Seismic Safety of Existing Bridges

• Head of Project: Prof. Dr. A. Dazio
• Assistant: M. Bimschas
• Partner: ASTRA

In many countries – including Switzerland – the majority of the bridges was built before the introduction of modern seismic design codes. As a consequence, the seismic safety of these existing bridges is uncertain and even for the moderate seismicity of Switzerland severe damages during an earthquake cannot be ruled out. For this reason, the Federal Roads Office (ASTRA) has developed a three-step strategy for the assessment of all highway bridges. Step (1) consists of a rapid screening procedure to identify the most critical bridges while steps (2) and (3) comprise an individual assessment of the critical bridges to a different degree of detail.

Recognizing that displacement-based approaches lead to a more realistic – and often more favourable – assessment result than force-based approaches this research project aims at providing the basis for the displacement-based assessment of bridges with special emphasis on Swiss conditions, i.e. typical construction details and moderate seismicity. Both seismic deformation demand and deformation capacity are specifically addressed. Past earthquakes and numerical analyses have shown that bridge piers not explicitly designed for ductile behaviour are particular vulnerable elements within the bridge structure. For this reason, a series of three large-scale (1:2) tests was carried out. The test units were designed to represent Swiss bridge piers from the 1960s and 1970s, featuring typical deficiencies that are no longer allowed in modern seismic design. This includes in particular very low transverse reinforcement ratios and lap-splices in potential plastic regions. The conclusions from the experimental results are now used to develop mechanical models focusing directly on the seismic behaviour of piers featuring these typical deficiencies.

An upcoming follow-up project will investigate further the cyclic-inelastic behaviour of existing bridge piers experimentally as well as theoretically. This will include a second test series in which important parameters of the test units will be varied in order to attain a reliable basis for improved mechanical models.