Structural Drawing

Use of Wrought Iron and Steel.

The great improvements in the manufacture of wrought iron, which followed upon the in troduction and development of the puddling furnace and the roll ing mill by Henry Cort towards the. end of the 18th century, led to its increased use in construction, while the investigations into the strength of cast-iron referred to above probably stimulated the more effective use of wrought iron, the nature of which ap proximated more closely to the ideal elastic material conceived by the mathematicians and physicists. Only the simplest sections were rolled in the solid, beams.

In 1847 Ferdinand Zores produced a small rolled joist or I-sec tion, the form being decided upon after consultation with French builders and iron manufacturers. Larger rolled sections were on view at the Paris exhibition in 1855 and these I-sections had been advocated by Fairbairn in England. The early small rolled joists were largely used in France as the basis of fire-resisting floor con struction, but while they were obviously of value in other forms of construction, the I-section came very slowly into general use. The first steel-joist section to be produced in England was rolled by Dorman and Long in 1855. The Harper building in New York built in 1854 was the first one to have wrought-iron beams set in masonry walls as lateral supports.

Cheap and reliable mild steel became available through the introduction of the Bessemer and Siemens-Martin processes of production but prejudice and early difficulties in manufacture delayed its use. As late as 1877 in England Board of Trade Regulations prohibited the use of mild steel in bridge construction.

In the construction of the Forth bridge (188o-9o) mild steel was, however, employed and the success of this venture constitutes a remarkable tribute to the skill of the structural engineers at that date and a testimony to the fitness and reliability of this material for bridge work and general constructional use. The extended use of steel for structural purposes was assisted and guided by the introduction of standard forms and dimensions, and by uniform specifications. The American Society of Civil Engineers, founded in 1852, in co-operation with the steel manufacturers contributed immensely to the development of standard formulae. In the Carnegie-Phipps company of Pittsburgh published a hand book of standard shapes with information and formulae as to their structural properties. These remained standard for years and were basic for many subsequent handbooks published. In American cities, where skyscraper construction started, building laws were passed giving effect to the conclusions of these early developments. In England specifications were issued by the

British Engineering Standards Association (formed in and the passing of the London County Council (General Powers) Act of 1909, provided the first authoritative regulations for the control of the design of steel-framed buildings.

Development of Framed Construction.—In the construc tion of various kinds of framed structures iron was being used to a considerable extent by the middle of the 19th century; thus the technique of this form of construction was being steadily de veloped. In some of the larger structures pin joints were used, but these were gradually discarded in England and the simpler riveted joint adopted. Of this latter type of construction Tueller's lattice girder bridge (1868) consisting of 30o ft. spans over the Mersey at Runcorn is a good example.

At a somewhat earlier period, similar developments in framed construction were taking place in America, where road bridges were being erected to meet a great demand. The majority of these were constructed in timber alone, or in timber and iron, but the well known forms of Howe truss (183o) and Pratt truss (1844) which were developed in this connection represented substantial progress in the design of framed structures. At a later date, the development of the wire rope by Robling, applied in the construc tion of the great suspension bridge at Brooklyn, indirectly in fluenced the introduction and perfection of the lift or elevator, which became, and is now, so vital an equipment of all tall build ings.

While a considerable amount of iron was used in the construc tion of buildings until well into the latter half of the 19th century, the construction of normal types was based on the assumption that the walls constituted the chief supports for transmitting structural loads to the foundation; also, since the thicknesses of these walls were regulated by building by-laws—which were based upon tradition and custom rather than upon scientific conclusion—the height to which buildings could be economically carried was lim ited, owing to the thick walls necessary to conform with the by laws. Moreover high building possessed no great advantage with out lifts. The first passenger elevator, a "vertical screw railway," was installed in 1859, by Otis Tufts in the Fifth Avenue Hotel in New York. In 1866 the first suspended elevator operated by a steam hoist was installed in the St. James Hotel in the same city.