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1547. Overhaul. Many contracts for railroad work
specify the maximum distance to which material shall be
transported at the given price per yard. When the distance
exceeds that specified in the contract, the excess is termed
overhaul, and a clause in the contract stipulates what addi-
tional compensation shall be made for each hundred feet of
overhaul. Free haul is commonly limited to 1,000 feet, and
for each hundred feet of overhaul an addition of 1 cent
per cubic yard is made to the contract price per yard. In
recent years the overhaul clause is omitted from most
contracts, as it is almost sure to involve litigation.


1548. The Location of Bridges. There are two
important factors in the location of a bridge, viz., first, the
determination of the angle which the center line of the
road shall make with the general direction of the channel,
and, second, the measurement of the span.

In all cases it is desirable that there should be a right-
angled crossing, and for bridges of large span the aline-
ment is often modified to obtain that result. The amount
of such modification, if any, will depend upon the impor-
tance and character of the traffic. If the line is for through
business where numerous passenger trains are to be run at
high speed, the angle of the crossing will be subservient to
the alinement; that is, a skewed bridge will be 'adopted
rather than to introduce curvature and mar the directness
of the line.

Skewed bridges are always more expensive and generally
less satisfactory than those crossing streams at right angles.

The character of the crossing being determined upon,
the next thing in order is the measurement of the span.
This may be effected in two ways, and, when practicable,
both methods should be used, the one serving as a check
upon the other. The first method is by direct measure-
ment; the second by triangulation. Before either method
is applied, the center line must be accurately checked and


established by fixed monuments set on both sides of the

1 549. Direct Measurement of Span. The direct
measurement of the span is made as follows: A light strong
steel wire is stretched from monument to monument,
spanning the stream. One end of the wire is fixed so that
the wire is either in actual contact with the point in the
monument or directly over it. To the other end a spring
balance is attached, which indicates by a dial the amount of
tension placed upon the wire.

The wire is then stretched until the sag is practically
removed, and the amount of tension noted. If the wire is
not in direct contact with the monument centers, the
measurement is found by plumbing from the wire to the
monuments. The points of measurement are then marked
on the wire and the measurement repeated. The measure-
ment should be made at least three times, the wire being
subjected to the same tension. As one end of the wire is
fixed, any variations in measurement will show at the free
end. If the measurements show any considerable variation,
the process must be repeated until three measurements
practically agree. Two supports are then erected upon a
level surface, at a distance from each other equal to the span
of the stream, and of such height that the wire will clear the
ground when stretched between the supports at the same
tension as used in the original measurement. The wire is
then stretched with the proper tension, and the points of
measurement transferred to the ground by plumbing. The
measurement on the ground is made with a standard steel
tape, and repeated three times. The average of the three
measurements, providing their discrepancy is slight, may be
accepted as the correct measurement of the span.

1550. Measurement of" the Span by Triangula-
tion. If practicable, the same monuments used in direct
measurement are used in triangulating. The first step is
the establishing of a base line, which should be of approxi-
mately the same length as the span, and laid out on as smooth


ground as the situation will permit. The measurement of
the base line is made with the greatest care and repeatedly
checked. When the ground is practically level the following
method is recommended (see Fig. 463): Strong stakes are
driven at A, B, C, etc., approximately 100 feet apart, their
tops being on the same level and pointed, with a small tack
in each stake. The spaces between the stakes are then
measured with a steel tape at a tension of about 15 pounds.
The measurements are made three times, and the average
of them taken as correct. Greater accuracy in measure-
ment is secured by having different persons read the tape
for each measurement, each recording his own reading, and
after the third reading place the three readings in a column

and take the average for the correct measurement. The
sum of the averaged measurements will be the length of the
base line A G. Suppose for this case that they are as







