Standard specifications for structural steel--timber--concrete and reinforced concrete

Front matter

ill! I

I

J

Preface

A serious attempt has been made to incorporate into one volume a set of ten specifications, which not only cover the most important materials used in construction work of any magnitude, but which are condensed so as to avoid unnecessary repetitions, are consistent throughout, and which, at the same time, conform in every essential to the latest experiments and investigations, to the best authorities, to modern practice, and to the author's own considerable experience.

Of new matter, particular attention is called to the specifications for reinforced concrete, as these are rather extensive, probably the first complete set m existence.

To facilitate the use of all specifications, the subject matter in each has been arranged, as nearly as possible, in the order, or rotation, in which the information is wanted.

Any designs made, or structures built in strict accordance with these specifications will insure first-class details, excellent materials, and creditable workmanship, as well as safety, durability, and economy. Hence they are designed to be equally well suited to the needs of engineers, architects, contractors, college professors and their students.

In the body of the specifications credit has been accorded to

are also due to Francis P. Wittmcr, M. Am. Soc. C.E., and others

for many valuable suggestions. J. C. 0.

New York, September, 1910.

V

Contents

TART PAGE

I. Steel Framework of Buildings 1

II. Highway Bridges # 13

PART 1 Dead, Snow, and Wind Loads

i. The Dead Load upon any part of a structure shall consist of the calculated weight of the materials and fixtures carried permanently by such part.

2. The actual weight of the different materials shall be computed as accurately as possible and, if necessary, a trial design shall be made to ascertain the weight of any portion in doubt.

3. Roof Covering. The weight of roof covering will average, for each square foot of superficial roof area, as follows:

Corrugated steel 2-3 lbs.

Shingles 1 6

Roofing files 6-8

Slate (J to i" thick) 6-8

Gravel and composition s LO

Terra-cotta, 2" thick (flat arch) 10 15

Concrete, per inch thickness (Hat arch).. LO 12

Plank sheathing, per inch thickness 3 I

4. Purlins. The weight of purlins will average, for each square foot of superficial roof area, as follows:

Steel purlins 2-3 lbs.

Wooden purlins 3-5 "

5. Roof Trusses. The weight of ordinary steel roof trusses, having a pitch of -^ to J, will average, for each square foot of horizontal area, as follows:

or for the total weight of one truss, where lT=total weight of truss in lbs.

c=weight in lbs. per sq.ft. on horizontal area carried by

truss.

d=distance in feet between trusses, provided that c/> —

L = span in feet of truss.

6. Snow Load. In latitudes between 40° and 50° the snow load shall be taken per horizontal square foot of roof area as follows :

Pitch from flat to ^ inclusive 25 lbs.

" over i to i " 20 "

" i and" more "

In other latitudes these snow loads should be modified according to local conditions.

7. Wind Pressure. The wind pressure shall be taken as 30 lbs. per sq.ft., upon the entire exposed area of the steel framework, or upon the vertical projection of sides and ends of roof and building, and as acting horizontally in any direction.

8. Minimum Roof Load. The minimum roof load, which will include the dead, snow, and wind loads, shall be taken as follows :

On roof covering and purlins, 40 lbs. per sq.ft.

of superficial area.

On roof trusses, 40 lbs. per sq.ft. of horizontal area.

Live Loads

g. Live Loads on Floors.

(a) The live loads on floors shall be considered as being uniform over their entire area and shall be taken, as a minimum, per square foot as follows:

hospitals 50 lbs. and up

Office buildings 60 "

Schools, theaters, churches 75

Ordinary stores, public buildings . . . 100

Warehouses, factories, armories 200

When designing flooring, joists, girders, etc., the probability of concentrated loads, such as pianos, safes, machinery, wagons, etc., must be considered and provided for.

(6) The above live loads, except in warehouses and other buildings exposed to simultaneous loading on all floors, may be reduced for columns as follows:

Columns supporting roof and top floor, no reduction.

" next ten floors, a cumulative reduction of 5 per cent for each succeeding floor.

" " remaining floors, a total reduction

of 50 per cent.

io. Crane Loads. The actual weighl of the crane, its dimensions and its lifting capacity shall l>r used if obtainable, otherwise the following data for Typical Electrical Traveling Cranes*

shall be used:

Capacity

Wheel Base.

Ft. In.

Maximum

Wheel

Load in

Pounds.

Weight of Rail per Yard for

Plate Girders.

