WEIGHT OF STEEL HIGHWAY BRIDGES. 1. Steel Through Plate Girders.—Through plate girder spans 36 ft. to 70 ft., roadway 20 ft. wide, without sidewalks, but including stringers. The weight of structural steel per lineal foot of span is W = 300 +3.8L. For sidewalks with steel joists add about 12 lb. per sq. ft. of sidewalks. (1) 2. Steel Low Riveted Truss Spans, with Timber Floor.-For low truss spans 36 ft. to 102 ft., with timber floors, the weight of structural steel per lineal foot of span, not including the weight of the stringers and the railing, is given approximately by the formula for a 16-ft. roadway and for a 20-ft. roadway W = 100+ 2.0L. (2) (3) W = 150+ 1.7L. 3. Steel Low Riveted Truss Spans, with Reinforced Concrete Floors. For low truss spans 36 ft. to 102 ft., with reinforced concrete floors, 5 in. thick with 6 in. of gravel at center and 3 in. of gravel at curb, the weight of structural steel per lineal foot of span, not including the weight of the stringers and the railing, is given approximately by the formula for a 16-ft. roadway W = 150 +3.5L. (4) and for a 20-ft. roadway W = 185 +3.5L. (5) 4. Steel High Truss Spans, with Timber Floor.-For high truss spans 104 to 204 ft., with timber floors, the weight of structural steel per lineal foot of span, not including the weight of the stringers and the railing, is given approximately by the formula for a 16-ft. roadway and for a 20-ft. roadway W = 250 +1.5L. (6) (7) Slabs IOWA HIGHWAY COMMISSION.-Steel Highway Bridges with Reinforced Concrete Floor.-Reinforced concrete floor slabs 6 in. thick for all spans in which stringers are used. for stringerless floors 7 in. thick for 8-ft. span, 8 in. thick for 9-ft. span, and 8 in. thick for 10-ft. Live loads for the floor and its supports a uniform live load of 100 lb. per sq. ft., and a 15-ton span. traction engine with two-thirds of the load on the rear axle; axles spaced II ft. centers, and rear Rear wheels 22 in. wide. The trusses are to be designed for the wheels spaced 6 ft. centers. uniform loads given in Table II. No allowance is made for impact. b = Let W weight of structural steel in lb. per lineal foot of span; L 1. Steel Beam Spans.-The weight of steel beam spans from 16 ft. to 32 ft. and with 16-ft., 18-ft., and 20-ft. roadway are given in Table I, Chapter XI. 2. Steel Low Truss Spans, with Stringers. For low truss highway bridges with spans of 35 ft. to 85 ft., not including the weight of the fence or the steel stringers, the weight of structural steel per lineal foot of span for a 16-ft. roadway is 3. Steel Low Truss Spans, without Stringers.-For low truss highway bridges with spans of 35 ft. to 100 ft., not including the weight of the fence, the weight of the structural steel per lineal foot of span for a 16-ft. roadway is 4. Steel High Truss Spans, with Stringers. For high through truss highway bridges with spans of from 90 ft. to 150 ft., not including the weight of fence or the steel stringers, the weight of structural steel per lineal foot of span for a 16-ft. roadway is WISCONSIN HIGHWAY COMMISSION. Steel highway bridges with reinforced concrete floor.-Reinforced concrete floor slabs 6 in. thick for all spans. Live loads for the floor and its supports a 15-ton road roller with two-thirds of the load on the rear axle, axles 10 ft. centers, rear rolls 4 ft. 10 in. centers, rear rolls 20 in. wide. The trusses designed for the loads given in Table II. No allowance is made for impact. foot of span, L = = Let W weight of structural steel in lb. per lineal length of span in feet; b = width of roadway in feet (without sidewalks). 1. Steel Beam Spans.—Weight of steel beam spans from 10 ft. to 38 ft. and for 16-ft., 18-ft. and 20-ft. roadway are given in Table II, Chapter XI. 2. Steel Through Plate Girders.-The weight of the structural steel in through plate girder highway bridges from 35 ft. span to 80 ft. span including floorbeams spaced 3 to 24 ft. apart, is given approximately by the following formula. For a 16-ft. roadway 3. Steel Low Truss Spans, with Stringers.-The weight of the structural steel in low truss steel highway bridges with spans of 35 ft. to 85 ft. span, not including the weight of the fence or the steel stringers, is given approximately by the formula. For a 16-ft. roadway 4. Steel High Truss Spans, with Stringers.-For high through truss steel highway bridges with spans of from 90 ft. to 150 ft., not including the weight of the fence or the steel stringers, the weight of structural steel per lineal foot of span is given approximately by the formula. For a 16-ft. roadway ILLINOIS HIGHWAY COMMISSION. Steel highway bridges with reinforced concrete floor. Reinforced concrete floor slabs 4 in. thick with a wearing surface assumed to weigh not less than 50 lb. per sq. ft. Live load for floor and its supports a 15-ton traction engine, supported on two axles spaced 10 ft. apart, with two thirds of the load on the rear axle; or a uniform live load of 125 lb. per sq. ft. The trusses designed for the loads given in Table II. No allowance is made for impact. Let W = weight of steel in lb. per lineal foot of span, L = span of bridge in feet, b = width of roadway in feet (without sidewalks). 1. Steel Low Truss Spans, with Stringers.