A balloon flange girder or (colloquially) balloon topper is a form of vertical
I-beam
An I-beam, also known as H-beam (for universal column, UC), w-beam (for "wide flange"), universal beam (UB), rolled steel joist (RSJ), or double-T (especially in Polish, Bulgarian, Spanish, Italian and German), is a beam with an or -shape ...
wrought iron
Wrought iron is an iron alloy with a very low carbon content (less than 0.08%) in contrast to that of cast iron (2.1% to 4%). It is a semi-fused mass of iron with fibrous slag inclusions (up to 2% by weight), which give it a wood-like "grain" ...
plate girder
A plate girder bridge is a bridge supported by two or more plate girders.
Overview
In a plate girder bridge, the plate girders are typically I-beams made up from separate structural steel plates (rather than rolled as a single cross-section), ...
, where the top flange, instead of being a simple flat plate, is extended into a hollow tube. When a girder is subjected to a positive bending moment the top flange acts in compression making a flat plate flange more susceptible to local buckling than the balloon flange is.
This type of girder was rarely used, its only common user being
Isambard Kingdom Brunel
Isambard Kingdom Brunel (; 9 April 1806 – 15 September 1859) was a British civil engineer who is considered "one of the most ingenious and prolific figures in engineering history," "one of the 19th-century engineering giants," and "on ...
in the 1840s and 1850s.
Design
Brunel was working at a period of increased theoretical and mathematical analysis of bridge and mechanical structures. Together with the work of
William Fairbairn
Sir William Fairbairn, 1st Baronet of Ardwick (19 February 1789 – 18 August 1874) was a Scottish civil engineer, structural engineer and shipbuilder. In 1854 he succeeded George Stephenson and Robert Stephenson to become the third pre ...
, particularly in relation to
Stephenson's
tubular bridge
A tubular bridge is a bridge built as a rigid box girder section within which the traffic is carried. Famous examples include the original Britannia Bridge over the Menai Strait, the Conwy railway bridge over the River Conwy, designed and teste ...
s such as
Conwy
Conwy (, ), previously known in English as Conway, is a walled market town, community and the administrative centre of Conwy County Borough in North Wales. The walled town and castle stand on the west bank of the River Conwy, facing Deganwy on ...
, there was an increased understanding of how beams in
compression
Compression may refer to:
Physical science
*Compression (physics), size reduction due to forces
*Compression member, a structural element such as a column
*Compressibility, susceptibility to compression
* Gas compression
*Compression ratio, of a ...
would fail by
buckling
In structural engineering, buckling is the sudden change in shape ( deformation) of a structural component under load, such as the bowing of a column under compression or the wrinkling of a plate under shear. If a structure is subjected to a ...
.
Brunel was known for his distrust of
cast iron
Cast iron is a class of iron– carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impur ...
as a material, at least for large beams.
This distrust of cast iron was vindicated when his friend Stephenson's adventurous cast-iron
Dee Bridge (1846) collapsed in 1847. Brunel gave evidence in his support at the following inquiry, but this was on the basis of Stephenson being a competent engineer within the bounds of current knowledge, rather than in support of large cast-iron beams.
Stephenson's Dee Bridge had used a truss girder, where an inverted T-shaped cast iron girder was trussed by applied wrought iron tension bars. This faulty design was instrumental in the bridge failure, where tension in the truss rods increased compression in the upper part of the girder such that it underwent
columnar failure. Despite this, although with the advantage of hindsight, Brunel would use similar applied tension chains for his
truss
A truss is an assembly of ''members'' such as beams, connected by ''nodes'', that creates a rigid structure.
In engineering, a truss is a structure that "consists of two-force members only, where the members are organized so that the assembl ...
design.
Around the 1840s, developments in the
puddling furnace
Puddling is the process of converting pig iron to bar (wrought) iron in a coal fired reverberatory furnace. It was developed in England during the 1780s. The molten pig iron was stirred in a reverberatory furnace, in an oxidizing environment, ...
reduced the cost of
wrought iron
Wrought iron is an iron alloy with a very low carbon content (less than 0.08%) in contrast to that of cast iron (2.1% to 4%). It is a semi-fused mass of iron with fibrous slag inclusions (up to 2% by weight), which give it a wood-like "grain" ...
and improvements to
rolling mill
In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, and/or to impart a desired mechanical property. The concept is simil ...
s allowed the production of large flat sections. This iron was now economic for the construction of girders, assembled by riveting of flat sections.