599.5689 feet,

which gives for the total length of the base line 599. 5689 feet.
Let Fig. 464 be the plan of the bridge crossing. A and
G are monuments in the center line on each side of the
river, and B G the base line, whose length we have deter-
termined by direct measurement to be 599.5689 feet. The
angles at A, B, and G are measured three times, and the
average of the readings taken as the correct reading. It is
desirable to use a transit which will read to 10 seconds. On


bridges of great length the angle readings are taken in three
sets of five readings each, and the average of all accepted
as the correct reading. This mode of angle measurement
was adopted in measuring the span of the Washington
bridge over the Harlem river, at New York. Suppose that
the average of three readings makes the angle at A,
57 29' 35"; at B, 59 01' 03.3" and at G, 63 29' 20". Their
sum is equal to 179 59' 58.3", which proves the angle meas-

urement to be practically correct. The length of the side
A G, i. e., the span, is determined by the principles of
trigonometry (see Art. 1243), as follows:

Sin 57 29' 35" : sin 59 01' 03.3" :: 599.569 : side A G.

sin 59 01' 03. 3" = .85733
599. 5G9 X .85733 = 514.028491
sin 57 29' 35" =.84333


= 009.522 ft. = side A G.

If the temperature of the air in this case were 60 Fahren-
heit, it may be considered normal, so that there need be no
allowance made for the expansion or contraction of the
tape. The base line B G is practically parallel to the direction


of the channel current as indicated by the arrow^ and
the angle at G of 63 29' 20" will be the angle of skew to
which the bridge will be built.

1551. The Location of Piers and Abutments.

The number of piers to be built will depend upon whether
the stream is navigable or not, and upon the cost of founda-
tions. If the stream is navigable there must be one channel
span of such width as the Government authorities shall

When no provision is required for navigation, the cost of
foundations alone will determine the number of piers. In
general, the cost of bridges will increase about as the square
of the span ; that is, if one bridge is of twice the span of an-
other, the first will cost about four times as much as the
second. If the stream is shallow and its bed of rock or com-
pact gravel or clay, suitable for foundations, it will be
cheaper to increase the number of piers, and shorten the
spans proportionately.

1552. Foundations. This subject is too broad for
any but general treatment. A bridge foundation must meet
two conditions, viz., stability and security; that is, it must
be able to safely support the maximum load imposed upon
it, and must be protected against those natural forces which
either periodically or continually attack it. The principal
enemies of bridge foundations are the erosive action of the
current and floating ice, both of which are most active at
high stages of water. Bridge piers, with few exceptions,
are of stone. Pier foundations may be divided into three
classes, viz. , rock or concrete foundations, pile foundations,
and caisson foundations.

1553. Rock and Concrete Foundations. When
the bed of the stream is rock or compact gravel, sand, or
clay, the pier site is prepared as follows: When of rock,
trenches equal in width to the thickness of the outside walls
of the pier are excavated to a depth of 12 inches. The
bottom of the trench is brought to the same general level,


and a layer of concrete added to furnish an even bed for the
masonry. As foundations are generally built at low stage
of water, the action of the current is but slight.

When compact sand or hard clay forms the bed of the
stream, a dam is built enclosing the foundation site. If the
water is stagnant and of a depth not exceeding 4 feet, a
trench is dug from 12 to 24 inches in depth, enclosing the
foundation, and the trench is then filled with clay and gravel,
well mixed and thoroughly rammed, forming a wall which
is carried ab'ove the surface of the water. The enclosed
water is then pumped out and the foundation area excavated
to a depth of from 12 to 24 inches, depending upon the
erosive force of the current and the weight of the proposed
pier. The pit is then filled with well-rammed hydraulic
concrete, and the masonry laid precisely as on shore. The
two lower courses of masonry are stepped outwards, that is,
they project beyond the main body of the pier, increasing
the bearing surface of the foundation. These footings or
offsets are made from 4 to inches wide. The foundation
courses should be of larger stones than those composing
the main body of the pier. The faces of the pier are usually
battered from ^ to 1 inch horizontal to 1 foot vertical.
Where the piers must resist heavy masses of floating ice,
the up stream ends are brought to an edge, forming ice

