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FIG. 12.

PLATE GIRDER HIGHWAY BRIDGE, BUILT BY AMERICAN BRIDGE COMPANY.

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BEAMS AND PLATE GIRDERS.-For spans of, say, 30 feet and under rolled beams are often used to carry the roadway, while for spans from about 30 to 100 feet plate girders are used. When the roadway is carried on top of the girders, the bridge is called a deck plate girder bridge, and when the roadway passes between the girders, the bridge is called a through plate girder bridge as in Fig. 12.

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FIG. 16. CLARITON-CLIFTON TWO-HINGED ARCH HIGHWAY BRIDGE OVER NIAGARA RIVER.

SWING BRIDGES.-Swing bridges may be made of plate girders or trusses, and may turn on a center pivot as in Fig. 13, or on a turntable supported on a drum as in Fig. 14. The center pivot swing bridge has two spans continuous over the pivot support, while the turntable swing

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bridge has three spans ordinarily continuous over the two turntable supports. When the swing bridge is open each arm acts as a simple cantilever span.

STEEL TRESTLES.-Steel trestles are used for carrying the roadway at a considerable distance above the ground, Fig. 15. The tower and intermediate spans are commonly built of

plate girders, whether the trestle carries a railroad or a highway roadway. The tower consists of two trestle bents as in (a) or (d), braced together by longitudinal bracing as in (b) or (c) Fig. 15. Bracing as in (a) and (b) is used with either adjustable or rigid diagonal members, while bracing (c) and (d) is used only for rigid members.

STEEL ARCHES.-Steel arch bridges are made (1) with three hinges, (2) with two hinges, and (3) without hinges, and may have solid webs, or spandrel or open webs. A two-hinged highway arch is shown in Fig. 16.

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FIG. 18. SUSPENSION HIGHWAY BRIDGE OVER NIAGARA RIVER AT QUEENSTON, ONTARIO.

CANTILEVER BRIDGES.-A cantilever bridge consists of two anchor spans, which support a suspended or channel span. The shore ends of the anchor spans are anchored to the shore pier and are supported on the river pier. A cantilever highway bridge is shown in Fig. 17.

SUSPENSION BRIDGES.-In a suspension bridge the roadway is supported by hangers attached to the main cables. Stiffening trusses are placed above the plane of the roadway to assist in distributing the live loads and for the purpose of increasing the rigidity of the structure. The suspension highway bridge over the Niagara River at Queenston, Ont., is shown in Fig. 18. Simple truss bridges, beam and plate girder bridges, only, will be considered in this book.

CHAPTER IX.

DATA FOR THE DESIGN OF STEEL HIGHWAY BRIDGES.

TYPES OF STRUCTURE.-The types of structure for steel highway bridges as recommended by the author are given in section 3, "General Specifications for Steel Highway Bridges," printed in Appendix I.

The following data will show present standard practice.

Illinois Highway Commission.-The types of highway bridge recommended by the commission are as follows:

Concrete Bridges.-For culverts requiring a waterway of 12 square feet or less, plain or reinforced concrete arch culverts or square culverts, reinforced concrete pipes or double strength castiron pipe.

For culverts having an area of more than 12 square feet, and for bridges having a span up to 30 ft., reinforced concrete slabs, plain or reinforced concrete arches.

For spans of 30 ft. to 65 ft., reinforced concrete through or deck girders, plain or reinforced concrete arches.

For spans greater than 65 ft., plain or reinforced concrete arches.

Steel Bridges. For spans of 12 ft. to 45 ft., steel I-beams; for spans of 30 ft. to 100 ft., plate girders or riveted pony trusses; for spans of 90 ft. to 160 ft., riveted trusses with parallel chords; for spans of 160 ft. and more, riveted or pin-connected trusses with parallel or inclined upper chords. Iowa Highway Commission.-The types of highway bridges recommended by the commission are as follows:

Concrete Bridges.-Box culverts for spans up to 16 ft.; slab bridges for spans from 14 ft. to 25 ft.; arch culverts and bridges for spans of 6 ft. and over; girder bridges for spans of from 24 ft. to 40 ft.

Steel Bridges.-Steel I-beams up to 32 ft. span; plate girders, 20 ft. to 80 ft. span; low truss 30 ft. to 100 ft. span; high truss 100 ft. span and over, riveted up to 140 ft. span.

Massachusetts Public Service Commission.—The types of highway bridge recommended by the commission are as follows:

Steel Bridges. For spans up to 20 ft., wooden stringers or rolled beams; for spans from 20 ft. to 40 ft., rolled beams or plate girders; for spans from 40 ft. to 70 ft., plate girders; for spans from 70 ft. to 100 ft., plate girders or riveted trusses; for spans from 100 ft. to 125 ft., riveted trusses; for spans from 125 ft. up, riveted or pin trusses.

