This viaduct is built over a rocky ravine on the railway from Port Alfred
to Grahamstown, at a height of about 200 ft. from the bottom. Its length
is 480 ft. 6 in., and the width of the platform is 15 ft., the gauge of
the railway being 3 ft. 6 in. The central span of the viaduct is an arch
of 220 ft. span between abutments, and about 90 ft. height; the remainder
of the space on each side is divided into two spans by an iron pier at a
distance of 68 ft. from the retaining wall. These piers are 36 ft. 2 in.
high, and carry girders 144 ft. long, balanced each on a pivot in the
center. One end of these girders is secured to the retaining walls by
means of horizontal and vertical anchorages, while the other end rests in
a sliding bearing on the top flange of the arch.
[Illustration: BRIDGE OVER THE BLAAUW KRANTZ RAVINE, CAPE COLONY.]
[Illustration: BRIDGE OVER THE BLAAUW KRANTZ RAVINE, CAPE COLONY.]
In designing the structure the following points had to be considered: (1)
That, on account of the great height above the ground, and on account of
the high price of timber at the site, the structure could be easily
erected without the use of scaffolding supporting it as a whole. (2) That,
on account of the high freights to Port Alfred, the quantity of iron in
the structure should be as small as possible. (3) That the single parts of
the principal span should be easy to lift, and that there should be as few
of them as possible. For this latter reason most of them were made in
lengths of 20 ft. and more. The question of economy of material presented
itself as a comparison between a few standard types, viz., the girder
bridge of small independent spans; the cantilever bridge, or the
continuous girder bridge in three large spans; the single girder bridge
with one large span and several small spans; and the arch with small
girder spans on each side. The suspension bridge was left out of question
as inadmissible. A girder bridge with small independent spans on rocker
piers would probably have been the most economical, even taking into
account the great height of the piers near the middle of the ravine, but
there would have been some difficulty in holding those piers in position
until they could be secured to the girders at the top; and, moreover, such
a structure would have been strikingly out of harmony with the character
of the site. On the other hand, a cantilever or continuous girder bridge
in three spans--although such structures have been erected in similar
localities--could not enter into comparison of simple economy of material,
because such a design would entirely disregard the anomaly that the
greater part of the structure, viz., the side spans, being necessarily
constructed to carry across a large space, would be too near the ground to
justify the omission of further supports. The question was, therefore,
narrowed to a comparison between the present arch and a central
independent girder of the same span, including the piers on which it
rests. The small side spans could obviously be left out in each case. The
comparison was made with a view not only to arrive at a decision in this
particular case, but also of answering the question of the economy of the
arch more generally. The following table contains the weights of
geometrically similar structures of three different spans, of which the
second is the one here described. The so-called theoretical weight is that
which the structure would have if no part required stiffening, leaving out
also all connections and all wind bracing. The moving load is taken at one
ton per foot lineal, and the strain on the iron at an average of four tons
per square inch. The proportion of the girder is taken at 1 in 8.
--------------+-----------------------+------------------------+
| Theoretical Weight. | Total Weight. |
Span in Feet. +-----------------------+------------------------|
| Arch. | Girder. | Arch. | Girder. |
--------------+---------+-------------+------------+-----------|
100 | 0.0724 | 0.1663 | 0.1866 | 0.2443 |
220 | 0.1659 | 0.4109 | 0.4476 | 0.7462 |
300 | 0.2414 | 0.6445 | 0.6464 | 1.2588 |
--------------+---------+-------------+------------+-----------+
|<------------Tons per foot lineal.------------->|
It can be seen from these results that the economical advantage of the
arch increases with the span. In small arches this advantage would not be
large enough to counterbalance the greater cost of manufacture; but in the
arch of 220 ft. span the advantage is already very marked. If the table
were continued, it would show that the girder, even if the platform were
artificially widened, would become impossible at a point where the arch
can still be made without difficulty. The calculations leading to the
above results would occupy too much space to make it desirable on this
occasion to produce them. Our two views are from photographs.--_The
Engineer._
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