Whatever may be the misgivings entertained by many engineers respecting
the future use of cast iron for structures of certain kinds, it is clear
that for architectural purposes this material is likely to be employed
to an extent hardly contemplated by many who have looked upon it with
disfavor. At the present moment many buildings may be seen in London, in
which cast iron has been introduced instead of stone for architectural
features, and the substitution of cast iron for façades in many
warehouses and commercial buildings seems to show that, notwithstanding
the prejudices of the English architect against the importation of the
iron architecture of our transatlantic brethren, there is a prospect of
its being largely employed for frontages in which ample lighting and
strength are needed. The extensive window space necessary in narrow city
thoroughfares, and the difficulty of employing brick in large masses,
such as pilasters and lintels, have chiefly led to the adoption of
material having less of the uncertain durability and strength of either
stone or terra-cotta in its favor. Architects would gladly resort to the
last-named material if it could be procured in sufficient size and mass
without the difficulties attendant upon shrinkage in the burning, and
the winding and unevenness of the lines thereby caused. They have also
an even more tractable material in concrete ready to their hand, if they
would seriously bring themselves to the task of stamping an expressive
art upon it, instead of going on designing concrete houses as if they
were stone ones. Cast iron has the advantage of being a tried material;
it is well adapted for structures not liable to sudden weights or
to vibration, and so it has come to be used for features of an
architectural kind, by a sort of tacit acknowledgment in its favor.
Those who are desirous of seeing examples of its employment in fronts
of warehouses will find instances in Queen Victoria Street, Southwark
Street, and Bridge Road, and Theobalds Road, where the whole or portions
of fronts have been constructed of cast iron. At some corner premises in
Southwark, the piers as well as the windows are formed of cast iron,
the former being made to assume the appearance of projecting pilasters.
There is nothing to which the most captious critic could object in the
treatment adopted here; the pilasters and other features have plain
moulded members, and there is no principle of design in cast work which
has been violated--the only question being the purely aesthetic one--is
it justifiable to copy features in cast iron which have generally been
constructed in stone or marble? The answer is obvious: Certainly not,
when those features suggest the mass and proportions or treatment
proper only for stone or marble; but when they do not so represent the
material, it is quite optional for the architect to build up his front
with castings, if by so doing he can obtain greater rigidity of bearing,
strength, and durability. He ought, of course, to vary the proportions
of his pilasters and horizontal lintels, and make them more in accord
with the material. It is the wholesale reproduction of the more costly
and ornamental features, such as we see in many buildings of New York
and Philadelphia, where whole fronts are manufactured of cast iron and
sheet-metal, which has shocked the minds of architects of culture and
sensitive feeling. Such imitations and cheap displays outrage the artist
by the attempt to produce in cast or rolled metal what properly belongs
to a stone front.
Bearing this distinction in mind, we are not presuming too much to
assert that architects have in cast iron, when properly employed under
certain restrictions, a material which might be turned to account in
narrow fronts where the use of brick or stone piers would encroach too
much upon the space for light. For warehouse fronts, we have evidence
for thinking that the employment of iron might be attended with
advantage, especially in combination with brickwork for the main
vertical piers. Plain classic mouldings, capitals and bases of the Doric
or Tuscan order, are well suited for cast-iron supports to lintels or
girders. In one attempt to make use of the structural features of the
latter, the fronts of the girders between the piers are divided into
panels, the flanges and stiffening pieces to the webs forming an
effective framework for cast or applied ornament to be introduced. The
iron framework thus constructed lends itself to the minor divisions of
the window openings, which can be of wood. In the new Leaden Hall and
Metropolitan Fruit and Vegetable Markets, cast-iron fronts have
been largely employed, consisting of stanchions cast in the form of
pilasters, with horizontal connections and other architectural members.
Regarding the more constructive aspects of cast iron, the employment of
it in fronts having numerous points of support and small bearings is
clearly within the capabilities of the material. So long as it is used
in positions in which its resistance to compression is the chief office
it has to fulfill, cast iron is in its right place. In the fronts of
buildings, therefore, where it is made to carry the floors and rolled
joists, and the lintels of openings, either as piers, pilasters, or
simply as mullions of windows, it is strictly within its legitimate
functions. So with regard to lintels and heads of openings where short
spans exist, cast iron is free from the objection that can be urged
against it for long girders. In fact, no position is better fitted for a
brittle, granular material than that of a vertical framework to receive
windows and ornamentation, and for such purposes cast iron is, to our
minds, admirably suited.
For bridge-building the value of this metal has lately been much
disputed, though we have several notable examples of its use in the
earlier days for such structures. In fact, the use of cast iron for
structural purposes is not older than the time of Smeaton, who in 1755
employed it for mill construction, and about the same time the great
Coalbrookdale Viaduct was erected across the Severn near Broseley, which
gave an impetus to the use of cast iron for bridge construction. The
viaduct had a span of 100 feet, and was composed of ribs cast in two
pieces; it was erected from castings designed by Mr. Pritchard, of
Shrewsbury, an architect, and this circumstance is worthy of note as
showing that an architect really was the first to employ this material
for important structural work, and that the same profession was the
first to reject it upon traditional grounds. It is quite certain,
however, the bridge-builder lost no time in trying his hand upon so
tractable a material; for not long after Telford erected a bridge at
Buildwas of even a greater span, and the famous cast-iron bridge over
the river Wear at Sunderland was erected from the designs of Thomas
Paine, the author of the "Age of Reason." Iron bridges quickly followed
upon these early experiments, for we hear of several being built on
the arched system, and large cotton-mills being erected upon fireproof
principles at the commencement of the present century, the iron girders
and columns of one mill being designed by Boulton and Watt. A little
later, Eaton Hodgkinson proved by experiments the uncertainty of cast
iron with regard to tensile strength, which he showed to be much less
than had been stated by Tredgold. Cast iron was afterwards largely
adopted by engineers. The experiments of Hodgkinson supplied a safe
foundation of facts to work upon, and cast iron has ever since retained
its hold. Thomas Paine's celebrated bridge at Sunderland had a span of
236 feet and a rise of 34 feet, and was constructed of six ribs, and is
remarkable from the fact that the arched girder principle used in the
Coalbrookdale and Buildwas bridges was rejected, that the ribs were
composed of segments or voussoirs, each made up of 125 parts, thus
treating the material in the manner of stone. Each voussoir was a
cast-iron framed piece two feet long and five feet in depth, and these
were bolted together. The Southwark bridge over the Thames, by Sir
John Rennie, followed, in which a similar principle of construction is
adopted. There is much to be said in favor of a system which puts each
rib under compression in the manner of a stone arch, and which builds up
a rib from a number of small pieces. At least, it is a system based on
the legitimate use of cast iron for constructive purposes. The large
segmental castings used in the Pimlico bridge, and the new bridge
over the Trent at Nottingham, from Mr. M. O. Tarbotton's design, are
excellent examples of the arched girder system. The Nottingham bridge
has each rib made up of three I-shaped segments bolted together and
united transversely; the span is 100 feet in each of the three openings,
and the ribs are three feet deep at the springing, diminishing about six
inches at the crown. We have yet to learn why engineers have abandoned
the arched bridge for the wrought iron girder system, except that the
latter is considered more economical, and better fitted for bearing
tensile stress. Cast-iron bridges constructed as rigid arches, subject
to compression and composed of small parts, have all the mechanical
advantages of stone without some of its drawbacks, while artistically
they can be made satisfactory erections.--_Building News_.
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