Development
Brunel had already experimented with simple bulb-headed girders in cast iron, for the relatively short 35 foot span of
Bishop's Bridge
Bishop's Bridge, sometimes known as Paddington Bridge, is a road bridge in the Paddington district of London which carries Bishop's Bridge Road across the rail approaches to Paddington station and across the adjacent Paddington Arm of the Grand U ...
canal bridge at
Paddington
Paddington is an area within the City of Westminster, in Central London. First a medieval parish then a metropolitan borough, it was integrated with Westminster and Greater London in 1965. Three important landmarks of the district are Padd ...
. These had a T-section lower flange in tension and a larger circular bulb at the upper edge, in compression. 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'.
Experimental girder
To develop a reliable truss girder for long-span bridges, Brunel carried out a remarkable experiment with a full size girder. This used a single wrought iron plate girder, 70 ft in length, which was loaded up to the point of collapse, first with 165 tons load then, after repair, to 188 tons.
Brunel was aware that the likely failure mechanism of this girder was by
buckling
In structural engineering, buckling is the sudden change in shape ( deformation) of a structural component under load, such as the bowing of a column under compression or the wrinkling of a plate under shear. If a structure is subjected to a ...
collapse in the upper flange, which would be under
compression
Compression may refer to:
Physical science
*Compression (physics), size reduction due to forces
*Compression member, a structural element such as a column
*Compressibility, susceptibility to compression
* Gas compression
*Compression ratio, of a ...
forces. To resist this, the flange was supported by triangulated plates and the flange was also slight curved. The experiment was a great success, the bridge eventually failing at a considerable load, representing an efficient use of construction materials for a bridge of this capacity, compared to previous designs.
South Wales Railway
Brunel made early and widespread use of this girder across his
South Wales Railway
The South Wales Railway ( cy, Rheilffordd De Cymru) was a main line railway which opened in stages from 1850, connecting the Great Western Railway from Gloucester to South Wales. It was constructed on the broad gauge. An original aspiration was to ...
. It was used for spans up to 100 feet, iron
truss bridge
A truss bridge is a bridge whose load-bearing superstructure is composed of a truss, a structure of connected elements, usually forming triangular units. The connected elements (typically straight) may be stressed from tension, compression, or ...
s being used beyond that.
Improvements in plate rolling allowed a change in the shape of the girder. Rather than merely a slightly-curved top plate with triangular gussets, it was now possible to roll a semi-circular plate. This allowed the fully developed 'balloon' shape to be used, as in the second cross-section illustrated.
The top web of the girder was semi-circular and riveted to the centre plate by an L-strip. The side gussets, also curved, were riveted parallel to the edges of this top plate, rather than through another L-strip, as used originally. Brunel (probably correctly) considered the smooth balloon profile to be a more efficient design, influenced by his geometric approaches to design rather than Eaton's mathematical analysis. More practically, the parallel lap joint halved the amount of riveting needed, compared to the L-strip.
None of these bridges are known to survive in their balloon form, although a girder from one was later re-used for bridge widening work (1861) across the
Coity
Coity is a village in Bridgend County Borough, south Wales, part of Bridgend town urban area. Coity is part of the community of Coity Higher and is notable for being home to Coity Castle, one of the best preserved castles in Glamorgan. Other bui ...
road near
Bridgend
Bridgend (; cy, Pen-y-bont ar Ogwr or just , meaning "the end of the bridge on the Ogmore") is a town in Bridgend County Borough in Wales, west of Cardiff and east of Swansea. The town is named after the medieval bridge over the River Og ...
and survived there.
The Coity Road bridge had been built before this date, but was widened to accommodate the new
Llynvi Valley Railway
In 1861 the Llynvi Valley Railway was opened in Glamorganshire, Wales, to convey mineral products to the Bristol Channel at Porthcawl. It adopted an earlier tramroad, the Duffryn Llynvi and Porthcawl Railway. The Llynvi and Ogmore Railway was o ...
. One side of the bridge was moved outwards to accommodate a new
siding and a balloon girder installed on that side. This girder (especially as it was installed after Brunel's death) is thought to have previously been used elsewhere on the
SWR,
although its original date and location is unknown.
Similar girders were used to cross the
Severn
, name_etymology =
, image = SevernFromCastleCB.JPG
, image_size = 288
, image_caption = The river seen from Shrewsbury Castle
, map = RiverSevernMap.jpg
, map_size = 288
, map_c ...
at
Over
Over may refer to:
Places
*Over, Cambridgeshire, England
*Over, Cheshire, England
*Over, South Gloucestershire, England
* Over, Tewkesbury, near Gloucester, England
** Over Bridge
*Over, Seevetal, Germany
Music
Albums
* ''Over'' (album), by Pe ...
.
Eastern Bengal Railway
The next development retained the semicircular top flange, but the side gussets were now abandoned altogether as it was considered that the depth of the flange, even if not supported by another plate, would be stiff enough. This also allowed better access to the inside, for painting. Intermittent cross diaphragms were placed across the flange, to maintain its position relative to the main web and avoid distortion by rocking sideways.
