Under-deck and combined cable-stayed bridges
Principal Investigator: Dr Ana Ruiz-Teran, Imperial College London
Professor Angel C. Aparicio, Technical University of Catalonia
Dr Alfredo Camara, Imperial College London
Dr Peter J. Stafford, Imperial College London
Dr M Ahmer Wadee, Imperial College London
Current Research Student:
Fernando Madrazo, Imperial College London (research scholarship funded by the Department of Education, Language policy and Culture of the Basque Government). PhD student, jointly supervised by Dr Ruiz-Teran and Dr Wadee.
Under-deck cable-stayed bridges and combined cable-stayed bridges are two innovative and efficient types of cable-stayed bridges. In under-deck cable-stayed bridges, the stay cables are located under the deck where they are deviated by one or more struts and are then self-anchored into the deck (Fig. 1a). On the other hand, in combined cable-stayed bridges the stay cables are located both above and below the deck, deviated by pylons and struts before being self anchored into the deck  (Fig. 1b).
Fritz Leonhardt (in 1978) and Jorg Schlaich (in 1991) designed the first under-deck (Fig. 2) and combined (Fig. 3) cable-stayed bridges, respectively. Around forty bridges of these types have been proposed or built . Many of them have been designed by internationally renowned structural engineers, and have stood out and received prestigious awards due to their innovative structural schemes and aesthetical features. Nevertheless, the full structural potential of this bridge type has not been used in practice yet .
By implementing the under-deck and combined cable-staying system, the structure is provided with two different load paths: the cable-staying system (with the stays working under tension and the deck, the pylons, and struts working under compression) and the beam system (with the beam working under bending and shear) .
When the structure is designed to enhance the axial response, these structural types become extremely efficient. This leads to a dramatic reduction in the deck depth and the amount of materials in comparison to conventional schemes, and hence allows for a more sustainable design [5-6]. These bridges are extremely efficient when using in-situ post-tensioned concrete decks for road bridges [5-6].
Under-deck cable-stayed bridges have an excellent performance under accidental situations, such as sudden breakage of stay cables  and seismic loading . They are very sensitive to vibrations induced by traffic action, with comfort for bridge-users being the limit state that determines the maximum deck slenderness for medium-span bridges . Our research continues with the current study of under-deck cable-stayed road bridges with composite decks .
Context and Methodology
Our research has been performed by means of sophisticated finite element models (static and dynamic, linear and non-linear), and also by means of analytical models. The response of these bridges under persistent and accidental situations has been studied, and design criteria for different configurations have been provided.
The first comprehensive research study in this area is Dr Ruiz-Teran’s PhD thesis , completed in 2005. Since then a significant amount of research has been, and continues to be, performed by members of the Bridge Engineering group at Imperial College London [2-10], in collaboration with the leaders of other research groups within the Structures Section at Imperial College London, and also in collaboration with other researchers at other universities.
The Federation Internationale du Beton (fib, The International Federation for Structural Concrete) awarded the 2009 fib Diploma for research to Dr Ruiz-Teran for her original and innovative research and technical contributions in this field .
 Ruiz-Teran AM (2005). Unconvential types of cable-stayed bridges. Structural response and design criteria. Doctoral thesis. University of Cantabria.
 Ruiz-Teran AM, Aparicio AC (2007). Two new types of bridges: under-deck cable-stayed bridges and combined cable-stayed bridges. The state of the art. Canadian Journal of Civil Engineering, 34(8): 1003-1015
 Ruiz-Teran AM, Aparicio AC (2010). Developments in under-deck and combined cable-stayed bridges. Proceedings of ICE Bridge Engineering. Special issue about 'Bridge Engineering: Cable-Supported Structures', June 2010, 163(2): 67-78
 Ruiz-Teran AM, Aparicio AC (2007). Parameters governing the response of under-deck cable-stayed bridges. Canadian Journal of Civil Engineering, 34(8):1016-1024
 Ruiz-Teran AM, Aparicio AC (2008). Structural behaviour and design criteria of under-deck cable-stayed bridges and combined cable-stayed bridges. Part 1: Single span bridges. Canadian Journal of Civil Engineering, 35(9): 938-950
 Ruiz-Teran AM, Aparicio AC (2008). Structural behaviour and design criteria of under-deck cable-stayed bridges and combined cable-stayed bridges. Part 2: Multi-span bridges. Canadian Journal of Civil Engineering, 35(9): 951-962
 Ruiz-Teran AM, Aparicio AC (2009). Response of under-deck cable-stayed bridges to the accidental breakage of stay cables. Engineering Structures, 31(7): 1425-1434
 Camara A, Ruiz-Teran AM, Stafford PJ (2013). Structural behaviour and design criteria of under-deck cable-stayed bridges subjected to seismic action. Earthquake engineering & structural dynamics, 42(6): 891-912
 Camara A, Nguyen K, Ruiz-Teran AM, Stafford PJ (2014). Serviceability limit state of vibrations in under-deck cable-stayed bridges accounting for vehicle-structure interaction. Engineering Structures, 61, 61-72
 Madrazo, F (2014). Behaviour and design of under-deck cable-stayed road bridges with steel and composite decks. Doctoral thesis Late Stage Review (jointly supervised by Dr Ruiz-Teran and Dr Wadee). Imperial College London
 Ruiz-Teran AM (2010). Unconventional cable-stayed bridges. Structural behaviour and design criteria. FIB Diploma 2009 for Research. Structural Concrete, 11(1): 25-34