BLOG | THE VALUE OF PERFORMANCE TESTING ON A GRAND SCALE

Structural engineering is all about selecting an appropriate structural system for the environment and applications a structure is likely to see over its lifetime. Engineers understand how materials behave and the loads a structure will have to resist, and they can make pretty good estimates about how the structure will respond.

Frequently, that’s as far as it goes – a good, safe estimate and a serviceable, reliable structure. But when the opportunity comes up to directly measure a structural system’s real-world response to its more severe loadings, well, everybody gets excited about that (all the engineers, anyway).

Typhoon season in Shanghai

With the approach of the 2018 typhoon season in the East China Sea, Motioneering’s engineers made plans to test the dynamic performance of the Shanghai Tower. The tower owner and developer of this supertall structure – the second tallest in the world at the time of its 2015 opening – had opted to include a tuned mass damper (TMD) to not only reduce the effects of wind, but also as an architectural feature.

The Shanghai Tower’s tuned mass damper extends through five floors at the top of the building. An observation area gives visitors a behind-the-scenes look at the gently swaying 1000-tonne steel mass and the cables from which it is suspended, highlighting the tower’s remarkable engineering.

TMDs are custom designed to counter a structure’s anticipated oscillation by moving out of phase with the building motion, which then requires dissipating the energy collected from its movement. In the case of the Shanghai Tower, we designed a 1000-tonne TMD for installation near the top of the building.

Carefully tuning the TMD close to the as-built frequency of the tower provides optimal damping. The tuning and commissioning of the Shanghai Tower TMD were completed in 2016 and it has been in operation ever since. The theoretical damping performance was tuned to increase the tower’s overall structural damping from 1% to 3% of critical. This has the effect of reducing by 45% the building horizontal accelerations – which is to say, the swaying – that building occupants would otherwise experience.

In addition to its large size, this TMD is notable for its use of an eddy current damping system. The Shanghai Tower is the first high-rise building TMD to use such a system and is also the world’s largest installation of this type. The system consists of a large array of rare-earth magnets attached to the bottom of the damper’s suspended mass which moves over a layer of copper plate affixed to the floor. As the mass travels back and forth, eddy currents are passively formed in the copper creating a force that resists the motion of the TMD mass relative to the tower, thus dissipating energy.

Other types of TMDs use controlled-flow hydraulic cylinders or tanks of sloshing water to dampen building movement. In the Shanghai Tower, a simple control system enables the tower’s eddy current damping to be easily engaged or disengaged. This made it an ideal setup for comparing the tower’s movement under typhoon conditions with and without the influence of the TMD.

Collecting and analyzing data

Two tests were conducted in the mid of typhoons in July 2018. Typhoon Maria was a devastating Category 5 storm that made landfall in Shanghai on July 11. Building and TMD acceleration data were recorded during Typhoon Maria and took advantage of an adjustable damping force device that previously had been prepared to create the conditions of locking out and releasing the TMD.

The TMD was locked out – which switched the damping force to near zero – and then released to provide maximum damping at 10-minute intervals. Because the wind speed and direction could vary greatly during each interval, the collected data illustrate the intuitive TMD effects but cannot be used to sharply quantify TMD performance. Although plotting the data shows that building peak accelerations with the TMD released are approximately 30% lower than with the TMD locked out, in other periods the building acceleration remains below 1.0 milli-g, which makes the TMD effect less significant because such low levels of vibration are generally imperceptible.

The accelerations of the tower and the TMD subsequently were recorded during Typhoon Ampil on July 22 for a total of 16,800 seconds (4.67 hours). The TMD was in normal working condition throughout the storm, with no released/locked-out comparison. The peak total acceleration of the tower was 2.915 mill-g, only slightly more than the predicted value of 2.8 milli-g. Data plots show that the acceleration of the tower and the TMD had a phase difference of 90 degrees, indicating that the TMD was working as expected.

These plots show the TMD and tower accelerations measured during Typhoon Maria in July 2018. The TMD was locked out and then allowed to operate at 10-minutes intervals throughout the duration of the test. The red areas of the plots correspond to times when the TMD was locked out, whereas the blue areas are when the TMD was operating normally.

Fortuitous weather events

In terms of data collection, we were fortunate in having two typhoons of very different intensities blow through within such a short time. Typhoon Maria provided a good chance to evaluate both the locked-out and active performance of the TMD under extreme conditions. On the other hand, Typhoon Ampil was a 1-year storm of the type expected to occur with some regularity in the area and thus provided a good baseline case. In both cases, the measured results agreed well with the predicted performance.

Modeling and heavy-duty theoretical analysis continue to play an important role in designing ever more sophisticated structures that stretch the limits of possibility while still keeping occupants comfortable and safe. By taking advantage of opportunities like those offered by the Shanghai Tower to compare expected results with real-world field observations, we continue to grow our understanding of how to optimize our designs.