Higher Loads Thanks to External Tendons: Reinforcement of an Office Building in Alabama

Recently, a new twelve story office building consisting of post-tensioned concrete beams and slabs was built in Montgomery, Alabama, USA. Once the building had been completed, the owner rented some of the office space to a tenant who had a computer server room requirement. Due to its size, the chiller system that was needed for the server room had to be installed on the roof. However, this turned out to be a problem because the weight of the chillers exceeded the design capacity of the roof structure.

DSI was contracted to help develop a design-build solution for the installation of a suitable external post-tensioning system to support the loads of the chillers. Five precast concrete beams had to be strengthened to support the new loads. To start, DSI conducted a ground penetrating radar (GPR) scan of the beams to locate and identify all internal reinforcing systems. These measurements were critical for the design because the installation of the end brackets and deviators for the external Post-Tensioning systems required that holes be cored through the beams.

For strengthening, DSI provided external 4-0.6” 270 KSI (1,860 N/mm²) DYWIDAG Strand Tendons in steel ducts. DSI installed a total of 10 tendons for the five roof beams, with two tendons used per beam. A tendon with an anchorage at each end was placed on either side of each beam . Two deviators were assembled on each of the beams in order to accommodate both beam tendons. The resulting eccentricity created the additional moment capacity that was needed to accommodate the higher load.

DSI fabricated and installed the brackets and secured them in place utilizing through bolts and anchors. The tensioning work was carried out by experienced DSI employees to make sure that the stressing sequence coincided with the new loads being applied to the roof. The DYWIDAG Post-Tensioning System was stressed in two phases: just before the chillers were placed and immediately afterwards. Upon final stressing, the ducts were grouted in order to secure the strands in place and provide a high degree of corrosion protection.

The project was implemented within a very tight schedule and required close coordination with other contractors. Thanks to the excellent co-operation of everyone involved, the strengthening of the office building’s roof was completed safely and successfully.