Forms used by the Chicago and Northwestern Ry. are shown in Fig. 6. The forms were built in sections 35 ft. long. The 2 in. x 8 in. braces were used to hold the sides of the forms apart and were removed as the concrete was put in place. The 2-in. pipe used to cover the rod bracing was old boiler flues and rejected pipe.

FIG. 5. FORMS FOR ILLINOIS CENTRAL R. R.
RETAINING WALL.

Concrete Footing o

FIG. 6. FORMS FOR C. &. N. W. Ry.

RETAINING WALL,

The forms for concrete arch culverts as prepared by the Michigan State Highway D ment are shown in Fig. 7.

Falsework for Arches.-The detail plans for the falsework used in the erection of a cor arch bridge on the joint track of the Colorado and Southern Ry. and the Denver and Rio ( R. R. in Colorado are shown in Fig. 8. The bridge consisted of twin arches each having an of 60 ft., and a barrel 112 ft. long. The falsework was designed to carry the actual loads! would come on the falsework during erection. The falsework was made very rigid in order there should be no appreciable settlement. The falsework was constructed with a barrel a length of 60 ft., so that one-half of each arch could be constructed at one time. The abut

Centers to be spaced 4-0"c. to c.

FIG. 7. FORMS FOR CONCRETE ARCH CULVERT.

were first constructed and then the falsework was constructed for one-half of each arch concrete was then placed on each arch beginning at the springing and proceeding upward t crown. The crown segments were constructed last. The arches were not reinforced, ex a small amount of reinforcing steel was placed near the extrados of each arch to make it pe for the ends of each arch to act as a cantilever until the crown segment was placed. The work proved to be very rigid, the maximum settlement noted in the arch sheeting was one dredth of a foot, with no appreciable distortion. The forms were lowered by means of the boxes shown in the drawing. After several years the arches show no cracks.

After the one-half of the bridge was constructed the falsework was taken down ant erected for the remaining half of the bridge. The additional cost required to make the false" very rigid was more than compensated for by the saving in cost of placing the concrete. arch was designed and constructed, by Crocker and Ketchum, consulting engineers. author was in direct charge of the design of the arch and the falsework.

Lagging. Lagging for concrete arches should be of surfaced lumber, preferably tome": and groove, and should be water tight.

Construction of Concrete Arches.-The arch ring should not be constructed until th around the abutments has been carried up to the skewback. The rings should pretes be concreted in one continuous operation, but if this is not practicable the arch ring nat divided into several sections by transverse bulkheads parallel to the roadway, each ring ber," such size that it can be concreted in a single continuous operation. The concreting shor carried on symmetrically about the crown of the arch. If the arch ring is heavy additional m forcement should be inserted near the extrados over the haunch so that the segments of the will act as cantilevers until the arch is closed at the crown. The spandrel walls should n: cast until the centers are struck, and the coping should not be cast until the spandrel wa completed. On very large arches it may be necessary to divide the arch ring into voussors * that the arch ring can be poured in such a manner as to load the centers symmetrically. Tr

extrados of the arch ring and the inside surface of spandrel walls snould be left smooth to receive the waterproofing. The surface may be waterproofed as described in § 75, Appendix II, or the membrane method may be used. Before applying the membrane the surface of the concrete should be clean and dry and not less than 15 days old. A primer coat should be applied cold. For asphalt the primer coat should be asphalt thinned with petroleum distillate; while for coal tar the primer coat should be creosote oil which shall be a pure tar distillate free from any sub stance foreign to a tar distillate. The membrane should be applied, so as to lap joints as for tar and gravel roofs. The surface of the concrete and of all laps are to be mopped with hot asphalt or hot tar. Especial care must be used to flash the concrete in angles and to provide the necessary expansion joints. For detai's of waterproofing concrete bridge floors, see the author's "Structural Engineers' Handbook."

FIG. 8. FALSEWORK FOR CONCRETE ARCH BRIDGE, C. & S. RY. AND D. & R. G. R. R.

