Establishing a comfort level in a variety of structures

Incorporation of supplementary dampers in signature structures such as 1,000-plus-foot-tall high-rise buildings has received significant attention in recent times. This solution to occupant comfort issues seems reasonable in such difficult design situations. However, supplementing the inherent damping of a structure is becoming more commonplace even for less unusual design challenges. Many shorter high-rise buildings are using this technology as well.

A Supplementary Damping System (SDS) is essentially an energy dissipation system that is incorporated into the design of a structure. It is optimally designed to absorb vibration energy, thereby reducing motion. There are many ways to integrate energy absorption into a structural system.

Technologies in common use today can be broadly classed as distributed, impact, active mass, semi-active mass, mechanical passive tuned mass, and liquid-based passive tuned mass.

Experts in structural dynamics and motion control encounter common questions and perspectives voiced by structural engineers regarding damping issues, as evident in the following examples.

Q: I can design my structure to avoid the need for dampers. Should this be my first goal?

A: It is a generally accepted practice in structural design to attempt to address all of the issues surrounding strength, deflection, and comfort through processes of optimizing the layout and makeup of the structural system. One could look to the analogous evolution of vehicle suspensions, which also serve the strength function of holding the vehicle up, the deflection function of controlling the vehicle, and the comfort function of absorbing bumps.

Prior to the 20th century, horse-drawn carriage suspensions were designed predominantly using multi-leaf springs that provided strength along with a certain amount of compliance in the interest of passenger comfort. Little attention to damping was paid, except for the friction inherent in the action of one leaf on another. This situation parallels current common practice in building design. We create a structural design and live with whatever inherent friction (damping) we get. However, it wasn’t long into the 20th century when automobile designers, needing better performance, realized that damping (shock absorbers) in their vehicles’ suspensions was a critical parameter in making the vehicles infinitely more controllable and comfortable. We have continued to use this technology for more than 100 years.

As structural engineers, we are on the threshold of coming to the same realization. Given the high sensitivity of the performance of many structures to damping, we can no longer leave it up to chance that we will have sufficient damping in the as-built structure. It should be considered early in the design process.

Q: How does incorporation of supplementary damping help me?

A: If we consider the common practice of designing structures, particularly dynamically sensitive structures, inherent structural damping is a critical consideration. Code provisions can indicate a potential for dynamic sensitivity and require detailed gust-factor calculations. Wind tunnel tests also require an estimate of the inherent structural damping. This value is generally expressed as a percentage of critical damping.

Much has been written in the literature about selection of a damping value to assume for design. Expensive and difficult research projects have been carried out to measure the as-built damping levels of a variety of existing buildings.

The findings indicate a wide range of answers. These are highly dependent on construction materials in the building, the structural system, the cladding system, internal partitioning, conditions under which the measurements were made, and foundation conditions. In an attempt to make the information useable, most codes and standards suggest values in the range of 1.0 percent for steel construction to 2.0 percent for concrete construction. However, in some measurements, values as low as 0.5 percent and as high as 5 percent are observed; see Figure 1.

What is not commonly appreciated is the importance of the damping parameter to the wind loading and serviceability issues that are central to the design process. In many cases, a change of 0.5 percent in the inherent damping assumption can have a greater effect on loading and comfort than can be provided by all of the practical optimization of the structural system. Quite often, major changes in stiffness and mass, on the order of 10 percent to 30 percent, are required to have the same impact on structural performance (see Figure 2).

It is in these situations that starting the design by incorporating a simple technology SDS, such as a sloshing water tank, can provide a great benefit to the structural engineer. The achieved total damping is a combination of the two components: inherent (natural) damping, and designed supplementary damping. Having incorporated supplementary damping into the design process, the inherent damping in the structure becomes of secondary importance. That is to say, the observed improvement in response is found to depend very little on the initial assumption of inherent damping and is now governed by the contribution of the damping system.

The opportunity here is to be able to start the design, not with an uncertain assumption of the expected damping, but with a known, achievable, high level of damping. If the design relies on inherent damping alone, and the assumption is inaccurate, then it is possible that the structural materials are not being efficiently utilized. Hence, we have wasted material and wasted money. Equally possible, the performance will not meet expectations, resulting in structural loads and motions experienced higher than anticipated.

When incorporating even a simple SDS, it is typical to be assured of a damping value of 3 percent of critical or more. With this as a reliable baseline, an optimal structural design can quickly be achieved with minimal iteration. This allows the structural engineer to use his time and expertise to realize an actual high-performing and efficient structure. The potential for cost savings, enhancing sustainability, and creating more useable floor space through reduced structure make enticing benefits for owners, developers, and architects, as well.

The result: In terms of its dynamic performance, the structure can be designed with total confidence since the mass, stiffness, and damping can all be carefully controlled and optimized together. This gives the structural engineer a competitive edge over others who avoid the latest technologies.

Q: Can you use supplementary damping to help with lateral drift or to reduce loads in addition to accelerations?

