odern engineering tools and tech- as partitions, glazing, and the façade. Beyond niques enable structural engineers any noticeable harm caused by a single large dis- Structural to continually redefine the limits placement, the accumulation of many cycles of of possibility. Nowhere is this amplitude can also cause fatigue failures. Mmore evident than in supertall buildings, where Wind-induced movement can cause two other Performance controlling wind-induced sway has become a significant problems that affect a building’s critical aspect of project success. usability. The first, audible creaking and groan- The use of tuned mass damping systems has ing, seems to be especially prevalent where there performance issues relative become a mainstay in attaining this control, in is the greatest amount of relative motion between to extreme events large part because each custom-designed system building parts as the building deflects. Often can be tuned to match the as-built characteristics of occurring on the lower levels, these potentially the building. They also provide a much more effi- loud noises can make even a new building sound cient solution than adding more mass or stiffness. like a rickety old ship. One recent example is the Tower which, The most common problem, however, is the when it opened in 2014, became China’s tallest perception of movement that comes from the building and the second tallest building in the acceleration of the ®building as it sways back and world. Even though the design of the 2,073-foot forth. This is an issue that designers must address (632-meter) tower was optimized to reduce wind to ensure occupants remain comfortable even as effects, the developer also chose to include a tuned the building moves. Although their homes can mass damper (TMD) to reduce accelera- be literally in the clouds, people want to feel like tions further and eliminate any feeling they are on solid ground. Feeling at Home in of structuralCopyright movement. The resulting The inherently low structural damping in 1,100-ton system is the world’s largest modern high-rise structures is a significant factor the Clouds eddy current TMD, discussed in more in managing occupant perceptions of movement. detail below. The challenge is made even more problematic because of the relatively high uncertainty in By Trevor Haskett, P.Eng. and Problems assuming an appropriate level of inherent struc- Andy Smith, P.Eng. tural damping. As buildings are designed to be taller and more slender, they also are designed to be lighter and, Challenges relatively speaking, not as stiff. As a result, wind tends to cause much more flexure in these struc- Adding movement criteria to the building design tures than magazinein shorter, more squat buildings. To process increases the complexity of coming up S Tput it anotherR way,U the taller Cand more Tslender a Uwith a goodR design. E Fortunately, a structure’s modern building is, the more lively it is likely to be. dynamic characteristics can be estimated using If left uncontrolled, excessive wind-induced the structural engineer’s computer model through building movement can cause various problems. a back-and-forth approach. For example, large oscillatory displacements may Many years ago, wind tunnel testing on an make it necessary to reduce the speed of elevators instrumented flexure was used principally to Trevor Haskett is the Senior during strong wind events. Displacements can come up with foundation loads to determine Technical Director and Principal also damage more brittle secondary elements such the building’s overturning moment. However, and leads RWDI’s team that works on structural and tuned mass damper projects. Andy Smith is the Engineering Leader of mechanical design for RWDI’s tuned mass damper projects.

Wind tunnel testing of the tall and slender played a key role in evaluating the effects of vortex shedding created by its very uniform shape.

