Keywords: static stiffness, modal stiffness, modelling, concrete building, frequency.

It is often beneficial to model a structure when measurements are available. This helps to identify possible modelling errors and to obtain enhanced understanding of the behaviour of the structure. A seven-storey in-situ concrete frame building was built at the Building Research Establishment's Cardington Laboratory. After each storey was constructed, the natural frequencies of the building were measured [1]. This paper investigates numerically the dynamic characteristics of the test building from construction to completion.

This study aims

• Examine the accuracy of the numerical models of the Cardington Concrete Building at different construction stages against the available frequency measurements.
• Identify quantitatively the efficiency of the bracing systems used in the building in terms of increased lateral stiffness.
• Check the validity of the relationship between the modal stiffness and static stiffness of the building.

The building consists of seven storeys of 3.75 m each in height, giving a total height of 26.25 m. The building has three bays of 7.50 m constituting a width of 22.50 m and four bays of 7.50 m making a length of 30.00 m. The building is shown in Figure 1.

The frequencies from the numerical modelling and measurements are compared for the seven construction stages.

The main findings from this study are

• The finite element models used in the paper give good representations of the building at different construction stages as the 31 predicted natural frequencies are close to these measured from experiments. Therefore the models can be used to investigate other related matters, such as the efficiency of the bracing systems and the stiffness of the building.
• The bracing systems used in the building can effectively increase the lateral stiffness of the building. The lateral stiffness of the building increases between 75% and 150% due to the use of the conventional bracing system. If the bracing pattern is altered to a global bracing system, the lateral stiffness of the building will be further increased without using extra materials.
• The relationship between the modal stiffness and static stiffness of a structure is assessed using the real structure. As the building is three dimensional, it provides an opportunity to examine the calculation of the modal mass of a real structure. The comparison of the two stiffnesses at the three selected stages shows that the newly derived relationship holds.

1

Bougard, A.J., Ellis, B.R., "Vibrating testing of the Cardington Framed Building", The 3rd Cardington Conference, 26-27 November, Bedfordshire, UK, 1998.

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