For the CCTV development site, the peak horizontal ground acceleration values associated with the three levels of design earthquake are 7 , 20, and 40 percent of gravity respectively.
Elastic Superstructure Design
With the structural bracing pattern determined from the initial concept work, a full set of linear elastic verification analyses were performed, covering all loading combinations including level 1 seismic loading, for which modal response spectrum analyses were used.
All individual elements were extensively checked and the building's global performance verified. Selected elements were also initially assessed under a level 2 earthquake by elastic analysis, thus ensuring that key elements such as columns remained elastic.
The elastic analysis and design was principally performed using SAP2000 (limited nonlinear structural analysis and design with static, dynamic, and push-over capability) and a custom-written postprocessor for the Chinese steelwork code, which automatically combined the individual load cases applied to the building for the limit-state design.
Capacity ratios were then visually displayed, allowing detailed inspection of the critical cases for each member. Due to the vast number of elements in the model — 10,060 elements representing nearly 300,000 feet (90,000 meters) of steel and steel-reinforced concrete (SRC) sections — and the multitude of load cases, four postprocessors were run in parallel, one for steel columns, one for SRC columns, one for braces, and another for the edge beams that together form the continuous tube.
The SRC columns used a modified postprocessor to account for the differences between the steel and SRC codes; section properties of these columns were determined using Xtract (nonlinear large strain composite cross-section analysis), which also computed the properties for the subsequent nonlinear analyses.
The postprocessor provided a revised element list which was imported back into SAP2000, and the analysis and postprocessing repeated until all the design criteria were met. As the structure is highly indeterminate and the load paths are heavily influenced by stiffness, each small change in element property moves load around locally.
Optimizing the elements only for capacity would result in the entire load gradually being attracted to the inside corner columns, making them prohibitively large, so careful control had to be made of when an element's section size could be reduced and when there was a minimum size required to maintain the stiffness of the tube at the back face.
To further validate the multidirectional modal response spectrum analyses, level 1 time-history checks were also made using real and artificially generated seismic records.
Design and Performance Verification
For the performance-based design, a set of project-specific "design rules" were proposed by the design team and reviewed and approved by the Chinese Ministry of Construction's expert review panel, creating a "road map" to achieve the stated seismic performance objectives.
Appropriate linear and nonlinear seismic response simulation methods were selected to verify the performance of the building under all three levels of design earthquake. Seismic force and deformation demands were compared with the acceptance limits established earlier to rigorously demonstrate that all three qualitative performance objectives were achieved.
Inelastic deformation acceptance limits for the key structural brace members in the continuous tube were determined by nonlinear numerical simulation of the postbuckling behavior. LS-DYNA (software for nonlinear explicit time history analysis), commonly used to simulate car crash behavior, was used for this work.
The braces are critical to both the lateral and the gravity systems of the building and are also the primary sources of ductility and seismic energy dissipation. Nonlinear numerical simulation of the braces was needed to establish the postbuckling axial force/ axial deformation degradation relationship to be used in the global 3D nonlinear simulation model.
This simulation was also used to determine the inelastic deformation (axial shortening) acceptance limit in relation to the stated performance criteria. Postbuckling inelastic degradation relationship curves illustrate the strength degradation as the axial shortening increases under cyclic axial displacement time history loading.
Discuss this article in the Architecture Forum...