Construction of the Penn Street Viaduct
The Penn Street reinforced concrete viaduct 1350 ft. long by 80 ft. wide was built under rather unusual conditions. When completed in 1913, the viaduct carried Penn Street across the Schuylkill River, the canal of the Schuylkill Navigation Company, the Schuylkill division of the Pennsylvania Railroad and two branches of the Philadelphia & Reading Railroad. The viaduct replaced a steel bridge which was built in 1885.
The old steel structure was of the Pratt truss type with a masonry approach at the east end and an earth fill at the west end. The total length was about 1128 ft., the river spans being 200 ft. long. It was built in 1885 and was designed merely for ordinary highway traffic. However, with the development of the western section of the city a considerable increase of traffic resulted and a traction company proceeded to lay tracks and run cars across the bridge without taking adequate measures to strengthen the structure.
When the danger arising from such a condition was brought to the attention of the county commissioners they made an investigation which showed that the structure was seriously overstressed. Subsequent to this investigation the street car traffic was abandoned and the bridge restricted to ordinary highway traffic. To relieve the conditions caused by such a curtailment of traffic accommodations it was decided to build a reinforced concrete viaduct.
Main Features of the Penn Street Viaduct
The viaduct has fourteen arches, five of 110-ft. span and nine of 48-ft. span. At the east end there is a reinforced concrete approach supported on columns, while the arches at the west end connect directly with the street grade.
The center line of the viaduct coincides with that of the old steel bridge. One of the problems of construction was to build the new structure without seriously interfering with traffic on the old one. The situation was further complicated by the three railroad crossings, all of which had to be kept clear of any obstructions interfering with the operation of trains. The new viaduct was designed for a total width considerably in excess of that of the old one, which made it possible to build a part of the new viaduct 24 ft. wide for its entire length by removing the downstream sidewalk of the old steel bridge. In this way traffic was handled without serious inconvenience. When the 24-ft. section was completed it was put in service, and the old bridge was dismantled and the remaining section of the viaduct built. A timber deck with board fences on either side was placed on the completed arched sections to care for the traffic.
Piers and Foundations
The foundations for the thirteen piers and the two abutments are, in part, supported by pedestal piles and, in part, are carried directly to an excellent bed of rock found at an average depth of about 10 ft. below ordinary low water in the river. The five 110-ft. spans across the river and the canal have their piers carried to rock in all cases. The river piers were built in three independent sections each supporting an arch rib. They have spread footings of widths varying with the conditions. Both end sections are pointed; the upstream section in each case being equipped with an ice breaker. Between these sections heavy reinforced concrete curtain walls 2 ft. thick were built. These extend from the tops of the footings to the springing line of the arches, thus giving the construction the appearance of a solid pier. This design was adopted partly to prevent the collection of drift in time of flood and partly for the sake of appearance.
The pier sections are reinforced with 1-in. square vertical bars spaced 18 in. on centers and 3/4-in. horizontal bars spaced 2 ft. on centers. This reinforcement is a continuation of that of the arch ribs to which it is bonded. One of the piers is located about 35 ft. from the east bank of the old canal and this condition necessitated dredging a new channel on the west side of the pier.
Abutment and Station Piers
Abutment piers, of a heavier section than the intermediate ones, were placed at either end of the five 110-ft. spans. They were designed to provide for the excess of thrust from the large spans over the lighter thrust of the adjacent smaller ones. They were likewise built in three independent sections, though not connected as in the case of the river piers, and are carried to rock foundation. At the point where the viaduct crosses the tracks of the Pennsylvania Railroad the proximity of the station necessitated a modification of the design for the other piers. This was accomplished by building arches, each with an 18-ft. span and a 4-ft. rise, between the sections of the pier located adjacent to the station.
The east end of the bridge is made up of a series of arch rings with 48-ft. spans and the piers supporting them are solid for the entire width of the structure. These piers are drained by means of 4-in. terra cotta pipes with outlets at ground level.
The eastern approach to the structure is carried on a series of concrete columns and curtain walls supported by MacArthur pedestal piles, about 25 ft. long, driven to rock. The columns are built directly on the pile sections. They are spaced 11 ft. on centers longitudinally, and are tied together with heavy concrete beams 16 in. wide and 3 ft. deep. The latter are reinforced continuously with seven 1-in. bars, in the bottom section, supplemented by vertical stirrups. The 12-in. reinforced concrete floor slab is carried directly on these beams. At the foot of the approach, about 80 ft. from Second Street, there is a transverse retaining wall about 6 ft. high and the remainder of the approach is a rock fill with light retaining walls on either side.
