Modern Engineering Tools and Tech

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 Shanghai 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 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 432 Park Avenue 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.

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