Damped Moment Frames – What They Are and Why You Should Consider Them

Resilience is quickly becoming as prominent a buzzword as sustainability in the construction industry. While sustainability focuses on preserving natural resources and maintaining balance, resilience complements it by ensuring structures can endure and recover from damage with minimal resource use. A resilient structure is inherently more sustainable. Among the tools available, dampers may be the most effective for enhancing structural resilience.

What is a damper?

The most common damper we encounter daily is a car’s shock absorber. Its job is to absorb energy from uneven road surfaces and “soften” the car’s response. Instead of bouncing repeatedly, the shock absorber cushions the impact and slows the car’s vertical acceleration—helping ensure you don’t spill your drink with every bump!

Some damping naturally occurs in building structures through energy dissipation in connections, non-structural components, and other inherent construction elements. Figure 1 illustrates how increased damping affects a structure’s response. In this simplified model, acceleration (vertical axis) is shown over time (horizontal axis). As the graph demonstrates, greater damping significantly reduces the structure’s acceleration—or force—within a short timeframe.

Figure 1 shows the difference between 2%, 5%, 10%, and 20% damping. 5% damping is typically used in the design of new structures and is inherent in the construction as explained previously. You can see how dramatic a difference increasing the damping just a few small percentages can make to the overall response on the building! By purposely installing actual dampers designed to soften the structural response we can dramatically reduce the force on a structure.

Dampers have long been used in specialized settings, particularly in high-rise and specialty structures. However, their implementation was historically limited by the need for complex analytical techniques. Over the past few years, code approval for a prescriptive analysis method has made dampers more accessible, enabling their broader application in construction. The most common approach is to pair dampers with moment frame systems. Thanks to the code-approved prescriptive method, designing these systems is now much more feasible for common structures.

Architectural considerations

The most common type of damper would appear in a structure similar to a braced frame. The locations of these dampers require coordination within the design team to ensure they can be placed in appropriate locations and not affect the building occupants flow or future modification. However, there is more flexibility in the location of dampers than for braced frames. Because they are not the primary lateral force resisting system, many location constraints do not apply. For example, they do not need to stack vertically and can be placed in different bays from floor to floor as necessary.

How much does a damped system cost?

A recent comparison of typical moment frame vs. a damped moment frame system for a 300,000 sq ft 8 story hospital structure yielded the following results:

In terms of structural cost, dampers and traditional systems are nearly identical, but the long-term benefits to a structure's performance are substantial. So why haven’t these systems been widely adopted?

• Complex Analysis Requirements: Until recently, the engineering analysis required was highly demanding.
• Architectural Limitations: Dampers are often installed diagonally, like braces in a braced frame, which can restrict future architectural flexibility and occupy usable space.
• Lack of Familiarity: Many contractors, owners, architects, and engineers are unfamiliar with the system, leading to hesitation simply because it hasn’t been widely used before.

Conclusion

At a minimum, a damped moment frame should be considered for use in the system selection process early in the schematic design. Especially for critical structures such as hospitals where resilience is of vital importance to the surrounding community. With the prescriptive methods no longer prohibitive and the cost of the system in line with other common systems we should be implementing this technology more often on our most important structures.

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