This page includes pictures of a new bridge over the river Severn at Gloucester, part of a by-pass to connect Hempsted with the A40, which it joins on Alney Island.

The entire by-pass is built on soft alluvial ground west of Gloucester, which is subject to more or less annual flooding.  The nature of this ground is the reason why Gloucester has not penetrated west of east channel of the river Severn, apart from some low lying houses on Alney Island.

The pictures are shown in order from south Llanthony Road) to north (A40), and were taken in January 2004.

Looking south, under the bridge at Llanthony road, showing the plate girders and the cylindrical concrete piers.  Note the camber of the bridge.  The steel piles behind the piers separate the bridge workings from the soil behind.  At the end of the bridge, a series of steel plates connects all the plate girders.  The bridge is built on a large skew angle to the river, as the picture shows.

These pictures show the end plates and the steel piles that were visible in the picture above, as well as the tops of the concrete piers.  Note the very small bearings on which the girders rest.  There is no point in providing large bearing surfaces, because even if they were surveyed and built very accurately, deflection of the bridge during use would reduce the contact area with every passage of traffic.  Better to use a small area with known and controlled behaviour, such as cylindrical or spherical bearings.

Here are views of the bridge, which has a span over the river and a span over the bank.  The cylindrical piers on the far bank are founded on a massive cuboidal pier which is founded deep in the ground.  The sides of the bridge are in shadow, because of the temporary footways bracketed out from them.

This typing of steel piling can be used to separate the workings from soil or water.   Each pile has a cross section like a rather angular S, and each interlocks with its neighbour.  The piles are driven using a pile driver, which is continually allows a heavy metal block to fall on the pile, which is protected by a cap.

A giant mechanical vulture is ripping into old concrete construction, which must be removed in order to build new.  Deformed steel reinforcing bars are visible: they are not broken by the machine.

Temporary footways are bracketed out from the bridge by means of standard angle-beams and adjustable struts.  Safety standards are of course as important for such work as for the main construction.  The use of tools and equipment on site are also the subject of strict control.

Near the new bridge, a very old disused railway track passes over the flood plain on a trestle bridge and an embankment.  Note the flood relief pipes in the embankment.

The plate girders are not continuous: the land span includes bolted joints.  Since there is only one joint in each girder, the joints do not have to transmit bending moment: they only have to carry half the weight of the land-spans and resist the shear force.  The second picture shows a place where cross-bracing is attached.

This picture shows one of numerous K-trusses that connect the I-beams.  They are needed because I-beams are stiff against the bending that they are designed to resist, but not against torsion about their long axis.  Their use in the decks of suspension bridges was discontinued for that reason.  In those bridges the long dimension of the I was in the horizontal plane.

The entire course of the by-pass lies across a flood-plain.  The embankment includes a number of culverts that allow flood water to pass through.  The second picture shows that ground movement has already occurred, even though the completion of the project is more than a year away.  The movement of these concrete restrainers is not, however, very significant.

Near the northern end of the by-pass another bridge crosses a low lying area of the flood plain.  It is a plate girder bridge with three spans.  The central span has a bolted-in centre section.