-The weight of the structural steel in low truss steel highway bridges with spans of 50 ft. to 85 ft., not including weight of the fence or the steel stringers, is given approximately by the formula. For a 16-ft. roadway, b W = 235 +2.35L. = 16 ft. (21) WEIGHTS OF STEEL HIGHWAY BRIDGES. and for an 18-ft. roadway, b W = 240 + 2.4L. (22) 2. Steel High Truss Spans, with Stringers.-The weight of structural steel in high truss steel highway bridges with spans of 90 ft. to 160 ft., not including the weight of fence or the steel stringers, is given approximately by the formula. For a 16-ft. span, b = 16 ft. W = 140 + 4L. and for an 18-ft. span, b = 18 ft. W = 180 + 4.5L. (23) (24) The weights given by formulas (21) to (24) are for bridges with concrete floors weighing 100 lb. per sq. ft. Calculations by Mr. Clifford Older, Bridge Engineer, Illinois Highway Commission, show that a variation of the weight of the floor of 10 lb. per sq. ft. makes a similar variation For the structural steel, not in the weight of the structural steel, including the joists, of 4.35 per cent for a 50-ft. span, of 3.75 per cent for a 160-ft. span, and proportional for intermediate spans. including the joists, an average value of 4 per cent may be used for each decrease of 10 lb. per sq. ft. of floor surface. BOSTON BRIDGE WORKS STANDARDS.*-The weights of steel highway bridges designed by the Boston Bridge Works are as follows: Through truss highway bridges without sidewalks designed for a live load of 80 lb. per sq. ft. The weight, w, of steel in lb. for the trusses, 100 lb. per sq. ft. and a 6-ton wagon for the floor. per sq. ft. of area covered by the floor, not including joists or fence, for a span of L ft., W = 5 + L/9.5 The weight of through truss highway bridges with two sidewalks is The sidewalks were 5 or 6 ft. wide, and the clear roadways were 16 to 20 ft. The total area covered by the roadway and sidewalk floors is to be used in calculating the weight of steel. Weights of Steel Highway Plate Girder Bridges.-The weights of highway plate girder bridges as designed by the Boston Bridge Works for the live loads shown are as follows. Deck plate girder highway bridges without sidewalks designed for a live load of 100 lb. per sq. ft. for girders, 100 lb. per sq. ft. and a 6-ton wagon for the floor. The weight, w, of steel in Ib. per sq. ft. of area covered by the floor, not including joists or fence, for a span of L ft., is w2.5L/3.4 The weight of deck plate girder highway bridges with sidewalks is w= 2.5+ L/4.4 (27) (28) The weight of through plate girder highway bridges without sidewalks is The weight of through plate girder highway bridges with sidewalks is = Weight of Electric Railway Bridges.-The Boston Bridge Works gives the following formulas total weight of steel in lb. per lineal foot of for the weight of electric railway bridges, where W bridge and L is the span of the bridge in feet. * Published by permission of John C. Moses, Chief Engineer. (31) The beam bridges were designed for 30-ton cars; the light truss bridges were designed for 15-ton cars or 1,500 lb. per lineal foot of bridge, and. the heavy truss bridges were designed for 30-ton cars, or 2,000 lb. per lineal foot of bridge. LIVE LOADS.-The live loads for highway bridges are usually assumed to consist of a uniform live load for the trusses and a uniform live load or a concentrated moving load for the floor and its supports. A few highway bridge specifications require that trusses be designed for a concentrated moving load as well as for a uniform live load, and also that the floor and its supports be designed for a concentrated moving load and that the portion of the floor of the bridge not covered by the concentrated load be covered with a uniform live load. In calculating the stresses in the truss members the uniform live load is commonly assumed as applied in full joint loads at joints on the loaded chord. Moving loads and loads suddenly applied produce stresses that are greater than the static stresses due to stationary loads or to loads gradually applied. This increase in stress due to moving loads or due to loads suddenly applied is called impact stress. IMPACT.-The effect of impact or increase in live load stresses over the stresses due to the same loads gradually applied, is very much less for highway bridges than for railway bridges. Experiments made by Professor F. O. Dufour and recorded in Journal of Western Society of Engineers, June, 1913, show that the effect of impact on steel truss highway bridges with concrete floors is very small. The effect of impact on steel truss bridges with plank floors is considerably larger than for bridges with concrete floors. The maximum impact percentages do not occur with maximum static stresses. Experiments made at the University of Colorado under the author's direction show that the effect of impact on highway bridges is very much less than for railway bridges. The specifications of the highway commissions of Illinois, Iowa, Michigan, Nebraska and Wisconsin do not add impact for highway bridges. The allowance for impact by the Massachusetts Railway Commission is as follows: For stringers, floor beams and hangers, when loaded with a 20-ton auto truck, 50 per cent; for all other loads, floor beams and stringers, 25 per cent; floorbeam hangers, 40 per cent; counters, 40 per cent; for all other members in trusses, and for main girders the percentage shall be 263 minus onetwelfth the loaded length