Pounds.

Beams.

Pounds.

5 Tons

40 -60

8 6

12.000 13.000

10 10

_

40 40

40 40

10 ••

19,000

-

.60

9 6

21,000

-

15 •

26.000

20.000

20 •*

33.000

-

10 6

36.000

S

9*1

40.000

-

-O

' 60

10 6

44.000

30 '•

10 6

48,000

' 60

52,000

40 ••

'40

64.000

' 60

70.000 where •> = Side clearance from center of rail. v= Vertical clearance from top of rail.

(a) A wheel-load shall be assumed as distributed in the top flange, over a distance equal to depth of girder, with a maximum of 30 inches.

In addition to the vertical load, the top flanges of the girder shall withstand a lateral loading of two-tenths (j-j) of the lifting capacity of the crane, equally divided between the four wheels of the crane. ii. Impact Loads. For beams, girders, and columns carrying traveling cranes an allowance of 25 per cent of the computed moments and shears due to same shall be added to compensate for the effects of impact and vibration.

Unit Stresses : Steelwork •

12. In designing the component parts of any building the maximum stresses due to the combined effects of axial stresses and bending stresses from dead, snow, wind and live loads, including impact, if any. shall not exceed, for structural steel and rivet steel, the following values in lbs. per square inch, except as modified in Pars. 14-16.

(a) Axial tension in main members, net section 16,000 lbs.

in wind bracing, " 20,000 "

(b) Bending, net section, on extreme fibers of:

Rolled shapes, built sections and

plate girders 16,000 lbs.

(c) Axial compression in main members, gross section, where *S=allowable unit stress.

/ = unsupported length of member in inches.

r = least radius of gyration of member in inches. (Per. 39.)

(d) Shear on: Turned bolts, and field rivets

when hand driven 7,500 lbs.

Shop rivets and pins 10,000 "

Plate girder webs, gross section. 10,000 "

(e) Bearing on: Turned bolts and field rivets

when hand driven 15,000 "

Shop rivets and pins . 20,000 "

13. Expansion Rollers.

Allowable pressure per lin. in 600d

where d= diameter of roller in inches.

14. Combined Stresses. When the effect of bending stresses due to wind loading is considered and added to the axial and bending stresses, due to other causes, the above unit stresses (Par. 12) may be increased 25 per cent.

15. Eccentric Stresses. Provision must be made for eccentric loading on columns and other members. Whenever angles used as web members or bracing are connected up by one legonly, the above unit stresses (Par. L2) shall be decreased 25 per cent.

16. Reversal of Stresses. Members and connections subject to reversal of stresses shall be proportioned for an equivalenl .stress equal to thai stress which, when added bo 50 per cent of the other, will give the greater section.

17. Cast Iron. The maximum stresses shall not exceed, in pounds per square inch, for

Tension _ 2.500 lbs.

Compression (on blocks) 'l2,000 "

Unit Stresses: Masonry, Foundations

18. Pressure on Walls. The pressure on walls caused by beams, girders, wall plates, etc., shall not exceed the following values in pounds per square inch.

On brickwork, laid in lime mortar. . . . 150 lbs.

cement mortar. 200 "

solid 150 "

On cinder concrete 1:2:6 125 "

On Portland cement concrete 1:2^:5. 275 "

1:2 :4. 350 ".

On ordinary rubble masonry, cement

mortar 150 "

On first -class stone masonry, cement

19. Pressure on Foundations. The pressure on masonry or foundations caused by column bases, etc.. shall not exceed the following values in pounds per square inch:

Dimension stones 125 lbs.

Portland cement concrete 1 :2| :o 300 "

1:2 :4 400 "

Other values as given above (Pars. 17. IS).

20. Pressure on Soils. The pressure on soils caused by foundations, walls, etc.. shall not exceed the following values in tons per square foot:

Earth, ordinary lh tons

" tamped or naturally solid 3 "

Clay, soft " 1 "

" dry or mixed with dry sand 2 "

" hard pan 3 "

" hard and mixed with coarse sand. . . 4 "

Sand and gravel mixed, when dry and

coarse 6 "

Rock, solid bed 200 "

21. Bearing Power of Piles. Wooden piles shall be spaced not less than 30 in. c. to c.

When the piles are driven through wet and loose soil to a good bearing the pressure shall not exceed 300 lbs. per sq. in. of their average cross-section. When driven through a firm soil this pressure may be increased to 600 lbs. (Pars. 270, 277.)

