(Above: The Tuned Mass Damper atop Tapei 101.Image source: Wikimedia Commons, author: travelwayoflife)
Ahmed Dwiddar discusses how advances in technology are helping engineers create more and more challenging structures.
Structural dynamics is the study of how structures react to highly varying loads over time. The topic has been historically linked to earthquakes and seismic design of structures, and for a long time was considered beyond the daily practice of engineering.
However, this is rapidly changing due to changes in materials, increasingly challenging architectural design, and sustainability considerations. The application of structural dynamics is now highly relevant in different scenarios, such as office floors subject to walking loads and tall buildings subject to wind and seismic loading - as buildings become slimmer and taller, and floors more slender, they exhibit a more significantly dynamic response which affects their functioning.
New bridges have also exhibited a more dynamic response to loads: a famous example is the Millennium Bridge in London. The architectural design dictated that the structure be very slender, which meant that its fundamental natural frequency was low enough to match the exciting frequency of pedestrians using the bridge - resulting in vibrations which caused London’s public to name it “the wobbly bridge
”. After studying the problem, structural engineers added dynamic control devices along the length of the bridge which dampened the vibration.
In very general terms, the research in this area can be split into two categories: the first is research which aims to understand the nature of loads and create loading models. The purpose of these models is to give a mathematical representation which reflects as close as possible these loads, and can help structural engineers produce designs which take account of them. The second area is to define the acceptable performance of structures under these loads and understand how the structures will respond to them.
Trying to understand the nature of the loads requires measuring them in actual structures, then trying to produce a model which represents them as accurately as possible. Technological advances in measurement devices has greatly improved our understanding of such actions, as has the ability to process and analyse huge amounts of data. Advances in mathematics, numerical methods and computer power have contributed hugely to our ability to model and idealize these actions.
Over time the approach to structural dynamics has shifted from designs that aim to make structures resist
these loads, to make structures react
to the loads - the application of this is vast, from pedestrian bridges and skyscrapers to a nuclear power plant.
Under this umbrella of “Performance Based Design” is an approach called dynamic control of structures, where devices are added to the structure to influence its behavior under dynamic loading.
(Base isolators under the Utah State Capitol - image source Wikimedia Commons, Author Mike Renlund)
These devices can be either passive or active. An example of a passive device would be base isolation, which is installed at the foundations of structures to absorb seismic energy and prevent it damaging the structure. Active devices are the more sophisticated, reacting to dynamic loading by imposing actions which counteract the external dynamic actions. An example of an active device would be the active mass damper (active control device) used in the Nanjing Communication Tower in China, which helps reduce the effect of lateral loads.
Structural dynamics is proving to be more and more an essential knowledge for structural engineers, as we are challenged by requirements like sustainability and planning restrictions. Better understanding of dynamic behavior will allow us to build longer bridges and taller buildings, while using less material – creating more remarkable structures in a more sustainable way.