1554. Cofferdams. For depths of stagnant water
greater than four feet and for less depths having a current,
the clay dam is replaced by a cofferdam. This construc-
tion consists of two rows of piles which are driven enclosing
the foundation site. The distance apart of these rows of
piles, as well as the spacing of the piles in the rows, will de-
pend upon the depth of the water surrounding the founda-
tion site, and the nature of the material into which the piles
are to be driven. The piles will also be required to support
a platform, upon which are placed the derricks, hoisting
machinery, and building material used during construc-



The usual form of construction of a cofferdam is shown in
Fig. 465. Two rows of piles P, Pare firmly driven, enclo-
sing the foundation area.
Longitudinal pieces of
squared timber IP, JFcalled
string pieces or wales
are bolted to the piles a
little above the water level.
Directly opposite the string
pieces on the inside of the
piles, guide pieces g, g
are bolted, the same bolt
passing through both string
piece and guide. The
guide pieces serve to keep
the sheet piles S > S in line
while being driven. Cross
timbers B, B called bind-
ers are notched down on
the string pieces to which
they are bolted. The de-
positing and ramming of
the puddle tend to cause
FIG - 465 the rows of piles to spread.

The binders prevent this and give strength and stability to
the structure. The plank flooring ^ supports the derricks,
hoisting machinery, building material, etc. The consistency
of the cofferdam filling must be such as to exclude the
water, and the weight and strength of the entire structure
must be sufficient to resist the pressure of the excluded
water. Taking the weight of water at G2 pounds per cubic
foot, the external pressure of the water against the sides of
a cofferdam is determined by the following rule (see Art.
975, Vol. I.):

Rule. The pressure upon any vertical surface due to the
weight of -the liquid is equal to the -weight of a prism of the
liquid whose base has the same area as the vertical surface,


and ivJwse altitude is the depth of the center of gravity of the
vertical surface below the level of the liquid.

Cofferdams are really retaining walls, which were fully
described in Arts. 1486 to 149O, inclusive, and the forces
acting against them are the same, though somewhat differ-
ent in application. In the case of retaining walls, the backing
being of earth or broken rock, only that part of the backing
included between the back of the wall and the line of natural
slope, extending from the inner foot of the wall upward at a
slope of 1 horizontal to 1 vertical, exerts any pressure upon
the wall. In the case of water, however, the particles, hav-
ing no cohesive force, all exert pressure against the dam.
The center of pressure of the water, like the center of pres-
sure of the forces acting against a retaining wall with
backing level with its top, is taken at one-third of the depth
of the water above the bottom. The direction of the water
pressure is at right angles to the face of the cofferdam, and
the moment of the water pressure is the product of the
pressure found by the above rule multiplied by one-third the
depth of the water. The moment of the resistance of the
cofferdam, that is, its stability, or resistance to overturning,
is the product of its weight multiplied by the distance from
the inner toe of the cofferdam to the vertical line drawn
from the center of gravity of the cofferdam.

EXAMPLE. If, in Fig. 465, the length of a cofferdam is 50 feet, its
height 7 feet, its thickness 4 feet, and the depth of water 6 feet, (a)
what is the pressure of the water against the side of the cofferdam ?
(b) What is the overturning moment of the water pressure, and the
resisting moment of the dam ? (c) What is the factor of safety of
the dam ?

SOLUTION. (a) 6 X 50 X 3 X 62.5 = 56,250 lb., the pressure against
the side of the cofferdam. Ans.

(&) In determining the moments of the water pressure and of the
resistance of the dam, we take a section of the dam 1 foot in length.
The pressure of the water against a 1-foot section of the cofferdam is
6 X 3 X 62.5 = 1,125 lb. Its center of pressure is at one-third the depth,
or 2 feet, above the bottom. The moment of the water pressure is,
therefore, 1,125 X 2 = 2,250 lb. Ans.

Taking the weight of the puddle filling at 120 pounds per cubic foot,


we have for the weight of a 1-foot section 7 X 4 X 120 = 3,360 Ib. The
moment of resistance of the dam is the product of its weight by the
perpendicular distance from the inside toe of the dam to the vertical
line from the center of gravity of the section. This perpendicular
distance is 2 feet. 3,360 X 2 = 6,720 Ib. Ans.

