Type of girder
A
box girder
or
tubular girder
(or
box beam
) is a
girder
that forms an enclosed tube with multiple walls, as opposed to an
I
-
or
H
-beam
. Originally constructed of
wrought iron
joined by
riveting
, they are now made of
rolled
or welded steel, aluminium
extrusions
or
prestressed concrete
.
Compared to an
I
-beam
, the advantage of a box girder is that it better resists
torsion
. Having multiple vertical
webs
, it can also carry more load than an
I
-beam
of equal height (although it will use more material than a taller
I
-beam of equivalent capacity).
The distinction in naming between a
box
girder and a
tubular
girder is imprecise. Generally the term
box
girder is used, especially if it is rectangular in section. Where the girder carries its "content"
inside
the "box", such as the
Britannia Bridge
, it is termed a
tubular
girder.
Tubular
girder is also used if the girder is round or oval in cross-section, such as the
Royal Albert Bridge
.
Where a large box girder contains more than two walls, i.e. with multiple boxes, it is referred to as a
cellular
girder.
Development
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]
The theoretical basis of the box girder was largely the work of the engineer Sir
William Fairbairn
, with the aid of the mathematician
Eaton Hodgkinson
, around 1830. They sought to design for the most
efficient
beam possible in the new material of riveted
wrought iron
plates.
Cellular construction
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Most girders are statically loaded such that one web is in compression, the other in tension. Fairbairn's original
cranes
used a cellular construction for the compression face for their jib, so as to resist buckling. This jib was curved, tapered and formed of riveted wrought iron plates. Three cells were formed inside the concave (lower) face of this girder, again of riveted plates.
[1]
Where a tubular girder is used as a bridge span (i.e. loaded in the centre rather than at one end, like a crane) the compressive force is in the top web of the girder and so the cells are placed at the top. Dynamic forces (moving loads, wind) may also require both faces to be cellular. (The preserved Britannia Bridge section shows that both top and bottom flanges were of cellular construction, but (according to Fairbairn) the cellular construction of the bottom flange was adopted, not because of the nature of the forces it had to withstand,
[2]
: 206
but because of their magnitude and the consequent "practical difficulties which would have been encountered, had it been attempted to achieve the requisite sectional area in a solid mass")
[2]
: 183
In some ways this isn't a "cellular girder" as such (compared to a
spaceframe
or
geodesic construction
) as the cells don't share loads from the entire girder, but merely act to stiffen one plate in isolation. Design of such complex integrated structures requires mathematical modelling techniques in advance of Fairbairn's day.
In bridges
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]
Fairbairn's theoretical girder appeared at just the right time for the increasing demand for long railway bridges.
Robert Stephenson
engaged both him and Hodgkinson as consultants to assist with his
Britannia
and
Conwy
bridges, both of which contained the railway track within a large tubular girder. Shortly afterwards
Brunel
also chose to use a pair of small diameter round girders as part of a larger
truss
at
Chepstow
. However, although many of the longest-span railway bridges in use in the 1860s used tubular or box girders
[3]
Benjamin Baker
in his
Long-Span Railway Bridges
was already dismissing the 'box girder with web plates' as 'the most unfavourable type for long-span railway bridges which it will be necessary for us to investigate'.
[4]
The
Coronado Bay Bridge
has the tallest box girder.
Box girder bridges of shallow rectangular cross-section and
aerofoil
characteristics became extensively used in road bridges from the 1960s onwards, such as the
Severn Bridge
, being much lighter than the deeper
truss
-type
girder
construction used on previous bridges such as the
Golden Gate Bridge
.
Safety concerns over box girder bridges
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]
In the early 1970s, a number of box girder bridges collapsed during construction: the
Cleddau Bridge
in Wales,
West Gate Bridge
in Australia and the Koblenz Bridge in Germany. That led to serious concerns over the continued use of box girders and extensive studies of their safety, which involved an early use of computer modelling, and was a spur to the development of
finite element analysis
in
civil engineering
.
See also
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Bridges
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References
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