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Read Ebook: Instructions on Modern American Bridge Building by Tower G B N George Bates Nichols
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INSTRUCTIONS
MODERN AMERICAN
BRIDGE BUILDING.
WITH
PRACTICAL APPLICATIONS AND EXAMPLES,
ESTIMATES OF QUANTITIES, AND VALUABLE TABLES.
Illustrated by four Plates and Thirty Figures.
BY G.B.N. TOWER,
CIVIL AND MECHANICAL ENGINEER,
BOSTON:
A. WILLIAMS & COMPANY,
PREFACE
This little treatise was written for the purpose of supplying a want felt by the author while giving instruction upon the subject. It was intended for an aid to the young Engineer, and is not to be considered as a complete substitute for the more elaborate works on the subject.
The first portion of this work mentions the various strains to which beams are subjected, and gives the formulae used in determining the amount of those strains, together with a few examples to illustrate their application, and also the method of calculating a simple truss.
The second portion names and explains the various members of a Bridge Truss, and, by means of examples, shows the method of calculating the strains upon the various timbers, bolts, etc., as well as their proper dimensions; and gives, in addition, several useful tables.
The explanatory plates, which are referred to freely throughout the work, are believed to be amply sufficient for the purpose intended.
So much has been written on this subject that it is next to impossible to be wholly original, and no claim of that nature is preferred. It is simply an arrangement of ideas, gleaned from the various works of standard authorities, and modified by the author's practice, embodied in book form. To give a correct list of all the books consulted would be simply impossible;--but it is well to state that the Hand-book of Railroad Construction, by Prof. G.L. Vose, under whom the author served as an Engineer, has been used as authority in many cases where there has been a difference of opinions among other authors. Some parts have been quoted entirely; but due credit has been given, it is believed, wherever such is the case.
It is not claimed that this little work covers the whole ground, but it is intended to describe, and explain thoroughly, three or four of the more prominent styles of Truss, leaving the other forms of Wooden Bridges to a subsequent volume.
Abutments and Piers, as well as Box and Arch Culverts, belonging more properly to masonry, will be treated of hereafter under that head.
Iron Bridges form a distinct class, and may be mentioned separately at some future period.
If this small volume should lead the student of Engineering to examine carefully the best Bridges of modern practice, and study the larger scientific works on this art, the author will feel satisfied that his efforts have not been entirely in vain.
TOWER'S
Modern American Bridge Building.
BRIDGE BUILDING
The simplest bridge that can be built, is a single beam, or stick of timber, spanning the opening between the abutments--but this is only of very limited application-- owing to the rapid increase in sectional dimensions which is required as the span becomes greater.
Next comes the single beam supported by an inclined piece from each abutment meeting each other at the middle point of the under side of the beam--or, another arrangement, of two braces footing securely on the beam and meeting at a point above the middle point of the beam, which is suspended from the apex of the triangle formed by them, by means of an iron rod--These arrangements may be used up to 50 feet. For any span beyond 50 feet, modifications of this arrangement are used which will be described hereafter. Now let us investigate shortly the different strains that the various parts of a bridge have to bear--and the strength of the materials used. The theory of strains in bridge trusses is merely that of the Composition and Resolution of Forces. The various strains, to which the materials of a bridge are subjected--are compression, extension and detrusion.
Wood and Iron are the materials more generally employed in bridge construction--and in this pamphlet we shall take the following as the working strength of the materials--per square inch of section.
Tension. Compression. Detrusion.
Wood, 2000 1000 150
Wro't Iron, 15000 11000
Cast Iron, 4500 25000
The greater the angle of inclination from the horizontal, the less the strain from a given load--and when the beam is vertical the weight causes the least strain.
But when wood, iron, or any other material is used for a pillar or strut, it has not only to resist a crushing force, but also a force tending to bend or bulge it laterally.
A post of circular section with a length of 7 or 8 diameters will not bulge with any force applied longitudinally, but will split. But if the length exceeds this limit--it will be destroyed by an action similar to that of a transverse strain.
A cast iron column of thirty diameters in length, is fractured by bending; when the length is less than this ratio--by bending and splitting off of wedge shaped pieces. But by casting the column hollow, and swelling it in the middle, its strength is greatly increased.
Barlow's formula for finding the weight that can be sustained by any beam, acting as a pillar or strut, before bending, is:--
now, having the weight given, and assuming the dimensions of the cross-section--we shall have
in the above formulae,
These two strains of compression and extension must be equal in amount--and upon the relative strength of the material to resist these strains, as well as its form and position, the situation of this axis depends. If wood resists a compression of 1000 lbs. per square inch of section, and a tension of 2000 lbs. the axis will be twice as far from the top as from the bottom in a rectangular beam.
The following table by Mr. G.L. Vose gives, with sufficient accuracy for practice, the relative resisting powers of wood, wrought, and cast iron, with the corresponding positions of the axis.
Dist. of axis Resistance Resistance from top in to to frac's of Material. Extension. Compression. Ratio. the depth.
Wrought Iron, 90 66 90/66 90/156, or 0.58.
Cast Iron, 20 111 20/111 20/131, or 0.15.
Wood, 2 1 2/1 2/3, or 0.66.
Thus we see that the resistance of a beam to a cross strain, as well as to tension and compression, is affected by the incompressibility and inextensibility of the material.
The formula for the dimensions of any beam to support a strain transversely is
The following is a table of ultimate and working strengths of materials, and factors of safety:
Weight Ult. Ult. Working Strengths Factor Safety. in lbs. Materials. Ext. Comp. Exten. Comp. Tension Comp.
Length Safety Length Safety Length Safety given in Weig't in given in Wt. in given in Wt. in Diameters. Pounds. Diameters. Pounds. Diameters. Pounds.
In tensional strains, the length of the beam does not affect the strength; but in the beams submitted to compression, the length is a most important element, and in the table given above, the safety strains to which beams may be subjected, without crushing or bending, has been given for lengths, varying from 6 to 60 diameters.
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