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An Interesting Type of Flat Slab Construction Lackawanna Applies This Design to Large Grade Separation

• Viaduct at Newark After Extensive Study

I N THE SEPARATION of the grades of the Newark Turnpike and the tracks of its Morris & Essex branch at a point known as Sanfords Crossing, N. J., the Delaware,

Lackawanna & Western is constructing a reinforced con-crete viaduct of the flat slab type reinforced in four direc-tions which will carry the roadway over the tracks. The unusual application of this type of construction to a high-way bridge in a grade separation project, which was de-cided on only after extensive studies had been made to de-termine the most advantageous type applicable to this loca-tion, resulted in the development of many interesting fea-tures in design and important economies in construction.

The Newark Turnpike is one of the two thoroughfares connecting the highly developed manufacturing center of Newark, N. J., and the Metropolitan district of New York City. It, and the so-called Plank road, the only other thor-

and the highway is to be carried over the tracks by means of a viaduct and earth fills sustained by retaining walls.

Crossing on a Very Flat Skew Referring to the general layout it will be noted that the

center line of the bridge forms the acute angle of crossing of 19 deg. 12 min. 20 sec. with the line of the tracks. The viaduct is divided into three sections, a center section 418.82 ft. in length spanning six tracks, a north approach 125.13 ft. long and a south approach 238.5 ft. in length. Each sec-tion is separated from its adjacent section by an expansion joint formed by a 2-in. break in the floor slab located be-tween a double row of columns.

The south approach was stopped at a height where re-taining walls and fill were found to be more economical. It was necessary to carry these walls for a length of 293 ft.

Foundation and Par/ Pile Pion

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Elevation,

General Plan and Elevation of the Viaduct

oughfare extending between Newark and New York, are numbered among the most important intersectional high-ways in this country.

The traffic over these two highways is extremely heavy and is moved almost exclusively by motor driven vehicles. For years the greater portion of it has moved over the Plank road because of the poor condition of the Turnpike. That highway has now become inadequate to meet the demands of the rapidly increasing traffic and the Turnpike is being im-proved so that the congested conditions on the Plank road may be relieved.

In connection with the highway improvement the road-way is being raised about five feet above its former surface from a point in Harrison, N. J., to the Hackensack river bridge in Jersey City, a distance of 33/4 miles. The road-way is to be 40 ft. in width between curbs and a 10-in. Tel-ford macadam road forms the base of the pavement. The base will serve as a temporary surface until the completion of the pavement which will consist of Belgian blocks laid on a one-inch sand cushion over the macadam.

As mentioned, the Turnpike intersects the railroad at Sanfords Crossing, 3 1/2 miles west of Hoboken, N. J. Through this district the highway and the railroad traverse the perfectly fiat country known as the Newark Meadows

to prevent the encroachment of the fill on adjoining prop-erty. The length of the north approach was fixed so that the approach fill fanning around the end corner would not encroach upon the future outside tracks.

The Morris and Essex branch of the Lackawanna car-ries a very heavy traffic, consisting largely of high speed suburban trains. At the point of intersection by the turn-pike it at present includes an east and a westbound main track and a westbound passing siding. In the plans for the improvement provision has been made for one additional westbound track and two future eastbound tracks. It will he noted that the tracks are arranged in pairs and the size of the slab panels of the center ,section over the tracks was determined in the one direction by placing the column lines on 32-ft. centers between each of the three pairs of tracks. In the other direction, lengthwise with the track, the spac-ing of the columns is 30 ft. 65/8 in., forming an almost square panel. This spacing was determined by placing on the curb line of the viaduct the diagonally opposite end columns of the two rows adjacent to and parallel with the future out-side tracks. The total distance between these two columns, one line projected on the other, is divided into 13 equal spaces of 30 ft. 6 in.

In addition to the columns of the center section placed be-

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tween the tracks, there is another row of four columns along the curb line which forms several irregularly shaped panels in order to carry out the triangular portion on each end of the center section, to a row of columns normal to the center line of the roadway in line with the last column of the outer row parallel with the tracks. With the exception of these few irregular panels all the panels of the center sec-tion are 30 ft. in. by 32 ft. This arrangement whereby the panels of the center section are placed normal to the track forms eight triangular portions of the slab which pro-ject beyond the sidewalk or balustrade line. These, how-ever, considering the acute angle of crossing, are compara-tively small, and little of the bridge floor is unused.

The columns or panels of the approach spans are placed in an unusual manner. Instead of being normal to the center line of the roadway, the panels are placed in a di-agonal direction. This was done in order to bring the out-side row of columns a sufficient distance from the face of the bridge so that the overhang formed thereby would balance the moments over the columns. The total lead load on the outside columns is eccentric on the side of the over-hang to such an extent that when the roadway live load is applied on the other side of the columns the resultant is a

ing required the same number of piles loaded to 18 tons per pile for the extreme maximum live load.

