® December 2017 December 2017

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

Reprinted with permission by STRUCTURE® magazine implications when you know you cannot expect much damping from the building structure, but you are going to reach for the sky anyway?

Setting Limits How people feel about perceived move- ment and acceleration is highly subjective, so trying to define how much acceleration is too much yields only a fuzzy threshold. However, there is a consensus that build- ing occupancy type and anticipated return periods (or mean recurrence intervals) factor into setting a reasonable range of such limits. Residential buildings have tighter limits on movement than other buildings, such as offices or commercial space. People in a condominium or apartment are going Located within blocks of New York’s Central Park, 432 Park Avenue is more than twice the height of any to be much more particular about how of its nearby neighbors, leaving the upper portion of the structure fully exposed to the wind. comfortable their residence is, on an around- the-clock-basis, than the same people would up with foundation loads to determine the to safety. Conventional thinking is that, as be in an office building. When acceleration building’s overturning moment. However, far as loads are concerned, more stiffness is guidelines for buildings are set, they also in more recent times, it was realized that the almost always better. include an anticipated recurrence interval. test data already being collected could also be Researchers have gleaned a significant For example, larger accelerations that the used to estimate accelerations. This is now a amount of data on building performance majority of people would sense might be routine activity. characteristics, including damping ratios acceptable if they occur only infrequently, For many current projects, the structural as a function of height and building type. such as once a year or once every 10 years. engineer begins by laying out the structural These characteristics help to estimate struc- For weekly or monthly occurrences, how- system to resist the gravity loads vertically tural performance. However, data is less ever, the acceleration would be limited to and the wind loads laterally – and sometimes prevalent for a ’s inherent a much smaller value that ideally should be earthquake loads, as applicable – based on damping except to know that it is going imperceptible to most people. the selected primary structural materials to be very low. In fact, the trend is that Traditionally, the industry has found that (which is to say concrete and steel) and their the typical amount of inherent damping is keeping residential building accelerations configuration. This initial layout includes the decreasing in new buildings as designs get below about 18 milli-g for the worst storm lion’s share of what determines the building’s leaner and more efficient, which brings us expected only once every 10 years heads off mass and stiffness. back to the observation that new buildings most complaints. For office towers, accelera- This typically leads to an initial design tend to be more lively. So, what are the tions of 25 milli-g might be acceptable. That based on a finite element model. The output from that model provides the dynamic characteristics of the building. Using that information coupled with wind tunnel data and analysis, the structural engineer is given a set of equivalent distributed static wind loads, based on the specific dynamic characteristics of the building and local meteorological climate. This data can be put back into the same finite element model to confirm the adequacy of all structural members for the ultimate design. Another check is conducted to ensure serviceabil- ity requirements are met during regularly occurring wind events. All of the secondary members in buildings – everything from the glazing to the interior drywall and partitions – also contribute to building stiffness in minor ways, but these additions are not taken into consid- eration by the structural engineer, making Two 660-ton opposed pendulum tuned mass dampers (TMDs), located near the top of the tower on the the findings a bit conservative with regard east and west sides of the core, provide supplemental damping for 432 Park Avenue.

® Reprinted with permission by STRUCTURE magazine  December 2017  www.STRUCTUREmag.org means, essentially, that weather patterns could a range of frequencies. By aligning these and forth indefinitely, unable to dissipate be expected to produce building swaying that limits with logarithmic graphs showing the energy that the wind transferred into would be noticeable and uncomfortable on a building’s total peak accelerations plot- it. The opposite behavior, known as critical the uppermost floors – causing chandeliers or ted against the typical mean time between damping, results in no oscillation at all, and draperies to move, or doors to swing on their these occurrences, when and to what extent the building simply returns to its at-rest posi- hinges – once in 10 years. structural performance improvements are tion after any perturbation, in the shortest Although 10-year acceleration targets have necessary can be determined. (This is a interval of time. Neither of these is the case proven to be useful guideposts for designers very simplistic description of the process, in real-world tall buildings. over the years, here again the liveliness of as several key assumptions go into the actual The amount of damping inherent in a newer buildings comes into play. Whereas, plot generation.) tall or supertall building is impossible to for older buildings, most plots of peak predict with any certainty. In fact, inherent acceleration versus average time between Resisting Wind Loads damping is the most uncertain structural occurrences typically had roughly the same variable. It, therefore, requires significant slope, such plots for lighter, more flexible The process of determining appropriate judgment and should be viewed together structures can have much flatter slopes. In wind loads for tall and supertall buildings is with other material behavior design assump- these cases, it is not unusual for the 1-month quite complex. It involves historical weather tions. Observed damping ratios for scores and 1-year accelerations to govern. Further data, usually from a nearby airport, which of buildings confirm that the damping is complicating the picture, the cyclic frequency may require interpretation to be more site- very low, and trends lower with every story of the building’s sway also affects occupant specific. Further extrapolation is necessary closer to the sky. A range of inherent damp- sensitivity. One level of acceleration that because weather data typically are collected ing, typically from 1% to 2% of critical, is is acceptable on a very slow swaying, low- close to the ground. The most critical wind used in the design. frequency building may be objectionable on speeds for a supertall building occur several It turns out that the challenge of designing a higher frequency building. hundred yards above the ground surface. a building to stay below specific accelera- This type of limit is reflected in the When determining how much a high-rise tion targets is very sensitive to the as-built International Standards Organization’s building will oscillate in the wind, the con- damping level in the structure, and that standard ISO 10137: 2007, which provides trolling factor is damping. At one extreme, is not known until the building has been acceleration criteria for residential and office there is little or no resistance to oscillation constructed. structures at the 1-year return period across and the building continues to sway back By way of example, if the assumption is 1.5%, it could easily be as high as 1.8% or NOTE – The accelerations experienced in a swaying building are most frequently expressed in thousandths as low as 1.2%. That sounds like an insig- of a G, the constant acceleration due to gravity, which is 9.81 meters per second squared. Applying the nificant absolute difference, but it can make metric prefix milli yields the term milli-g. a 20% relative difference in the acceleration

