Fan. 12, 1882] 
NA TURE 247 
m= 180 Hoan 22a n= BIS nme 
| ee are a aac 2 iam Al 
The Forth Bridge. 
z 
“ELEVATION 
SECTIONAL PLAN AT RAIL LEVEL. 
ALLISON 
attainable. It caused no little surprise therefore, and not 
a few expressions of incredulity, when Messrs. Fowler and 
Baker announced that the span of 1700 feet was no reason 
at all for departing from the old and well-tested principle 
of girder construction, but that on the contrary a girder 
bridge of proper design would be not only infinitely stiffer, 
but also considerably cheaper, than a structure on the 
suspension principle. This conclusion was however con- 
firmed by Mr. W. H. Barlow and Mr. T. E. Harrison 
after careful independent investigation, and a design was 
finally agreed upon by all the engineers and accepted by 
the four railway companies finding the funds for its 
construction. 
The Forth Bridge as intended to be constructed may 
be briefly described as a continuous steel girder bridge, 
of varying depth and with fixed points of contrary flexure. 
In several essential points it is analogous to the well- 
known continuous tubular girder bridge across the Menai 
Straits, as in both cases the continuous girder traverses 
two main spans and two end spans of about half the width 
of the former. In the Forth Bridge, however, the main 
spans are 1700 feet, and the end spans 675 feet, whilst in 
the Britannia Bridge the respective dimensions are but 
460 feet and 230 feet. It is hardly necessary to say, 
therefore, that though the principle is the same the pro- 
portions and general details of the two structures present 
no points of resemblance. 
The continuous girders of the Forth Bridge are 340 
feet, or one-fifth of the span, in depth at the piers, and 
only 45 feet deep at the centre of the 1700 feet span. In 
ordinary continuous girders the points of contrary flexure 
are situated about one-fifth of the span from the piers, 
but in the Forth Bridge they are fixed at the most econo- 
mical distance, which was found on investigation to be 
about two-fifths. Every continuous girder may, for pur- 
pose of calculation, be regarded as made up of a central 
girder supported by two cantilevers. In ordinary cases 
the central girder will have an effective span about equal 
to the total span +/%3. If the Forth Bridge were an 
ordinary continuous girder the effective span of the 
central girder would thus be about 982 feet, but as a 
matter of fact it will be made 350 feet, and it is to this 
deviation from the ordinary proportions that the economy 
of the design is largely due. By keeping the central 
girder of moderate span, or in other words by removing 
the points of contrary flexure farther from the piers, the 
central portion of the girder is lightened, the bending 
moment correspondingly reduced, and the mass of 
metal is concentrated near the piers, where it acts with 
the least leverage. Thus the lower member of the great 
girder, where it springs from the masonry piers at a 
height of 20 feet above high water, is a steel tube 12 feet 
diameter and about 2 inches thick, whilst at the centre of 
the span, where the bottom member has been cambered 
upwards to a height of 150 feet for navigation purposes, 
it is of ordinary trough section, about 3 feet in width. 
On plan as well as in elevation the Forth Bridge is a 
continuous girder of varying depth. To resist high wind 
pressures the tubular lower members are spaced 120 feet 
apart, centre to centre at the piers, whereas the trough 
lower members of the central girder are spaced but 27 
feet apart. By varying the effective depth of the girder 
both on plan and elevation a great saving is effected in 
the bracing, as the shearing stresses are to a large extent 
taken up by the main members of the girder. Ina bow- 
string girder the bracing is similarly lightened by the 
arching of the top member. There are other sources of 
economy in the proposed design which we have not space 
to refer to at present. 
It is intended, we understand, to give such proportions 
to the several members of the structure that under the 
combined stresses resulting from the maximum rolling 
load and a wind pressure of 56 lbs. per square foot, 
the stress per square inch will in no case exceed one- 