Design of Framing

22. Roof Trusses. Roof trusses shall preferably be of such type as will allow the purlins to be placed only at the panel points. Wherever this is neither practicable nor economical, the top chord shall be designed for both bending and direct stresses.

Roof trusses shall be riveted throughout, except in special cases for long spans where the field splices and main joints may be pin-connected.

23. Spacing of Roof Trusses. The spacing of roof trusses shall preferably be such as to allow only the use of single rolled shapes as purlins (Par. 26). Whenever this limit be exceeded intermediate jack-rafters may be used. For ordinary roofs the most economical spacing of trusses will generally be as follows:

tor spans up to

16'

to ( u

" from

17'

0"

it a a

55 70 "

IS'

0"

it it a a

70 85 "

0"

n a a

85 LOO"

20'

0"

24. Pitch of Roofs. The pitch must vary according to circumstances, but shall preferably be made as follows:

For tar and gravel roofs from ^ to J-j-

" tile, corrugated steel and

shingled roofs " i to J

" slate roofs " J and up

25. Bracing. Roof trusses shall be braced in pairs; in the plane of the top chord the bracing may be made adjust able , whereas in the plane of the bottom chord the bracing must be rigid.

Bracing in planes of the sides and ends of buildings may also be made adjustable.

Knee-braces must be provided wherever practicable, for instance, between trusses and columns and between crane girders and columns.

26. Purlins. Both steel and timber purlins shall be attached to the roof trusses by means of lug angles. Steel purlins shall be composed of single-rolled shapes (Z-bars, Cs, I-beams and Ls) whenever possible, and be of a depth 5: than one-thirtieth (3^) of the span. Otherwise plate girder or lattice girder purlins shall be used, and no trussed purlins will be permitted.

Purlins shall generally be spaced from 4 ft. to 5 ft. apart for all forms of roof covering except reinforced concrete, where the spacing may be increased to 12 ft.

27. Girts. Girts shall be attached to columns by means of lug angles and shall be composed of single rolled shapes. They shall generally not be spaced more than 4 ft. apart.

28. Beam Girders. Beams composed of single rolled shapes (I-beams and r» shall be proportioned by their moment of inertia.

Xo beam shall be used whose flange width is less than one-twentieth ( t l) of the span length, unless its compression flange is properly supported side wise against buckling. AVhen more than one beam is used to form a girder, they shall be connected by bolts and separators, so as to bring the unsupported flange width within the above ratio, spaced with a maximum limit of 5 ft. Beams used in floors shall have a depth of not less than one-twentieth Q^) of the span length; they shall be riveted to the columns, whenever such occur, by means of connection angles which must carry the entire load. Shelf angles may be provided for convenience during the erection. The free end of such floor beams must be securely anchored to the walls.

29. Plate Girders (see Part IV).

Details of Design

30. Minimum Size of Material. No rolled shape or metal of less thickness than \ in. shall be used except for fillers. No angles shall be less than 2%"X2 ,, Xi" and no rivets, except in beam connections and lacing bars, shall be less than f in. in diam. (Pars. 33, 38, 45, 46.)

31. Details of Joints. Main members of trusses shall be so arranged around the joints that their neutral axes will meet in a common point.

All joints in riveted work, whether in tension or in compression, shall be fully spliced.

32. Details of Connections. The strength of connections shall be such as to cause the main member to fail should the combination be tested to destruction. No connection, except for lattice bars, shall have less than two rivets.

33. Rivets. In main members composed of angles the maximum diameter of rivet used shall not exceed one-quarter (J-) the width of leg through which it passes. Three-quarter (f) in. rivets shall not be countersunk in plates less than f in. in thickness and J-in. rivets shall not be countersunk in plates, less than ^ in. in thickness.

Except in bed plates, no rivet shall be of loss diameter than the thickness of the thickest plate through which it passes.

34. Rivet Spacing. The minimum pitch shall never be less than three diameters of the rivet and, if possible, not less than 3" for J" rivets, 2£" for \" rivets, 2J" for |" rivets, and If" lor \" rivets.

The maximum pitch of rivets, in the direction of the stress, shall not exceed 6 in., or 1(> times the thinnest outside plate connected, and not more than 10 times that thickness at right angles to the stress, except when two or more plates in contact are to be held together, where the pitch may be L2 in. in any direction.

The maximum pitch at ends of built up compression members shall not exceed four diameters of the rivet for a length equal to two times the width of the member.

