This precast prestressed concrete pedestrian

Transcription

1 The Rockach Boulevard Pedestrian Bridge by ELIEZER SHAMIR Partner, E. Shamir - H. Posner Consulting Engineers Tel-Aviv, Israel This precast prestressed concrete pedestrian bridge crosses the Rockach Boulevard, which is one of the main arterial highways entering Tel-Aviv (the largest city in Israel). It is located at one of the most congested regions of the city, near the exhibition areas. The bridge provides pedestrian access from the large parking lot to the exhibition areas, spanning over a dual highway with a very high traffic capacity during both day and night, especially during the rush hours. The total bridge span, including ramps at both sides of the structure, is about 160 m (525 ft) of which 60 m (197 ft) are above traffic flow and sidewalks, providing 4.80 m (15.7 ft) net clearance. The total width of the deck is 6.0 m (19.7 ft). Circular ramps of 8.0 m (26.2 ft) average radius, as well as circular stairs of 3.0 m (9.84 ft) average radius, each having 3.0 m (9.84 ft) net width, are located at both ends of the main structure. These ramps provide easy and comfortable access for pedestrians. They slope longitudinally at 7 164

2 PCI JOURNAL/November-December

3 clan OT peaestrian nrlage (dimensions in metric units). METRIC UNITS) PRECAST REINFORCED CAST IN SITU HIGH STRENGTH CONCRETE COLUMN CONCRETE JOINT is 900 RAMP a STAIRS CAST IN SITU G ASS PARAPET INCLINED PRECAST PRESTRESSED GIRDER h^ I I \ DECK: HOLLOW CORE PREFAB. PRESTRESSED UNITS+TOPPING PILES SOCKET TYPEPILES CONNECTION PILE CA P Elevation of pedestrian bridge (dimensions in metric units). 166

4 30 CM. PREFAB. PRESTRESSED HOLLOW CORE UNITS / 4.30,t65 TOTAL DEPTH 4CM CRANOUTE CAST IN 51TU 39 CM. TOP COVER 5?7 CM. TOPPING H. iwrau INCLINED PRECAST HIDDEN TRANSVERSE PRESTRESSED DYWNA6 PRESTRESSED GIRDER REINFORCED CONCRETE BARS BEAM Typical cross section of deck (dimensions in metric units). PCI JOURNAL/November-December

5 percent average, while the main structure is inclined from only 4 to 6 percent. Architectural considerations required the structural depth of the deck to be a maximum of 40 cm (15.7 in.) (including surface cover). All exposed structural surfaces are treated as smooth architecturally exposed concrete. Deck top cover, including also side curbs along deck edges, is coated with a special concrete mix, using large quantities of colored aggregates, exposed by manual brushing (granolite finish). Parapets are made of modular frames, composed of steel rectangular hollow section tubes, with 12 mm ('h in.) glass panel infill. These parapets are aligned along the entire length of the bridge, including the circular ramps. The structure is supported on deep foundations, i.e., bored piles, cast in place using bentonite slurry. This is because the water table is only 3.0 m (9.8 ft) below ground level, and the local soil is predominantly organic clay. The main part of the bridge, above the highway, is made of precast components. The structure was erected without interrupting traffic flow. The main frames are comprised of precast reinforced concrete vertical columns and precast prestressed concrete inclined girders. These are connected to each other by welded wire reinforcement and cast in place high strength concrete to form four triangular shaped plane frames (see plan, elevation and typical deck section). The columns are connected to pile caps by socket type connections. The main frames are located at both sides of the deck, one pair at each side. Prestressing of each inclined girder was done by four Freyssinet tendons of 12 - lvz in. diameter (270 K). The girder was cast and prestressed horizontally, and then lifted into place as a complete precast unit. Lateral connection of the two pairs of frames (located at both sides of the deck) to each other is done by four reinforced concrete beams, cast in place. Each of the transverse beams is prestressed axially by two Dywidag bars of 32 mm (1 1/4 in.) diameter, providing a 100 ton effective prestressing force in the connections. Supported on the transverse beams, the deck is composed of hollow core pre- 168

6 stressed concrete slabs, 30 cm (12 in.) thick, spanning 12.8 and 13.5 m (42 and 44.3 ft) parallel to the bridge axis, and a 5 to 7 cm (2 to 3 in.) reinforced concrete topping layer. The deck acts as a continuous monolithic slab. Due to the two intermediate supports "hidden' behind the inclined girders, this configuration gives the impression of two clear 26.3 m (86.3 ft) main spans above the highway lanes. The combination of the above mentioned components and a total deck thickness of 40 cm (16 in.) contributes to the general light appearance of the structure; the deck seems to "fly over" its supports. The most dominant supports, therefore, are those located in the median and at both ends of the bridge. Transverse stability of the structure, and the ability to resist lateral and longitudinal forces, is achieved by its continuity. The deck is assumed to act as a rigid monolithic plate, bending in its own horizontal plane and transmitting reactions to the pier frames in the median and at both ends of the bridge. Due to the smooth Teflon-coated bearings, no lateral reactions are transmitted to the inclined girders. Construction was completed within 4 months, at a total cost of about $750,000, including additional site development. The project was opened to the public in May 1988, and it is now being fully utilized. The main advantages of using precast components were in minimizing construction time, avoiding interruption to traffic flow and keeping a low construction cost. During the last 6 months, the bridge has been enjoyed by the public and has proven to be a popular landmark. Credits Owner: Netivey Ayalon Ltd., Tel-Aviv, Israel. Structural Engineer: E. Shamir - H. Posner, Consulting Engineers, Tel-Aviv, Israel. Architect: J. Rechter and Associates, Tel-Aviv, Israel. General Contractor: Solel Boneh Ltd., Tel-Aviv, Israel. Prestressing Subcontractor: Freyssinet Israel Ltd., Tel-Aviv, Israel. Photographer: G. Gurfine, Tel-Aviv, Israel. PCI JOURNAL/November-December