Two story post-tensioned office building acquired by a new tenant. They wanted to retrofit the entrance creating a grand staircase connecting the two floors within the building. This required a large opening be created in the post-tensioned floor slab.
To accomplish the architect’s concept Restruction Corporation used GPR, X-ray and rebar detectors to locate and map out the post-tensioned tendons in the slab. A structural engineer was hired by the Restruction Corporation to verify original design drawings, perform structural analysis for the opening, and design strengthening options for consideration. Restruction Corporation, structural engineer, and architect worked together to determine the best option from both a cost and constructability perspective while accomplishing the architect’s goals. Conceptual drawings were refined and the contractor generated a budget for the owner’s approval. Once approved the final drawings were produced for construction.
Restruction Corporation removed all the existing finishes in the way of the new opening. The curtain wall was partially removed and was alternatively supported to facilitate opening construction. New concrete beams were engineered to be placed around the new stair opening to take up stresses from the post tension strands and supporting the staircase. The beams were bonded to the floor slab and extended to encapsulate the building columns. The beams had to transfer post tension forces of 390 Kips in one direction and 650 kips in the other direction to accommodate the post tension system that had to be maintained for the rest of the floor.
Shoring was installed below the 1st and 2nd levels to take up loads from the new beams. Shrinkage-controlled, self-consolidating concrete was used for the concrete beams. The concrete was placed from above the slab through holes core in the slab. A total of 24 cubic yards was placed. Beam tightness and bond to the underside of the slab was critical to the design. When the forms were striped it was evident that we had accomplished this requirement. Once the concrete beams gained required strength the post tension strands around the stair opening were cut and re-tensioned. Carbon fiber reinforcement was used to additionally strengthen the slab at the opening corners and adjacent slab sections. Carbon fiber was also used for additional strengthening at the first floor where the stair case sat.
Shoring was installed below the 1st and 2nd levels to support the concrete slab being removed. The slab was cut into 3-foot square blocks and removed from the building. The concrete blocks were hauled off the site for recycling. The opening perimeter was modified to accept the new staircase which was installed by general contractor.
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