Dynamic forces on buildings cause movement and cyclic loads. Is damping the secret to more resilient structures? Engineers explore the basic theories of dampers, damping and motion control.
What causes structures like buildings and bridges to move?
Structures move under the influence of wind, pedestrian loading or seismic action. While this movement is natural and occurs in every structure, it’s only a problem when the movement interferes with the activity for which the structure was designed.
As engineers build more daring structures at greater heights and use more aggressive construction techniques or higher tech materials, this movement can become more pronounced. This is more likely to cause discomfort for people inside the structure or compromise structural integrity of the building materials.
The types of structures that might move too much, or move enough that it becomes troublesome to people, include high rise towers, bridge decks, stay cables, floors (of all types), stairs, canopies, antennae, communications towers, spires, and other light & slender architectural features.
Up until now, the majority of public attention about structures that move has been placed, rightfully so, on high-rise buildings or skyscrapers. And that is because, given their height, skyscrapers infamously tend to move around in the wind.
What can engineers do to control structural motion and movement?
Historically, in order to control structural movement and motions, structural engineers would add either more stiffness or more mass. As you make a structure heavier using these two properties, it tends to react less to the loads that are being placed on it. Thus, there will be less motion.
What’s often missed is another property called damping. Structural engineers can augment the damping of the performance of the structure, usually with very modest costs, in order to dramatically improve the structure’s performance.
Controlling motion: understanding inherent damping vs. supplemental damping
Structural motion can be a problem, not just for building occupants, but also for the integrity of the structure. Building materials can degrade over time from movement that is not adequately controlled. Damping is the result of the energy that is produced by motion. So, in order to understand just how and why motion causes significant problems for structural engineers, it first requires an understanding of the two different types of damping: inherent (or intrinsic damping) and supplemental damping.
Inherent damping occurs when energy is built up as a result of wind or pedestrian loads placed upon a structure. This energy needs to go somewhere and is often dissipated through friction resulting in heat. This friction can include movement in the beams and columns, or in the drywall partitions rubbing against the ceiling. Inherent damping is not part of the design, it simply happens when there is a buildup of excess energy. Inherent damping is therefore accidental, a consequence of the materials that are used for strength, such as the concrete used in tall skyscrapers.
The second type of damping is known as supplemental damping, which refers to the damping that can be added to a structure by engineers. Supplemental damping includes the additional systems that engineers can intentionally design into the structure to much more efficiently extract the motion–the energy–out of the building so it can ultimately dissipate without affecting building material or structural integrity.
What type of damping systems should I choose?
When the traditional approach of adding mass or changing structural materials are prohibitively expensive or perhaps not even feasible, damping makes sense.
There are a variety of damping systems that structural engineers use in high-rise buildings or long-span bridges. The type of system that works best will vary with each project. Ultimately, by engineering one of these systems into the structure, it enables engineers and architects to build even more ambitious buildings.