Year: 1874




In report on the Illinois and st. Louis
bridge IN 1863 the author advanced the proposition that
nm upright arched bridge could be more cheapy con-
sruced for railroad purposes than was possible by the
suspension system. This postulae called forth some
dissenting views from engineering journals soon after,
10 which had not at the time the leisure to reply In
this paper will endeavor TO show that upright arched
bridges can be more economically constructed than is
possible by any other method whatever no matter what
length Of span may be required AS the cost OF the
spap excepting the cost Of roadway will by any system
increase a, least as rapidly as the square OF the
distance spanned, the a length Of span will be
limited In all cases by financial difficulties
before grave engineering ore encountered. will


frat point out OF the disadvantages OF the system
adopted for the supersructure OF the st. Lons BRIDGE
which was the best method OF arch construction then
known 10 me- and will respectfully venture 70 indicate
such improvements as have suggested themselves in
OF these difficulties being 80 foroiby thrust
upon my attention IN the progress of that work. In the
st. Louis bridge the upper ana lower members which
constitute single rib OF the span, are tubular In form,
the tubes being each inches outside diameter The rib
thus formed feet deep from center TO center Of the
tube and the lines OF tubing braced together by


single triangular system OF vertical bracing formed OF
fat bars secured the at points about II feet dis
tapt from each other throughout the length Of the rib
the bars being placed IN pairs one on each side OF the
tubes and secured to them by pms passing through the
ends OF the bars and the couplings OF the tubes An
upper and lower line OF tubes thus braced constitutes
one rib Of an span The curve selected for the
ribs was greater convenience of manufacture, the
segment OF circle. Each individual tube ia straight
the curvature rib being accomplished at the juno
tiou Of these individual members each OF which is
about 70 feet long, The curve Of the rib differs from
parabOa but few inches.


EFFECTS OF TEMPERATURE.


II the effects Of temperature could be avoided and the
curve parabola, equally distributed load on
nh 0f hA hy 10 aNd W ar
tubes equally. e. half each and the only strain
bracng would be in transferring half Of the
load the lower tube In this the tubes could be
together and the bracDg dispensed with. We should
SECTION requisite to sustain this equally distributed load
To the arch against unequal loading the tubes
was assumed 10 60 F. The effect Of temperature
(ranging from 400 TO 1t0o FahrenheO increases the
tha crata 10 rise which relieves the lower tube OF COM
abniments and hence that tube does not
osS iasa so' are borne Wholly by the upper
tube at the abutments, aud therefore cts section
eram the stratus are Likewise changed. There
however the lower tube does al tne duty.
the in the upper tube at this point hence
the lower must be increased at the center OF the


s,oer tSbs at the and the lower one
at The crsm must, when the temperature rises ABOVE
RIB and its load The strains in the respective tube:
decrease from these points toward tne hauhcnss oi
through the ,1idde e Of the rib Al the launches
tho compression Is sustained IN neaI5 ,nnna edua
Inc extra section required by temperature. therefore,
becomes smaller In the tubes leave the cram and
r,hs are formed for medium Temperature contraction
by cold reverses these strains The OF the arch
,ss ;stose.d because it must then sustain the
entire compression which at median temper
in,'.e"t. tro lower tubes bsyo " carry
S tLo load Thus II Is seen thai p'f Al .ee "'.'-,