Shotcrete, which became widely used after the Second World War, is suitable for the construction of bridge structures in inaccessible terrain or for buildings with large spans. The project designer from Valbek, Ing. Ondřej Matoušek.
What are the biggest advantages of the segmental casting technology?
Segmental concrete structures are built in hard to reach places where it is a problem to set up support structures for the construction of the bridge itself. These are mainly deep valleys, watercourses, building sites, etc. The main advantage of this type of bridge is that there is no need to build any support structures on the ground, which could be very complicated or even impossible to build. At the same time, segmental casting makes it possible to construct bridges with large spans, such as the longest such construction – the Chinese Shibanpo Yangtze Bridge with a span length of 330 m. Only the pier foundation and the pier itself are built on the ground. The superstructure is then constructed from the built-up pier, without the need for additional support structures that would have to stand firmly on the ground.
Another advantage is that the structure is structurally certain for virtually the entire construction period, which brings a number of additional design advantages.
Conversely, what are the pitfalls and typical problems associated with the design of cast-in-place concrete structures?
The pitfall of cast-in-place concrete, from my point of view, is the cost. In bridge construction, the tensile stresses that arise in the upper fibres of the structure need to be balanced. This is achieved by prestressing reinforcement, which is expensive and more so than in bridges constructed using conventional technology.
At the same time, extra care must be taken in the foundation design of the structure for two reasons:
- Another pitfall can be the long-term increase in deflections in the middle of the bridge spans. This needs to be considered in the design of the structure;
- due to the construction process, where almost the entirety of one half of the bridge span is constructed from a single pier, care must be taken to ensure the stability of this “sway bar” to prevent it from collapsing before it is connected to the rest of the structure.
Another pitfall can be the long-term increase in deflections in the middle of the bridge spans. This needs to be considered in the design of the structure.
You also used the technology of segmental casting in your graduation thesis Design of the Danube River bridge on the Bratislava bypass. The jury of the Czech Concrete Society even awarded it the title of Outstanding Master’s Thesis in Concrete. What exactly did you design?
I wrote my thesis at the time when the D4 motorway project in Slovakia – the Bratislava bypass – was being prepared. For the Danube river crossing I originally designed 3 bridge variants: extradosed, suspension and cast-in-place concrete bridge. From these options, the segmental concrete structure was chosen.
The designed bridge had a main span of 230 m. The height of the superstructure was 12 m at the piers and 5 m in the middle of the span. Several large, two-storey houses could be built inside the chamber structure designed in this way – just to give an idea of the possible robustness of the cast-in-place concrete bridges.
The thesis won an award, but unfortunately the bridge itself is not implemented directly according to my design.
Master thesis, Ing. Ondřej Matoušek
The bridge mentioned above is not realized according to your design, but in the end it is not only design-wise but also technologically very close to your design. How does the final realisation differ and how do you evaluate it in retrospect?
Technologically, it really is practically the same bridge. I can only judge from the available photographs, but the bridge is being built with cast-in-place concrete on approximately the same span as in my design. As in the design, the superstructure is concreted as a backbone beam, which in the final static state will be supplemented by very long cantilevers supported by auxiliary precast struts. The bridge piers are designed in approximately the same locations as in my work.
The significant difference is in the connection of the superstructure to the piers themselves. In my thesis I designed the piers as pairs of leaf piers that were framed to the superstructure at the main span and then further from the centre of the bridge the piers were designed as swinging uprights. In contrast, the implemented structure is placed on bearings. Since the bridge can swing freely on the bearings, auxiliary piers were used during construction to ensure the stability of the swinging piers until they are connected.
You are also involved in the flyover of the Křimická city ring road in Pilsen. Which of the expansion units are you responsible for? What are the key aspects of such flyovers?
Flyovers are built to bridge rivers, urban developments or, for example, in floodplains – which is the case with the aforementioned flyover on the Křimická ring road in Pilsen.
The flyover at the Křimická MO is composed of three expansion units: SO 1202A, SO 1202B and SO 1202C. SO 1202A and SO 1202B are designed by colleagues from Valbek’s Pilsen branch. They are built on a sliding frame. The length of each of them is about 510 m, the span of each bay is about 33 m. The supporting structure in this part of the flyover has a constant height of 1.6 m and is designed as a prestressed beam.
I am dealing with the dilatation unit SO 1202C, which has a bridging length of 188 m. It is a cast-in-place concrete bridge with a rather smaller span of 85 m. The height of the chamber is 5 m at the pier location and then 2.1 m in the middle of the span.