Wisconsin Highway Commission.-The types of highway bridge recommended by the commission are as follows:

Concrete Bridges.-Spans of 1 ft. to 10 ft., slab culverts and bridges; spans 10 ft. to 18 ft., slab bridges; spans 10 ft. to 40 ft., through girders.

Steel Bridges.-Spans 10 ft. to 38 ft., rolled beams; spans 35 ft. to 80 ft., Warren riveted low trusses or plate girders; spans 80 ft. to 135 ft., Pratt riveted high trusses; spans over 135 ft., riveted high trusses with curved chords.

WIDTH OF ROADWAY.-The following data will show standard practice.

Illinois Highway Commission.-The widths of roadways are specified for State Aid Routes, Principally Traveled Roads, and Secondary Roads.

On Designated State Aid Routes.-Bridges up to and including 10 ft. span, 20 to 30 ft. roadway; bridges over 10 ft. up to and including 60 ft. span, 18 to 24 ft. roadway; bridges over 60 ft. span, 16 to 20 ft. roadway.

On Principally Traveled Roads.-Bridges and culverts 10 ft. or less in span, 20 to 30 ft. roadway; bridges over 10 ft. and up to and including 60 ft. span, 16 to 20 ft. roadway bridges over 60 ft. span, 16 to 18 ft. roadway.

On Secondary Roads.-Bridges and culverts 10 ft. or less in span, 18 to 24 ft. roadway; bridges over 10 ft. span, 16 ft. roadway.

Culverts Under Fills.-The barrel of the culvert shall have a .ength that will permit of side slopes of horizontal to I vertical, and a top width of 20 to 30 ft. on State Aid Routes, 20 to 30 ft. on Principally Traveled Roads, and 18 to 24 ft. on Secondary Roads.

Iowa Highway Commission. The widths of roadway for highway bridges as recommended by the commission are as follows:

Concrete Bridges. For box or arch culverts with spans of 2 ft. to 16 ft., 24 ft. roadway for county roads, and 20 ft. for township roads; for slab bridges with spans over 16 ft. span, 20 ft. roadway for county roads, and 18 ft. for township roads; for girder bridges over 16 ft. span, 20 ft. roadway; for arches over 16 ft. span, 24 ft. roadway for county roads, and 20 ft. for township roads. The slopes on fills shall be 1 horizontal to I vertical.

Steel Bridges. A roadway of 20 ft. on county roads, for all spans, and 18 ft. on township roads for all spans. The minimum legal width of roadway is 16 ft.

Association of State Highway Departments.-The following minimum widths of concrete bridges are recommended.

For First Class Roads.—Culverts under 12 ft. span, 24 ft. roadway; slab bridges over 12 ft. span, 20 ft. roadway; all other spans 20 ft. roadway.

For Second Class Roads.—Culverts under 12 ft. span, 20 ft. roadway; slab bridges over 12 ft. span, 18 ft. roadway; all other spans, 18 ft. roadway.

For Third Class Roads.—Culverts under 12 ft. span, 20 ft. roadway; slab bridges over 12 ft. span, 18 ft. roadway; longer bridges, 16 ft. roadway.

The above widths of concrete bridges have been adopted by the Wisconsin Highway Commission.

LOADS. The loads carried by a bridge consist of (1) fixed or dead loads, (2) the moving or live load, and (3) miscellaneous loads.

The dead load consists of the weight of the structure and is always carried by the bridge; the live load consists of the moving load which the bridge is built to carry, while the miscellaneous loads include wind loads, snow loads, etc. Data on dead loads are given in the "Specifications for

Steel Highway Bridges" in Appendix I.

WEIGHTS OF BRIDGES.—The weight of a bridge is composed of (1) the weight of the steel in the steel framework, consisting of the vertical trusses, the upper and lower lateral systems, the floorbeams, the portals and sway bracing; (2) the weight of the joists and the fence; and (3) the weight of the floor covering.

WEIGHTS OF STEEL HIGHWAY BRIDGES.-The following data may be used in calculating the dead loads in the design of highway bridges or as a basis for preliminary estimates.

AMERICAN BRIDGE COMPANY.-Standard Steel Highway Bridges with Timber Floor. Timber floor, 3-in. plank on roadway and 2-in. plank on footwalks. Live loads for floor and its supports, 100 lb. per sq. ft. of floor surface, or 6 tons on two axles 10 ft. centers and 5 ft. gage, or a 15-ton road roller. For trusses 100 lb. per sq. ft. of roadway up to a span of 75 ft., 75 lb. per sq. ft. of roadway for spans of 168 ft. and over, and proportional for intermediate spans. No allowance is made for impact. Designed for allowable stresses given in specifications in Appendix I. Let W = weight of the structural steel per lineal foot of span; L length of span in feet, b width of roadway in feet (without sidewalks).

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