Spandrel filled arches should be drained by French drains 15 in. square provided with suitable tile outlets. Drains should be provided for all abutments and retaining walls. Filling of spandrel filled arches should be deposited in layers 6 in. to 8 in. thick, and thoroughly compacted by ramming. The fill should be made symmetrically from both ends of the arch.

Striking Centers.-Centers should be gradually and uniformly lowered in such a manner as not to produce injurious stresses. The forms for small span arches should be supported on hard wood wedges, while sand boxes should be used for large span arches. In mild weather, centers should remain in place under arches of less than 60 ft. span for at least 21 days, and under arches over 60 ft. span for at least 28 days.

Depositing Concrete Under Water.-The depositing of concrete under water should be avoided if possible, as results are somewhat uncertain even when the work is done under strict supervision. The methods that give the best results are:

1. The concrete is lowered in large buckets having a hinged bottom which sets sufficiently far above the lower edge of the bucket that it may open freely downward and outward when the

bucket reaches the surface upon which the concrete is to be deposited. The top of the bucket is left open. The bucket should be completely filled before lowering.

2. The concrete is deposited through a vertical tube or "tremie" reaching down to the surface upon which the concrete is to be deposited. The tremie should be kept filled and the flow of concrete should be continuous. In beginning the operation the tremie should be filled with concrete in such a manner that the concrete is not permitted to drop through the water. This may be accomplished by plugging the tremie with sacks which will be forced down as the tremie is filled with concrete, by plugging the end of the tremie with a cloth sack filled with cement, or by other

means.

If the charge is lost the tremie should be filled before proceeding.

3. The concrete may be deposited in loosely filled porous cloth or jute bags. These bags are placed so as to bond together. The mortar working through the porous bags cements the mass. 4. Premoulded concrete blocks of large dimensions may be used.

5. A canvas bag may be used as a depositing bag in place of a bucket. After filling, the mouth of the bag is closed by one turn of a line so looped that a pull on the line will release it. The bag is lowered mouth down to the surface upon which the concrete is to be deposited, and a pull on the line opens the bag and releases the concrete.

The following precautions should be taken in depositing concrete under water:

(a) The concrete should be made with aggregate smaller in size than for concrete deposited in air. The aggregate should be carefully graded so as to make a dense mixture. The mix should be not less than 1-2-4 mix, and should contain more cement than for concrete deposited in air. The concrete should be thoroughly mixed in a batch mixer with only sufficient water to make a stiff mass. Concrete should never be deposited in running water. In running water a cofferdam should be constructed in such a manner as to insure still water within the cofferdam. The concrete shall be deposited continuously in order that laitance may not form between the layers of

concrete.

(b) Before beginning concreting after an interruption the laitance should be removed from the surface of the concrete already placed. It is impossible to prevent the formation of laitance, but great care should be taken to reduce the amount of laitance and also to prevent the formation of horizontal cracks. Concrete should not be deposited in water the temperature of which is cold enough to retard setting.

Placing Reinforcement.-The vertical steel in abutments and piers should be in place and be rigidly supported before concreting is started. The horizontal steel should be wired in place in advance of the concrete as indicated on the plans. All the steel in the superstructure should be wired in place before any concrete is deposited in the forms. Great care should be used to see that the steel is located exactly as shown on the drawings. Reinforcing steel should be supported on metal or other approved supports to hold it at the proper distance above the forms. The practice of laying reinforcing steel directly on the forms and attempting to raise the steel during construction is pernicious and should not be permitted.

Inspection of Design and Construction of Concrete Structures.-The construction of concrete structures should not be separated from the design, but the engineer who prepares the design should supervise the construction.

The design drawings and specifications should give the dead, live and wind loads, the allowance for impact, the working stresses, and the arrangement of all details. The drawings should show the size, length, location of points of bending, and exact position of all reinforcement, includ ing stirrups, ties, hooping and splicing. The specifications should state the qualities of all materials and the proportions that are to be used.

Plans should also be prepared by the engineer for all falsework and forms. Alternate plans for falsework and forms should be invited from experienced contractors.

Inspection during construction should be made by the engineer's inspectors, and should cover the following:

.

1. Tests and inspection of materials. 2. Construction and erection of falsework and forms. 3. Sizes, arrangement, position and fastening of reinforcement. 4. Proportioning, mixing, consistency, and placing of concrete. 5. Tests of concrete made on work. 6. Testing concrete to see if it is sufficiently hardened before supports are removed. 7. Protection of finished parts of structure from injury. 8. Comparison of dimensions of all finished parts of structure with plans. 9. Inspection of finish of concrete.

ERECTION OF STEEL HIGHWAY BRIDGES.-The details of the operation of erecting steel highway bridges will depend upon the type of bridge, length of span and character of the crossing. Short span plate girder and riveted truss bridges may be riveted or bolted up on the bank, and then swung in place by means of a gin pole (a long pole held solidly at the bottom and held in place at the top by guy ropes; the load is lifted by blocks and falls fastened to the top and bottom of the pole, while the load is swung into place by manipulating the guy ropes). Pinconnected bridges of all spans and long span riveted truss bridges are erected on falsework, usually constructed of timber.

Through truss bridges are usually erected by means of a gantry overhead traveler which runs on a track supported on the falsework. Details of a through bridge traveler are shown in Fig. 9. The falsework may be made of framed bents as shown in Fig. 10, or pile bents may be used. Falsework. Falsework for the erection of bridges is built up of bents made of three or more posts or piles, braced transversely in the same manner as for permanent trestles. Framed bents are carried on mudsills, or on piles when the foundation is inadequate or where there is flowing water. Where piles cannot be driven in running water or where there is danger of flood, it may be necessary to use spread footings which are anchored in place. Where it is practicable to obtain piles of sufficient length they may be used for the full height of the falsework. The timber used in building falsework should be sound, strong, free from defects that will affect its strength or interfere with its use. Since the structure is temporary, durability is not an important element in selecting timber. for falsework unless it is to be used several times. For examples of timber trestles, see Chapter XVI.

Plans of typical four-legged falsework as used by the American Bridge Company are shown in Fig. 10. When trains are to be carried and 2-8 in. X 16 in. stringers are used under each rail, bents must not be spaced over 18 ft. centers for the falsework as shown.

Piles. Timber piles may be driven with a drop hammer or with a steam hammer. A spool roller pile driver with a drop hammer is shown in Fig. 11. The hammer is raised to the top of the leads by the hoisting engine; the hammer is then permitted to fall on the top of the pile, dragging the hoisting rope down with it. The force of the blow of the hammer depends upon the weight of the hammer, the height of free fall, and the resistance of the hammer in the leads. By catching the hammer as it descends the operator can cushion the blow so that the safe bearing power of a pile as calculated from the penetration may be very misleading.

The safe load on piles may be calculated by the Engineering News formula given in § 82 of the "General Specifications for Concrete Highway Bridges and Foundations," Appendix II. Piles should have a penetration of not less than 10 ft. in hard material and not less than 15 ft. in soft material.

The following specifications may be used for false work piles. All piles are to be spruce, yellow pine or oak, not less than 8 in. in diameter at the tip and not more than 14 in. in diameter at the butt. Piles are to be straight and sound, and free from defects affecting their strength. Piles are to be driven into hard ground until they do not move more than in. under the blow of a hammer weighing 2,000 lb. and falling 25 ft.

Erection of a Through Truss Bridge.—The following description of the erection of a 409-ft. Petit through pin-connected highway bridge will illustrate the method of erecting truss bridges. The falsework was constructed by driving 5 lines of piles to a good refusal. The piles were sawed off and capped with 12" X 12" timbers. The longitudinal sills for the traveler were 10" X 12"