The reconstruction of historic bridges has its pitfalls. These are multiplied if you are going to change the design concept of the original. And the variations of possible solutions can significantly reduce the requirements of conservationists. An outstanding example of such a realisation is the reconstruction of the historic bridge at the Hořín lock.
Ing. Jan Blažek, the head of V-CON, the company involved in the project.
The Hořín lock is the highest navigation level in the Prague – Mělník section. Can you tell us what helps the ships to overcome it?
The lock was built in the time of the Austro-Hungarian Empire. It is located at the confluence of the Vltava and Elbe rivers at the end of the Vraňany – Hořín lateral canal, whose mission is to enable large ships to navigate the section without complications. The Hořín lock compensates for the difference in river levels, which is over 8 m.
What has changed during the modernisation of the lock change and why?
The modernisation of the Hořín lock is part of a larger project to raise the underpass heights of the structures on the Vltava Waterway from the original 4.1 m to 7 m. The intention of the project is to make the section navigable also for large passenger and cargo ships.In the case of the Hořín lock, this led to the modification of the lower head. The gates at the ends of the lock were widened and the fixed bridge over the lock became a drawbridge.
What changes did the reconstruction require from the bridge at the entrance to the lock?
The investor’s brief was: “to convert the original stone bridge, which is over 100 years old, into a lifting bridge – which can also be raised up to 5 m higher when a larger ship passes through.”
The technical requirements were not the only ones that had to be met…
The entire lock is a technical monument, which equates to strict oversight by conservationists during the permitting process. From the outset, any interventions that would even marginally alter the external appearance of the bridge were completely impermissible. For example, a solution with a lighter steel structure would have been technically preferable for the required lift – but there was no possible discussion with the conservationists.
How did you reconcile the new project with the need to maintain the appearance of the original 1905 bridge?
In the end, we decided to create an internal steel structure, using the original stone structure as the cladding. And the connection between the steel and the stone is provided by a layer of reinforced concrete.
The inner steel frame is lifted by four hydraulic pistons. The position of the structure and the safety of its movement during lifting are ensured by steel guides.
The steel structure of such a bridge is certainly not the lightest. How was it installed in the lock?
The steel skeleton of the bridge weighs approximately 100 tonnes and was set into its final position by a mobile crane in one piece. Stone cladding was fitted and concreted onto the temporary support structure in the raised position.
How much do the original stone pieces weigh? And how were they actually attached back to the structure?
The complete stone cladding, including the railings and roadway, weighs approximately 240 tonnes. The original stones were numbered and dismantled – the largest ones weighed several tonnes. Next, the stones were cleaned and machined. The heaviest ones were fitted with stainless steel anchors, which were then attached to the internal steel frame. And the space between the stones and the steel was concreted.
And how does one lift such a colossus of weight?
The entire bridge is lifted by four special hydraulic pistons from the American company Eaton – each with a capacity of 200 tonnes. The lifting of the bridge has 3 speeds:
normal, operational 5 minutes;
slow, service 15 minutes
and, if necessary, it can be raised express in 2.5 minutes.
How will tall ships be handled at the Hořín Lock?
Larger ships will request in advance the filling of the locks and the lifting of the bridge by the operator at the lower headhouse. And beyond that, the plan is to raise the bridge regularly several times a week at fixed times.
The modernisation has affected the entire Vraňany – Hořín navigation channel. What has it brought to water transport?
Among other things, the modifications in the section have resulted in the raising of bridges and the conversion of ordinary bridges to drawbridges in a total of seven cases. As a result, high cruise ships and cargo ships can now sail along the Vltava from Prague to the Elbe near Mělník without problems. Moreover, it should soon be possible to sail from České Budějovice.
In your opinion, what would surprise the builders of the original bridge the most if it were to rise before their eyes?
I think they would be surprised that although it looks the same from the outside, it is completely different from the inside – both statically and structurally. And they would certainly be surprised that we have actually used the original stone structure of the bridge – and made it into the lining of the new bridge – completely without any remnants. This makes the new bridge look just like the old one.
And what aspect would you like to draw their attention to?
The construction of a drawbridge is a watchmaker’s job. Much more than with conventional bridge structures, it is necessary to ensure that the production and assembly of the individual parts are millimetre-precise so that nothing cracks when moving.
It was challenging to spatially design and coordinate the fitting of the shape of the complicated bridge into the stone substructure. The entire V-CON team in collaboration with Valbek had the opportunity to demonstrate their engineering skills on the edge between civil and mechanical engineering.