C.N.R. builds more long span concrete bridges
Precast and poured-in-place slabs, up to 90 ft. in length, employ no waterproofing, ties or ballast—Ten-slab deck is erected in eight working hours.
Setting in one of the ten 80-ft, 130-ton slabs of the Eighteenth Street crossing at New Toronto, Ont.
Continuing a practice started in 1933, the Central region of the Canadian National has been making extensive use of precast and poured-in-place concrete deck slabs for grade separation structures, expanding the application to bridges with multiple tracks and involving continuous slabs up to 90 ft. in length for main line loadings and 112 it. 9 in. in length for electrified. interurban loadings. Also, with several modifications, it has continued. with two exceptions, the practice of laying the track rails directly on the concrete slabs, without crossties or ballast. The more-recently-built structures of this character are of special interest both because of the increased length of slabs involved and because of the improved technic [sic] employed in constructing and erecting the slabs in the interest of economical construction and minimum interference with train operation.
Precast slabs carry single rail load
Between the two types of slabs employed, precast deck slabs have been used essentially where it was found inadvisable or uneconomical to attempt to detour traffic during construction, whereas poured-in-place slabs have been used primarily where two or more tracks were involved, one or more of which could be taken out of service at a time to permit the construction of the slabs in place. In other words, the two types of construction are considered interchangeable on the Central region, except for the factor of traffic. Furthermore, in spite of the relatively heavy power and high speeds employed on the main lines of the region, all of the main-line bridge slabs, both precast and poured-in-place, have been designed for Cooper's E-60 loading. For those bridges on its electrified interurban lines, Cooper's E-40 loading has been used.
It should be noted also that the precast slabs are designed of a width, usually about 6 1/2 ft. wide, to carry only one track rail, whereas the poured-in-place slabs, unless for some special reason, are made to a full single-track width of approximately 13 ft. Thus, in the case of the precast slab decks, each single-track width of deck usually involve5 two slabs, structurally independent of each other, which divide the train loads equally between them. This feature of design simplifies the handling of the slabs during erection, but its primary purpose is to minimize the concentrated crane loads on the span approaches back of the abutments while setting the slabs in place. That it has been desirable to take this factor into consideration is seen in the fact that the longest precast mainline slabs built to date, 80 ft., weigh approximately 130 tons.
Among the more interesting grade separation structures built recently that employ precast slabs for clear spans are a single-track structure over Station avenue at Shawinigan Falls, Que., and a single-track structure over a state highway near Groveton, N. H. The former of these, which is at an angle of 88 deg. 6 min. with the center line of the roadway, involves slabs 60 ft. long and 3 ft. 9 in. thick, providing a clear roadway and sidewalk opening between abutment faces of 46 ft. The structure near Groveton, on the other hand, crosses the highway at an angle of 45 deg.; involves slabs 60 ft. 6 in. long and 3 ft. 6 in. thick; and provides a clear roadway and sidewalk width of 32 ft.
Long continuous precast slabs
Among the more important structures involving precast slabs continuous over a center pier and providing two separate roadways, are a single-track structure over King's Highway, on the Stratford division, near Malton, Ont., a three-track structure over Mann avenue, at Ottawa, Ont., and a five-track structure over Eighteenth street, at New Toronto, Ont. The bridge over King's Highway crosses the roadway at an angle of 72 deg. 21 min. The two, half-width, single-rail slabs in this structure have an overall length of 67 ft. 11 in. and a thickness of 2 it. 6 in., and provide a double roadway opening of 57 ft. 6 in. between abutment faces.
The three-track structure over Mann Avenue, Ottawa, involves eight precast slabs continuous over a center pier and crosses the roadway at right angles. Here the slabs have an overall length of 78 it., a width of 5 ft. 6 in. and a thickness of 3 ft., and provide two clear roadway openings, each 32 ft. wide. In the case of the five-track structure over Eighteenth street, New Toronto. the crossing, at an angle of 72 deg. 37 min. with the street, involves ten slabs, each 80 ft. long, 6 1/2 to 7 1/2 ft. wide, and 2 ft. 8 in. thick These precast slabs, the longest yet constructed on the Canadian National, provide an opening of 66 ft. between abutment faces for two roadways and two sidewalks.
A part of the deck of the Victoria Park Avenue bridge.
Getting ready to pour a precast deck slab.
Building the precast slabs
All of the precast slabs for the different bridges were built in special forms at the track level, as near the point of erection as possible, the forms consisting of 2-in. tongue and groove sheathing lined with resinated Silvaply, supported on a foundation of 12-in. by 12-in. timbers placed sufficiently close together to prevent any possibility of their sagging under load. The joints between adjacent slabs are of the lap type, with the top half of one slab projecting three inches over the bottom half of the other, and with the bottom halves of the slabs abutting, while the top halves are separated 1/2 in. To insure these conditions and the true relationship of one slab to another, the slabs for each bridge were poured in the same relative positions that they were to occupy in the finished structure, beginning with one of the outside slabs and working progressively across the deck. Following this procedure, the inside face of each slab as poured formed in itself the joint face form for the abutting slab.
