Balloon flange girder

Together with the work of William Fairbairn, particularly in relation to Stephenson's tubular bridges such as Conwy, there was an increased understanding of how beams in compression would fail by buckling.

[1][2][i] This distrust of cast iron was vindicated when his friend Stephenson's adventurous cast-iron Dee Bridge (1846) collapsed in 1847.

[4] Around the 1840s, developments in the puddling furnace reduced the cost of wrought iron and improvements to rolling mills allowed the production of large flat sections.

These had a T-section lower flange in tension and a larger circular bulb at the upper edge,[ii] in compression.

[7][8] As was his habit, Brunel hydraulically tested samples of these girders for strength in 1838 and recorded the results in one of his books of 'Facts'.

[10][iii] Brunel was aware that the likely failure mechanism of this girder was by buckling collapse in the upper flange, which would be under compression forces.

The side gussets, also curved, were riveted parallel to the edges of this top plate, rather than through another L-strip, as used originally.

More practically, the parallel lap joint halved the amount of riveting needed, compared to the L-strip.

This girder (especially as it was installed after Brunel's death) is thought to have previously been used elsewhere on the SWR,[13] although its original date and location is unknown.

Intermittent cross diaphragms were placed across the flange, to maintain its position relative to the main web and avoid distortion by rocking sideways.

[10] This was of centre-pivot construction, but were highly asymmetrical, the outboard side being nearly three times longer than the landward, balanced by a large cast-iron counterweight.

[10] As the bridge was for a light roadway and did not have to carry the weight of a railway or train, its girders were of a lightweight construction that simplified manufacture.

The upper arch girder uses the triangulated form of the early experiment, with a flat top plate and without any vertical web below the flange box at all.

[27] The bridge as constructed though used the C-shaped open form of the upper flange, as used for the Eastern Bengal Railway.

[23][24] The line of 1857 through Devizes in Wiltshire down Caen Hill was carried over a road on an unusual development of the balloon flange, combining it with a lenticular plate girder.