The construction of the cast-in-place concrete part of the flyover is still in its early stages. Recently a load test of the concrete truck was carried out. I have currently handed over the elevation for the third lamella, for which reinforcement work is now underway.
There are no major problems with the construction of the bridge itself, so far it is behaving as per the design assumptions. The only thing that was a little bit of a problem was the need to modify the foundation of abutment 36 compared to the tender documentation. The geological conditions did not fully correspond to the results of the geological survey and it was necessary to change the foundation of the abutment from flat to deep foundation with large-diameter piles ø 1500 mm.
The footbridge leading to the island village of Lužec nad Vltavou is visually very interesting. Its subtlety is said to be due to special materials and technology…
That is exactly right. The footbridge is interesting not only for its “simple” appearance, but also for the technologies used. It is a suspended footbridge 130 m long with a span of the main span of 99 m. The very notion of a suspended footbridge is a rarity in the Czech Republic. The subtlety of the footbridge was achieved by using ultra-high-performance concrete (UHPC) for the concreting of the individual segments of the footbridge.
The project called for C110/130 concrete. In the end, in 28 days we managed to mix concrete with a compressive strength of 140 MPa, which is 3 times higher than the value of commonly used concretes. In addition, UHPC concrete has very low water absorption and high resistance to chemical attack, and therefore a long service life. It is still technologically quite difficult to mix concrete of such parameters. This is reflected in its price, which is many times higher than for conventional concretes.
The actual implementation was carried out by cursory assembly of pre-concreted segments. These were concreted on Rohanský ostrov in Prague and then floated down the river to Lužec nad Vltavou. There, they were suspended from a 40 m high steel pylon by means of cursory assembly. The footbridge project was an interesting and, in retrospect, a nice experience for me. The complicated construction of the supporting structure was managed by my colleague Lukáš Vráblík, for whom it was also – dare I say it – a challenge.
Lužec nad Vltavou footbridge
To return to the fleeting concrete construction, did it also help bridge the deep valley at Velemyšlevsi?
A relatively deep valley had to be bridged at Velemyšleves. In this case, a light concrete pour was directly offered. The completed bridge has high piers and fits naturally into the valley. In fact, I have heard nothing but praise for it.
Velemyšleves Bridge
Personally, I only came across this project in passing (laughs). And that was when I was writing my thesis. I was helping my colleagues with auxiliary design work and I had the opportunity to see what the design of such a bridge contained. I have not only applied the experience I gained in my diploma thesis, but I am also drawing on it now – for example, in the design of the segmental concrete bridge on the MO Křimická flyover.
Near Ružomberok, on the D1 Hubová-Ivachnová section, the bridge was built using the staggered method. Why is progressive alignment chosen in such locations and what are its advantages?
D1 Hubová – Ivachnová, Slovakia
Bridges constructed using the staggered drawout method generally have smaller spans compared to bridges constructed with cast-in-place concrete. They are chosen where it is possible to build a larger number of piers. This in turn requires the final bridge to have a regular geometry.
In principle, the idea is to build a fabrication plant at one location where the formwork is prepared and where the reinforcement of the superstructure is prepared. In the production plant, a section of the bridge is concreted, which is connected to the already concreted section, and then the bridge is shifted by the length of the concreted section using presses.
The whole bridge is gradually built in this way. The construction of these bridges is very fast due to the reduced need for formwork and the repeatability of the reinforcement. At the same time, as with cast-in-place concrete structures, there is no need to build formwork under the entire bridge.
The bridge at Ružomberok is one of the longest progressive drawbridges in Europe. I myself had the honour to participate in its design with experienced colleagues from Valbek and I learned a lot from this project.
Now you yourself are reviewing the work of future engineers at the ČVUT. How do you view the future of segmental casting?
Opposing sounds rather lofty given my experience, but it is true that I have had the opportunity to write my opinion on several works of ČVUT students.
For example, one of these papers dealt with the problem of long-term deformation of cast-in-place concrete structures. Its aim was to build a detailed computational model of the mentioned bridge near Velemyšlevsi and to compare the long-term evolution of deformations on this bridge with the real measured values during the operation of the bridge, taking into account the actual construction schedule.
It turns out that the measured evolution of deformations on the structure corresponds to the calculated values, which indicates that the structure behaves as calculated. Thus: if the design is correct, the structure works without any problem.
I think there is definitely a future for cast-in-place concrete structures in our country, but they need to be built where they fit in.