(c) This moment opposes the moment of the water pressure, which
we found to be 2,250 Ib. The factor of safety of the dam is, therefore,
the quotient of 6,720 -H 2,250 = 2.99, nearly. Ans.

In this calculation we have ignored the weight of the
piles and timber composing the cofferdam, as well as the
resisting power of the piles. These would considerably in-
crease the factor of safety of the dam. The water pressure
per square foot upon the bottom of the enclosed area will be
equal to the depth of the water (6 feet) multiplied by 62.5,
or G X 62.5 375 pounds. This pressure is resisted by the
material composing the bed of the stream and the sheet

If the bed of the river is composed of compact sand or
clay, little trouble need be anticipated. If, however, the
bed consists of loose sand and gravel, special provision must
be made to exclude the water.

An effective device used by French engineers is the fol-
lowing (see Fig. 466) : Two rows of piles P, P are firmly
driven. Wales W, IV and guides G", G are bolted to the
piles. A row of close piles C of square timber is driven
and bolted or pinned to the outside guide. The foundation
area and the space to be covered by the cofferdam filling
are dredged to the depth of 3 or 4 feet and the entire pit
filled with concrete. Before the concrete has had time to
set, the inside row D of close piles is driven, their feet pene-
trating 2 feet into the concrete. The clay filling is deposited
to the depth of one foot upon the fresh concrete and rammed,
so there may be a perfect connection between the puddle
and the concrete. This work must be done with dispatch.
The remainder may be deposited more gradually. After
sufficient time has elapsed to allow the bed of concrete to
become thoroughly hardened, the water is pumped out of
the enclosure. If the pressure of the water is great enough


to lift the concrete foundation, additional weight must be
added to keep it secure until the weight of masonry insures

FIG. 466.

1555. Pile Foundations. When the river bed is
composed of soft, yielding alluvium extending to a consider-
able depth, but underlaid by a firm soil of ample depth, a
pile foundation is commonly adopted. The piles should not
exceed in length thirty times their butt diameter, and
should be cut from live straight trees. Oak piles are the
most durable and strongest; rock elm, spruce, and yellow
pine are of about equal strength and durability. The out-
line of the proposed pier will to some measure determine the
arrangement of the piles, 'but the general arrangement is
always the same, and is as follows: The piles are driven in
rows, spaced not less than two and a half feet, center to
center, and cover the entire foundation area.

In some special cases the outside row of piles is made
double, the outer piles projecting beyond the outlines of the
pier. The calculation of the bearing power of piles and the
various methods of driving are fully explained in succeeding
pages. The piles, after being thoroughly driven, are sawed


off at a uniform level at a suitable depth below low water

A general plan of the pile foundation and the masonry
usually adopted for bridge piers is shown in Fig. 467. The
dimensions of the foundation from center to center of out-
side piles are width 9 feet and length 33 feet, the pier being
for a standard double-track roadway. The piles are cut off
4 feet below low water. A timber platform, or grillage,
of heavy timbers is built upon the piles, to receive the
foundation. First, a course of cap timbers is laid crosswise
upon the heads of the piles. The caps are commonly 12 by
14 inches, and notched down 2 inches upon the pile heads,
leaving 12 by 12 inches of solid timber above the piles.
The caps extend six inches outside the piles, to which they
are fastened with 1 inch square drift bolts. Care must be
taken that the tops of the caps are on a uniform level. The
second course of timbers is stringers 12 by 14 inches laid
lengthwise of the pier, and notched down 2 inches on the
caps to which they are drift-bolted at each intersection.
They are laid close together, forming a complete flooring.
A third course of 12 by 12-inch timbers is laid at right angles
to the stringers to which they are securely drift-bolted. The
top of the grillage should be at least 1 foot below low water.
Upon it the masonry is started.

In Fig. 467 A shows the side elevation of the foundation
and pier; B, the elevation of the up-stream end of the
foundation and pier; C, the arrangement of piles in the
foundation, and D, the plan of the pier. The courses g and
h are the coping courses, the latter forming the seat upon
which the bridge rests. The foundation piles are spaced 3
feet center to center. The grillage of timber extends on all
sides 12 inches from the centers of the outside row of piles.
The first course of masonry is laid flush with the outside of
the grillage, and extends on all sides 6 inches beyond the
second course. The second course projects on all sides 6
inches beyond the main body of the pier.