To make the type of joint called for—tight through its lower half and 1/2 in. open through its upper half—building paper was laid up against the lower half of the slab first constructed, and a greased board, 1/2 in. thick, was set up against the upper half. At the same time, in. clearance was provided between the horizontal faces of the joint by separating them during construction by a strip of sheet lead, 1/2 in. thick and 3 in. wide, this strip being replaced in position when the slabs were erected.
To insure the water tightness of these joints, 12-in. by 1/8-in. strips of sheet lead were anchored in the tops of the slabs as constructed, paralleling the joint seam. When the slabs were erected, the joint gap was first calked with oakum and then filled with asphalt mastic, following which the lead sheets were lapped over each other and soldered together. Other than at the joints, none of the slabs were specially waterproofed, either integrally or externally, reliance being placed solely in the density of the concrete itself to prevent the absorption of moisture.
As might be expected in such long spans, the main reinforcing is heavy, weighing, for example, approximately 20,500 lb. in each of the 80-ft., half-width slabs of the Eighteenth Street bridge at New Toronto. In all cases, the main reinforcing is of square deformed bars of high-carbon or rail-steel specification, laid parallel with the longitudinal center line of the slabs. A feature in this connection is that specifications required that many of the bars extend the full length of the slabs, without welded or otherwise formed joints. Another feature in connection with the reinforcing is the practice employed of assembling and arc welding all of the main bars and their vertical ties into frames or trusses before placing them in the forms. This practice was developed both to simplify the assembling of the reinforcing and to insure the accuracy of its position in the finished slabs.
Three thousand-pound concrete was called for in the construction of all of the precast slabs, and all of them have been constructed under the close supervision of representatives of reliable testing companies. The pouring of the slabs was done with concrete buggies and was carried out in one continuous operation, although the practice followed called for building up the concrete in layers of 10 to 12 in. at a time over the entire area of the slabs. Specifications also called for the internal vibration of the concrete throughout the pouring operations, employing mechanically-operated vibrators.
Rails anchored to concrete
As already stated, all of the concrete deck slabs, with but two exceptions, are without waterproofing, ballast or crossties, the rail being supported on metal plates, usually resting directly on the field. The only bridges employing crossties are the single-track structure over Station avenue, at Shawinigan Falls, where 1-1/8 in. superelevation was required in the track, and the five-track structure over Mann avenue, Ottawa, where the track layout involves non-parallel tracks and a turnout, a situation which would have made the proper placing of the rail plate anchor bolts particularly difficult. Even in these cases, no ballast was used, the ties resting directly on the concrete.
The rail plate assemblies that have been used are of two kinds—the GEO type of assembly, involving a sturdy base plate, two rail clips with holding bolts, and two plate anchor bolts; and a type developed on the Central region, also involving a base plate or chair, two bolt-held clips, and two anchor bolts. Both types of assemblies provide 1-in-20 canted rail seats. Where no track signal circuits were involved, the plates were made to rest directly upon the concrete, with no insulation or cushion beneath them. In track circuit territory, however, it was found necessary to insulate the plates from the deck, which was done by seating the plates on thin rubber mats and fitting the anchor bolts with insulating fibre bushings and washers.
As to be expected under the arrangement employed, the greatest accuracy has been necessary in alining [sic] the anchor bolts during the construction of the1 slabs and in preparing uniformly level seats for the rail plates to insure uniform surface for the track rails. Whether the slabs were precast or poured-in-place, the problems involved and methods used in this regard were. essentially the same.
Setting the rail plate anchors
For setting the anchor bolts, special steel templets [sic] were employed, supported from the tops of the side forms, which held the anchor bolts in correct alinement [sic] and elevation during the pouring of the slabs. One of these templets was supplied for each line of bolts and consisted essentially of a continuous 2-in. by 2 1/4-in. structural steel angle, to the upright leg of which were welded 1-in. pipe sleeves, 6 in. long, at the proper anchor bolt centers. Through each of these pipe sleeves, which determined the longitudinal spacing of the anchor bolts, was placed a 1-in guide bolt, threaded both top and bottom. These guide bolts were equipped on top with a take-up nut, and on the bottom with a standard pipe coupling, 2 in. long, thus completing the templet arrangement.
The slabs of this four-track bridge at Oakville, Ont. All poured-in-place, have an overall length of 61-ft.
Placing one of the eight 75-ft. slabs of the three-track structure over Mann Avenue, Ottawa, Ont.
