Written by director and partner Einar Thór Ingólfsson
The use of castings in civil structures is often associated with historic buildings, old bridges or pipes. In the civil engineering society, structures such as the cast Iron Bridge in the UK, or the wrought iron columns at the St. Pancras Station in London have become symbols of old outdated technologies. In my civil engineering community, castings are often (wrongfully) perceived as a brittle and non-reliable with reference to traditional gray cast-iron which has been extensively used throughout history. With this writing, I intent to change this perspective and discuss some of the benefits of using castings for civil engineering applications.
The old Iron Bridge, UK (left) and a modern Wind Turbine Hub (right) – pics from Wikipedia.
It is worth mentioning that castings are already being used in several civil engineering applications. Amongst others, these include cable end-fittings, cable clamps and saddles for large suspension bridge, large bridge bearings and glass facade brackets. These components are made from various alloys including stainless and black steel, aluminium, gray iron and the relatively new material Ductile Iron. Ductile Iron is also referred to as Spheroidal Graphite Iron or simply SG-iron referring to the microstructure of the material which consists of a nodular graphite structure rather than graphite flakes known from traditional gray iron. Invented in the 1940’s ductile iron features, as the name suggests, ductility which is an essential property in the civil engineering design philosophy. Now, there is a cast-iron material with completely comparable mechanical properties to low-alloy steel (S235J2). Yes – it passes the Charpy-V notch test at -20 deg. C. In fact, ductile iron has become a preferred material in the wind-turbine industry because of its low-cost and superior fatigue resistance compared to fabricated steel.
National Grid T-Pylon, UK (left) and cast central node of ductile iron (right) at the foundry prior to machining.
Having overcome the potential problem of ductility, we can turn to discussing the benefits of using castings in structural design. One of the major benefits is the freedom to shape the component in any way imaginable. Tubes can be joint seamlessly and geometrical fillets, double curved surfaces and variable material thicknesses are easily integrated into a casting. Seamless, smooth design is not only desirable for aesthetic benefit, but it can also be designed for an optimal flow of stresses and for design against fatigue.
Tomahawk Overhead Line structure which is designed for HS2 in UK has a cast-corner node. The prototype structure is currently in fabrication.
In several of our recent projects, we have successfully used castings to combine the whish of the architects to maintain a seamless design with invisible connections and our own design requirements to minimise the stress concentrations, enhance fatigue life and reduce cost. The stress design is achieved through an iterative process in close collaboration between the architects, the 3D modeller and the FE-designer. Here, having the correct tools is essential to maintain an efficient process.We have successfully used SolidWorks for the 3D modelling and the preliminary stress design and ABAQUS for the final verification.
Cast steel nodes for a 220 kV tubular overhead line tower prior to fit-up and welding.
Models are easily exported from SolidWorks to ABAQUS and the design process is very smooth. During the design process, the foundry is usually involved as well. A well designed casting, does not only look good and ensures efficient flow of stresses, it must also be designed for optimal castability. By this, I mean the process of creating the cast component in the foundry, including the pattern making, the design for optimal solidification with minimum risk of internal imperfections, specification of allowable tolerances and NDTs and subsequent machining. The design for optimal castability can become a driver of the cost and in this process we involve the foundry to develop the casting in a close collaboration with our design team.
I hope that this article was helpful and inspiring.
If you share my enthusiasm about castings for civil engineering applications, wish to share your knowledge or collaborate on new exciting projects – feel free to send me an e-mail at firstname.lastname@example.org
I would love to discuss this topic further.
– Einar Thór Ingólfsson