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Professor claims engineering breakthrough with indestructible bridge

A university professor has come up with an optimised design for an arch bridge with unlimited bearing capacity.

 

Optimised arch bridge

Above: Optimised arch bridge

It is claimed that her work at the University of Warwick could make possible a new generation of indestructible bridges.

Emeritus Professor Wanda Lewis in Warwick’s school of engineering has used a design process called ‘form-finding’, inspired by the natural world. Form-finding enables the design of rigid structures that follow a strong natural form – structures that are sustained by a force of pure compression or tension, with no bending stresses, which are the main points of weakness in other structures.

This could lead to the design of bridges and buildings that can take any combination of permanent loading without generating complex stresses – something that engineers have never before achieved.  These structures will have enhanced safety, be more durable and have reduced maintenance needs.

Professor Lewis, a fellow of the Institution of Civil Engineers, has been studying forms and shapes in nature for 25 years: the outlines of a tree or a leaf, the curve of a shell, the way a film of soap can suspend itself between chosen boundaries. In all of these natural objects, she observed that they develop simple stress patterns, which help them to withstand forces applied to them.

Professor Lewis has been developing mathematical models that implement nature’s design principles and produce simple stress patterns in structures. The principles behind her mathematical models are illustrated using physical form-finding experiments involving pieces of fabric or chains, for example.

Her published work explains how a piece of fabric is suspended, and allowed to relax into its natural, gravitational, minimum energy shape; then that shape is frozen into a rigid object and inverted. She finds the coordinates of this shape through computation by simulating the gravitational forces applied to the structure. This produces a shape (a natural form) that can withstand the load with ease.

Professor Lewis argues that “nature’s design principles cannot be matched by conventional engineering design”.

While classical architectural designs may be appealing to the eye, they are not necessarily structurally sound. “Aesthetics is an important aspect of any design, and we have been programmed to view some shapes, such as circular arches or spherical domes as aesthetic,” she says. “We often build them regardless of the fact that they generate complex stresses, and are, therefore, structurally inefficient.”

How to build the optimal arch has been debated for centuries. In the 17th century, Robert Hook demonstrated to the Royal Society that the ideal shape of a bridge arch is that resembling the line of an upside down chain line – the catenary form. The only other form proposed by classical theory is the inverted parabola. Each of these shapes can only take a specific type of load without developing complex stresses, which are points of weakness. Professor Lewis’ ‘form-finding’ process fills the gap in classical theory, it is claimed, offering a new mathematical solution in the pursuit of the optimal arch subjected to general loading.

 

The Constriction Index, 14th July 2016

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