Determining factors for the design

In August 2019, the Princess Amalia Bridge near West-Knollendam was put into operation. The bridge is the first of the two bascule bridges to be replaced as part of the ‘N246-N244 route maintenance project’. An important criterion for the replacement of the bridges was to minimise maintenance. In addition, the province of North Holland had stated that the route section of this road may not be altered. This meant that both bridges needed to be constructed at the same location as the existing bridges and that the alignment of the road was not be adjusted. Minimising hindrance and limiting and shortening construction activities on location was also a determining factor for the design.

How the bridges are constructed

The bridges comprise of two approach ramps, a bascule chamber and a steel movable deck. The Princess Amalia Bridge has two traffic lanes and a separate cycle path. The Kogerpolder Bridge has three traffic lanes and a separate cycle path. The architectural design of both bridges has been designed equally in many areas to clearly convey that both bridges belong to one another in accordance with the site plan. By opting for a rejuvenated bridge deck and pointed pillar heads, a slender appearance has been created.

Pile foundation design was a huge puzzle

The existing bridges were completely demolished with only the existing pile foundation remaining thereunder. Designing the new piles was a huge puzzle due to the new bridges being placed in exactly the same position as the old bridges. The existing pile plan has been mapped using archived documents and as-built drawings. The new pile plan has been designed around this and incorporated into a 3D model with assumed deviations. This model turned out to be very much in line with that in practice, which meant that only one pile needed to be repositioned and supported mathematically.

No maintenance-sensitive joint profiles required

The basis for the chambers was a construction pit without underwater concrete. This limited the construction period and provided the Princess Amalia Bridge with a relatively shallow chamber. The arm of the counterweight was, therefore, limited and had to be provided with a high-density weight (steel ballast). This wasn’t an issue with the Kogerpolder Bridge as the bridge deck is higher and the counterweight can be filled with concrete. The spans of the approach ramps of the Princess Amalia Bridge are limited to allow for the application of concrete girders. Tubular girders have been used on the Kogerpolder Bridge. On both bridges, the decks are firmly connected to the chamber and the abutments (by way of wet joint/flexural joint). Due to this, no maintenance-sensitive joint profiles are necessary.

Limited hindrance to shipping

The pillars were constructed using precast (concrete) box culverts. The (concrete) box culverts with round openings were placed over the steel tubular piles onto welded brackets. The tubular piles were prefilled with concrete and reinforcement at the head. Following installation, a reinforcement cage was applied to the pillar and the (concrete) box culverts were poured with concrete. This is how the piles, the precast (concrete) box culverts and the pillar are structurally connected. Following the placement of the precast girders of the deck and the pouring of the top layer, the deck is also structurally connected to the pillar. This construction method allowed for the pillars to be built without the need for a construction pit or drainage, thus limiting hindrance to shipping.

The reinforcement has been completely modelled in 3D and was, as much as possible, assembled into cages in the factory and placed as a whole. The 3D model and prefabrication produced very precise reinforcement cages that fitted together well.

Movable parts are mostly identical

Both bridges will be electromechanically driven by means of a crank mechanism. A series-connected drive mechanism has been chosen for this. This is widely compatible with: the motor - gearbox 1 - intermediate axle - gearbox 2 - crankshaft – connecting rod – buffer - movable deck. By opting for a long intermediate axle, the rigidity of the operating mechanism is reduced, and the occurring forces are limited.

To minimise maintenance, the steel movable decks are designed as sealed, box-shaped steel structures and are welded airtight. Sealing the underside leads to a reduction in the preservation of the surface when comparing to that of a conventional implementation with an open underside. Moreover, the bridges are easier to maintain due to the smooth finish. The base plates not only fulfil a practical function as a sealed plate but also contribute from a constructive point of view as the lower flange of the main girders and cross girders.
The deck surface that is directly driven upon is a ‘conventional’ version: the thickness of the deck plate and the welding details are in accordance with NEN-EN 1993-2. The troughs protrude through the cross girders, whereby the cross-girder webs close tightly around the troughs. This type of trough allows the trough webs to be welded to the deck plate over the full length of the deck by way of an automatic welding process. Two main girders form the connection between the closed deck structure and the counterweight. The narrow bascule chamber has necessitated the application of tight vertical curves in the lower flanges of the main girders. The stress-increasing effects resulting from the curvature have been minimised by the choice of wide and narrow flanges.

A direct force transfer from the main pivot points to the foundation piles was chosen by means of continuous columns between the pivot point brackets and the chamber floor. In respect to the free rotation of the movable deck; this is coupled with single-sided bearings on the outer side of the main girders. The main pivot points are interconnected by a robust connecting pipe, which prevents twisting in the girders between the deck and the counterweight.

The Princess Amalia Bridge was put into operation on the 10th of August 2019. Extensive work is currently being carried out on the Kogerpolder Bridge which is expected to be completed in early 2020.

Would you like to know more about this project? Pieter will be pleased to tell you all about it. Contact him by email or call 088 943 3200.

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