in feet, with a maximum of 25 and a minimum of 10 per cent. = Mr. J. A. L. Waddell in "Bridge Engineering" specifies that highway bridges shall be designed for the impact allowance, I 100/(nL + 200), where L is the loaded length of the bridge in fect that produces maximum stress and n is total clear width in feet of roadway and footwalks divided by twenty. The above impact allowance is made for motor-truck loadings but not for road-roller loadings. General Specification for Steel Highway Bridges adopted 1918 by the Engineering Institute of Canada specifies impact as follows: Impact shall be added to the maximum computed stresses produced by the specified motor-truck and electric-car loads only. For motor-truck loads, the impact shall be taken as 30 per cent of the statically computed stresses produced thereby. electric car loads, the impact shall be determined by the formula = For I S.150/(L + 300), S = = statically computed maximum stress in member considered length of load in feet producing maximum stress in member con The specifications of the West Virginia Highway Commission and the Oregon Highway Commission specify the impact factor, I = 100/(L + 300), where L is the loaded length of the bridge in feet that produces maximum stress in the member. The Montana Highway Commission specifies 25 per cent impact. LIVE LOADS. The U. S. Bureau of Public Roads specifies 30 per cent impact. The Department of Public Roads of Kentucky requires no impact allowance for bridges with concrete floors, and 25 per cent for bridges with wooden floors. The Utah Highway Commission specifies 25 per cent impact for floors, and 15 per cent for trusses. For concrete highway bridges the impact allowance varies from no impact allowance, as specified by the highway commissions of Illinois, Iowa, Michigan, Nebraska and Wisconsin; an allowance of 15 per cent of the live load, as specified by the highway commission of West Virginia, to an allowance of 30 per cent of the live load, as specified by the U. S. Bureau of Public Roads. Watson's "General Specifications for Concrete Bridges," third edition, 1916, uses an impact allowance of I: 150/(L + 300), where L is the loaded length of the bridge in feet that produces maximum stress. = Ketchum's Specifications for Impact.-The author has adopted the following impact factors for concrete bridges and steel bridges. (a) For concrete arches with spandrel filling or culverts with a minimum filling of one foot, no allowance for impact. (b) For concrete slab and girder bridges and trestles, and arches without spandrel filling, 30 per cent for impact. (c) For steel bridges the following allowance for impact. For the floor and its supports including floor slabs, floor joists, floorbeams and hangers, 30 per cent. = For all truss members other than the floor and its supports, the impact increment shall be length of span for simple highway spans (for trestle bents, towers, I = 100/(L+ 300), where L movable bridges, arch and cantilever bridges, and for bridges carrying electric trains, L shall be taken as the loaded length of the bridge in feet producing maximum stress in the member). CONCENTRATED LIVE LOADS.-Traction engines weighing 20 tons are quite common in The heaviest motor truck in common use has a capacity of 7 tons and the west and northwest. a total weight of 13 tons, with nearly 10 tons on the rear axle. With an overload of 50 per cent, which is not unusual, this truck would carry 14 tons on the rear axle. The maximum road roller weighs 20 tons. The highway commissions of the different states have adopted concentrated live loads as follows: Illinois specifies a 15-ton traction engine; Iowa specifies a 15-ton traction engine for bridges with reinforced concrete floors; Wisconsin specifies a 15-ton road roller; Michigan specifies an 18-ton road roller; Nebraska specifies a 20-ton traction engine; Minnesota specifies a 20-ton traction engine; New York specifies a 15-ton road roller; all loadings to be used without impact. Utah specifies an 18-ton road roller with 25 per cent impact; Oregon specifies a 15-ton road roller for medium traffic and a 20-ton road roller for heavy traffic; Montana specifies a 20-ton traction engine with 25 per cent impact; the Massachusetts Railway Commission specifies a 20ton motor truck with an allowance of 50 per cent for impact on the floor and its supports; Mr. J. A. L. Waddell in "Bridge Engineering" specifies for class A bridges an 18-ton motor truck with impact allowance as given above. The Ohio State Highway Commission specifies a 20-ton concentrated load on two axles spaced 10 ft., wheels with gage of 6 ft. with two-thirds on rear axle on roads in industrial communities, and 15 tons with same spacing and distribution on country highways. Impact for bridges with concrete floors is one-half that given for steel bridges in Appendix I. The U. S. Bureau of Public Roads specifies a 15-ton truck for all types of highway bridges except timber bridges for which a 10-ton truck is specified. Each truck has axles 10 ft. centers, and wheels 6 ft. centers, with two-thirds of the total load carried on the rear axle. All loads with 30 per cent impact. General Specification for Steel Highway Bridges adopted 1918 by the Engineering Institute of Canada specifies motor-truck loadings as follows: For city bridges a 25-ton motor-truck with |