Cross head installed on raker beam
Schematic of beam strengthening solution
Hydraulic jacks system used to lift installed precast "tubs" during repairs
Installed CFRP composite strengthening system

Strengthening of Arena's Beams

Project Description 

As a large university arena was under construction, cracking was observed in the raker beams that support the overhag of the second level seating areas. With the arena opening approaching, the cracking halted a large portion of the project. Structural Group and its companies joined the owner's project team to rapidly develop a design-build solution.

Investigation of the beams revealed a need to add tension reinforcement to resist the applied loads and control cracking. The project team evaluated several techniques that lead to a solution consisting of external post-tensioning and externally bonded FRP reinforcement. Design and installation details of the strengthening system were modified to accommodate actual conditions of each of the raker beams being upgraded.

At locations where pre-cast, seating deck "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.

External Post Tensioning
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.

Externally Bonded FRP
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.

Working as part of a design-build team, the beam strengthening assisted in the state-of-the-art arena opening on schedule.