35. Edge Distance of Rivets. The minimum distance from the center of any rivet hole to a sheared edge shall be H" for i" rivets. 1J" for f" rivets, \\" for f" rivets, and 1" for \" rivets, and to a rolled edge, except in flanges of I-beams and channels, 1}". 1J", 1" and \" respectively. The maximum distance from an}- edge shall be eight (8) times the thickness of the outside plate, but shall not exceed 5*in.

36. Tension Members. In calculating the net area of tension members the area of rivet holes must be deducted, assuming the diameter of the hole \ in. larger than the nominal size of the rivet.

Main tension members shall be composed of sections placed symmetrically about the central plane through the truss. Secondary tension members may be made of a single shape. (Par. 15.)

Pin-connected riveted tension members shall have a net section through the pin hole 25 per cent in excess of that through the body of the main member. The minimum net section back of the pin hole, parallel to the axis of the member, shall not be less than that through the body of the main member.

37. Eye-bars. Heads of eye-bars shall be forged and so proportioned as to develop the full strength of the bar. (Par.

The eye-bars shall be placed in the truss so as to be, as nearly as possible, parallel to its central plane, the maximum inclination being limited to 1 in. in 16 ft.

38. Rods. The minimum size of bracing rods, or laterals, shall be f in. in diameter and they shall be upset at the screw ends. The minimum size of sag rods shall be f in. in diameter, but these need not be upset.

39. Compression Members. Main compression members shall be composed of sections placed symmetrically about the central plane through the truss. Sub-struts may be made of a single shape. (Par. 15.)

Xo compression member shall have a length exceeding 120 times its least radius of gyration, except those used for wind and lateral bracing, which ma}- have a maximum length of 140 times their least radius of gyration.

40. Tie-Plates. The open sides of compression members shall be stayed by diagonal lattice having tie-plates as near each end as practicable and at intermediate points where the lattice is interrupted. In the main members the tie-plates shall have a length of not less than the width of the main member on the connected side, provided that no tie-plate be less than 10 in. long near the ends of such members.

Their minimum thickness shall be one-forty-eighth (Jg) f the distance between center lines of the connecting rivets.

41. Columns. In proportioning the area of columns the effect of bending stresses due to wind or eccentric loading shall be included.

Columns shall have as few splices as practicable and, whore spliced, the splices shall be strong enough to resist the direct as well as the bending stresses.

42. Lattice Bars. Single lattice bars shall have an inclination of approximately 60° with the axis of the main member and shall have a minimum thickness of one-fortieth (-j-^) of the distance between the rivets connecting them to the member.

Single lattice bars may be connected with one rivet, except in flanges more than 5 in. wide, where two rivets shall be used. Lattice angles shall be connected by at least two rivets.

Double lattice bars shall be used where the distance between rivet lines in the flanges exceeds 15 in. They shall have a minimum thickness of one-sixtieth (-^ ) of the distance between the rivets connecting them to the member, shall have an inclination of approximately 45°, and shall be riveted at their intersection.

The minimum width of lattice bars shall be:

lor 15-in. channels, or built sections with 1 _, .

. _ . ' 2\ in. ( fr-m. rivets)

angles over 3 in J "

For 12-, 10-, and 9-in. channels, or built sec- 1 rt1 . . . , N tions with ->-m. angles 1

For 8- and 7-in. channels, or built sections 1 . . . . . . 2 -in. (f-in. rivets)

with 2\-m. angles j

For (j-in. channels or less I , : in. I'-in. rivets)

43. Pins. The minimum diameter of pins shall be eighttenths (, s ) of the width of the widest eye-bar attached to it.

Members shall be packed on pins, using filling rings where necessary, in a manner that will prevent any lateral movement.

44. Pin Holes. Pin holes shall be reinforced by plates where necessary. The plates shall be of such size as to distribute properly, through sufficient rivets, the pin pressure to the webs and their flanges in each segment of the main member.

45. Wall Plates and Column Bases. The minimum thick- — -hall be \ in. for wall plates and § in. for base plates. The

wall plates and column bases shall be detailed and placed in such a manner that the load will be evenly distributed, using cement mortar or grout for filling if necessary.

They shall be of sufficient size and thickness so as not to exceed the allowable unit stresses. Pars. 12. 14. 17. 18 and 19.)

