120 SECTIONAL ADDRESSES 
steel wire and rods laid in them, the proportion of the steel being so 
entirely inadequate in comparison with the concrete that its addition 
suggests a magical rather than a mechanical influence. 
For the longest spans, reinforced concrete has now superseded masonry ; 
but fine masonry arches of 300-ft. span have been built. The construction 
of spans of increasing length has been made possible by improved tech- 
nique in building. ‘To avoid high stresses arising at the springing and 
key stone, as a result of the settlement or elastic deformation of the 
centering, as weight is added during building, and as a consequence of the 
initial deformation of the arch itself when the centering is removed, gaps 
are left in the arch, and special forms of construction are now introduced 
to act as temporary hinges, so that when the bridge is completed and 
the gaps filled in, the position of the line of thrust is fairly definitely 
known. 
To economise in the quantity of masonry and make it possible to use 
higher compressive stresses, arch bridges are now made with ribs of a 
width comparable with the depth of the arch, instead of the arch being 
made continuous for the whole width of the bridge. 
In reinforced concrete arches either permanent hinges of steel are 
introduced or else all the reinforcing bars are drawn together at the 
critical points to form a temporary hinge, and the surrounding concrete 
is filled in only on completion. 
Reinforced concrete arches with spans as great as 590 ft. have been 
constructed. 
The stability of a masonry dam is a problem that has exercised the 
minds of engineers and mathematicians for many years. The failure of 
the Bouzey dam in France in 1895 gave prominence to the problem. The 
Bouzey dam was straight with a length of 1720 ft., and the water held up 
had a maximum depth of about 40 ft. When the dam failed, the upper 
30 or 35 ft. of its height for a length of 560 ft. was swept away, and the 
flood, passing down the valley, caused great havoc, and eighty-six people 
lost their lives. 
Investigations after the disaster revealed many points of interest. In 
the original design, the maximum pressure on the masonry was the only 
factor considered in calculating its proportions. In the course of the 
investigations after the disaster it was shown that the resultant of the 
thrust combined with the weight of the masonry was so placed that a 
tensile stress of 1-3 tons per sq. ft. must have been imposed on the 
masonry. Laboratory tests proved that the maximum tensile strength 
of the masonry was only 60 per cent. higher. In opposition to the theory 
that the parts that failed had overturned by virtue of this weakness, it 
was held by some that failure was by shearing. ‘The shearing stress being 
calculated as 1-32 tons per sq. ft. by some, and as 3-2 tons per sq. ft. 
by others. 
Rankine, in 1871, had recommended that no horizontal joint in a dam 
should be expected to withstand any tensile stress, in other words, there 
should be no uplifting tendency. After the Bouzey disaster—it was con- 
sidered advisable that at the upstream face there must always be a definite 
compressive stress, and the French Government introduced the regula- 
