|March 23, 1940
|Vol. 108 No. 12
First introduced in new line construction several years ago, they are now being installed widely to replace obsolete masonry sub-structures
One of the two steel-pile piers that were installed at the Nith River crossing to replace old brick masonry piers
The use of H-section steel bearing piles on the Central region of the Canadian National two years ago in the construction on a new line of bridge piers under extremely adverse conditions, involving piles ranging up to 210 ft. in length, proved so successful that steel-pile piers are now being employed extensively on this region in the replacement of existing masonry piers. Primarily responsible for this trend is the conviction that the H-pile pier is more economical. than the conventional masonry structure, this being a particularly important consideration where the use of masonry piers would entail the construction of cofferdams.
Moreover, experience has shown ·that bridge piers of steel-pile construction maybe built with relative rapidity and that, where used in the replacement of masonry piers that have been washed out or otherwise extensively damaged by high water, delays to traffic are reduced to a minimum. In fact, it is now a practice on the Central region to maintain a quantity of steel piles in stock so that they will be readily available for use in such emergencies.
In the steel piers that are being constructed on the Central region of the C. N. R, the H-piles not only act as bearing piles but also form the shaft of the pier. Generally speaking, the piers are rectangular in plan and are formed by transverse rows or bents of piles, the number of such bents and the number of piles in each of them being dependent on the -loading and the foundation conditions. For its entire height above the water level, or above the ground as the case may be, each pier is thoroughly braced in both directions with steel angles placed both diagonally and horizontally.
At their upper ends, the piles in each pier are capped with steel channels, which are placed transversely with respect to the long dimension of the pier. One such channel is placed on each side of each row of piles, with the upper legs of the channels even with the tops of the piles. Bearing on these channels is a pier cap composed of two or more parallel wide-flange beams which are placed longitudinally on the pier and extend its entire length. The cap beams are liberally reinforced with stiffener angles and with steel diaphragms. Surmounting the cap at each bearing point is an I-beam grillage, in which the beams are formed into a unit by means of diaphragms and spacer bolts. The foregoing statements apply in a general way to most of the steel-pile piers that have been built to date, although there has been one notable exception as to the arrangement of the piles, which is described later in this article.
Practically all field connections involved in the fabrication of the piers are made by arc-welding, care being exercised to minimize the effects of internal stresses in the welds. Frequently it is necessary to splice the piles in the field to obtain units of adequate length and these connections, involving the use of splice plates, are generally also made by welding.
Steel piles were first introduced in bridge work on the Central region of the C.N.R during the construction in 1937-38 of the 100 mile line between Noranda and Senneterre in Quebec, where on two occasions their use served to simplify greatly problems imposed by the presence of extremely unstable sub-soil conditions. One of
On the other hand, the alternative site, which was three miles north of the location described above, presented extremely difficult foundation problems but in other respects it constituted a more suitable location for the structure. In fact, it was felt that the advantages of this site far outweighed its disadvantages and it was decided to locate the bridge at this point.
The predominating characteristic of this crossing was that the river bed was composed, to a depth of about 80 ft., of a soft "soupy" clay that appeared to have scarcely any bearing power. Below this material there was approximately 50 ft. of stiffer clay and then about 50 ft. of quicksand overlying gravel. The east bank of the river was underlaid by a ledge of solid rock which, however, dropped away steeply and quickly disappeared altogether. The maximum depth of the river at the average water level was about 25 ft.
Before deciding on a type of structure for this location, careful consideration was given to various types of substructures to determine their suitability in the light of the severe subsoil conditions prevailing. This survey involved first an investigation of the possibilities of a conventional substructure design embodying concrete piers on pile foundations. However, in view of the considerable depth of the water and the even greater depth of the soft clay, necessitating the construction of cofferdams or caissons under extremely unfavorable conditions, it became apparent that the cost of such piers would be prohibitive. Indeed, the practicability of this type of construction under the conditions imposed was open to considerable doubt. Next an investigation was made of the possibilities of a form of construction involving piers built of H-section steel piles, and it was decided that this type of construction was not only practicable but that it offered considerable savings as compared with the cost of conventional masonry piers.
