Dynamic Performance of a Tall Building to Earthquake Loading : Finding the optimum mass and stiffness

Abstract: With the increasing cost of land, the development of ultra-high performance concrete, and the technological advancements in equipment and machinery, we are able to construct and refurbish skyscrapers like never before. How they are built, what materials they use, or what structural system defines them, all affect their dynamic response, which in the end is the combination of the stiffness and mass, two key parameters in engineering of tall buildings.At an early design stages, engineers attempt several building configurations, seeking the best dynamic performance whilst keeping in mind cost and use of materials. Since dynamic properties are inherent to structures, we focused on the modal analysis of the towers with a granted static equilibrium, facing not only earthquake acceleration but also wind loading.A 60-storey, 210-metre-tall residential concrete building was proposed, serving as the basis for the various structural modifications, looking to study the changes in certain parameters such as the natural periods, total mass, modal participation factors, or maximum top displacements.First, mass and stiffness were put to the test with the use of outrigger floors at mechanical levels. Here, 20 cases were studied, having an optimal return on investment for a solution with 3 single-storey outrigger floors at the 30th, 45th and 60th, being the usable floor area inside the apartments a decisive contender in the balance.Then, the effect of mass distribution was assessed through the thickening of floor slabs, from a two-way flat slab of 300mm, to a deeper 400mm. Done at the right storeys, the upgrade achieved an optimisation in the modal participation. As such, increasing the depth of floors from the 27th to the 59th resulted in a 29mm cut in top side sway.Finally, human perception of motion was examined, proposing the use of a 780-ton tuned mass damper for the first vibration mode, and a 980-ton tank for the second one, aiming to attenuate peak accelerations.