Visitors to the observation area at the top of the Shanghai Tower can see the slow movements of a 70-ton jade sculpture mounted atop the tower’s pendulum- like tuned mass damper.

® Reprinted with permission by STRUCTURE magazine  December 2017  www.STRUCTUREmag.org dissipating the energy, 432 Park Avenue and an appropriate system of attachment to This slender, taller-than-all-neighbors resi- the structure. The mass is dential tower in the heart of New York City specifically sized for each offered an extreme challenge in managing building according to the wind effects. Despite extensive attempts to demand for improved reduce wind effects through reshaping, which performance; for supertall led to including wind floors at several levels buildings, this is typically of the structure, the need for a supplemental several hundred tons. damping system was a foregone conclusion. Liquid dampers use a The building’s long period, together with mass of moving water in the required large movement of the damper various configurations, mass, eliminated sloshing damper technol- including tuned liquid ogy from consideration. To meet the space column dampers and constraints, two 660-ton opposed-pendulum tuned sloshing dampers. TMDs, one on each side of the building Although water damp- core, were ultimately used. ers are usually somewhat Shanghai Tower less expensive than their solid counterparts, they This tower was one of those rare cases with take more space and are accelerations below the ISO standards to not as high performance begin with, but the owner wanted them to per ton of installed mass. be even lower – and was prepared to spend Solid TMDs usually significantly more to achieve that. The goal consist of multiple steel was to give the impression that the structure plates that are transported simply does not move. to the TMD location and That led to the installation of a 1,100- assembled in place. The ton TMD, which the owner also wanted Supported by the crown structure of the tower, the simple pendulum of mass can be suspended by to display as an architectural feature vis- the tuned mass damper at the top of the Shanghai Tower is suspended cables, much like a simple ible within the observation levels. Also, over an eddy current damping system by 12 cables, pendulum, or supported a unique form of damping was added to three on each of four corners. by other low friction the system. Typically, TMDs have sizeable means. Other configura- viscous damping devices (VDDs), similar levels. Instead of the target 18 milli-g at a tions in common use include a dual-stage to shock absorbers, which are used to drain 10-year return period, it could end up being pendulum, which requires only about half energy from the TMD and also control its as high as 22 milli-g or as low as 14.5 milli- the vertical clearance, and an arrangement response in high winds. For the Shanghai g, which is quite a wide range of response. of opposing pendulums. In the latter case, Tower, a large array of rare earth magnets So, even though it is understood that mass held aloft by struts is linked to a was attached to the pendulum, and a layer damping levels are low, the uncertainty in second mass supported pendulum-style. This of copper plate was fixed to the floor. As the predicting real-world accelerations is still configuration can be used to create a long TMD travels back and forth, electrical eddy very significant. period set up in the relatively low headspace. currents are passively formed that create a After the building is structurally complete, force that resists the motion of the pendu- Staying in Control the TMD must undergo a tuning and com- lum mass relative to the tower. This system missioning phase. With the TMD locked replaced the eight large, inclined VDDs that This essential uncertainty concerning a struc- out, the final as-built frequency of the build- otherwise would have been used, making ture’s damping characteristics can be greatly ing must be measured. The TMD is then the installation much more aesthetically reduced with the addition of a tuned mass tuned to best interact with that frequency pleasing. This installation is the world’s damping system. Engineered to operate pas- and release it to do its work of steadying the largest eddy current TMD. sively in response to building movement, tower, keeping even its highest occupants these types of damping systems exert forces feeling as stable and sure-footed as if they Conclusion opposing the building’s movement. were on solid ground. A TMD system should be as high up in Supplemental damping technology is some- the building as possible to be most effec- TMDs at Work thing that should be in every tall building tive. Most damping systems are designed designer’s toolbox. Especially when used in to be adjusted, or tuned, once the building Selecting a specific type of TMD for a conjunction with shaping techniques that is substantially complete to accommodate given building is accomplished through an reduce wind effects, TMDs can make living the uncertainty of the structure’s as-built implementation assessment. Primary con- and working in high-rise buildings every bit sway frequency(ies). siderations include the force required and as comfortable as more traditional, shorter These TMDs consist primarily of a large space constraints, although other factors also buildings. And that allows people to relax mass, either liquid or solid, some means of come into play. and enjoy the spectacular view.

® Reprinted with permission by STRUCTURE magazine  December 2017  www.STRUCTUREmag.org