The 10 slabs of this 10-track structure over Victoria Park Avenue, Toronto, Ont., which were poured-in-place, are 72-ft. long.
When ready to set the templet in place and to begin the pouring of the slab, one of the anchor bolts was screwed up into the lower end of each pipe coupling a distance of exactly one inch and then the entire assembly was locked tight in the pipe sleeve by drawing up on the take-up nut on top of the guide bolt. With all couplings and pipe sleeves exactly the same length, it is evident that this arrangement insured the uniform top level of all of the anchor bolts.
With all of the anchor bolts thus secured in the templet arrangement, the templet as a whole, along with the other anchor bolt templets necessary for the other lines of bolts, was carefully set to the proper line and elevation within the forms and secured rigidly in place. Pouring of the slab was then started and, as the concrete was brought to full height it was screeded off to the proper level, employing screed guides which had been set previously to true elevation, with support on the slab reinforcing. A little later, when the concrete began to solidify, finishers removed the screed guides and worked on the surface to insure a perfectly level seat for each rail plate. This work, which required great skill, especially in view of the obstructions caused by the templets and anchor bolts, was constantly checked throughout by an engineer's level. Two or three days following the completion of this work, the templets were removed and further attention was given to smoothing off the rail plate seats and to cleaning the anchor bolts. The plates were then set in position over the anchor bolts and checked for alinement and surface, but were not bolted down into final position until a later date when the concrete had developed the major part of its strength.
The two slabs of this bridge, each 60-ft. long and weighing 127-tons, were erected in two hours.
Continuing the care with which each individual phase of the slab construction was carried out, the greatest care was taken to insure ideal curing conditions for the concrete, thus involving the wetting of the slabs at intervals for a period of at least ten days, and keeping them covered with straw, burlap or other suitable material during at least the first 28 days. Evidence of the effect of these precautionary measures in preparing and curing the concrete employed in the deck spans is seen in the fact that, whereas the specifications called for a 3,000-lb. strength in compression at the end of 28 days, the concrete invariably tested far in excess of this figure, usually somewhere between 3,800 and 4,500 lb. at the end of this period.
Precast slabs placed by wrecking cranes
In the case of the single-track bridges, the erection of the precast deck slabs was carried out in two principal operations, the first involving the temporary placement of the slabs directly alongside their final location, on lateral extensions of the falsework at each end of the opening, and the second, the lifting of the slabs to their newly-constructed supporting structures. Where two or more tracks were involved, the intermediate slabs, that is, those other than the outside slabs, were set directly from cars run out on adjacent tracks.
All of the lifting of the slabs was done with two 160-ton wrecking cranes. These were given a hook hold on special lifting stirrups provided near the ends of the slabs during their construction, and so spaced as not to introduce objectionable stresses in the slabs. Since the heaviest of the slabs weighed 130 tons, the cranes had no difficulty in lifting and placing them. However, because of the heavy concentration of the loads on the front trucks of the cranes during the lifting operations, the advance precaution was taken to consolidate as much as possible the embankment approaches to the bridges to prevent settlement of the track.
At the time appointed for the final placement of the slabs for any individual track, which was invariably between scheduled train movements, the bridge track and temporary deck, and frequently much of the supporting falsework, were removed by locomotive cranes, and then, one at a time, the slabs were raised, swung over, and set in final position on their new supporting structures. In each case, the slabs were given bearing at piers and abutments on continuous lead or zinc pads.
The precast slabs in single-track structure over King's Highway, near Malton. Ont., are 67 ft. 11 in. long and 2 ft. 6 in. thick.
As soon as adjacent track slabs were in position, several crews set about to complete the few remaining finishing-up operations. One of these filled the joint gap between slabs with oakum and asphalt mastic and then completed the seal by lapping and soldering the lead joint sheets provided in the construction of the slabs. Another group of men proceeded to set in and secure the track rails. and to connect them up with the approach rails, while still other men. assisted by locomotive cranes, continued with the work of removing such falsework as still remained.
Following this procedure, occupation of tracks was kept to a minimum, amounting usually to two hours or less for individual tracks, whether involved in single-track structures or in multiple-track structures. The erection of the five-truck deck structure of the bridge at Eighteenth street, New Toronto, involving ten slabs. required four working days for completion, but the actual setting of all ten slabs, and interference with the normal use of any track, required only eight working hours.
