Profitability versus safety of high-rise office buildings

The paper applies a computer-based method involving stochastic search, multi-criteria optimization and color filtering to investigate the tradeoff between the life-cycle profitability of highrise commercial office buildings and their load-path safety against progressive collapse under abnormal loading. The study was motivated by the progressive-collapse failure of the twin towers of the World Trade Center in New York on September 11, 2001. The assessment of life-cycle profitability is based on annual lease rates for office space that reflect the amounts of open floor area and natural window lighting that a building has. The assessment of load-path safety against progressive collapse is based on the degree of force redundancy that the structural system of a building has. A Pareto-optimal tradeoff surface formed by a population of conceptual designs for a particular office building project is established in the 3D-space of capital cost, operating cost and income revenue. Computer color filtering of the optimal cost-revenue tradeoff surface is employed to highlight the relative profitability and safety of the different building designs. It is shown that design concepts with the greatest profit potential and those with the greatest safety potential correspond to buildings that also are the least safe and the least profitable, respectively. It is further shown that the capital and operating costs for safer buildings are less than those for more profitable buildings. Computer color filtering of the cost-revenue tradeoff surface is also employed to identify building designs having different window ratios and bay areas, and it is shown that there are multiple compromise building concepts with quite reasonable window ratios and bay areas that are significantly safer than the most profitable buildings while being significantly more profitable than the safest buildings. The paper concludes with some general remarks concerning the design of buildings to withstand or delay progressive collapse under abnormal loading.