Vibration Damped Composite Girders
In building construction, the most common static system is a single span beam, the vibration behavior of which can be simplified by a simple degree of freedom system (image 1: dynamic model). The self-damping (logarithmic decrement) is only d=0.06 in regular composite structures.
Given this relatively low self-damping factor, it is necessary to increase the natural frequency of the system in order to avoid perceptible vibrations. Structures below congregation areas, sport or exposition halls should have a natural frequency of at least 5 Hz without additional vibration-damping measures. To meet this requirement, one could, for example, weld a massive steel cover plate to the bottom flange of the wide span steel beam (image 1).
A significantly more efficient approach, however, would be to reduce the perceptible vibrations by means of minimizing the acceleration amplitude “az,” instead of increasing the natural frequency “f” of the system:
The maximum acceleration amplitude, “az” in a normal workplace is set in the existing technical literature to 0.03 m/s2.
In order to reduce the acceleration amplitude, it is crucial to increase the damping “d.” This can be realized by means of vibration dampers (image 2). These improve the system behavior significantly, and smaller natural frequencies “f” are admissible. However, additional vibration dampers come with increased costs, require space, and are visually disruptive.
The vibration damped composite girder (image 3) was conceived with the idea of using the elements already needed for resisting the loads to achieve additionally vibration damping.
Brief description
The static system of a vibration damped composite girder corresponds to an underspanned beam. The tie bar is fixed to the beam at its ends and participates in the resistance of the system accordingly. It is connected to the main beam through springs with their own damping in intermediate points. The bar acts in this sense as a damping mass and, by means of the springs, increases the vibration damping “d” of the system.
Vibration damped composite girders can be performed in several forms:
- “Genuine” underspanned girders
- Truss girders
- Solid-web girders
Example:
During the second construction phase of the two-story exhibition hall A in Berlin, 229 girders were constructed as vibration damped composite girders.
To improve the natural frequency to 5 Hz, the girders' bottom chord was reinforced with an additional cover plate during the first construction stage so that steel consumption rose to approx. 18 tonnes per girder. Despite the natural frequency of 5.2 Hz that was achieved, the vibration behaviour was not ideal. The low system damping d = 0.06 resulted in acceleration amplitudes that were felt to be unpleasant.
For that reason, a different approach was adopted during the second construction stage: we used vibration damped composite girders for the first time.
As a result of the system damping, which we measured as ranging between delta = 0.36 to 0.40 on the construction, we achieved a natural frequency of 3.8 Hz. The steel weight decreased by around 45 percent to approx. 10 tonnes per girder.
At the same time, solid-web trusses may be constructed both in an architecturally appealing manner, for instance, as fish belly girders used for the Rudower Strasse building project in Berlin, and as ordinary welded sections with parallel chords.
In doing so, the tie member is either passed below the bottom chord or on both sides of the web. The second option lends itself in particular to easy fastening of a box-shaped, fire-resistant lining to the entire girder cross section.
The system of vibration damped composite girders may be individually adapted to both the aesthetic as well as economic requirements of the building project.
Our vibration damped girders are subjected to patent protection.
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