direct sunlight, the deck can become extremely hot, which causes expansion. The joists are secured at both ends so this expansion mani-
fests itself as a similar upward camber in the joists.When the joists heat up, and expand, the stress can result in a tearing of the connection with the beam. When the concrete at the top expands and the concrete on the bot- tom is unchanged, a knuck- le-effect of cambering and bowing creates small sur- face spalls on the beam. As the joist bows, the concrete that it is resting on becomes
Carbon fiber wraps installed to strengthen beamat connection area and painted for aesthetics.
Solution 2: CFRP Typically, the damage of the spalling beamis caused because
When exposed to direct sunlight, the deck can become extremely hot, which causes expansion.
damaged. If the spall is severe enough, the joist could fail because of the downward pressure.Where the joist is cast into the concrete and the concrete is stronger than the joist, the stem could crack at the interface of the embedded portion. Primarily a challenge in warmer climates, the condition needs to be reme- died to ensure that failure does not occur.
Solution 1: Cazaly Hangers The first solution is the use of joist hangers, commonly
referred to as Cazaly hangers, when the garage is constructed.A Cazaly hanger is ametal hanger that is cast into the joist and then cast into the beamin a
slip.The installation of the hanger accom- modates the movement that results from the sun camber; there- fore, it prevents spalling.
24 MARCH 2009 • PARKING TODAY •
www.parkingtoday.com
of the additional unanticipated downward force on the beam. In this case, it is often advantageous to strengthen the beam with carbon fiber reinforced polymers (CFRP). These systems are paper-thin fabric sheets bonded to concrete mem- bers with an adhesive to significantly increase their load-carrying
capacity.
Installation typically is achieved by applying an adhesive to the prepared surface, installing the FRP fabric into the adhesive and
then applying a second layer of the adhesive to encapsulate the
fiber.After curing, the CFRP composite will add considerable capacity to the element despite the fact that it is a very thin lami- nate. This is because the CFRP has tensile strengths up to 10 times that of steel.
Solution 3: CFRP and Supplemental Joist Hangers In some projects, the best solution is to incorporate two
repair
strategies.As previouslymentioned, in some cases, the sun camber results in the joist stem cracking at the interface with the beam. Strengthening of the beam is not necessary in these cases since it is already outperforming the joists. The crack in the joist has compromised the structural integrity of the joist within the beam so the bearing point needs to be moved outside the beam.
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