A new 16,000-seat basketball arena required an emergency strengthening of the main raker beams that support the overhang of the second level. Cracks observed on the beams during construction lead to an in-depth investigation of the existing condition and revealed that the existing raker beams
required additional tension reinforcement to effectively resist the applied loads and control cracking. To rectify this structural issue, SPS and the strengthening design Engineer-of-Record formed a design-build team and evaluated several techniques that lead to an optimum solution consisting of an external post-tensioning system and externally bonded FRP reinforcement. The external post-tensioning system was composed of high strength T-headed steel bars and special steel cross heads for anchorage. The post-tensioning system provided vertical and horizontal "clamping" forces that actively engage this reinforcing system and reduce beam cracking and deflection. Core holes were drilled through existing transverse beams to install the vertical T-headed bars. To avoid damage to existing steel bars, small areas were chipped open on the topside of the beams prior to coring. Locations of the core holes were then adjusted to minimize damage to existing steel reinforcement. At less deficient locations, externally bonded FRP system was used to provide the additional tensile reinforcement. To address issues related to FRP development length and anchorage requirements, bonded concrete block was cast-in-place on the underside of each raker beam. Specifications for surface roughness, steel doweling, and material selection were used to ensure composite behavior of a concrete block with the raker beam. At locations where pre-cast concrete tubs were already installed on the raker beams, a special jacking system was used to lift the tubs during strengthening installation. To achieve this, several hydraulic jacks were operated simultaneously to avoid overstressing the precast concrete elements. As the arena construction was halted due to this issue, the design-build team was able to design and install the repair in a fast track manner. Design and installation details of the strengthening system were immediately modified to accommodate actual conditions of each of the raker beams being upgraded.