, 1921-08-11
BRIDGE WORK IN WYOMING

BRIDGE WORK IN WYOMING
Estate Highway Dept. News Lottery
The work on bridge construction
in the State of Wyoming is OF par
tlcular interest and Importance be-
cause several factors affecting the
design of structures. which do not
call for special study In the more
fully settled and flatter states of the
East, here demand the most careful
consideration. For example, while
the annual rainfall in this state is
very light, cloudbursts of the great
est intensity occur from time to
time, with the result that streams or
draws which are dry for eleven
months in the year carry enormous
floods for very short periods. Num-
erous irrigation works also affect the
flow Of our streams to a consider
able extent. The disposition of ma
terials further affects the design in
a number OF cases, as do the ex-
tremely long distances from railroad
shipping points, which in some in
stances increase the cost of concrete
work from five to eight times that
mother localities.
The Bridge Department in its
work of designing structures for the
various highways which are being
constructed in this state keeps in
mind two things-permanence and
economy. In considering the first,
particularly with reference to struc-
lures of concrete or reinforced con-
crete, the design is made on the
C n kL-k 4
serve not for a period of TO-, 20 or
SO years. but for all times. In
working on this assumption, there
are at least three important elements
to cons<ier:
First, waterways required; second,
provision for maximum loads which
the future may bring, and third,
protection FOR any part OF the struc-
ture subjected to wear.
In a comparatively new state like
Wyoming, where data on the flow
of streams are more or less limited,
the first element in connection with
waterway required, offers by all
means the most difficult problem. of
the whole design, but this problem
is net in the, best way possible, by a
study of the drainage area, character
of soil and slopes, and elevations of
high water marks. For the latter,
dependence has to be placed largely
on the statements of the. oldest
settlers in the community, and 1t is
readily appreciated by one who has
had experience along these lines that
very frequently the information
thus obtained proves to be of little
value.
If, in the design, sufficient water,
way is secured to care for the ey-
tremes Of flood, the structure ma
be considered safe from a stand
poin of foundation ~security. proy;ded1
d course, that ordinary practitn KS
TO stability has been follow Cd. II
the waterway adopted proves to be
inadequate, and il the roadway Ills
resist the flood tide, then uatutIiiy
there IS A great increase of head al
the restricted opening, With 4 con-
ssGuent velocity increase. This in
creased velocity results In !sc9ur of
the stream bed and this scour con
times until the area OF waterway
IS decreased sufficiently 80 radUe thr
Felccity % a D1inE Where seouy
Goa 5Cs. Now, cha foundation
nave heeI pr3p4TlJ designed fhr|
stability, and are carried bsio# the
plane Of TGTimum sc0ur. the ssTJc-
tOre viii stand; II mott failure is like
ly.
n considering the second A.emsut
entering INTO the permanenee OF tn
structure, i. e, meEimam loads which
t&e future NAV briag, IL is.the prac II
tee OF this department. to consider
% concentrated load OF Is tons, or a.
uniformly distributed LOAF or 10
pounds per sQu&T6 foot, with allow
ances. in general, Of Is per cent for
stresses developed by impact. Thus
It is seen that provision is made for
d load of about half the capacity Of
the average railroad freight car. On
all but the shortest spans, the uni
form loading usually governs, and
since this loading is caused only by
q closely packed crowd of people, or
q herd of cattle passing over the
structure, it would seem that the.
future could not bring forth any. in
cas- in such l%oadings. AS to great
increases in concentrated loadings,
it Is the firm belief of the depart
ment that no economy can result
from the introduction of heavier
motor vehicles or trucks than are in
use at the present time; that in
crease in tonnage hauled will be, ac-l
conplished by the use of t~railers and
that the uniform loading designed
for will be ample to provide for
such rattle; and finally that the
tendency toward the use of pnep-.
matte tires and better and smoother
bridge approaches and bridge tioors
viii further reduce impact, so that
'presept lQad allowances will perhaps
lie aYen u1ore liberal for future
ttraffic than now. Nevertheless, IN
localities Of especiafly heavy traffic,
( PROVISION is egen made for loading
ioG 20 tons ana 125 pounds per square
fhot. with the same allowance for
t1apaet. Thus It wenla appear that
7he oegnfanenee oiltha- strurture IL
esSiiraa In this respect,
In considering the third element.
tthat Q2 &ear on the structure, pro-
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