The weekly Union times. [volume] (Union C.H., South Carolina) 1871-1894, 1879-02-07
For the Time.
Building Bridges. 70


built stone abuttnents on the piling wooden
foundation piaa on Fair Forest, nenr Gen. r.
Glenn .. ashort distance above the County line.
The situation had no hard foutiation at a depth
OF some fifteen feet, the mire soundngs showed
the banks to be very soft. sandy and deep. The
many bridge builders around that place, Frank
West, James Williams, Mr. Hare, Maj MoDow-
ell, Gen. Glenn, and persons generally, strongly
asserted that these stone abutments, could not be
made to stand, extending as they do about two
feet into the water's edge, resting on very deep,
soft sand. The June and ether freshets have not
moved the abutmsnts the eighth of an inch. The
first cost of this improved plan was no more
than the old wooden plan, while the future cost
to rebuild the wood will not exceed one-fifth
cost, to rebuild on the old plan The frss lead.
ing want in bridge economy is stone and dirt
sbutments, to shorten the bridge as much as
practible as above said; next to this is stone
piers, their distance apart to vary as the varied
circumstances Of the situation may demand
The Howe Truss, or the Lattice. or the Utley
bridge, can be made to span from so 70 l60 feet.
l have seen many wagon bridges with 100 feet
spans, the older ones built on the Lattice plan,
thAmore recent built on n 1--we truss plan,
which, for some reasons, like be(ter than the
Utly or the Lattice bridge, sns adviSs Its adopl .
tion Where the situation calls for.h hspan l60,
feet, the bottom cords, especially, should made
much larger the braces, however, not to be
increased in size but little, as il would give them
an unnecessary weight on the long span, they to
be much longer, the top cord to be much higher
on long than short spans, and much less than
the bottom cord in any length of span, as its
functions require much less strength than the
bottom cord. When the lower cord is IL inches
square the top cord should be about by 10.
The usual practice, however, is 10 by IL for the
top. I introduced several of the Utly span
bridges in Spartanburg County in 1866, but
in every instance specified the top cords, braces,
needle beams and short sleepers to be much
lighter than Utly used in this County,sto dis
pense with what considered dead weight
Sosn after the freshet Of '52, Northern man
built sort of self sustaining bridge at the
Hurricane Shoals, Pacolet River, using very
long, heavy and very fiat running braces, rising
only about 81 feet in 20; soon after this the same
plan was adopted at two other places ON tho
river, ono close above, the other close below
Trough Shoals, and all three Of them were re-
built anew on the same plan. It did badly in
both the first and second building, as soon as the
work began to get little old, and it took TWEEN =
ty years to school these people, that the plan
was very inferior one. 1 mention this to show
how long time is frequently required to teach
people what is worthless or inferior. 1t is now
entirely abolished, and the once lovers of it can
now look back upon it and their blindness.
never did like it, because the plan was contrary
to the clear teachings of the science of mechanics
1t is laid down by On able author that the
strength of n stick Of timber is as the square of
the depth, directly as the width, and inversely
rs the length. l presume tho author means
these laws of course only to apply to short
length sticks, as the ,ceight of Al. or so feet
long would SO largly enter into it as to materi-
ally change the above rule. Therefore, it follows
according to my theory, that great lengths,
either in sleeper or selfsupporting span on the
Howe truss, the Lattice, the Utly plan that al
dead weight should be discarded that the di.
mensions and proportions OF the different parts
should be such rise only as to p ,rform the res
pcctve functions Of the different parts The
bottom cords of any Of the truss spans performs
o different function to that Of the top cords
The foot of the braces the former strains
. them to make them longer, while with the latter
tho braces push to make them shorter. With
this difference, the bottom cords require to be
. much stronger to make them safe from piyipp
, down or breaking.
. very similar principle exists with regard
> to sleeper 40 or 90 feet long. My theory is
that sleeper Of great lenght will be stronger t.
4 inches
= thick and the bottom edge 8 or 10. The effect
of weight OH the under edge is to strain it apar,
where the break first begins, while on the tor
edge it is pressed together. Therefore, with
f these quite different functions, much less ma.
terial is required to perform the demands of the
! upper than the lower edge. Some may differ
with mo on this, but am very sure 1 am cor
rect. got out my Trough Shoal bridge sleepers
q in this shape in '6l. But, however, the old
. sleeper bridge is in many places passing away,
and have mainly referred TO it as above to call
attention, that very similar principle applies
| to the Truss bridge, not to weight it down with
disproportionate weights, SO. If this be not ut.
tended to, with other defects in the planning
, with sorry workmanship, the material q littI,
D inferior, put up on wooden trusels and they on
bad foundations the money had nearly as we'll b,
thrown in the river or the fire. But, on the 9th
er hand, with stone abutments, and in some
. places stone piers with Truss bridge on this
4 covered in, with all parts we'll planned and tn,
0 work well executed, would make bridge that
1 would last the county half q centusy while Tho
first cost would little more than double the cos
: of the inferior, the latter being much more sub
! ject to freshet damages, in connection with its
rapid decay, and much more exposes the trustls
Q to accident. But some will say, to invest sc
: much in q bridge and it so liable to wash away
t would not pay. In reply to this beg to say
that to no part Of mechanics have devoted q
higher degree of research and study, than Hy
draulics, or, in other words, the laws Of water IL
motion md full ~ that have - on n,