Bridge at Kirchkein, Germany

This bridge carries the Frankfurt-Eisenach-Dresden Autobahn across the Frankfurt-Hassel Autobahn. It is a double bridge. It was constructed in 1949.

The total width of double bridge is 2 by 36 ft. Each bridge carries a 29 ft roadway and two sidewalks. Each consists of two spans, one 78,49 ft. and the other 83,75 ft. in length. The primary supporting elements in each span are six simply supported prestressed beams spaced at 5,84 ft. and braced laterally by crossbeams 14 ft. on centers and by the reinforced slabs of the roadway deck. The prestressed reinforcing in each beam consists of seventy-six 0,4 in. round bars having an ultimate tensile strength of 150,000 psi.

The amount of material required for the superstructure, per square foot of bridge area, is as follows:

prestressed steel	- 5,85 lb,
structural-grade steel	- 4,55 lb,
concrete	- 1,715 cu ft.

This system of prestressing is not the most economical for field operations. The massive presressing bed required by this system is expensive, and thus it is undesirable to erect more than one or two such at any construction site. Therefore the manufacture of several beams requires a fairly long period of time. Moreover, a heavy crane is needed for placing the beams. These constructions are sustained by the beds received for this type of work, in which the cost of steel bridges and conventional reinforced bridges were in the same general range as the cost of this type of prestressed bridge.

The George Washington Bridge bus terminal, New York

One of New York’s most striking new buildings was opened on 17^th January by the Governors of the States of New York and New Jersey. This is the bus terminal at George Washington Bridge. On the New York side of the Hudson river. It has been constructed for the Port of New York Authority; its roof was designed by Pier Luigi Nervi.

The new building, which straddles the twelve-lane depressed George Washington Expressway forming the approach to the bridge, is designed to distribute the various bus lines, and some 2,000 busses, which terminate at this point, and to provide a passenger station which will facilitate the daily movement of some 50,000 New Jersey commuters, replacing a number of small terminals scattered over half a mile radius of the bridge head. It will thus bring about two practical improvements: suburban buses to and from New Jersey will be removed from the New York streets, and passengers’ journeys will be shortened by anything from five to twenty minutes.

The three-level bus station, at right angles to the road, is 460 ft. long and 187 ft. wide. The lowest level provides terminal facilities for long-distance buses; the main concourse is at the second level, and the “computer’ bus terminal, with 36 bus-loading island platforms and a continuous unloading platform the full length of the building, is at the top level. The terminal is directly connected by ramps with the upper level of the George Washington Bridge.

The site of the new building is a striking one and the George Washington suspension bridge is itself a striking structure. The terminal had thus to be worthy of its prominent position, and only a designer of the caliber of a Nervi was considered qualified to combine the aesthetic aspect with the solution of the undoubted practical difficulties involved in its construction.

One of the foremost requirements was for constant natural ventilation, which would ensure the removal of bus exhaust fumes even in the lightest breeze. The original scheme was drawn up by the Port of New York Authority to provide for this, with a roof formed of series of units, alternately inclined and horizontal, and with side openings for ventilation. This scheme was, in its broad outlines, retained in the final design.

The lower portion of the structure incorporates structural steel framing to tie in with the suspension bridge approach; from second floor upwards comes Nervi’s striking concrete structure with its wing-like roof. The association of the two materials, however, in itself created an added difficulty in design and construction; special methods of joining the steel and reinforced concrete parts of the structure had to be carefully studied with particular reference to the different elastic characteristics of the two materials.

The final scheme comprises two large reinforced concrete lattice beams along either edge of the building, and a longitudinal spine beam supported on a central row of columns. Spanning diagonally between spine beam and edge beams are lattice trusses, each a right-angle triangle in elevation, so placed that their high points meet on the edge beam. The triangular spaces thus formed between them are in filled by the roof slabs – alternately flat, carried on the lower flange of the trusses, and up tilted, carried on the top flange. Each of these large triangular roof units is made up of a series of smaller triangles, carried on a network of intersecting beams, and topped with a continuous 4 in. concrete slab.

The whole roof structure is thus a complex of triangles: triangulations in the edge lattice beams; triangles of the diagonal lattice beams; triangles of the wing-like roof sections, triangles making up each of these sections.

The central row of columns carrying the main spine beam is of the subtly twisted section beloved of Nervi, which besides its aesthetic appeal, has the advantage of presenting a minimum obstruction at the base and a maximum supporting surface in the required direction at the top.

The building has an expansion joint midway along its 460 ft., length, and is divided across the width into two 93 ft. 6 in. bays by the central row of columns, which are placed at approximately 65 ft. 6 in. centers.

In construction, the 10 ft. deep spine beam, the bottom flange of the diagonal roof trusses and the horizontal roof sections were cast monolithically, some 550 sq. yds. concrete being placed continuously.

The triangulations of the trusses and their outer end posts were then placed individually on the bottom flanges, and the final stage was to place the concrete of the up tilted roof sections with their network of beams.

The whole structure was cast in situ, and all the concrete is exposed, and left as it came from the forms. Very great care was therefore taken in the choice of mix and the choice of formwork. Experiment and testing of concrete mixes was begun almost two years ago before actual concreting began on site. Altogether 129 different mixes were tested, and nearly 100 test specimens made up with variations of mix, type of formwork, form liners and coatings, and methods of compaction. Finally a specimen roof panel was made up and tested, using plastic-lined forms; these tests showed some instability in the plastic linings under temperature changes, and eventually plastic-coated plywood was used. A test section of the lattice edge beam was also made up for approval of the finished surface.

Formwork for the columns and trusses was designed to give the concrete a definite board-marked pattern, using butt-jointed pine boards. The roof covering units, on the other hand, were given an extremely smooth surface by casting them against the plastic coated plywood.