Beginning with the third course, the north end of the
pier gradually develops into a conical-shaped ice breaker,



and in construction consists of the intersection of a cone
with a wedge. The curve of intersection is shown in the

elevation by the curves/, and in the plan by the curve e'f.
The arrangement of the stone in each course is shown in the


elevation A. It will be observed that in no instance is the
bond less than 12 inches, and the proportion of headers
(stones showing their short side at the face of the pier) to
stretchers (stones showing their long side at the face of the
pier), and their arrangement is such as to form one compact
mass of masonry.

Where there is a rapid current, causing frequent changes
of channel, as is the case with many Western and Southern
rivers, it may be necessary to rip-rap the foundation. This
process consists of depositing stone about the piles to a
depth of 4 or 5 feet, the deposit extending several feet be-
yond the piles in all directions. Any action of the current
tending to undermine the foundation is promptly checked
by the rip-rap, which fills any cavity worn out by the

In making working drawings for bridge piers, the arrange-
ment of the stones in each course should be carefully planned
before the masonry is started. If only two or three courses
are planned beforehand, confusion is sure to follow. By
furnishing quarrymen with complete plans, they have a
wider range of sizes, and will be enabled to take better ad-
vantage of the stone as it comes from the quarry. The
probable result will be better prices and prompter delivery
than when only partial plans are furnished.

In giving dimensions to quarrymen, no allowance is made
for mortar joints, which in bridge masonry are usually one-
half inch in thickness. When the given dimension is taken-
from an angle to the middle of a joint, the stone cutter will
deduct one-fourth inch from the dimension for the neat
length of the stone. When the dimension is from center of
joint to center of joint, the stone cutter deducts one-half
inch. Detailed plans are usually sent to the quarry where
the stone is cut to dimension, and the several stones for each
conrse numbered, the courses being designated as Course
A, Course B, etc., or in some other way. The quarry fore-
man lays out the work, deducting the allowance for joints,
and the stone is cut, marked, and shipped to the bridge site
in shape for laying. Stones of irregular and intricate form


are drawn to a large scale of from 1 to 3 inches to the foot,
and often full-sized drawings are made, from which tem-
plates of sheet zinc are cut for use in the quarry.

1556. Stone Suitable for Bridge Masonry.

Stone for bridge foundations and piers must be free from
seams and defects common to surface stone. Stones con-
taining free iron are objectionable, as they are sure to be-
come discolored from the action of the elements. Granite
is to be preferred, but limestone, hard sandstone, bluestone,
and marble are all suitable. In ordinary bridge work, the
stone is laid rock face, i. e., with undressed faces, and
pitched to line at the joints. By giving the corner stones a
draft of two inches, i. e., so as to show two inches of
dressed surface on each side of the angle, an effect of much
higher finish is imparted to the entire work at compara-
tively small additional cost.

In cutting the stone, great care should be taken to make
the beds even and the stone of uniform thickness through-
out, so that when laid the beds will be truly horizontal. In
coursed masonry all the stones in each course have the same
thickness throughout. A variation of % inch in the thick-
ness of the stones is readily detected, even by an unpractised
eye. All mortar used in bridge building should be prepared
under the direction of a competent inspector, and under no
circumstances used after setting has commenced.

1 557. Backing. The space inclosed by the face stone
is usually filled with a less expensive material, called back-
ing. In large piers, concrete is much used. It forms a
homogeneous mass, and when well rammed, as it always
should be, fills all the space between the faces. Rubble
masonry is largely used for backing, and in laying, care
must be taken to secure proper bond, especially between the
backing and the headers which reach from the face into flie i
body of the pier. Piers erected on caisson foundations are,
on account of the great cost of sinking, given as small
dimensions as are consistent with safety. To give increased

Online LibraryInternational Correspondence SchoolsThe elements of railroad engineering (Volume 2) → online text (page 26 of 35)