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41. Wind Loads for Railroad Bridges

For steam railway bridges the wind loads per lineal foot of span for both the loaded and the unloaded chords are to be taken from the curves given in Fig. 96. The wind loads for the loaded chords include a pressure of three hundred (300) pounds per lineal foot on the train, the centre of which pressure is applied at a height of eight (8) feet above the base of rail. For determining the requisite anchorage for a loaded structure, the train of empty cars shall be assumed to weigh one thousand (1,000) pounds per lineal foot.

In trestle towers the columns and transverse bracing shall be proportioned to resist the following wind-pressures in addition to all other loads:

First. When the structure is loaded, six hundred (600) pounds per lineal foot on stringers and cars, concentrated at a height of one foot above base of rail, and two hundred and fifty (250) pounds for each vertical foot of each entire tower.

Second. When the structure is empty, three hundred and fifty (350) pounds per lineal foot on stringers, assumed to be concentrated one foot above the centre of stringer, and three hundred and fifty (350) pounds for each vertical foot of each entire tower.

The wind loads for longitudinal bracing are to be taken as seventenths (0.7) of those for the transverse bracing.

In figuring greatest tension on columns and anchor-bolts, computations are to be made for both the loaded and the unloaded structure, in double-track trestles placing the train of empty cars on the leeward track.

The wind loads of the upper lateral system shall generally be assumed to be carried to the ends of the span by the said lateral system, no part thereof being considered to travel down by the intermediate vertical sway-bracing.

All wind loads are to be treated as moving loads. No percentage of impact is to be added to wind loads.

Wind loads for swing spans are specified subsequently in this chapter, as are also those for the design of the machinery of vertical lift and bascule bridges.

In vertical lift bridges the towers are to be figured for a wind load of fifteen (15) pounds per square foot with the movable span in its highest position and for one of thirty (30) pounds per square foot with the said span in its lowest position, the longitudinal wind load on the span being taken as seven-tenths (0.7) of the transverse.

In bascule bridges the structural portions shall be designed for a wind load of thirty (30) pounds per square foot with the span closed, and for one of fifteen (15) pounds per square foot when the said span is in any other position.

42. Wind Loads for Highway Bridges

For highway and électric-railway structures the wind loads per lineal foot of span for both the loaded and the unloaded chords are to be taken from the curves shown in Fig. 9d. The wind loads for the loaded chords of bridges carrying electric railways include a pressure of two hundred and fifty (250) pounds per lineal foot on the cars, the centre of which pressure is applied at a height of seven (7) feet above the base of rail. These diagrams were figured for a clear roadway of twenty (20) feet. For wider structures, the wind loads for the loaded chords are to be increased two (2) per cent for each foot of width in excess of twenty (20). The wind loads given on the diagram have been computed from detailed designs for simple spans up to seven hundred and fifty (750) feet in length, but beyond this limit they have been assumed; consequently, in designing spans of greater length than this, it will be necessary to check the assumed wind-pressure after the sections are proportioned, using an intensity of twenty-five (25) pounds per square foot. The intensities employed in preparing the curves varied from forty (40) pounds for very short spans to twenty-five (25) pounds for very long ones.

For viaducts carrying highway traffic only, the wind-pressure on the empty structure is to be assumed as three hundred (300) pounds per lineal foot on the spans at the level of the floor, and two hundred and fifty (250) pounds for each vertical foot of each entire tower. The wind loads for longitudinal bracing are to be taken as seven-tenths (0.7) of those for the transverse bracing.

For elevated railroads and for viaducts carrying electric trains, the wind loads are to be taken as eight-tenths (0.8) of those specified for railroad bridges.

All wind loads are to be treated as moving loads.

For all highway structures the live load and the wind load shall not be assumed to act together, excepting only that the electric-railway live load must be taken as acting in conjunction with the wind.

Wind loads for swing spans are specified subsequently in this chapter, as are also those for the design of the machinery of vertical lift and bascule bridges.

The wind loads for the design of the towers of vertical lift highway bridges and the structural portions of bascule highway bridges are to be the same as those specified for railway bridges.

43. Indirect Wind Load or Transferred Load

For through truss spans with inclined end posts, even with polygonal top chords, the transferred load is to be assumed to produce a tension. in the leeward bottom chord that is constant from end to end of span and a similar release of tension on the windward bottom chord. For trusses with parallel chords this assumption is correct, provided that all

the wind-pressure travels directly to ends of span by the horizontal bracing; while for trusses with polygonal top chords the assumption is a compromise, the travel of wind-pressure being ambiguous. The transferred load is to be found by multiplying one-half of the total wind load on the top chord by the vertical distance between the point of contraflexure of the inclined end post and the hip apex and dividing the product by the perpendicular distance between central planes of trusses.

