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I'm currently designing a small reinforced concrete bridge on private property. The project is in Scotland.

It's very small scale, a single lane, with a span of around 4.5m. Service life is taken as 120 years.

Using Eurocode design for standard road bridges seems far too over-the-top (there will not be any lorries crossing this bridge for example), so I'm looking for a good load design precedent (with backup documentation). The largest vehicle that may foreseeably cross the bridge would be, say, a 30 tonne concrete mixer.

If there's a good code that relates to, say, loading on temporary works or construction traffic loading then we feel that would be perfect for use on this bridge. Lest we over-engineer the thing and it costs a fortune...

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    $\begingroup$ Unless you are going to restrict usage to "light vehicles" the maximum legal road vehicle weight in the UK (and almost all of the EU) is 44 tonnes, and the maximum weight per axle 11.5 tonnes. See internationaltransportforum.org/IntOrg/road/pdf/weights.pdf for more information. $\endgroup$
    – alephzero
    Sep 25, 2015 at 16:19

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I'm going to take a slightly unconventional approach, and suggest you design to an assessment standard; namely BD21/01 [pdf link].

My reasons are:

  1. It's a UK standard, and you're in the UK
  2. It provides loading details for vehicle classes which are lighter than the full 40/44 tonnes.

The rest of this answer probably won't make much sense to those not familiar with BD21. I thought it useful to include this advice, but I don't think a full explanation of all terms and loading to BD21 is appropriate here.

I would suggest that seeing as it's a private bridge, the surfacing maintenance will be minimal, and such the design should be for Poor surfacing. Clearly the traffic flow is Low, so you're looking at Figure 5.4 for your K factor.

As an absolute minimum, I think your bridge needs to be sufficient for all fire engines (i.e. group 1 FE loading to BD21). Beyond that, you've suggested a concrete mixer truck might need to use it for access. A quick bit of googling and I've found two different links which suggest a concrete truck might be 26 tonnes or 32-33 tonnes. The difference between 26 tonnes and 40 tonnes loading for a 5m span is a K factor of 0.85 vs 0.87. Given the small difference, and that you wanted to allow the larger concrete trucks on your bridge, I think you should design for the 40 tonnes loading.

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I'm not particularly familiar with the Eurocode (I work in the US). However, a 30-tonne concrete truck doesn't seem much smaller than what we call the AASHTO Design Truck in the US. The Design Truck has a loading as shown below (ref. AASHTO LRFD Bridge Specification, 2012).

design truck

By my calculations, the metric equivalent of the Design Truck has 14.5-tonne axle loads (32 kip is about 14.5 tonne). AASHTO also uses the Design Tandem, which has a pair of 25-kip (11.3-tonne) axles spaced 4ft (1.2 m) apart.

As I'm sure you know, concrete is heavy, and the trucks are heavier than one might expect given their footprint.

I would assume that your standard lorrie has a similar loading to the Design Truck since they both are designed to carry shipping crates. By this logic, designing for a 30-tonne concrete truck isn't really all that different than designing for a lorrie. Depending on the wheel configuration of your concrete truck, its loading could actually be more critical than the lorrie.

As such, doing your bridge design for a lorrie load seems appropriate to me.

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  • $\begingroup$ This seems fair to me. Following Eurocode design practices for road bridges, the following two load models are used. As you can see, they absolutely dwarf the loads you could expect on this bridge. Unfactored Load Model 1 (LM1) to Eurocode (This would in all likelihood be the governing loadcase): 600kN Tandem System + 5.5kN/m2 UDL (Note that even 40/44 tonnes MGVW is dwarfed by this loading) Unfactored Load Model 2 (LM2) to Eurocode (this is generally used for localised rather than global checks e.g. around expansion joints, but can govern for structures of 3m clear span or less): $\endgroup$
    – Smeato
    Sep 23, 2015 at 15:49
  • $\begingroup$ @grfrazee Don't forget the AASHTO Design Tandem! (A pair of 25 kip axles spaced 4 ft. apart.) Though I think this represents a tank driving down the interstate. Probably not applicable to a private development. $\endgroup$
    – CableStay
    Sep 23, 2015 at 17:11
  • $\begingroup$ @CableStay, unfortunately AASHTO doesn't have a nifty illustration of the Design Tandem, but I'll add that info to the Answer. Good catch. $\endgroup$
    – grfrazee
    Sep 23, 2015 at 19:27
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For any structure with a service life of 120 years I would always design it to the current, applicable design standard or code. You don't know what to expect during the service life. The owner could sell, a new settlement may develop, or some industry or mining activity may change the requirements of the bridge.

Unless there is a clear sign on the bridge limiting the use, most engineers would look at the design code applicable during the time it was constructed to establish safe working loads. They would not be able to deduce that you have employed a "simple design method" and may make a bad judgement call.

The other thing is, you already mentioned a 30 ton truck could possibly cross it. So are you be designing for it in any case?

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  • $\begingroup$ Under normal circumstances, I'd completely agree. However based on the awkward location of this bridge, I'm certain of it's future use. Thank you for your input though! I wish I could go into more detail, but to be brief, it's not even connected to the road network and I highly doubt it ever will be. $\endgroup$
    – Smeato
    Sep 29, 2015 at 14:48

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