Using second-hand steel spans, the structure that was chosen for the location embodied three through-truss spans, each 154 ft. 7 in. long center to center of main bearings, and a deck plate-girder span, 74 ft. 10 in. in over-all length, which was placed at the easterly end of the structure. The center truss span and the adjacent ends of the adjoining spans are supported on two steel-pile piers, while both abutments and the other pier are of reinforced concrete. As a means of reducing the length of the crossing, an approach fill of rock was constructed at the west end of the bridge and at this end the concrete abutment is supported on spread footings. The other abutment and the concrete pier rest directly on the rock ledge mentioned previously.
Each of the steel-pile piers is comprised of 48 piles of 12-in. by 12-in. 53·lb. section, which are arranged in four transverse rows of 12 piles each. Pier No.2 (the more easterly of the two), in which the piles project about 30 ft. above the river bottom, embodies a cap and grillages similar -in. general to those described at the outset. The channels at the tops of the piles are of 15-in. 33.9-lb. section, while the cap is composed of three 36 3/8 in. 182-1b. wide-flange beams. The grillage at each truss bearing point consists of five 18-in. I-beams. Since the Kinojevis river as used for logging operations, it was necessary to protect the pier against damage by floating
This steel-pile pier at the Decew's Creek bridge was installed to replace a failed masonry structure
The substructure of the Lake Lemoid Narrows bridge includes three steel-pile piers.In the construction of Pier No.1, in which the piles are considerably longer than in Pier No.2, it was considered desirable to take additional measures for imparting stability to the pier. This pier, in which the piles extend about 25 ft. above the river bottom, was likewise enclosed to a height somewhat above the high-water level with a crib of steel sheet piling, which was then filled with concrete above the ground line, using a tremie. Above the top of the crib, the piles were enclosed in a concrete cap on which the bridge trusses are supported directly.
As a part of the preliminary investigation work at the bridge site, a steel test pile was driven to refusal, requiring a pile 210 ft. in length. However, because the last 30 ft. of the test pile required about 100 blows per foot of penetration, using a 7,000-lb. steam hammer, it was considered unnecessary to drive the remaining piles to the same depth as the test pile. Those in pier No. 1 were driven to an average depth of about 175 ft. and those in Pier No.2 to an average depth of approximately 145 ft. below the out-off elevation, the piles in the latter pier being landed on the rock ledge.
The piles used at this location were furnished in 70-ft. lengths, and as an indication of the unstable character of the subsoil it is interesting to note that frequently the first sections of the piles penetrated for their full length into the river bottom under their own weight and without the help of any blows from the hammer. In fact, in some instances it was necessary to support the first section while the splice was being made, to keep it from sinking out of sight.
In the design of the pile piers, a comparatively low unit load (30 tons per pile) was used. Consideration was given to the question as to the depth that the piles would have to be driven before they would receive definite lateral support. In this connection it was realized that the slenderness ratio would have been high if the piles had been considered as being fixed only by the stiffer clay underlying the soupy material. However, in view of the well known consolidating effect of even the least stable soils around newly-driven piles, lit was considered that the piles would be supported laterally for a considerable part of their height. Also, when the fact that the piles were to be fixed at the top was considered in the light of the foregoing consideration, it was reasoned that the slenderness ratio would be reduced to a conservative figure.
Another difficult bridge construction problem, involving subsoil conditions somewhat similar to those prevailing at the Kinojevis river, was encountered where the Noranda-Senneterre Line crosses the Lake Lemoine narrows, and here also piers built of H-section piles were constructed to carry the main spans. This is a navigable stream in which the bottom is also composed of a soupy clay, although the depth of the latter (averaging 20 ft.) is considerably less than at the Kinojevis River site. At the Lake Lemoine narrows, soundings indicated the presence of solid rock at a reasonable depth on both sides of the stream, but here also the rock on both sides dropped away steeply within the limits of the .stream and disappeared altogether.