44. Vibration Load

In railway bridges the vibration load is a transverse loading, generally in excess of the wind load, applied to the lateral bracing only. The stresses which it produces are not to be added to any other stresses, its sole object being to ensure sufficient sectional areas for lateral members in order to attain proper rigidity for the structure as a whole. For the loaded chords of through and deck spans and for viaduct towers its value is to be taken at seven hundred (700) pounds per lineal foot for singletrack structures and eight hundred and fifty (850) pounds per lineal foot for double-track structures. For the unloaded chords the corresponding figures are, respectively, three hundred (300) and three hundred and fifty (350). In computing the stresses caused by vibration loads, they are always to be considered as advancing.

Highway bridges and electric-railway bridges are not to be figured for vibration loadings.

45. Traction Load

The total traction load on any portion of a structure is to be taken as a certain percentage of the greatest live load that can be placed on that portion of said structure. For elevated railroads and electric-railway bridges this percentage is to be taken as twenty (20); and for railway bridges it is to be determined by the formula,

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and L= loaded length in feet.

The values of T may be taken from Fig. 9e.

In proportioning the towers and columns of railway trestles and elevated railroads, the said towers and columns between consecutive expansion points are to be assumed to receive no aid from neighboring towers and columns, but must be figured for the greatest possible traction load between the said consecutive expansion points. No percentage of impact is to be added to traction loads. There is to be no traction loading for highway bridges unless they carry electric-railway tracks.

46. Centrifugal Load

The centrifugal load is to be computed for the greatest probable velocity of trains by the formula,

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where C is the centrifugal load per lineal foot, W is the equivalent live load per lineal foot, R is the radius of the curve in feet, and V is given by the formula,

V = 60

2.5D

where D is the degree of curvature.

The values of C for curves up to

twenty (20) degrees can be taken from Fig. 8b.

All portions of the structure affected by the centrifugal load are to be figured to carry properly the stresses induced by the said load in addition to all other stresses to which they may be subjected. It is to be assumed as applied five (5) feet above the base of rail, the average centre of gravity of the moving load. The transferred load on the stringers, girders, or trusses due to the transference of the centrifugal load to the plane of the lateral bracing shall be considered, as well as the stresses produced in the laterals and chords forming the horizontal truss for carrying this load to the ends of the span. The overturning effect of the centrifugal load on the structure as a whole shall also be duly considered. The effect of the shifting of the centre of gravity of the load due to the superelevation of the outer rail shall also be taken into account, as well as the effect of the eccentricity of the load due to the curvature of the track. No percentage of impact is to be added to centrifugal loads. There is to be no centrifugal loading for highway bridges unless they carry electric-railway tracks.

47. Effects of Changes of Temperature

In ordinary structures changes of temperature will not affect the stresses in the members, provided, of course, that proper precaution be taken to permit unrestricted expansion and contraction. But in all arches, excepting only those hinged at both ends and at the crown, the stresses caused by the assumed extreme changes of temperature must be computed and duly considered. Temperature stresses must also be given proper consideration in all steel trestles in which the expansion points are placed farther apart than the length of two consecutive bays.

WORKING STRESSES

48. Intensities of Working Stresses

The following intensities of working stresses (ie., pounds per square inch of cross-section) for medium and rivet carbon steels are to be used for all cases, except as hereinafter specified to the contrary.

Tension on gross sections of eye-bars and reinforcing
bars, on net sections of all built members, and on
net sections of flanges of all beams..

Bending on pins..

Bearing on pins.

Bearing on shop rivets.

Bearing on end stiffeners of plate girders (outstanding

legs only).....

Shear on pins.

Shear on shop rivets.

Shear on plate-girder webs, gross section.

Bearing on expansion rollers, in pounds, where d is the

diameter of the roller in inches...

16,000 lbs.

27,000 lbs.

22,000 lbs.

20,000 lbs.

16,000 lbs.

15,000 lbs.

10,000 lbs.

10,000 lbs.

600 d.

For field rivets the intensities for bearing and shear are to be reduced twenty (20) per cent.

Turned bolts with driving fit are to be stressed the same as field rivets.

Compression in pounds on struts with fixed ends, 16,000 60

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r

80

Compression in pounds on struts with hinged ends, 16,000 Compression on gross section of flanges of rolled beams 16,000 lbs. Compression in pounds on gross section of flanges of built beams, 16,000 – 2001.

300/

Compression in pounds on forked ends, 10,000 300

In these compression formulæ is the unsupported length of strut, flange, or jaw-plate in inches, r is the least radius of gyration of the strut in inches, b is the width of the flange in inches, and t is the thickness of jawplate in inches.

The intensities of working stresses for nickel steel, established on the basis that the least allowable elastic limit (determined by the drop of the beam) in specimen tests is 55,000 pounds per square inch for plateand-shape steel and 60,000 pounds per square inch for eye-bar steel, are to be as follows. In case that a still higher grade of nickel steel is procurable, all the intensities, excepting those on rivets, are to be multiplied by the ratio of the higher elastic limit to 55,000 or 60,000, according to the character of the steel under consideration.

Tension on gross sections of eye-bars

Tension on net sections of all built members, and on

net sections of flanges of all beams..

Bending on pins...

Bearing on pins.

28,000 lbs.

26,000 lbs.

45,000 lbs.

35,000 lbs.

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