The form of girder was widely used in Brunel's work for the
Eastern Bengal Railway.
Cumberland Basin bridges
When Brunel rebuilt the entrance locks of the
Cumberland Basin in
Bristol Harbour, between 1848–1849, he also constructed a '
swivel bridge' – Brunel's first moving bridge.
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
A counterweight is a weight that, by applying an opposite force, provides balance and stability of a mechanical system. The purpose of a counterweight is to make lifting the load faster and more efficient, which saves energy and causes less wea ...
.
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. A full balloon upper flange was used, similar in shape to the South Wales Railway bridges, but the flange sat above the main web of the girder and the web did not span the flange and reach to the top. This simplified construction as it avoided the T-joint, the necessary L-strips and thus several rows of riveting.
The lower flange was of an entirely novel form, being triangular in section, although with concave sides. Again, the main web did not span the flange. All three joints were now simple lap joints with single-row riveting.
Like a number of early Brunel bridges, Brunel's involvement with them was largely forgotten and only recorded in obscure works. At one point they were under serious threat of demolition until their historical significance was re-recognised. A £1 million appeal was launched in 2014 to restore the bridge.
A very similar swivel bridge was built a few years later as the in
Kronstadt
Kronstadt (russian: Кроншта́дт, Kronshtadt ), also spelled Kronshtadt, Cronstadt or Kronštádt (from german: link=no, Krone for " crown" and ''Stadt'' for "city") is a Russian port city in Kronshtadtsky District of the federal city ...
, Russia.
Windsor Railway Bridge
Windsor Railway Bridge
Windsor Railway Bridge is a wrought iron ' bow and string' bridge in Windsor, Berkshire, crossing the River Thames on the reach between Romney Lock and Boveney Lock. It carries the branch line between Slough and Windsor.
The Windsor Railway B ...
(1849) is a
tied-arch
A tied-arch bridge is an arch bridge in which the outward horizontal forces of the arch(es) caused by tension at the arch ends to a foundation are countered by equal tension of its own gravity plus any element of the total deck structure such grea ...
or bowstring girder bridge. The span is composed of two girders that form a truss. The upper girder is an
arch and carries the weight of the bridge. The lower girder is suspended from this by vertical rods and is not required to support its own weight. The main function of the lower girder is to act as a tie; this counteracts the side-forces of the arch, avoiding the arch's usual side-forces on its foundations. As this is a railway bridge, where suspended deck bridges are a problem owing to swaying of their deck, this girder also has a stiffening function.
Brunel used a form of his balloon flange girder for both girders. 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. The lower girder uses the 'open' form of the flange, slightly curved and with no gusset plates. As the lower girder is not carrying its weight, it is not subject to the usual buckling forces.
Chepstow Railway Bridge
Chepstow Railway Bridge
Chepstow Railway Bridge was built to the instructions of Isambard Kingdom Brunel in 1852. The "Great Tubular Bridge" over the River Wye at Chepstow, which at that point forms the boundary between Wales and England, is considered one of Brunel' ...
(1852) was a complicated bridge that made the first use of
Brunel's truss design to produce a suspension bridge with a wide uninterrupted span at high level above a shipping channel. The west bank of the gorge was shallow and muddy though, so half of the bridge's total span was provided by three 100 foot spans of a girder bridge, carried on cast iron cylindrical piers. These girders (''illus'') were of a form and size very similar to the original experimental girder.
The landward girders were replaced in 1948 and the main truss, with its girders beneath, in 1962.
Portions of the girders survive today.
Crathie Bridge
Crathie
Crathie ( gd, Craichidh) is a village in Aberdeenshire, Scotland. It stands on the north bank of the River Dee.
Abergeldie Castle is away. It was built around 1550 and had 19th century additions. It was garrisoned by General Hugh Mackay in ...
Bridge (1854–1857) is a 125 foot single span across the
River Dee to the royal
Balmoral estate.
The bridge had first been drawn with a form of the
Brunel truss echoing his
Royal Albert Bridge
The Royal Albert Bridge is a railway bridge which spans the River Tamar in England between Plymouth, Devon and Saltash, Cornwall. Its unique design consists of two lenticular iron trusses above the water, with conventional plate-girder app ...
at
Saltash
Saltash (Cornish: Essa) is a town and civil parish in south Cornwall, England, United Kingdom. It had a population of 16,184 in 2011 census. Saltash faces the city of Plymouth over the River Tamar and is popularly known as "the Gateway to Corn ...