Due to the greater width of the stream, the bridge at the Lake Lemoine narrows is considerably longer than that at the Kinojevis river and differs also to the extent that it embodies a swing span. The latter, which like the other steel in the bridge was second-hand, is of the through-truss "bob-tailed" type, in which the spans are 87 ft. 9 in. and 140 ft. 3 in. long, respectively. Both end rest piers of the swing span are of steel-pile construction, while the center pier is of reinforced concrete and is founded on the rock ledge on the west side of the stream, which at this point lies at a level approximately 50 ft. below the river bed. The depth of the water at the center pier is about 10 ft.
On the west the swing span is approached by a frame trestle 246 ft. long, in which the bents are founded on timber piles driven to rock. Immediately east of the swing span are two through plate-girder spans, each slightly more than 88 it. long. The adjacent ends of these spans are carried on a steel-pile pier, while a timber-pile pier provides the support for the east end of the easterly span. From the east, the steel spans are approached by a timber trestle 411 ft. long, which is partly of standard pile construction and partly of frame construction supported on mud sills. For its entire length, this approach trestle is constructed on an earth fill, which also extends under the two plate-girder spans.
Except for variations in detail and in the number of piles per pier, the steel-pile piers in the Lake Lemoine Narrows bridge are essentially similar to Pier No. 2 in the Kinojevis River bridge, a further difference being
However, at the location of the two other steel-pile piers in this structure, the rock foundation was not accessible, and the piles in these piers were driven for a considerable distance into a blue clay underlying the soupy material. The maximum length of the piles in these piers is about 153 ft., although their average length is about 90 ft. In the pier at the east end of the swing·span the piles have a height of about 33 ft. above the ground line, although in the other steel-pile pier the height of the pier was reduced to about 12 ft. by the fill at this end of the bridge.
Close up view of Pier No. 2 in the Kinojevis River bridge. The sheet-piling crib protects the pier against floating logs
The cofferdam that was constructed for this purpose embodied two concentric rings of interlocking steel sheet piling, the inner ring having a diameter of 29 ft. 10 1/8 in. and the outer ring a. diameter of 32 ft. 2 3/4 in. Between the rings were inserted, in a vertical position and with the webs placed radially, a series of 12-in. H-piles, which were placed at alternate piles in the inner ring in such a manner as to give the maximum bearing power against both rings. To achieve this end the piles in the inner wall were driven with their webs outward, while those in the outside wall were placed with the webs inward, thus presenting flat surfaces on the interior between which the H-piles could be inserted. The inner wall of sheet piling in the cofferdam was cut off at the elevation of the lower edge of the coping on the pier, but the H-piles and the outer ring of piling were cut off at a level about 1 ft. above low water level. The height of this pier above the water level is about 20 ft.
To strengthen the cofferdam against external pressure, an interior bracing system was devised, which consisted essentially of circular girders or collars of laminated timber construction, the out-side diameter of which corresponded to that of the interior of the cofferdam. These girders, each of which was built in segments of the circle to facilitate handling, were 9 3/8 in. by 1 ft. 11 ½ in. in cross-section and were reinforced on both the top and bottom sides with a 10-in. channel, pre-formed to the desired curvature. When the girders were in position in the cofferdam, they were separated by a number of 10-in. by 10-in. timber struts, the ends of which fitted into sockets formed by angles and plates bolted and welded to the channels on the girders.
There were four of the reinforcing girders and as the excavation proceeded they were lowered successively into the cofferdam, each ring carrying the timber struts by means of which it was separated from the next higher ring. To make allowance for the variation in the water head, the rings were so spaced as to provide the greatest protection near the bottom of the cofferdam. Thus, with the lower ring near the bottom of the cofferdam, the spacing between the successive rings was approximately 10. ft., 15 ft., and 25 ft., respectively, the upper ring being about 6 ft. below the water level. As the placing of the concrete proceeded, the rings were lifted progressively
View of the operations involved in shifting the steel spans in the Nith River bridge into position on the new steel-pile piers and concrete abutments
A noteworthy example of the use of steel-pile piers to replace existing substructures that have reached the end of their service life was afforded in the renewal of the piers and abutments of a bridge across the Nith river near New Hamburg, Ont. Originally, this structure consisted of four deck plate-girder spans, each 64 ft. long between the end bearings. The substructure, which was built about 1860, was comprised of three piers and, two abutments of brick-masonry construction, the piers being about 30 ft. high above the footings. Disintegration of the lime mortar in the old piers had taken place to such an extent that it was decided to replace them with steel-pile piers.