46. Anchor Bolts. Columns shall be anchored to the foundations, by means of anchor bolts, when stressed in tension at their base. The minimum diameter of anchor bolts shall be t in. upset: they shall be of sufficient length to engage a mass of masonry, the weight of which shall be lh times the tension.

The anchor bolts shall in all cases be of sufficient size to resist in shear any horizontal force acting thereon.

47. Temperature. Where necessary provision shall be made for expansion and contraction covering a range of 150 c F

48. Expansion Rollers. The minimum diameter of expansion rollers shall be 4 in.

PART II General Requirements

49. Classification. Highway bridges may, based upon traffic conditions, be divided into three classes, viz.:

Class A — City bridges, subject to heavy traffic.

Class B — City, Interurban or Country bridges, subject

to medium traffic, and Class C — Country bridges, subject to light traffic.

50. Type of Bridge. The following types of bridges shall preferably be used:

For spans up to 20 ft., wooden beams or rolled beams.

" " from 20 to 40 ft., rolled beams or plate girders. " " " 40 to 70 ft., plate girders.

" " " SO to 100 ft., plate girders or riveted t russes. " " " 100 to 160 ft., riveted trusses.

" long spans, 160 ft. and over, pin-connected trusses.

Pony trusses shall be avoided wherever possible, but may be used for spans from 40 to 90 ft. (Par. 80.)

51. Materials. All parts of the superstructure, except the flooring and paving, shall be of structural steel, or rivet steel.

Cast iron may be used for minor parts and for ornamental purposes; all other castings shall be of steel.

52. Clearances. All through bridges carrying electric cars shall have a clear head-room, above the top of the rail, of at least 15 ft., for a width of 6 ft. over the center of the track. Where the track is straighl there shall be clear width of at least

7 ft. on each side of the center of the track at a heighl of 12 in.

above the top of rails. Where the tracks are curved, the additional clearance shall be computed by assuming the extreme length of car as 45 ft., width 8 ft., and distance between centers of trucks 20 ft.

Through bridges, not carrying electric cars, shall have a minimum head-room of 14 ft. above roadway, for classes A and B, and of 12 ft. 6 in. for class C, unless otherwise required by local ordinances.

53. Paved Floors. Pavements consisting of stone blocks, paving bricks, asphalt, etc.. resting upon a bed of concrete, not reinforced, shall be supported upon buckled or corrugated plates. The minimum thickness of this concrete bed shall be 3 in. for the roadway and 2 in. for the sidewalks. Such floors shall be pitched transversely and proper provisions for their thorough drainage shall be made.

Pavements consisting of wooden blocks may rest on a timber floor, consisting of planks laid transversely and at least 4 in. thick.

54. Wooden Floors. (See Pars. 271, 279 to 281.)

55. Cross-Ties. (See Par. 283.)

56. Guard-Rails. (See Par. 284.)

57. Handrailing. A handrailing 3 ft. 6 in. high shall be placed on each side of the bridge, except where plate girders serve the same purpose. Where the handrailing is of rolled steel or cast iron it shall be of pleasing design and shall be rigidly attached to the superstructure. For wooden handrailing see Par. 282.

58. Approaches. All floor-timbers, rails, guards and handrailings shall extend over all piers and abutments and shall make suitable connection with the embankments at either end of the structure.

Loads

59. Dead Load. The dead load consists of:

(a) The weight of the steelwork:

(b) The weight of the paving, if any;

(c) The weight of the wooden flooring, if any. and

(d) The weight of the electric railway tracks, if any.

The approximate weight of the steelwork shall be obtained either by trial design or otherwise.

The weight of the paving shall be taken at 160 lbs. per cu.

ft. for stone blocks, at 150 lbs. per cu. ft. for paving bricks, and at 130 lbs. per cu. ft. for concrete and asphalt.

The weight of the wooden flooring shall be taken at 4J lbs. per foot-board measure for oak, yellow pine and other hard woods, at 3-J- lbs. per foot-board measure for white pine and other soft woods.

The minimum weight of cross-ties and guard rails shall be taken at 200 lbs. per lin.ft. of each track, and the weight of rails, fastenings and splices at 100 lbs. per lin.ft. of each track.

6o. Live Loads. All bridges shall be designed to carry certain concentrated and certain uniformly distributed loads, as specified below, placed so as to give the greatest stress in each part of the structure.