. The bridge as constructed though used the C-shaped open form of the upper flange, as used for the
Eastern Bengal Railway. As the bridge was only for light road traffic, it was also possible to replace the solid web of the girder above the roadway level with an openwork lattice, making the view from the bridge visually more appealing for its illustrious resident. Despite this, Her Majesty was 'not amused' by the bridge.
Devizes 'Fish Bridge'
The
line of 1857 through
Devizes
Devizes is a market town and civil parish in Wiltshire, England. It developed around Devizes Castle, an 11th-century Norman castle, and received a charter in 1141. The castle was besieged during the Anarchy, a 12th-century civil war between ...
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. This lenticular fishbelly shape gave its popular name of 'Fish Bridge'. From his work on his truss design, Brunel was already familiar with the advantages of the lenticular form.
The original Fish Bridge was replaced in 1901 with a
bowstring truss. The name remained in common use, even after the railway was removed during the
Beeching Axe.
Surviving examples
* Over Junction Bridge, the railway bridge across the
Severn
, name_etymology =
, image = SevernFromCastleCB.JPG
, image_size = 288
, image_caption = The river seen from Shrewsbury Castle
, map = RiverSevernMap.jpg
, map_size = 288
, map_c ...
at
Over
Over may refer to:
Places
*Over, Cambridgeshire, England
*Over, Cheshire, England
*Over, South Gloucestershire, England
* Over, Tewkesbury, near Gloucester, England
** Over Bridge
*Over, Seevetal, Germany
Music
Albums
* ''Over'' (album), by Pe ...
. (1848)
*
Cumberland Basin,
Bristol Harbour (1849) One fixed span has been recently restored for use as a footbridge.
*
Windsor Railway Bridge
Windsor Railway Bridge is a wrought iron ' bow and string' bridge in Windsor, Berkshire, crossing the River Thames on the reach between Romney Lock and Boveney Lock. It carries the branch line between Slough and Windsor.
The Windsor Railway B ...
(1849)
*
Chepstow railway bridge
Chepstow Railway Bridge was built to the instructions of Isambard Kingdom Brunel in 1852. The "Great Tubular Bridge" over the River Wye at Chepstow, which at that point forms the boundary between Wales and England, is considered one of Brunel' ...
(1852)
Sections of the girders survive at
Brunel University
Brunel University London is a public research university located in the Uxbridge area of London, England. It was founded in 1966 and named after the Victorian engineer and pioneer of the Industrial Revolution, Isambard Kingdom Brunel. In June ...
,
Uxbridge
Uxbridge () is a suburban town in west London and the administrative headquarters of the London Borough of Hillingdon. Situated west-northwest of Charing Cross, it is one of the major metropolitan centres identified in the London Plan. Uxb ...
.
*
Crathie
Crathie ( gd, Craichidh) is a village in Aberdeenshire, Scotland. It stands on the north bank of the River Dee.
Abergeldie Castle is away. It was built around 1550 and had 19th century additions. It was garrisoned by General Hugh Mackay in ...
Bridge (1854–1857) to the royal
Balmoral estate.
* Coity Road Bridge,
Bridgend
Bridgend (; cy, Pen-y-bont ar Ogwr or just , meaning "the end of the bridge on the Ogmore") is a town in Bridgend County Borough in Wales, west of Cardiff and east of Swansea. The town is named after the medieval bridge over the River Og ...
(1861)
* Stoodleigh Bridge, Devon. Semi-circular open-bottom top flange,
lattice truss
A lattice bridge is a form of truss bridge that uses many small, closely spaced diagonal elements forming a lattice. The lattice Truss Bridge was patented in 1820 by architect Ithiel Town.
Originally a design to allow a substantial bridge to be ma ...
girder, c. 1863.
* Peel Street Bridge, Bristol. Semi-circular open-bottom top flange,
lattice truss
A lattice bridge is a form of truss bridge that uses many small, closely spaced diagonal elements forming a lattice. The lattice Truss Bridge was patented in 1820 by architect Ithiel Town.
Originally a design to allow a substantial bridge to be ma ...
girder, 1878.
* The canal portion of
Bishop's Bridge
Bishop's Bridge, sometimes known as Paddington Bridge, is a road bridge in the Paddington district of London which carries Bishop's Bridge Road across the rail approaches to Paddington station and across the adjacent Paddington Arm of the Grand U ...
,
Paddington
Paddington is an area within the City of Westminster, in Central London. First a medieval parish then a metropolitan borough, it was integrated with Westminster and Greater London in 1965. Three important landmarks of the district are Padd ...
, may be Brunel's first use of this technique. It was dismantled and stored in 2004.
See also
*
Brunel truss
References
{{Brunel
Plate girder bridges
Girders
Bridges by Isambard Kingdom Brunel