In this work, which was carried out in 1938, the bridge was reduced to three spans, utilizing the old steel, which were arranged in a staggered position relative to the old arrangement. Thus, each of the new steel-pile piers was constructed midway between two of the old brick piers, and the new abutments, which are of reinforced concrete, were each located midway between the existing abutment and the adjacent brick pier. Each of the steel-pile piers at this location embodies 18 H-piles, arranged in three transverse bents of six piles each. The piles have an average length of 60 ft. and project about 35 ft. above the ground line.
In another instance, involving a two-span through-truss bridge at South River, Ont., the masonry pier was replaced with a steel-pile pier, which was constructed at a point 13 ft. from the old pier and the existing steel spans were shifted longitudinally. At the same time the old masonry abutments were replaced with concrete structures, that at one end being founded on rock while the other was supported on H-section piles.
A third example of the use of steel-pile piers to replace existing masonry structures is provided by a project involving a bridge across Decew's creek near Northwood, Ont. This is a double-track structure embodying a 70-ft. through plate-girder span and two 28.5-ft. deck plate-girder spans, both of which were placed at one end of the longer structure. Originally, the substructure of this bridge was comprised of stone-masonry piers and abutments, but when the pier under the ends of the main span and of the adjacent approach span failed it was replaced with a steel-pile pier.
The Kinojevis River bridge embodies two steel-pile piers, one of which (in foreground) has a concrete cap
With the ends of the adjacent spans being supported on falsework, the new pier was constructed ,in the location of the old masonry unit. The pier that was constructed for this location differs from the typical design to the extent that, instead of being arranged in a rectangle, the piles were so placed as to give the greatest bearing power under the girders. Specifically, the pier is comprised of a single transverse row of 16 uniformly-spaced piles, the intermediate units of which receive the load from the girders in the adjacent approach span, and an additional row of ·four piles at each end (under the main girders).
In this pier the grillage under the web of each main girder, consisting of four transverse 24-in. I-beams, rests directly on the cap channels on the tops of the piles. The ends of the girders in the approach span are supported on a bed plate laid directly on the cap channels on the transverse row of piles. This pier is about 10 ft. high above the ground level, and the piles have a penetration of approximately 25 ft.
An example of the installation of steel-pile piers to replace masonry structures washed out by high water was afforded in the case of the bridge across the Portneuf river at Portneuf, Ont. The original structure at this point embodied a deck plate-girder span 74 ft. 10 in. long, which was flanked at each end by a 50-ft. deck plate-girder span. The substructure, consisting of two piers and two abutments, was of concrete construction.
This section along the center line of the Kinojevis River bridge shows the great depth of the steel-pile piers
In September, 1938, the east abutment and pier were washed out completely, leaving only the westerly girder span in position. In the reconstruction of the bridge, the latter 50-ft. span was allowed to remain in position, the two spans that had been washed out were salvaged and re-erected in new positions, and another deck plate-girder span, 57 ft. 7 in. long, was added to the structure, thus considerably augmenting the waterway area.
The substructure for the reconstructed portion of the bridge consists of a new concrete abutment carried on H-piles, and two piers of steel-pile construction. In each of these piers, the piles are arranged in three rows of seven piles each. The average penetration of the piles is about 20 ft., while the height of the piers above the ground is about 21 ft. in one case and 15 ft. in the other. A feature of these piers is the cut-water that was provided at the upstream end of each of them as a protection against ice and other floating objects. In each case the cut-water is comprised of interlocking steel sheet piling driven· to form an angle pointing upstream, at the apex of which is placed a steel H-pile.
We are indebted for the information contained in this article to C. P. Disney, bridge engineer of the Central region of the C. N. R.