14 December 2017 Located within blocks of New York’s Central Park, 432 Park Avenue is more Two 660-ton opposed pendulum tuned mass dampers (TMDs), located near ® than twice the height of any of its nearby neighbors, leaving the upper portion the top of the tower on the east and west sides of the core, provide supplemental of the structure fully exposed to the wind. damping for 432 Park Avenue. in more recent times, it was realized that the performance. However, data is less prevalent a much smaller value that ideally should be test data already being collected could also be for a supertall building’s inherent damping imperceptible to most people. used to estimate accelerations. This is now a except to know that it is going to be very low. Traditionally, the industry has found that routine activity. In fact, the trendCopyright is that the typical amount of keeping residential building accelerations For many current projects, the structural inherent damping is decreasing in new build- below about 18 milli-g for the worst storm engineer begins by laying out the structural ings as designs get leaner and more efficient, expected only once every 10 years heads off system to resist the gravity loads vertically and which brings us back to the observation that most complaints. For office towers, accelera- the wind loads laterally – and sometimes earth- new buildings tend to be more lively. So, tions of 25 milli-g might be acceptable. That quake loads, as applicable – based on the selected what are the implications when you know means, essentially, that weather patterns could primary structural materials (which is to say you cannot expect much damping from the be expected to produce building swaying that concrete and steel) and their configuration. This building structure, but you are going to reach would be noticeable and uncomfortable on initial layout includes the lion’s share of what for the sky anyway? the uppermost floors – causing chandeliers or determines the building’s mass and stiffness. draperies to move, or doors to swing on their This typically leads to an initial design based Setting Limits hinges – once in 10 years. on a finite element model. The output from magazineAlthough 10-year acceleration targets have that model provides the dynamicS characterT - RHow peopleU feel aboutC perceived T moveU- proven R to be usefulE guideposts for designers istics of the building. Using that information ment and acceleration is highly subjective, over the years, here again the liveliness of coupled with wind tunnel data and analysis, so trying to define how much acceleration newer buildings comes into play. Whereas, the structural engineer is given a set of equiva- is too much yields only a fuzzy threshold. for older buildings, most plots of peak lent distributed static wind loads, based on However, there is a consensus that build- acceleration versus average time between the specific dynamic characteristics of the ing occupancy type and anticipated return occurrences typically had roughly the same building and local meteorological climate. periods (or mean recurrence intervals) factor slope, such plots for lighter, more flexible This data can be put back into the same finite into setting a reasonable range of such limits. structures can have much flatter slopes. In element model to confirm the adequacy of all Residential buildings have tighter limits these cases, it is not unusual for the 1-month structural members for the ultimate design. on movement than other buildings, such and 1-year accelerations to govern. Further Another check is conducted to ensure service- as offices or commercial space. People in complicating the picture, the cyclic frequency ability requirements are met during regularly a condominium or apartment are going of the building’s sway also affects occupant occurring wind events. to be much more particular about how sensitivity. One level of acceleration that All of the secondary members in buildings comfortable their residence is, on an around- is acceptable on a very slow swaying, low- – everything from the glazing to the interior the-clock-basis, than the same people would frequency building may be objectionable on drywall and partitions – also contribute be in an office building. When acceleration a higher frequency building. to building stiffness in minor ways, but guidelines for buildings are set, they also This type of limit is reflected in the International these additions are not taken into consid- include an anticipated recurrence interval. Standards Organization’s standard ISO 10137: eration by the structural engineer, making For example, larger accelerations that the 2007, which provides acceleration criteria for the findings a bit conservative with regard majority of people would sense might be residential and office structures at the 1-year to safety. Conventional thinking is that, as acceptable if they occur only infrequently, return period across a range of frequencies. By far as loads are concerned, more stiffness is such as once a year or once every 10 years. aligning these limits with logarithmic graphs almost always better. For weekly or monthly occurrences, how- showing a building’s total peak accelerations Researchers have gleaned a significant ever, the acceleration would be limited to plotted against the typical mean time between amount of data on building performance NOTE – The accelerations experienced in a swaying building are most frequently expressed in characteristics, including damping ratios as thousandths of a G, the constant acceleration due to gravity, which is 9.81 meters per second a function of height and building type. These squared. Applying the metric prefix milli yields the term milli-g. characteristics help to estimate structural

STRUCTURE magazine15 December 2017 these occurrences, when and to what extent Staying in Control is linked to a second mass supported pendu- structural performance improvements are neces- lum-style. This configuration can be used to sary can be determined. (This is a very simplistic This essential uncertainty concerning a struc- create a long period set up in the relatively description of the process, as several key assump- ture’s damping characteristics can be greatly low headspace. tions go into the actual plot generation.) reduced with the addition of a tuned mass After the building is structurally complete, damping system. Engineered to operate pas- the TMD must undergo a tuning and com- Resisting Wind Loads sively in response to building movement, missioning phase. With the TMD locked these types of damping systems exert forces out, the final as-built frequency of the build- The process of determining appropriate wind opposing the building’s movement. ing must be measured. The TMD is then loads for tall and supertall buildings is quite A TMD system should be as high up in tuned to best interact with that frequency complex. It involves historical weather data, the building as possible to be most effec- and release it to do its work of steadying the usually from a nearby airport, which may tive. Most damping systems are designed tower, keeping even its highest occupants require interpretation to be more site-specific. to be adjusted, or tuned, once the building feeling as stable and sure-footed as if they Further extrapolation is necessary because is substantially complete to accommodate were on solid ground. weather data typically are collected close to the uncertainty of the structure’s as-built ® the ground. The most critical wind speeds sway frequency(ies). TMDs at Work for a supertall building occur several hundred These TMDs consist primarily of a large yards above the ground surface. mass, either liquid or solid, some means of Selecting a specific type of TMD for a given When determining how much a high-rise dissipating the energy, and an appropriate building is accomplished through an imple- building will oscillate in the wind, the con- system of attachment to the structure. The mentation assessment. Primary considerations trolling factor is damping. At one extreme, mass is specifically sized for each building include the force required and space constraints, there is little or no resistance to according to theCopyright demand for improved perfor- although other factors also come into play. and the building continues to sway back mance; for supertall buildings, this is typically 432 Park Avenue and forth indefinitely, unable to dissipate several hundred tons. the energy that the wind transferred into Liquid dampers use a mass of moving water This slender, taller-than-all-neighbors residential it. The opposite behavior, known as critical in various configurations, including tuned tower in the heart of offered an damping, results in no oscillation at all, and liquid column dampers and tuned slosh- extreme challenge in managing wind effects. the building simply returns to its at-rest posi- ing dampers. Although water dampers are Despite extensive attempts to reduce wind tion after any perturbation, in the shortest usually somewhat less expensive than their effects through reshaping, which led to includ- interval of time. Neither of these is the case solid counterparts, they take more space ing wind floors at several levels of the structure, in real-world tall buildings. and are not as high performance per ton of the need for a supplemental damping system The amount of damping inherent in a tall installed mass. was a foregone conclusion. The building’s long or supertall building is impossible to predict Solidmagazine TMDs usually consist of multiple steel period, together with the required large move- with any certainty. In fact, inherentS dampingT Rplates thatU are transported C to theT TMD locaU- ment R of the damperE mass, eliminated sloshing is the most uncertain structural variable. It, tion and assembled in place. The mass can damper technology from consideration. To meet therefore, requires significant judgment and be suspended by cables, much like a simple the space constraints, two 660-ton opposed- should be viewed together with other mate- pendulum, or supported by other low fric- pendulum TMDs, one on each side of the rial behavior design assumptions. Observed tion means. Other configurations in common building core, were ultimately used. damping ratios for scores of buildings confirm use include a dual-stage pendulum, which that the damping is very low, and trends lower requires only about half the vertical clearance, with every story closer to the sky. A range of and an arrangement of opposing pendulums. This tower was one of those rare cases with inherent damping, typically from 1% to 2% In the latter case, one mass held aloft by struts accelerations below the ISO standards to of critical, is used in the design. It turns out that the challenge of designing a building to stay below specific acceleration targets is very sensitive to the as-built damping level in the structure, and that is not known until the building has been constructed. By way of example, if the assumption is 1.5%, it could easily be as high as 1.8% or as low as 1.2%. That sounds like an insig- nificant absolute difference, but it can make a 20% relative difference in the acceleration levels. Instead of the target 18 milli-g at a 10-year return period, it could end up being as high as 22 milli-g or as low as 14.5 milli- g, which is quite a wide range of response. So, even though it is understood that the damping levels are low, the uncertainty in predicting real-world accelerations is still Visitors to the observation area at the top of the Shanghai Tower can see the slow movements of a very significant. 70-ton jade sculpture mounted atop the tower’s pendulum-like tuned mass damper.

