Building dynamics laboratory

The use of non-destructive testing methods in the examination of building components, buildings and supporting structures is playing an increasingly important role in the building industry. They are used especially in the assessment of construction defects and structural damage. Another area in which non-destructive testing methods are used is the investigation of dynamic effects such as vibrations from road and rail traffic, drilling, blasting or wind excitation on structures and / or people in structures.

At a certain intensity and in certain frequency ranges, dynamic loads can cause considerable annoyance to people and damage to buildings. Causes of vibrations can include road/rail traffic, blasting and drilling work. The intensity of the vibration is not determined by the pulse size but by a number of parameters, including the distance from the source and some soil properties. In order to avoid nuisance and damage, vibrations caused by the above-mentioned work can already be measured during the construction preparation phase. This involves measuring the effect of vibration, vibration velocity or acceleration on the structure. On this basis, equipment and methods can be determined that generate only low levels of vibration.

Another use of vibration measurements is vibration monitoring, where the measured vibrations can be documented as part of any preservation of evidence. Exceeding the permissible values, which are harmless for the structure, can be signalled optically and acoustically.

Furthermore, the monitor can be informed automatically, e.g. by fax. The duration of the vibration monitoring can be selected between individual events and continuous monitoring over months.

An interesting task arose during the tunnelling of Askanischer Platz in Magdeburg. To prevent vibration damage to the oldest building in Magdeburg - the Lukasklause - it was monitored and documented for the entire duration of the construction work.

Particularly interesting investigations were carried out at Magdeburg Cathedral. The northern bell tower with two belfries was examined with regard to its vibration behaviour. First, the bell tower was measured with the help of a laser scanner. With the help of a CAD programme, the dimensions of the tower and the supporting beams of the belfries can be easily determined from such scans. The resulting CAD drawings in turn form the basis for finite element modelling for numerical vibration analysis of the tower. After the laser measurement, velocity measurements were carried out at a total of five measurement points distributed over the height of the tower. The measurements are used to identify eigenvalues, eigenmodes and damping parameters of the bell tower.

The vibration measurements were carried out for different states of the tower, e.g. bell ringing and "environmental noise" (vibration that occurs without excitation by bell ringing due to wind, road traffic, etc.). After the evaluation of the measurement results, a calculation model of the tower is prepared.

Parallel to the vibration tests, non-destructive measurements are carried out to determine the wood material values (E-modulus) of the belfries. The calculation model is adjusted with the help of all the measurement results in order to finally be able to carry out further vibration simulations.