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 can be a problem when the movement interferes with the activity for which the structure was designed.
As engineers build more daring structures at greater heights or 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 enough to become perceptible to people include: bridge decks, shopping centers, high-rise buildings and even office floors. Other types of structures are unoccupied but might also move enough to interfere with proper function (e.g. cellular communication towers) or cause latent damage due to fatigue (e.g. chimneys or stacks).
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 tremendous height, skyscrapers infamously tend to move around in the wind.
What can engineers do to control structural motion?
Historically, in order to control structural movement, structural engineers would add either more stiffness or more mass. As you make a structure sturdier using these two properties, it tends to react less to the transient loads that are being placed on it. Thus, there will be less motion.
What has been overlooked 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 term we use to describe the removal of vibration energy that would otherwise linger as vibrational, resonant 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 main categories of damping: inherent (or intrinsic) damping and supplemental damping.
Inherent damping occurs as a natural byproduct of the use of strong primary structural materials, and also due to the attachment of nonstructural materials such as cladding and internal partitions. As the structure oscillates under transient loading, the build-up of vibration energy is limited by the rate at which this energy is subsequently converted to heat. Some sources of energy dissipation include heating by 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.