When it comes to skyscrapers, wind can have an intense effect on the top levels of a super tall, slender building. Strong winds can cause the building to sway, increasing the discomfort of tenants who live on the higher floors.
Mitigating the wind-induced motion of a super tall building is crucial, as it can increase occupant comfort as a result and improve overall building performance. Building project teams use a device called a tuned mass damper (TMD) to reduce motion.
However, when a TMD is installed, it is vital to know that it will perform as necessary and provide the intended level of damping. That is where monitoring is crucial and a step that should not be missed.
What is a Tuned Mass Damper?
A tuned mass damper (TMD) is a type of mechanical device that helps reduce the effects of wind and other dynamic loads on a super tall building. The result of this is a reduction in the overall motion of the building. A TMD is usually a large, suspended mass with viscous damping devices that transform the energy from vibrations into heat.
In most cases, tuned mass dampers are installed at the top of a super tall building. A damper works by moving out-of-phase with the motion of the building and providing sufficient force that directly opposes this motion. The energy created will move from the building to the TMD, which is where it will then dissipate, ultimately reducing building motion.
Super tall buildings are particularly susceptible to excessive motion, due to their height as well as their slenderness. Tenants, especially those on the topmost floors, can feel the motion of the building and often feel discomfort as a result. Tuned mass dampers are an especially effective method of mitigating this motion and restoring occupant comfort – and this has been proven.
Once installed, it is equally important to monitor the effectiveness of the TMD and make adjustments if necessary. This is equally, if not more important, than including the TMD in the building design as it will ensure the TMD is operating as intended.
What is Long-Term Structural Monitoring & How Does it Ensure TMD Effectiveness?
When dampers perform as necessary, occupant comfort and safety in super tall buildings is ensured. One notable method of demonstrating a damper’s effectiveness is employing long-term structural monitoring.
A structural monitoring system can be installed to continually record bi-axial responses of both the building and the TMD. The monitoring records building and TMD accelerations during both ambient (calm) and significant wind conditions along with monitoring critical dynamic building parameters, including the building’s natural frequencies, mode shapes, and damping ratios.
Long-term structural monitoring can continually capture responses of both the damper as well as the building itself, which when compared over a significant time span can give us an indication of the effectiveness of the TMD.
Monitoring In Action
The best way to illustrate the effectiveness of long-term structural monitoring is with an example. Recently, Motioneering installed a long-term monitoring system in a super tall and slender building. Wind tunnel tests at the design stage revealed building occupants were likely to experience discomfort during wind events, and therefore a supplemental damping system was necessary to meet occupant comfort by increasing the total effective damping.
In this particular building, a tuned mass damper was selected after studying multiple designs, as it required the least amount of space in the building. Structural monitoring was installed after the damper to allow for the continuous recording of responses from both the building and the damper.
The monitoring started with recording accelerations during calm conditions as well as very windy conditions with maximum wind speeds reaching 77 km/hr within the first few months of installation. The data from calm conditions helped reveal the building’s natural frequencies as well as inherent damping, even with the tuned mass damper installed. The monitoring system revealed the damper was functioning precisely as intended when the approximate total effective damping in combination with the spectral shape of the responses demonstrated a visible change from the calm condition results to those from extreme wind conditions.
In addition to this, the structural monitoring system recorded responses from a series of consecutive events during a TMD maintenance activity when it was locked out before being released again. Specifically, one morning the wind speeds reached 56 km/hr while the damper was briefly locked out. When the damper was released later that morning, the wind speeds reached 57 km/hr from the same general direction. Upon comparing the two responses, the results clearly demonstrated the effectiveness of the damper through the total effective damping it introduced.
When a long-term structural monitoring system is in place, building designers and engineers can accurately identify critical dynamic building parameters, such as the building’s natural frequencies, mode shapes, and damping ratios. We can also determine parameters associated with any supplemental damping systems, including achievement of effective added damping.
For more information on damping and motion control, check out our post Why Structures Move: Dampers, damping, and motion control.