A: Yes. A damping system can reliably help to reduce lateral drift and loads. Drift and load can be thought of as being comprised of two parts: A static (or mean) component and a dynamic (or fluctuating) component. Supplementary damping attacks the dynamic component of the drift or loads by absorbing energy from the structure. It is especially effective for dynamically sensitive buildings. A damping system can reliably reduce the dynamic component by one-third to one-half. To address drift and loads, the damping system simply needs to be designed with the higher return periods in mind.

Q: What does supplementary damping look like and where does it go in the building?

A: There is a wide range of possibilities, generally categorized as Distributed Damping Systems and Mass Damping Systems. Distributed damping needs to be located in areas of greatest relative movement between structural elements, where these locations depend highly upon the structural system. The most effective location for a Mass Damping System (TSD, TLCD, TMD, or AMD) is close to the area of peak amplitude of vibration, which is typically near the top of the building.

Distributed Damping Systems

These types of systems typically take the following forms:
* viscoelastic materials to dissipate energy that is placed within multiple structural connections;
* cross-bracing that incorporates fluid or viscoelastic elements to dissipate energy in multiple frame locations; or
* outrigger, column, or shear-wall systems that incorporate hydraulic or viscoelastic elements to dissipate energy.

Mass Damping Systems

Tuned Sloshing Damper (TSD)—A specifically shaped tank of water incorporating engineered baffles to dissipate energy can be effective in two lateral axes simultaneously. Water can also be used for other purposes in the building (see Figure 3).

Tuned Liquid Column Damper (TLCD)—A U-shaped tank of water incorporating adjustable gates for dissipating energy is effective in one axis only. Water can be used for other purposes in the building (see Figure 4).

Simple Pendulum Tuned Mass Damper (TMD)—A mass—typically steel or concrete—suspended on cables, tuned to be effective in two axes simultaneously, incorporates hydraulic cylinders to dissipate energy (see Figure 5).

Coupled Pendulum Tuned Mass Damper (TMD)—Two masses—typically steel or concrete—suspended on a combination of cables and struts to minimize vertical height requirements, is tuned to be effective in two axes simultaneously, and incorporates hydraulic cylinders to dissipate energy (see Figure 6).

Active Mass Damper (AMD)—This is a special case of a Mass Damping System that reacts to the building motion in real time. It is mechanically similar in general layout to the TMDs described above, but with the distinction of having a drive mechanism instead of energy dissipaters. Computer control based on building motion sensor input and power are required.

Each of these technologies has its own unique characteristics and applications, and therefore it is suggested that the optimal solution be evaluated based upon cost, space, and performance factors.

Q: This sounds like it will add complication to my design process, and I don’t have the time or experience to carry this out!

A: There is assistance available in this area. Structural dynamics and motion control consultants exist. What is important is a practical knowledge of the design integration and the entire construction process.

The goal of the consultant is to integrate seamlessly into your design process. Although he or she uses some advanced tools to perform their portion of the design scope, these are intended to work with the information provided by the software tools used by most engineers and architects.

During the interaction that occurs with the consultant as part of the design team, your design information is obtained, combined with the structural response data from the wind tunnel laboratory, and a coupled-motion analysis is performed to optimize damper performance. A fully-integrated damping solution is provided.

Q: Will the general contractor know what the damper is? Is it difficult to construct?

A: The general contractor will probably have a number of questions at the outset. However, it is not difficult for a general contractor to obtain bids and build a damping system. The consultant can assist with the entire process, from tender to fabrication and construction, to tuning and commissioning.

Most of the components are familiar to contractors and include such materials as concrete, rebar, waterproofing, structural steel, and steel plate. Other items, including baffles, adjustable gates, cables, and hydraulic cylinders are also used. Some parts are preassembled and pre-tested by the fabricator. When the site is ready, final assembly can occur in the building in parallel with completion of tower erection.

Q: I design bridges. Does this apply to bridges too?

A: Yes, much about bridge design is governed by dynamics, and damping is a key parameter. Bridge motions are produced by wind, traffic, pedestrians, and earthquakes and are important both during construction and for the completed bridge. Inherent damping of bridges is subject to similar uncertainties as buildings. Supplementary damping is often used to mitigate many of these issues.

Scott L. Gamble, B.A.Sc., P.Eng., is vice president of Motioneering Inc., and RWDI Group in Guelph, Ontario, Canada. He can be reached at 519-763-3870, ext. 2256 or at scott.gamble@motioneering.ca. Jamieson K. Robinson, P.Eng., is an associate and senior specialist for Motioneering Inc., and RWDI Group. He can be reached at 519-763-3870, ext. 2355 or at jamieson.robinson@motioneering.ca.

FIGURES AND CAPTIONS

Figure 1: Observed inherent damping for concrete and steel buildings
Caption: A wide range of variables and measuring techniques has led to a wide range of observed inherent damping levels in buildings. Generally, inherent damping decreases with increasing height.

Figure 2: Relative effects of changing mass, stiffness, and damping
Caption: For dynamically sensitive structures, increasing the total damping can have a greater effect on reducing motions and loads than increasing mass and/or stiffness.

Figure 3: Tuned Sloshing Damper

Figure 4: Tuned Liquid Column Damper

Figure 5: Simple Pendulum Tuned Mass Damper

Figure 6: Coupled Pendulum Tuned Mass Damper