Launching Girder by Martyn Gomersall
Good day readers!
Last month we started talking about the construction phase of a modern day railway or more specifically – a Metro. We looked at how construction happens underground.
I recently came into possession of a few pictures of the machinery used in below surface construction. I will post these at the end of this blog.
Today, we look at construction above ground – more specifically the elevated sections of a railway (like viaducts for example).
Martyn Gomersall takes us again through the basics of the process. All photos are by Martyn, unless otherwise stated and used with his permission. Why don’t you give his Blogpage a gander: https://safetyvisionblog.wordpress.com
Before, I had discussed the development of an urban railway underneath our feet and now I want to go through the construction of metro railways above ground.
The first thing to note is that the construction of an elevated metro is much more disruptive to vehicular movements and to the public in general; however it is usually less costly to build above ground and presents less problems in many, many ways.
The first step is to plan the route and then begins the land acquisition issues. Many people live with the idea that their land is more valuable than it is and consider the fact that because the railway needs the ground they can ask any amount as a purchase price… I saw parts of some projects delayed for years as courts deliberated on what is a fair price and even then some recalcitrant land owners still refused to sell… this has led to some violence and even murder in the past. (One site where I currently work has a permanent police presence of 5 or 6 officers due to threats of violence by people who consider themselves to have been cheated out of land).
That be it as it may, once the land is purchased then the construction can begin. This first step is to build the vertical piers which will support the railways. To do this deep piles are driven into the ground, steel reinforcement bars (rebar) are constructed in a grid pattern and then the hole is filled with concrete to create a pile cap. From the pile cap further rebar is installed in a highly controlled pattern, a mould placed around this rebar and then again concrete is poured to fill the mould.
Once the vertical piers have been completed then the overhead structure needs to be built. The design of this can follow many different forms which for simplicity are:
1. Segmental Post Stressed
2. Cast in Situ
3. Pre-cast Beam
4. Steel Deck
Segmental Post Stressed
In this method short hollow segments are cast in an off-site plant and delivered to site. The segments are lifted into position with specialised equipment and then steel ropes are threaded through the full length of the segments. At this point specialised glue is placed onto the matching faces of the segments and then the steel ropes are tensioned and then left for the glue to bond. (See Photo).
This method consists of once again building a framework of rebar which rests on the Pier Caps and then this is surrounded with shuttering boards which have to be supported on scaffolding (known as false-work or cribs). Once the shuttering is complete concrete is poured into the moulds and we wait for the concrete to set (usually within 28 days full strength is acquired). There is another method based on this process which is known as “Balanced Cantilever” which dispenses with the cribs and advances at a slow pace and in simultaneously in two directions. New Sections being built are supported and balanced on the previous sections which have been cast. This is useful where there are crossings over wide roads or other areas where it is not possible to place any scaffolding to support the shuttering; this is however a ratherspecialised construction method.
Here we have long pre-stressed beams cast at a specialised casting plant and transported to site where they are lifted onto the Pier Caps. Usually 4 or 5 beams are used parallel to each other and then a deck is constructed which rests on these beams. Pre-stressed means that they contain steel ropes which are tensioned before the concrete is poured into the moulds and only released once the concrete has reached sufficient strength.
In some instance, especially other railway crossings, it is wiser to fabricate a steel truss bridge and lift it into position partly assembled. This is usually advisable where time constraints prevents long working sessions or waiting for concrete to gain full strength. Another option is to construct the steel span bridge next to the railway and then jack it out over the railway. This is a very specialised operation and requires the resources of many design and experienced engineers to ensure the structure doesn’t topple whilst it is being “launched”.
Depending on the method used to build the railway spans the next step is to erect parapet walls, install track work and all the other additional equipment required to operate a railway safely.
Here are a few pictures of a platform extension currently underway at the Golden Railway that I’ve taken recently:
These are some of the underground pictures: