Influence of concrete time-dependent effects on the performance of top-down construction

The influence of time-dependent concrete behavior on the performance of top-down excavations is evaluated in light of the observed responses during construction of One Museum Park West (OMPW), a 53-story reinforced concrete tower. Performance during construction was recorded at five instrumentation stations, which included strain gauges in the concrete floor slabs used as lateral bracing during the top-down excavation sequence and inclinometers adjacent to the walls. Top-down construction was simulated with a 3D numerical model of the basement system in which concrete material time-dependent effects were considered, but not the stress relief caused by excavation or other construction activities. Time-dependent concrete response in the floor slabs and the passive resistance developed by the soil as the wall deforms due to concrete time-dependent effects are included in the model. The concrete time-dependent properties related to shrinkage, creep, and aging of the material used in the basement floor slabs are determined by means of standard concrete practice methods. The development of concrete strength and stiffness with time is compared with compressive strength tests on concrete cylinders from construction concrete reports for the concrete used in the floor slabs. Computed lateral deflections of the perimeter pile walls and strains in the floor slabs arising only from concrete material time dependence are compared with the results of performance data collected from inclinometers and strain gauges installed at five instrumentation stations. In this way, the relative contribution of concrete time-dependent effects and excavation-induced movements to the observed lateral wall movements during the top-down stages are determined and conclusions are drawn concerning the design and performance of the top-down support system at OMPW.