Great lengths to form bridges

Materials World magazine
,
29 Oct 2018

A new bridge form could help engineers span greater distances, providing previously impossible over-land links. Ellis Davies reports.

The building of bridges is not something that gets a lot of mainstream airtime these days, with the construction of walls tending to take the spotlight. However, a new theoretical bridge form developed at the University of Sheffield, UK, could allow for the construction of longer bridges of up to 10km per span, according to researchers. The study chose to assess the geometry rather than the materials used, in order to maximise efficiency, and came up with a form that involves split towers, potentially 1km in height.

Professor Matthew Gilbert of the University of Sheffield, and lead author of the study, Theoretically optimal forms for very long-span bridges under gravity loading, published in Proceedings of the Royal Society, sees a problem with the current methods used in the design of bridges. ‘They do not take self-weight into account in a vigorous way, which means they can’t be easily applied to very long span bridges,’ he says.

Floating in the Forth

The UK has an abundance of suspension bridges, having become accomplished at designing and building these during the 1960s and 1970s. Examples of the trusty bridge include the Humber Bridge in Humberside, the old Severn Bridge spanning the River Severn between England and Wales, and the old Forth Bridge in Firth of Forth, Scotland. These structures comprise a thick main cable that stretches between towers, from which thinner cables hang down and are connected to the deck. This form is currently being used for the longest span bridges.

The newer Forth Bridge, known as the Queensferry Crossing, takes a more modern, yet widely used approach to the long span bridge – cable-stayed. Here you have direct connection between the deck and the towers, which is generally considered to be more structurally efficient.

The common denominator in most bridges is the material they are made of steel. The principle benefit of steel is its toughness and resilience, and the challenge would be to replicate that toughness in any other materials, considered for use. For this reason, the Sheffield team used steel as the material for their modelling of forms.

Building on old design

In these new forms, the team has expanded on the use of vertical towers. ‘We showed that by having a number of pylons radiating out from the central tower you significantly reduce the volume of material needed to construct the bridge,’ says Gilbert. The theory builds on 20th Century theories from Cornish engineer Davies Gilbert, who showed the optimal shape of a cable in a suspension bridge accounting for the presence of gravity loads.

The most optimal design is a wheel-like tower with spokes but it is an impractical shape as it would be difficult to build. The alternative is to use a split tower approach, comprising two-to-three spokes, which would be 12% heavier than the optimal design over a 5km span. This, however, is far better than a traditional suspension bridge, which is 73% heavier.

Load paths on the new bridge design are kept short, meaning that forces from the deck are efficiently distributed through the superstructure to the foundations of the bridge. It also avoids sharp corners and compressive elements.

Going modern

To create the form, the team used computational techniques to model the optimum design. ‘We used layer customisation,’ M Gilbert explains. ‘You want to get a load from a given point in space to a given set of supports, so you populate the design space with nodes or points, and then connect each of those points together to form a dense grid of potential members or structural element. You then use optimisation to find the minimum volume subset, and let the computer find the best, or minimum volume, way to get the load to the supports.’ Using these techniques often results in some complex structures, not necessarily familiar to engineers.

The team wanted to address a number of key issues in the building and design of very long span bridges. ‘One is self-weight, because if you have a very long span you’ve got a lot of structure to carry. You can end up with an inefficient form that carries more material than is necessary, which requires more structure – so you end up with a spiralling decline,’ says Gilbert.

An efficient form that is lighter needs less structure, leading to greater spans with a given material. ‘Civil engineers have tended to be quite conservative, so generally speaking very long span bridges are constructed of steel,’ says M Gilbert. ‘Most of what we studied assumed the use of steel. However, all the methods we’ve used can be used with any material.

‘From a materials point of view, normally engineers would say that if we’re hitting the buffers with suspension bridges, that may be because of the material not having and adequate strength-to-weight ratio – so we change it. We’ve looked at it from the point of view of changing the geometry and form. However, you could do both. You can change the material and form you can get double the benefits.’

Improving and advancing

The team will continue to work on the project, building into the optimisation of the form. ‘There are still some issues – dynamic issues connected with wind loading and constructability. We’re talking with people with specific expertise in various areas, and hopefully we can carry this work on,’ says M Gilbert.

Around the world, the same basic concepts for long span bridges have been widely replicated, with many similar suspension buildings around the 2km mark. ‘If we want to build a longer span our work could feed into it,’ says M Gilbert. ‘The suspension bridge isn’t very structurally efficient. There have been suggestions of bridges between UK and France, Scotland and Northern Ireland and [over the Strait
of Gibraltar].’

For the crossings Gilbert mentions, the longer the span the better. This is chiefly due their use as shipping lanes. In particular, the English Channel is an extremely busy lane, with the Dover Strait being the world’s busiest, recording around 500-600 ships per day. Having longer spans means fewer towers to cause obstruction, therefore decreasing the bridges’ effect on shipping traffic.

Getting political

Both the English Channel and Scotland to Northern Ireland bridges have been endorsed and recommended by Boris Johnson – the former back in January 2018, and the latter in October. The bridge to France was quickly passed over by the government due to expenses, construction challenges and shipping obstruction. ‘Shipping moves 95% of the UK’s trade – it is vital that its movement remains as frictionless as possible, without disruption of any kind. The government and the EU should concentrate on keeping trade moving freely through our ports, which is in the economic interest of both sides of the Channel,’ UK Chamber of Shipping Chief Executive, Guy Platten, told the Guardian in January. As Gilbert points out, the new bridge forms could circumvent the issues of shipping obstruction, but the expense would remain.

The Scotland–Northern Ireland route, however, is being considered. In March 2018,  Scottish government spokesperson told BBC programme The View that it will ‘initiate discussions to explore improving connectivity between our two islands’. Boris Johnson reiterated the call in October.

For this, there are two bridges proposed – Larne–Portpatrick and Torr Head–Mull of Kintyre. The second would be the cheapest option as it spans the lesser distance, whereas the first has been calculated to cost £20bln to span the 30 miles. The bridge is being peddled as a physical link for Northern Ireland to the UK, to firm up its position following Brexit.

Alan Dunlop, Visiting Professor at Robert Gordon University, Scott Sutherland School of Architecture, and the University of Liverpool, UK, asked the Scottish, UK and Irish governments to take the prososal seriously in a talk he gave at a conference at Robert Gordon University in September 2018. ‘I have done quite a bit of work looking at major infrastructure projects worldwide and I think on that basis there is a justifiable case about whether it is economically viable. It definitely can be done economically. There is concern about the cost of the project, but you know what they say – you can have three economists you’d have four conclusions. There are opinions either way about whether it can stack up or not,’ he told the Belfast Telegraph.

On the topic of these bridges, M Gilbert offered his opinions, saying, ‘We seem to be in a world that is talking more about building wars rather than bridges. But, we as engineers are here to talk about what’s possible. From a personal point of view, I think bridges are the way to go.’