Girder design for suspension bridges has remained largely unchanged for the past 60 years. However, for future super-long bridges, aiming at record-breaking spans beyond 3 km, the girder weight is a limiting factor. Here we report on a design concept, inspired by computational morphogenesis procedures, demonstrating possible weight savings in excess of 28 percent while maintaining manufacturability. Although morphogenesis procedures are rarely used in civil engineering, often due to complicated designs, we demonstrate that even a crude extraction of the main features of the optimized design, followed by a simple parametric optimization, results in hitherto unseen weight reductions. We expect that further studies of the proposed design, as well as applications to other structures, will lead to even greater weight savings and reductions in carbon footprint in a construction industry, currently responsible for 39 percent of the world's CO 2 emissions.
In the last six decades closed-box orthotropic steel girders have been widely used in cable supported bridges. Several parametric studies were previously carried out in order to reduce inherent fatigue stress problems and in general to improve the bridge girder designs. However, in most cases, only one or two parameters were studied simultaneously, hence the full potential of orthotropic girders is not achieved. In the present work, a multi-scale FE model of a suspension bridge is established with sophisticated boundary conditions applied to a local parametric sub-model of a bridge girder. With this local model an automated gradient-based parametric optimization is carried out with the goal of minimizing weight or price of the girder. It is thus possible to optimize several design variables simultaneously, and concurrently fulfilling constraint functions on fatigue stresses, deformation and buckling. The results show potential weight savings of 6-14% and price savings of 9-17%, mainly found by thinner plates and narrower troughs. Besides the explicit savings, the results indicate the potential of applying gradient-based optimization in civil engineering designs.
Major cable-supported bridges with a span exceeding 600m would not normally be feasible without a steel box girder to carry the traffic load. These box girders generally comprise a structural steel deck plate stiffened longitudinally by open or closed ribs and transversely by diaphragms. The stiffened deck plate supports the local wheel load and distributes it to the diaphragms from where it is transferred into the box girder. The stiffening ribs also increase the total cross-sectional steel area and thereby contribute to the overall bending capacity of the box girder and finally, the stiffeners increase the resistance of the plate to buckling. This type of deck structure is generally called an "orthotropic bridge deck" due to the orthogonal nature of the stiffened deck plate and was first developed and utilised in 1950. The orthotropic steel deck is considerably lighter than a concrete slab in a composite girder and therefore makes longer span bridges possible, by reducing the self-weight forces that would otherwise dominate. The selfweight of the girder is a key driver in major bridge designs since it must be carried by all other bridge components, such as cables, towers, anchorages and foundations. The orthotropic steel girder is usually a fully closed box girder with all stiffening ribs placed inside the girder giving a smooth outer surface that is easy to paint and maintain. The interior of the closed box is often dehumidified without being painted giving an environmentally-friendly fabrication that is safe to operate and maintain. However, the orthotropic steel deck is highly prone to fatigue meaning that the number of passing lorries, their axle/wheel configuration and total vehicle weight have a significant impact on the bridge deck design and fatigue life. The orthotropic deck therefore has higher fabrication costs when compared to conventional steel structures due to the high-quality steel manufacturing and welding procedures that are necessary to achieve the required fatigue performance. The overall layout of an orthotropic bridge deck is basically unchanged since the 1960s, however the detailing has been refined considerably to a level where it is now seems difficult to improve it much further. However, the introduction of new materials e.g. sandwich steel deck structures in combination with new welding or gluing techniques could push box girder designs to new heights.
Designing and reviewing suspension bridges with world record breaking main spans in countries like Turkey, China and Norway requires a new and advanced method for fatigue verification. COWI's in‐house software IBDAS (Integrated Bridge Design and Analysis System) is used to perform the full fatigue verification including load modelling, finite element modelling, stress history analysis and damage evaluation based on the damage accumulation method. Discussions of boundary conditions are avoided by utilizing a global IBDAS beam model for the overall static system. A few segments of the girder in the global model are replaced by a semi‐local shell model in which a fine local shell model with all local geometry is placed at the location of interest. Fatigue data points used for verification are defined in the local model, typically based on hot‐spot extrapolation methods. In IBDAS, the extrapolation can be defined whereby the fatigue data point stress histories can be used directly for fatigue evaluation with the reservoir cycle counting method. Hereby, time‐consuming bookkeeping of FE node output is avoided. This paper presents technical aspects of the fatigue verification method used for the latest suspension bridge designs including the advantages of utilizing a fully integrated system.
The main design principles for girders in cable-supported bridges have not changed significantly over the past 60 years, and are limited in further development. The design concept suffers from substantial fatigue issues, and will be challenged by self-weight in future very-long bridges with main spans beyond 2 km. In this work, truss topology optimization, including global and local stability, is applied in a conceptual study of new weight-reduced designs for girders in cablesupported bridges. The methods are based on finite element limit analysis and convex optimization.A single section of a continuous girder, subject to local and global loads, is optimized to minimize weight while fulfilling constraints on stresses as well as global and local stability. The optimized designs, significantly different in layout from the conventional, show initial weight savings of up to 45% compared to the present design. Further parameter studies indicate potential weight savings of up to 54%.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.