Concrete-filled Steel Tube Installation for Additional Load Transfer to Retrofit an Existing Building

In 1997, a 39-floor reinforced concrete building was constructed in Bangkok, Thailand. During construction, the rooftop was modified, which led to additional loads. This required unusual design details to retrofit the building to provide the necessary structural support. In particular, two concrete-filled steel tubes (CFST) were installed, extending from the ground up to the 37^th floor (FL). While the CFSTs were being built, construction of the upper floors continued. Provisions were made to transfer the loads to the CFSTs when they were completed. Despite careful provisions to align the CFSTs and balance the loads, there were concerns regarding the transfer of loads. A displacement monitoring system was designed and installed to measure static displacements. The data show that total displacements at the 37^th FL for the north and south CFSTs are 18.42 mm and 20.47 mm, respectively, which are similar to 11%, and are within the requirements of compression members in the code consulted, New Zealand Stand NZS 3404: 1992, which allows vertical deflections between connection points of less than 1/1,000 of length, which in this case equals 126.65 mm. The CFSTs were monitored by the displacement monitoring system to control verticality and displacements, and to balance loads during construction. The transfer of the additional loads from the rooftop to the CFSTs was successful. The building was opened in 1998.