2012 Munich, Lothstrasse
A turning loop was constructed for tram service 20/21 at the Munich Technical University in Lothstrasse. The line passes directly below the university buildings with a direct structural connection to the roof slab of the underground parking garage. In order to reduce the vibrations in the building caused by the trams, the engineers determined a tuning frequency of f0 = 8 Hz for the system, a value which presents a challenging task in providing vibration protection.
A "heavy" mass-spring system (MSS) with point bearings is the usual design when the system needs to have a natural frequency as low as this. In this case, the bearings are usually arranged to allow inspection and, if necessary, replacement.
However, from practical considerations, full surface bearings under the whole of the slab were specified because the backs of the elastic mats act as lost formwork and ensure proper drainage of the trough.
Calenberg Ingenieure offered the most suitable product with its proven USM® 1000 W. The robust backs of the sub-ballast mats form a resilient substrate for fixing the slab reinforcement. The voids between the cone-shaped studs ensure an adequate cross-sectional area for drainage, even under traffic loads.
A double layer of studded mats was used to achieve the required natural frequency of f0 = 8 Hz. The anti-vibrational effect of the cone-shaped studs is due to their deflection under load.
With a double-layer of mats, the designer must ensure that both layers deflect the same amount. Using one layer of glass-reinforced plastic (GRP) board on the back of the lower USM® 1000 W distributes the load adequately and means the mats do not have to be precisely placed to ensure axial alignment of the studs.
USM® 4010 was used for the side mats. These are stiff enough to transfer the centrifugal and side impact forces of the vehicle and so avoid detrimental effects on the behaviour of the MSS.
The designer of the slab track placed great importance on having an effective drainage system. Surface water drains through the voids between the studs of the side mats to the invert of the trough. The cone-shaped studs on the invert mats ensure an adequate cross-sectional area for drainage, even when trams run over them.
The falls on the surface of the trough slab cause the water to flow to drainage chambers in the centre and from there out through drainage pipes. The pump effect created by the backs of the mats when a tram passes over increases the speed of flow on the invert and flushes any dirt that may have entered the system into the drains.
Cutting the mats to shape using a sharp knife proved very effective when laying the sub-ballast mats on the tightly curved tracks. The laying team had the benefit of supervisory visits from Calenberg application engineers.
The finely detailed design concept produced by these experts ensured that not every strip of mat had to be cut to shape and was implemented to everyone's satisfaction. In spite of the adverse weather, including rain, snow and severe frosts, laying the mats went well and provided the required preconditions to achieve the expected level of protection against structure-borne noise and vibrations.
Calenberg Ingenieure GmbHAm Knübel 2-4
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