STRUCTURE magazine16 December 2017 ®

Supported by the crown structure of the tower, the simple pendulum of the tuned mass damper at the top of the Shanghai Tower is suspended over an eddy current damping system by 12 cables, three MAKING NEW AND EXISTING STRUCTURES on each of four corners. Copyright STRONGER AND LAST LONGER begin with, but the owner wanted them to ADVERTISEMENT–For Advertiser Information, visit www.STRUCTUREmag.orgADVERTISEMENT–For Information, Advertiser be even lower – and was prepared to spend significantly more to achieve that. The goal INNOVATIVE PRODUCTS was to give the impression that the structure simply does not move. STRUCTURAL TECHNOLOGIES’ V-Wrap™ FRP is a lightweight, high-strength, That led to the installation of a 1,100- code approved composite system for concrete and masonry structures and ton TMD, which the owner also wanted structural elements. These lightweight, high-strength materials are used to to display as an architectural feature vis- restore and upgrade load-carrying capacity. ible within the observation levels. Also, a unique form of damping was added to magazinePERFORMANCE the system. Typically, TMDsS have sizeableT R • ULong-term C durability T U R E viscous damping devices (VDDs), similar • ICC-ES approved to shock absorbers, which are used to drain • UL-approved fire-resistant finishes available energy from the TMD and also control its response in high winds. For the Shanghai FLEXIBLE AND EFFICIENT Tower, a large array of rare earth magnets • Utilized on a variety of structural elements was attached to the pendulum, and a layer • Ideal for complex geometries of copper plate was fixed to the floor. As the • Result in faster schedule and cost savings TMD travels back and forth, electrical eddy currents are passively formed that create a force that resists the motion of the pendu- RELIABLE SUPPORT lum mass relative to the tower. This system replaced the eight large, inclined VDDs that STRUCTURAL TECHNOLOGIES combines comprehensive, no-cost, technical support from industry experts with extensive and relevant structural otherwise would have been used, making engineering experience, including expertise in seismic applications. the installation much more aesthetically pleasing. This installation is the world’s EXPERTISE largest eddy current TMD. • Product selection • Specifications Conclusion • Preliminary design • Construction budgets Supplemental damping technology is some- thing that should be in every tall building Our Strengthening Solution Builders ensure V-Wrap™ systems are engineered to designer’s toolbox. Especially when used in meet a project’s specific requirements with components that optimize application conjunction with shaping techniques that performance. Quality you can trust from a rock solid team you can rely on. reduce wind effects, TMDs can make living and working in high-rise buildings every bit as comfortable as more traditional, shorter www.structuraltechnologies.com | 410-859-6539 buildings. And that allows people to relax and enjoy the spectacular view.▪ STRUCTURE magazine17 December 2017