Slab of Minimum Thickness

The depth of the slab in the approach spans is 10; in. increased to 15% in. with the drop panel. The drop panel over the outer row of columns is carried for one-half its width along the edge of the slab to give the outer diagonal band the proper strength. In other words, the slab for a 5 ft. 7 in. strip along either edge has a depth of 1572 in. This outside diagonal band is further strengthened by the continuation of the bent-up bars of the direct bands (22 ft. 6 in. span) over the columns to the edge of the slab. In this manner the area of the cantilever action over the columns is increased, thereby effecting a greater stiffness in this long band by extending the point of contraflexure. The effect of approximately balancing the moments over the outer row of columns was a reduction in the amount of steel in the so-called interior panel.

Twenty-two feet is the minimum vertical clearance from the top of rail to the underside of the drop panel, the edge of which is practically the edge of equipment. The same clearance could safely have been made to the underside of the slab but this additional clearance will in time be partially absorbed by raising the track due to re-ballasting.

The columns between the tracks are protected to a con-siderable extent in case of a sideswipe from a derailment by

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April 23, 1920

RAILWAY AGE 1235

placing them on a pier 3-ft. wide 6 ft. above the top of rail. The piers are reinforced by four lines of old rails. This high pier has another function in acting as a girder for the distribution of the loads on the piles which are spread out from column to column in narrow confines between the tracks, obviating the necessity of driving piles beneath the tracks.

The panels of the concrete balustrade are to be 2 in. in thickness and are to be precast in a horizontal position dur-ing the construction of the viaduct. After they are thor-oughly cured they are to be set in position and the posts will be cast in place around them. The precast panels will project into the posts 172 in.

This type of construction was decided on only after ex-

Details of Expansion Joint in the Roadway Slab

tensive studies had been made to determine the most ad-vantageous type applicable to this location. It has the primary advantage of giving the most shallow floor system that could possibly be obtained. This was of particular advantage in this location as it gave the minimum length to the approaches which were to be built over perfectly flat ground. In addition the flat slab offered a flexibility not found in any other type of bridge design except the arch type, which could not advantageously be used here in that the floor slab could be warped slightly or the grade changed between the columns to fit the surface of the slab concen-trically to a vertical curve of the highway formed at the in-tersection of the approach grades at the center of the struc-ture over the tracks.

As compared with all other forms of reinforced concrete

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Details of the Concrete Railing

construction the flat slab also offers the advantages of simpli-city in the form work and in the arrangement of the rein-forcing steel. In this type of construction there are two flat planes of the underside of the forms, the upper plane forming the underside of the slab and the lower plane around the columns forming the drop panel. The saving in mate-rials in the flat slab type of construction is appreciable and increases with increased loading. In this structure, which had to be supported on piles, the use of this light type of construc-tion resulted in a reduced cost of substructure and founda-tions.

By reason of its uniform cross-section and continuity of the reinforcement there is no type of reinforced concrete

construction that is better able to resist shrinkage and thermal changes than the flat slab. Where expansion joints are necessary they are easily incorporated by forming a double row of columns, splitting the structure into independent sec-tions with a clean cut joint, the slab being cantilevered be-yond the end columns to balance the negative movement.

The wooden piles used in the work were reclaimed from repairs made to ferry boat racks on the water front of the Hoboken terminal. These piles, after 15 years of service, were broken down by impact of ferry boats and by weather-ing. The failure however was confined to the upper 18 ft. of the piles exposed above the water. The original length of piles was approximately 80 ft. so that by cutting off the defective ends after the piles were pulled a length in excess of the 35 to 50 ft. required for the viaduct construc-tion remained in a perfect state of preservation.

Construction With a limited space to carry on construction, the tower

and chute method of depositing the concrete was chosen. Three towers, each 120 ft. in height, have been erected, one on the north side and two on the south side of the tracks. In laying out the plant due consideration was given to a minimum construction operation over the main line tracks.

The construction of this viaduct, which was begun late in 1919 and which is expected to be completed early in the summer of this year, involves 1,800 cu. yd. of concrete in the retaining walls, 2,010 cu. yd. in the column footings and protection piers, 2,530 cu. yd. in the columns, slab and parapet as well as 1,150 wood piles and 176 tons of rein-forcing steel. The work is being carried on under the di-rection of Gw. J. Ray, chief engineer and L. L. Tally-n, until recently acting chief engineer of the Lackawanna and M. H. Doughty, division engineer, A. H. Henckel, resident engineer is in charge in the field, and A. B. Cohen, con-crete engineer is in charge of the design. H. F. Curtis, New York, is the contractor.

From the Baltimore Sun.

No, We Do Not!

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