The main arch spans are made up of three arch ribs tied together with transverse spandrel walls, the latter supporting the floor system of the bridge. The middle arch rib is 16 ft. wide and the two outer ones are each 10 ft. wide. The curve of the intrados is an ellipse with a clear span of 110-ft. and a rise of 24 ft. The thickness of the center rib at the crown is 3 ft. 6 in. and that of both outside ribs 2 ft. 9 in. The thickness at the haunches is approximately 7 ft. These ribs are reinforced as shown in the accompanying drawing. Spandrel walls 2 ft. thick are placed at intervals on the arch ribs, the height of these walls depending upon their position in the structure. The 12-in. floor slab is supported directly on these spandrel walls.
The 48-ft. arches are likewise elliptical and each has a rise of 9 ft. 6 in. and a crown thickness of 1-ft. 5 in. The reinforcement is of the same type as that used in the larger arches. No stirrups are used in this reinforcement and all rods are lapped at least forty diameters.
Sidewalks and Roadways
The structure has a 55-ft. roadway and two 12-ft. 6-in. sidewalks. In the center of the 55-ft. roadway provisions were made for two trolley tracks and terra cotta thimbles placed in the concrete section of the floor system for the trolley poles. The surface of the original roadway was a 3-1/2-in. creosoted wooden block pavement laid on a sand cushion, resting on a concrete base. The top surface of the floor slabs and the extrados of the 48-ft. arches are waterproofed with a four-ply layer of Hydrex felt. The sidewalks are given a 3/4-in. surfacing made up of one part of cement and two parts of sand and the curb lines are protected with Wainwright curb bars.
On each side of the bridge there is an ornamental reinforced concrete railing with posts for lighting purposes. These posts are placed above alternate piers in the short spans and at the middle of the longer spans. The wires are carried in terra cotta conduits below the sidewalks.
The bridge is built on a rising grade from each end to the river spans, the grade from the east being 2.65 per cent and from the west 2.00 per cent. The west approach connects with a macadam roadway having a 3 per cent grade, and this roadway is given a crown of 6 in.
The piers were constructed in open caissons from a timber trestle built on the upstream side of the steel bridge. An industrial railway was laid on this trestle with spurs extending to the different piers, and when the arch sections were started a cableway was installed immediately below the downstream side of the steel structure. The cableway had a span of 750 ft. and was used for erecting the steel centers and placing the concrete in all of the arch sections within its range. The east approach of the structure and those arches east of the cableway tower were built by means of a traveling derrick. This derrick handled the construction materials required for the falsework and also the concrete from the 3/4-yd. Smith mixer used on that end of the work.
The concrete plant supplying the cableway was located on the west bank of the river a short distance upstream from the end of the old bridge. It consisted of two Mc-Kelvey 3/4-yd. mixers supplied by gravity from wooden material platforms. The sand used was brought in by cars on a spur track of the Reading railroad to a point easily accessible from the concreting plant, and the limestone was obtained from a quarry adjacent to the site. Owing to these favorable conditions, the cost of handling material was decidedly reduced.
An industrial track extended from the cableway tower to the concrete mixers and the concrete was handled on this track in 1 1/2-yd. Koppel cars by means of a continuous cable connected with a hoisting engine. This cable carried the empty cars into a position in front of the mixer and returned the loaded ones to the cableway. In this way the amount of concrete placed depended upon the speed with which it could be handled by the cableway.
Falsework and Centering
The falsework and centering for the 48-ft. arches, except in the case of the arch across the Reading track, on the east side of the river, was of the ordinary wooden type. For the exceptional case the need of adequate clearance for the full span required a special design. The centering for the 110-ft. arches consists of trussed steel arch sections hinged at the crown and held together by horizontal tie rods. These tie rods are provided with turnbuckles which enable slight discrepancies in the position of the centering to be corrected without resorting to the use of wedges. The ribs are supported on wooden sills resting on oak lowering wedges carried by steel I-beams. The I-beams are in turn carried on steel columns supported on the top footing courses of the piers.
In all of the form work, good quality, planed, well-matched yellow pine lumber was used and particular attention was given to securing good workmanship.
The reinforcing bars are of Corrugated and Mono types furnished by the Philadelphia Steel and Wire Company under a contract which calls for the placing of all steel in the forms. The bars in the arch rings are about 34 ft. long and are carried through holes bored in the bulkheads of the forms, thus holding the steel to its proper position in the concrete section.
Three different concrete mixes were used. That in the piers and footings is I:13:6, that in the arch ring, spandrel walls and floor is I:2:4, and that in the balustrade and lampposts, I:2:3. The exposed concrete surfaces are bush-hammered and the edges are chamfered. Allentown Portland cement was used throughout the work.
The construction was carried out under the direction of Mr. Charles F. Sanders, county engineer, and of Mr. B. H. Davis, who, as consulting engineer, also designed the viaduct. Messrs. L. H. Focht & Son, of Reading, Pa., were the contractors.