On Some Common Errors in Iron Bridge Design
Forfatter: W. C. Kernot
År: 1898
Forlag: FORD & SON
Sted: Melbourne
Sider: 49
UDK: 624.6
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which means a valuable gain in strength. There would be no
difficulty and but little expense in altering these structures now,
removing the cross pieces one or two at a time when the bridge
was free from live load, and replacing them by square plates as
in lig. 28. An even better but slightly more expensive arrange-
ment would be to insert a complete triangulation similar to that
shown for a different purpose in Fig. 8.
lig. 29 represents a type of braced strut appearing in. the late
Sir John Hawkshaw’s great railway bridge over the Thames at
Charing Cross, Fig. 15, and copied thence extensively in Europe
and in New South Wales. It requires but little consideration
to detect the weakness of this arrangement. Suppose there is a
tendency for the member to bend to one side, the rectangular
panels will become rhomboidal, one diagonal being extended, and
the other reduced in length. Now if these diagonals are straight
they will oppose the maximum resistance to such distortion,
but if bent or crooked, as in the Charing Cross Bridge, the
extended one will tend to straighten, and the compressed one to
become more crooked. Experiments have been macle at the
University on different types of compression members, and have
shown that great advantage ensues from replacing this unsatis-
factory double system of crooked flat bars, by a single diagonal
system of straight bars, as shown in Fig. 30. This improvement
might easily be applied to actual structures, taking advantage of
times when they are free from live load to remove the one and
insert the other.
Fig. 31 is a photograph of a number of experimental com-
pression pieces tested at the University, arranged in order of
merit. The best of these carried, in proportion to the metal it
contained, rather more than twice the load that the worst did.
These specimens were free to bend in their own plane, but braced
at three points in a direction at .right angles to their own plane,
and represented compression diagonals in closely latticed girders
intersected and kept from bending in the plane of the main
girder by the tension diagonals. The figures at the bottom
indicate the ratio of the load carried, to the weight of the
structure, and thus express the relative values of the various
systems. The first and third of these models have angle bars
and the second and fourth channel bars for their sides.