Thanks for all the replies. I originally wanted to ask a general question about a checker’s responsibilities. The example I cited above was to give some context however some posters seem to have been irked by this. It wasn’t my intention to and I apologise if this is the case. Also, it wasn’t my intention to discuss the finer details of the project. However, at the risk of going off topic it has thrown in some interesting discussion for me.
I should explain that my background is a designer and have been designing railway structures for the last 10 years. I’m aware I work in a safety critical industry. At the risk of repeating myself I work for a government owned entity that is owner/client, designer, project manager, operator and maintainer of railway infrastructure as a single organisation. So yes it’s my own organisation at fault.
I actually know the designer personally. He admitted that his beam design is well under-utilised with respect to structural capacity (~35%). As mentioned by someone the problem was the onerous deflection limit of L/300 (80mm for 25m span). Loading is actually specified by the UK National Annex to Eurocode BS EN 1991-1-7 for a single horizontal 825kN free point load. My own back of envelope calculations are:
MEd = PL /4 = 825 x 25 / 4 = 5160kNm approx maximum applied design moment
(load factor = 1.0 for accidental situation),
Grade 355 MPa steel, section depth 750mm -> Second moment of area required for ultimate moment
I (ULS) = My/f = (5160x10E6 x 375 / 355) / 10^4 = 545,000 cm^4 approx.
Second moment of area required to satisfy deflection limit d = PL^3 / 48EI
I (SLS) = PL^3 / 48Ed = (825x10E3 x 25000^3 / 48x210000x80) / 10^4 = 1,600,000cm^4 approx.
One might argue that the beam should be allowed to deflect closer to its elastic limit or even allow some plastic deformation to dissipate impact force perhaps with some replaceable components if it does not pose a maintenance liability or danger to other road users.
I agree it isn’t just about saving material or labour costs on the railway, site time and access is severely limited and dictates the cost. The design solution and thus the construction method is a major factor. Work has been cancelled or put on hold because a long enough service shutdown cannot be obtained or the local authorities are not willing to grant permission for sufficient road closures. A lighter beam may have only required a smaller crane lift and removed the need for disruptive heavy foundation work. The load needs to be transferred safely into the ground as well.
As a designer I’m used to being challenged on the solutions I propose to problems. For example, I’ve just designed a scheme for a protection wall to stop road vehicles from driving off the edge of a retaining wall and crashing onto the railway cutting below. The project managers think my design is too expensive to build and propose an alternative. I’m writing a paper for senior management justifying why I chose my solution over their alternative which I don’t believe will work for this particular site. Even though it’s a pain for me but I believe they’re entitled to a sensible discussion with designers.
Yes risk needs to minimised, but engineers are also working in a practical and economic framework. When I said risk averse I meant avoiding risk at any cost and covering every conceivable angle which I don’t believe is good engineering. From a legal standpoint, the UK the approach to risk is the ALARP principle “As Low As Reasonably Practicable” there is only a finite pot of money and resources. The UK rail industry has established procedures in assessing risk against the costs to prevent deaths and to justify whether the work needs to be done or not. There are potentially hundreds of sites we have to deal with and the government can’t afford to build a Rolls Royce solution for all of them. But say if it was up to the designer to choose design loading, I would possibly be looking at road conditions (minor/major roads/urban areas) and analysis of past bridge strikes with possibly some full scale testing, to make a reasonable site specific assumption on vehicle mass, speed and deformation.
Only a portion of the cost overruns can be attributed to the original design assumption as the remainder is down to poor management decisions in forcing design changes but the subsequent chain of events could have been avoided.
As for thinking out of the box and removing the hazard at source, raising bridge levels is problematic when it comes to changing track alignment, raising embankments, modifying station platform heights and tunnel headwalls. Raising ballast levels could overload adjacent bridges which would then require strengthening. We deal with a lot of old infrastructure some which are 150 years old. The Highway authorities are rarely agreeable with closing or altering their roads. They don’t see it as their problem and money is tight for them too. All our bridges in question have adequate road signage forewarning road users but it doesn’t prevent bridge strikes from occurring. We document these events and some bridges have a long history of hits even though the majority of incidents cause no damage or have only caused superficial damage so far. For many years this risk has been deemed acceptable but in recent years there is a policy change.
