There has been a bit of dialogue between our structures engineer and other engineers reading the blog, which was very enjoyable for all parties concerned.
Following a question raised by a reader about the drip strips we have had added, Bridges Engineer John Sreeves of CH2M Hill has sent us a longer, and very interesting reply which we think will interest our readers. Here is what he said:

During the restoration of these bridges, we became aware of various shortcomings in the original design that were conspiring to impair future durability. Most of the bridge problems are caused by water penetration and poor drainage which we hope to address, along with improved accessibility to some areas for inspection and painting. The brick ballast walls at the deck ends were in such a degraded and collapsed condition that they had to be totally rebuilt. So for the first time since 1903, the ends of the rail bearers and main beams were exposed to reveal extension corrosion. Where the brickwork had been in direct contact with the steel for 111 years, it had not been possible to apply any paint to the interface, which had attracted water through capillary action. The tops of the ballast walls were in contact with the underside of the deck plate, where similar wet conditions prevailed.

As the ballast walls were to be completely renewed, the opportunity arose to make improvements to access and drainage details without in anyway compromising the original design or affecting the heritage of the bridge which we were keen to preserve. To this end, drip strips 25mm x 8mm were welded on to the ends of the deck plates so that water can drain freely off the end of the bridge deck and not percolate along the underside of the deck plate. Furthermore, the addition of a drip strip serves to strengthen the end of the deck plate which is only 3/8” (9.525mm) thick and is quite flexible. It should be appreciated that the deck plates were never intended to carry the weight of ballast, track and live load. In the original design, all of these bridge had directly fixed track on longitudinal timbers mounted over the rail bearers for direct vertical load transfer. The deck plates were simply non-structural coverings over the underlying grillage. Handwritten additions to the drawings show that these bridges together with the others on the railway were converted to ballasted track in 1928, which would have been about the time when the original longitudinal timbers had become life expired. Consequently the additional load applied and flexure has caused fatigue cracking in the plates which we have had to repair.

To ensure accessibility for future inspection and repainting, the ballast walls as rebuilt are not all uniformly 9” (229mm) thick as per the originals. We have notched around the ends of the rail bearers and main beams to ensure a 75mm gap. Also the tops of the ballast walls are kept about 15mm down from the soffit of the deck plate. The intention is that all metal surfaces on the entire bridge shall be repaint-able with at least a fine nozzle spray with the exception of the undersides of the bearing plates. Unfortunately some areas of the main girders remain extremely difficult to access for cleaning and repainting where masked by the pilaster walls, but it should be possible to get a spray nozzle into the gaps in the future. The three-coat epoxy based paint system we are using is the best available that complies with Highways Agency and Network Rail specifications, and a 25 year life is expected.

Further enhancements to durability have been attained by the installation of transverse filter drain pipes at the back of the abutment walls. It had been noticed that seepage through the abutment in the vicinity of the bearing plinths was causing permanent damp conditions and frost spalling. The bearing plinths are highly loaded but surprisingly they are cut from soft sandstone unlike the indigenous limestone of the area. Being highly susceptible to weathering it is essential to keep them dry, so the installation of cut off drains will intercept and prevent rainwater getting to the upper areas of the abutments.

Finally, bird mesh has been fixed to the abutment shelf pockets between the beams as the accumulation of nests, feathers and faeces is very unpleasant and has a deleterious affect on paintwork!

John C Sreeves 

Thank you John !