rebuild and strengthen the city's historical bridges. Now engineers are just about to start construction on a major project to rehabilitate one of the city's oldest river crossings.

The choice of technical solutions, techniques and methods of reconstruction and repair for historical bridges needs to respect not only the practical requirements of contemporary traffic loads but also aspects of heritage preservation, such as layouts, aesthetic considerations and architectural décor of the original structure. This approach has been used for design development of the Lieutenant Schmidt Bridge reconstruction project, which is about to start construction, and is due to be completed in October 2007. On completion the structure will not only be wider to accommodate more traffic, it will also look more like the original design.

The 331m-long bridge will be closed for ten days at the end of this year. A temporary bridge is being built next to the old structure, onto which traffic will be diverted while the old bridge is rebuilt.

This eight-span bridge was the first permanent bridge over the River Neva, and when it was built in the 1850s, it was also the longest bridge in Europe. It was previously called the Nikolayevsky Bridge and was considered one of the city's most beautiful structures from an architectural point of view.

Stanislav Krbedz, a Russian engineer of Polish descent, developed the design. Originally it was a cast-iron bridge with a bascule section adjacent to the bank of St Basil's Island at the fairway. The bridge is close to Blagoveshchenskaya (Annunciation) Square, which was reflected in its original name. It was renamed Nikolayevsky Bridge in 1885 in honour of Emperor Nicholas I, and in 1918 was renamed again in memory of Lieutenant Schmidt.

It was reconstructed in 1936-1939 because the bascule span could no longer meet the navigation requirements on the Neva. When it was first built, the bascule span was close to the bank, to make it easier for sailing boats to pass through. With the arrival of steam ships, however, bigger draw spans were needed at the middle of the river, where it was deeper. The abutment was also suffering from progressive strain, causing the mechanical lift equipment to become stuck.

The new bridge was built to a totally different configuration and architectural outlook; unfortunately it was no match for the original lightweight façade, with its lace-like arch spans and impressive silhouette. At the time, financial constraints meant that the old piers and beam spans had to be reused; the new bridge was an efficient engineering solution, but from an architectural point of view it was a conspicuous failure, and was completely at odds with the surrounding architecture.

Over the intervening 70 years, the technical condition of the main structure and mechanisms of the bascule span became extremely unsatisfactory and now the bridge no longer meets the bearing capacity and safety requirements.

The main goal of the latest Lieutenant Schmidt Bridge reconstruction is to recover the structure's high-performance properties and its original historical appearance. This work will aim to contribute towards efforts to preserve the historical buildings on both banks of the Neva and keep the adjacent embankments and squares intact for the period of the works.

The project intends to recreate the appearance of the bridge as it was originally designed by Kerbedz, restoring the shape and outlines through use of modern construction techniques and sustainable materials. Technical solutions for restoring the bridge were determined by a thorough analysis of historical and archival studies and detailed survey of the structure of the existing bridge.

One thing that is peculiar to this project is the complicated ground conditions, which include deep layers of banded liquid and high-plasticity loamy soil. Kerbedz was knowledgeable about construction techniques appropriate to banded loamy clay under pressure, particularly in regards its disintegrating capacity in the horizontal plane in the free state and its shear resistance in compression - ie when lateral expansion is not possible. The timber piles of Kerbedz's foundations serve as elements of soil compaction, restricted by the two rows of sheet piles. A 3m-deep layer of concrete inside the sheet piles has further compacted the banded clay. In this way he created a structure whose foundations performed in the same way as the combined slab and pile foundations that are currently widely used in Europe for construction of high-rise buildings. During bridge construction, Kerbedz carried out preliminary loading of each pier with 3300t of rails over a six month period, and installed rock infill around the piers - this meant that the foundations gave a good performance throughout one and a half centuries. The satisfactory technical condition of the piers and underwater substructure, combined with the absence of settlement, led to a decision by engineers that it was both feasible and practicable to continue to use them.

Reconstruction of the main structural elements of the existing bridge also includes repair works on the piers and replacement of the superstructure to make the carriageway one and a half times its original width. One of the main engineering aims of the project is to keep the load on the pier foundations the same, despite the significant increase in the bridge width. This is achieved by using steel superstructure with orthotropic carriageway plate, by using steel during reconstruction of the bascule span piers and by decreasing the height of the piers located in the riverbed.

The permanent superstructure of the bridge will consist of a continuous through-truss with a curved lower chord. The exterior of the truss will as much as possible be recreated to the original design, using photographs and drawings of the former Nikolayevsky Bridge before its reconstruction in the 1930s.

The girders are made up of welded low-alloy steel elements, and these girders are interconnected on top of the orthotropic carriageway plate and at the bottom by means of cross-ties 6m apart. In cross-section, the bridge consists of four main girders spaced 7.2m at the centre and 9.4m on each side.

The 14mm thick bridge deck is supported by flat, longitudinal stringers with a cross-section of 180mm by 14mm, spaced 300mm apart, and by T-shaped transverse ties spaced at 3m. The transverse ties of the plate are fixed to the top flanges of the girders between the lattice joining points.

The lower chord of the girder is a box-like welded beam open on top at the point where the cross-ties are attached. To avoid dirt getting into the open upper parts of the chord they are provided with dust-tight steel covers fixed with bolts to the chord elements. The shape of the lower chord repeats a circular curve.

The upper chord of the girder also has a box-type cross-section open on the underside. In the mid-span it is very close to the lower chord while at the piers it is supported by diagonals of the double lattice. The upper and the lower chords are connected above the piers by a rigid insert web that at the bascule abutment also acts as a support unit. The pivot axes, bearings and the bascule driving gear elements are fixed to it.

The lattice elements are H-shaped in section and flange-connected directly to the webs of the upper and lower chords without gusset plates, using high-strength bolts. It was this solution that allowed the original architectural appearance of the old cast-iron Nikolayevsky Bridge to be recreated. The facades of the main girders are decorated with high-strength bolts with semi-circular heads that imitate rivets. The lattice diagonals in the crossing points are designed to be connected using patch plates bolted to the flanges. The web of the interrupted diagonal is not joined but brought to the flange along the free line and terminated short of it. The breaking point is treated mechanically.

The bascule span of the bridge will be a double-draw retractable system. In its closed position it is a beam structure with a clear span of 45m. The proposed technical solutions for the bascule span of the reconstructed bridge were aimed at preserving as much as possible of the existing pier structures and their foundations. As a result, the permanent superstructure rests on the pier of the bascule span along the foundation axis, while the draw superstructure is connected to the permanent one through the shared units: pivotal axis and the positive bearing axis. This connection results in the fact that the bascule span actually becomes a cantilever part of the permanent bridge.

In the closed position, each deck rests on the pivotal axis on the upper chord level and individual bearings on the lower chord level. Balancing of the superstructure is achieved using counterweights located between the girders of the permanent superstructure. The locking mechanism of the span is a finger-type and is able to absorb bending moments in the mid-span caused by an unbalanced bascule weight, as well as temporary loads including the span behaviour of the continuous deck.

The hydraulic cylinders of the drawbridge are located in front of the pivotal axis and fixed to the transverse beams of the bascule and the permanent superstructure. In the drawn position the bridge bascules are rigidly fixed by the gate key which will be installed on the permanent superstructure in the support area. To avoid spontaneous opening, locking of the bascule is possible using a retractable key catch.

A mechanical damping device is provided on the counterweight part of the bridge for use in the closed position. It ensures stress control of the damping device on the counterweight and suppresses vertical vibrations. The rated force of damping is 7t per beam and the damper is hydraulically driven from a pump plant designated for the bascule key locking in a drawn position.

The mechanisms of the bascule span will be provided with automatic control and status analysis of electric power equipment, hydraulic systems, mounting groups and the main steel structures of the bridge.

The reconstruction project also includes the works on recreation of the original embankments and ramps as well as restoration and rehabilitation of the architectural décor of the bridge. The whole scope of works under the project is planned to be completed within a two-year period. The reconstruction project design was developed by engineers from ZAO Institute Strojproect in St. Petersburg, Russia.

Tatiana Kuznetsova is chief project engineer and Yury Krylov is chief engineer for steel structures at ZAO Institute Stojproect

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