(a) Class A. For the floor system and local truss members a concentrated load of 40,000 lbs., distributed on two axles 8 ft. centers and 5 ft. gauge (occupying a length of 20 ft. and a width of 10 ft.), and upon the remaining area of the floor, including sidewalks, a load of 100 lbs. per sq.ft.

For the trusses or girders, 100 lbs. per sq.ft. of entire 1 roadway and sidewalks for spans of 100 ft. or less, 80 lbs. for spans of 200 ft. or over, and proportionally for intermediate spans. (6) Class B, For the floor system and local truss members a concentrated load of 30,000 lbs., distributed on two axles 8 ft. centers and 5 ft. gauge (occupying a length of 20 ft. and a width of 10 ft.), and upon the remaining area of the floor, including sidewalks, a load of 90 11 >s. per sq.ft.

For the trusses or girders, 00 lbs. per sq.ft. of entire roadway and sidewalks for spans of 100 ft. or less, 70 lbs. for spans of 200 ft. or over and proportionally for intermediate spans. (c) Class C. For the floor system and local truss members a concentrated load of 20,000 lbs., distributed on two axles 8 ft. centers and 5 ft. gauge (occupying a length of 20 ft. and a width of 10 ft.), and upon the remaining area of the floor, including sidewalks, a load of 80 lbs. per sq.ft.

For the (russes or girders, 80 lbs per sq.ft. of entire

16 STANDARD >PE< IFICAT: -

roadway and sidewalks for spans of 100 ft. or less lbs of 200 ft. or over, and proportionally

for intermediate spans. (d) Electric Railways. Any bridge carrying electric railway epting those for exclusive railroad use» shall in addition to one of the above loadings be designed to :ry on each track a series of cars, each weighing 100.000 lbs., unless otherwise specified. This load to be distributed equally on two trucks 20 ft. centers, each having two axles 5 ft. centers and 5 ft. gauge (occupying a length of 40 ft. and a width of 10 "

6i. Impact. An impact allowance shall t the

computed maximum five load stress - - follows:

Tor bridges of all classes earning hiehwav l _

traffic only .."... P^^U+I

For bridges, or part of bridges, carrying T _ ...■ 200 \

electric railway traffic J ' 1—300/'

where 5 = computed maximum five load stress, moment or shear; L = loaded length of span in feet.

N •:- impact allowance shall be added to sti ss s produce wind, centrifugal or traction force -

62. Wind Pressure. The wind bracing shall be designed to one of the following lateral loadings, whichever produces the greater stress:

(a) Structure unloaded. 50 lbs. per sq.ft. on the exp - surface of all trusses and the floor as seen in elevation, or

(6) Structure loaded. (Bridges of all classes carrying high

traffic only), 30 lbs. per sq.ft. on the exposed surface of all trusses and the floor as seen in' elevation in addition to a uniform load of 150 lbs. per lin.ft. of structure applied on the " loaded " chord, or

(c) Structure loaded. (Bridges of all classes earning electric railway traffic the same loading as under xcept

that the additional uniform lead is 300 lbs. per lin.ft. of structure and is applied 7 ft. above the base of rail.

The minimum value of the above pressures shall be 250 lbs. per lin.ft. for the " loaded 77 and 150 lbs. for the " unloaded ;; chord of the structure.

Trestles shall in addition to one of the above wind loadings be designed to resist a pressure of 200 lbs. for each vertical foot of bent in height.

The above wind pressure shall in all cases be treated as moving loads. (Par. 61.)

63. Centrifugal Force. Any structure on a curve carrying an electric railway shall be designed to resist a lateral force of 10 per cent of the equivalent live load per lin.ft., applied 5 ft. above the top of rail. (Par. 61).

64. Traction Force. Any structure carrying an electric railway shall be designed to resist a longitudinal force of 20 per cent of the greatest live load placed upon the same.

Unit Stresses

65. Structural Steel. In designing the component parts of any highway structure the maximum stresses due to the combined effects from dead and live loads, including impact, or due to wind pressure, centrifugal and traction forces, shall not exceed for structural steel and rivet steel, the following values in lbs. per square inch, except as modified in Pars. 71 to 74.

(a) Axial tension, net section 16. 000 lbs.

(6) Bending, net section, on extreme fibers of rolled

shapes (Par. 77), built sections and plate girders 16,000 Joists, under concentrated loads, when flooring

is non-continuous (Par. 77) 20,000

Pins, on extreme fibers 24,000

(c) Axial compression, gross section, where S allowable unit stress.

1 = unsupported length of member in indie-.