Track, Structure and Interaction
Track Structure Interaction (TSI to it’s friends) is the latest of a series of unfortunate analyses that I have been subjected to following the adoption of the Eurocodes. The culprit behind this new addition is the adoption of CWR (continuously welded rails). Compared to the higher line speeds, and decreased maintenance costs, a little bit of extra stress at design doesn’t even factor on any client’s radar.
If you do need to do a Track Structure Interaction analysis, do it early on. The criteria surrounding deflection are onerous and could easily dictate your articulation, sub-structure and foundation design.
The problem with these great lengths of joint-less rails is that they form a structure in their own right, and will happily transfer forces across one bridge and into another. This can lead to over-stress as the rails pick-up forces, or track instability as the ballast moves under excessive deformation.
As with a lot of rail actions, the Eurocode TSI analysis has its beginnings in UIC 774-3 R (Track/Bridge Interaction, Recommendations for Calculations), which has been copied almost verbatim into BS EN 1991-2:2003 (Actions on Structures – Traffic Loads on Bridges). As with all Network Rail structures, however, NR/L3/CIV/020 (Design of Bridges) enjoys throwing a few curve-balls into the mix.
Before you start make sure you need to do a TSI analysis. It might sound an obvious thing, but Network Rail have their own list of exclusions beyond those of the Eurocode- typically you won’t need to do anything if the bridge is less than 30m long. There are also simplified calculations available for common cases- although the majority of these are for structures that would be excluded anyway.
If you do need to do a TSI analysis, do it early on. The criteria surrounding deflection are onerous and could easily dictate your articulation, sub-structure and foundation design. Keep an eye on the stress requirements too.
I have yet to see any definitive guidance for TSI criteria on curved bridges, an omission that will need discussion early on.
Actually doing the TSI analysis is a topic best saved for a later post; however I can recommend the original UIC guide over the Eurocode. Just make sure you give the actual standards a once-over too, as there are some (minor) differences.