Poured-in-place deck slabs
Among the grade separation structures built on the Central region for main-line loading, employing poured-in-place, simple span deck slabs, are two similar single-track structures over Fort Erie superhighway and Webster road, near Stoney Creek, Ont., on the London division, each of which involves one slab, 58 ft. 10 in. long, 13 it. wide and 4 ft. 6 in. thick; a single-track clear-span structure over Route 121, on the St. Lawrence division. near Mechanic Falls, Me., which involves one slab 60 ft. 6 in. long, 13 it. wide, and 3 ft. 7 1/2-in. thick; and a four-track, clear-span structure over Seventh Line road, on the London division, at Oakville, Que.Ont., which has four poured-in-place slabs, each 81 ft. long, 13 ft. wide and 4 it. 1 in. thick. The last mentioned of these structures crosses the roadway at an angle of 55 deg. 46 min. 30 sec., and provides a clear opening between abutment faces of 40 ft.
GEO Track Fastenings Hold the Rails on Several of the Bridges- Note the Guard Rail Construction. Also Used on Several of the Structures
Even more striking than these simple-span structures are several involving poured-in-place deck slabs continuous over a center pier. Two of the more important of these structures are a single-track bridge on the Laurentian division, near Bout De L'Ile, which involves a slab 74 ft. 6 1/2-in. long, 13 ft. 3 in. wide, and 2 ft. 8 in. thick, providing an opening of 60 ft. between abutment faces and two 25-ft. roadways ; and a two-track structure on the London division, at St. David, Ont., which involves two structurally independent slabs, 90 ft. long. by 13 ft. wide and 3 ft. thick, set on a sharp skew, to form two independent roadways, each being about 25 ft. wide.
Ten-track structure
Still another structure involving long poured-in-place slabs over a center pier, and the largest bridge of its kind to be built by the Canadian National, is the ten-track bridge over Victoria Park avenue, at Toronto, Ont. The deck of this bridge is made up of ten single-track slabs, seven of which are at an angle of approximately 90 deg. with the center lines of the two roadways beneath, while the other three are at an angle of approximately 81 deg. with the center lines of the roadways. With minor deviations, the individual slabs in this bridge are 72 ft. long, 13 ft. 3 in. wide, and 2 ft. 8 in. thick, providing two 22-ft. roadways and two 7 1/2-ft. sidewalks. The overall length of the subway, which is located directly at the throat of a yard, varies from 152 ft. 9 in. to 162 ft. 9 in., the difference in length being brought about by the different angles of crossing of the slabs. In addition to the ten track-supporting slabs, there are two sidewalk slabs, one on each side of the structure, and four light cover slabs which extend between track slabs that are not directly adjacent.
Just after the placing of two of the slabs in the Eighteenth Street bridge, at New Toronto. Ont. showing the rail chairs and men completing one of the watertight joints between slabs
Aside from the fact that the poured-in-place slabs required heavy falsework to support them during construction, there is relatively little difference between the design and construction features of these slabs and the 7 precast slabs. In other words, in the case of the poured-in-place slabs. as in that of the precast slabs, the reinforcing was weld-assembled into frames and set in the forms ;the concrete for each slab was poured in one continuous operation; and the same type of templets were used to insure the accurate placement of the anchor bolts. The major difference in the work of constructing the poured-in-place slabs as compared with the precast slabs, was that, whereas the latter were built entirely clear of the bridge site, and without interruption in the use of the truck except during the few hours required for erection, the latter required the detouring of traffic around the slabs being constructed for the full period involved in the construction of the formwork and the pouring and curing of the slabs themselves. Allowing 28 days for the curing process before subjecting the slabs to unrestricted loading, required that for the construction of each slab the track involved be out of service from 40 to 45 days. depending upon conditions.
Simple, Paneled Substructures
There is little unusual about the piers and abutments constructed to support the slab deck structures referred to in this article, except that, through special paneling and surface treatment they were made generally pleasing in appearance. All of the abutments are of the gravity type, with spread footings or timber pile supports as seemed best suited to the local ground conditions. All construction and expansion joints were separated with two-ply asphalt-impregnated ready-roofing and were flashed with copper, and the backs of all abutments were waterproofed with two moppings of asphalt emulsion and carefully drained. The drainage provided consisted generally of a stone filling up the back face, usually about 18 in. thick, above a line of 6-in. drain tile, supplemented by weep holes, as necessary, through to the front face.
The center piers, where used. are generally of the reinforced, open-spandrel type. with footings of a character dictated by local conditions. In all cases, with but few exceptions where traffic could be detoured at small expense and little inconvenience, the abutments and piers were constructed under traffic, while the track was supported on falsework. Equally as great care was exercised in the construction of the substructures as in the deck slabs themselves, specifications calling for the careful selection of materials and proportioning of the concrete mixes, internal vibration throughout pouring operations, and minimum compressive strength of 3,000 lb. at 28 days.
The construction of all of the bridges referred to in this article, with the exception of certain of the falsework and the placing of the precast slabs. which was done by company forces, was carried out under contract. We are indebted for the details concerning this work to Chas. P. Disney, bridge engineer, Central region of the Canadian National, at Toronto.