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SV Loads in Archie-MA note for users

October 2016

Bill Harvey Associates Ltd

sv loads in archie-m 3

Contents

0.1 Introduction 3

0.2 Vehicle and bridge widths 3

0.3 Factors 4

0.4 Liftoff 5

0.5 Loads 6

0.5.1 SV80 6

0.5.2 SV100 7

0.5.3 SV150 8

0.5.4 SVTrain 9

0.5.5 SVTT (Tank Transporter 10

0.1 Introduction

This is an attempt to interpret the rules in BD86 for users of Archie-M and provides some background to how the load files were devel-oped.

SV vehicles are mostly relatively simep, but the rules for ap-plication are much more complex, and certainly not as simple asappeared when they were first introduced.

0.2 Vehicle and bridge widths

Archie-M can only analyse for a single vehicle across the widthand, for the time being, the distribution model is that from BD21

and is naive in the extreme when it comes to dealing with complexbridges or loads.

If there is room on the bridge for two notional lanes and the loadcan fit entirely within one, the code requires that an HA load is ap-plied in the next lane. BD21 says that HA loads are not appropriatefor arches so the interpretation of this is difficult, but essentially itmeans limit the SV vehicle to its own space. The "Bridge Width"should be set to the appropriate width to be occupied by this loadalone, which might be the whole bridge. In any case, the bridgewidth must ot be set to less than the vehicle width, which appearsas the last item in the load for Archie-M.

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However, there is a further complication. The track of the axlesin general SVvehicles is 2.65m, which means that the influenceof two wheels will not overlap until the depth of fill at the loadposition is 1.15m. For most bridges, the critical axle is likely to bea smaller distance above the ring so it is necessary to analyse forsingle axles and an appropriate bridge width (fig 1).

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“Bridge width” = min(A,B)

Figure 1: Bridge width for distribution

0.3 Factors

SV vehicles have systems for distributing load between axles whichare remarkably effective. Never the less, there is an axle lift offfactor (Cl 3.36). This is applied to a 3 axle bogie and not for slowvehicles which is yet further confusing, it sounds very much likedynamic impact but is regarded separately (See DAF below). So,for lift off, we must apply 1.2 to the first and 0.8 to the last of thethree axles.

An Overload Factor (OF) is also required. This is given in Clause3.16 and is 1.2 for the critical axle and 1.1 for all others.

There is then a Dynamic Amplification Factor (DAF), given in3.17, which varies with the load, presumably on the basis that big-ger loads travel more slowly (fig 2).

Figure 2: Dynamic amplification factor

DAF = [1.7 ∗ (qka)/10)−0.15] >= 1.05

sv loads in archie-m 5

where qka is the basic axle load in kN. Note that this can be dif-ferent for different axles.

The order in which these clauses appear seems to imply that theDAF is calculated from the basic load but then applied to the loadafter OF is applied.

It is more than a little surprising that, despite the reasons givenin BD21 for using a factor of 2 overall, BD86 Cl 3.35 insists that thefactor of 2 is in combination to the factors above

0.4 Liftoff

Te issue of liftoff is an interesting one. I suspect that modern vehi-cles are so well articulated that liftoff is very rare. The geometry ofthe process is illustrated in figure 3. The minimum requirementsare a long stiff body, a long bogie and a relatively steep rise and fallsuch that the other end of the vehicle is on a noticeably differentvertical alignment from the bogie. Figure 3 shows the most com-mon form, though it can also happen if there is a nong steel riseand fall from a crest so that the tractor in this layout is headingdown one slope while the trailer is still rising on the other side.

I hope this sketch also clarifies exactly what is meant by a bogie.

Figure 3: The geometry of Liftoff

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0.5 Loads

0.5.1 SV80

The SV80 vehicle is shown in figure 4.

Figure 4: Layout of SV80 vehicle

So, here we have 2 groups of three axles with 2.4m spread in thegroups. Under current guidance, even for viaducts, this will onlybe critical if the whole length fits in half the span, so 6m overall or12m span. Indeed a trial with Archie will probably show that thesmallest span for a 6m spread to be critical is rather more than 12m.We therefore need to consider a single bogie for most cases.

The dynamic factor for a 130kN axle is 1.157, so the axle loadsin Tons for Archie are 15.34 tonnes. The overload factor is either1.1 or 1.2 yielding loads of 18.56 or 20.24 tonnes. With the furtherapplication of liftoff, factors of 1.2 and 0.8 are applied to axles 1 and3. Giving 24.29 for the critical axle (which must surely be the onewhich already has a 1.2 factor) and 14.85 for the trailing axles.

To deal with loads on single wheels, it is necessary to take intoaccount the calculation that Archie does, which is illustrated infigure 1 but with the assumption that two wheels on an axle alwaysdeliver overlapping loads. we must therefore (for highway wheelloads) set the axle length to zero. The loads for this are therefore:

Figure 5: Load specification for SV80

vehicle

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0.5.2 SV100

THe SV100 pattern is shown in figure 6

Figure 6: Layout of SV100 vehicle

Which results in this loading.

Figure 7: Load specification for SV100

vehicle

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0.5.3 SV150

The SV150 load is produced with smaller loads on longer bogiesthan for SV100 and so is much less likely to be critical (fig 8 ).

Figure 8: Layout of SV150 vehicle

Figure 9: Load specification for SV150

vehicle

sv loads in archie-m 9

0.5.4 SVTrain

The train is essentially just one of the previous patterns with atractor to pull it. The tractor load distribution is different herethan for the Tank Transporter. The drawbar places the last axleof the tractor and the first of the trailer 4m apart so they will not becritical in combination on any but the biggest arches.

Figure 10: Layout of SV Train

Figure 11: Load specification forSVTrain-Tractor

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0.5.5 SVTT (Tank Transporter

This vehicle is simply a very heavy low loader. The rear bogie,8m behind the tractor, is far enough away for them not to need tobe applied together. The TT load should therefore be just the rear25tonne axles.

Figure 12: Layout of SV Tank Trans-porter

Figure 13: Load specification forSVTT-Tractor

Figure 14: Load specification forSVTT-Trailer

sv loads in archie-m 11

Bill Harvey Associates Limited

Gairloch, Hensleigh Drive, Exeter EX2 4NZM: 07979 425 884 E: bill@obvis.com

Copyright © 2016 Bill Harvey Associates Limited