Axle Loading vs total weight

[G-BOAF]

Been pondering for a while (I'm not a strutural engineer)

Why are structures measured by axle load and not toal weight?

A 9F has an axle load of 17t, but its total weight (without tender ) is 89t with RA9

A BR 6MT Clan has an axle load of 19t but its total weight (without tender) is 88t with RA8

 

So, given the total weight of a the loco is all but the same, why does Axle load matter? I can undersand for a short overbridge the weight of one axle is important, but for a longer bridge or viaduct, surely the total potential weight bearing down on the middle of the structure is more important?

 

Also, why does the 9F have a higher (less permissive RA) with a lower axle load but same total weight as a Clan?

 

[Wheatley]

Because in many cases the weight concentrated on a single point (axle load) is of more concern than the total weight evenly(ish) distributed over a longer distance. Although bridges are fairly obviously long  structures supported only by the piers, so is the track - the bit in between each sleeper is unsupported and is in effect a mini bridge. So the extra 2 tons axle weight on the Clan will break rails which the 9F will not. The Clan will also produce a greater hammer blow per wheel as there are fewer of them and they are heavier, I think, that is getting into dynamic loads and I was mostly asleep for that bit of Applied Maths. 

 

 

 

 

[johnb]

It's not that simple, yes for structures the total weight is important . But it is the weight or point load on the track itself from any axle that is also important. The heavier the axle load the more robust the track, rails and ballasting must be.

 

As for the RA difference I would suspect that things like total coupled wheelbase and the ability of  a 2-10-0 to go round curves. However on that matter no doubt an expert will be along shortly!

[John-Miles]

It's even more complicated than that. How the weight is distributed is taken into account by structural engineers. This is turned into a series of curves which show how a bridge deflects in different loading cases as the loco or whatever, moves across the bridge. Limiting deflections is important. The life of a bridge is a function of how much it deflects under load. Lots of large deflections results in cracking and ultimately failure.

[LMS2968]

You still need to consider weight per foot run, another important factor.

 

As to hammerblow, 9F v Clan, I'd suggest that the 9F would be the worse offender. The weight of the wheels has nothing to do with it (although unsprung weight is another undesirables issue) but hammerblow is the result of reciprocating masses and what percentage of these is balanced at the wheel rim. Wheel rpm is another factor, and the small wheels of a 9F rotate considerably faster than those of a Clan at any given speed.

[Wheatley]

Thanks LMS, I was working on "bigger wheels weigh more = greater unsprung mass" but I hadn't even thought about RPM. Told you I was was asleep for that bit !

 

 

[rab]

3 hours ago, Wheatley said:

Thanks LMS, I was working on "bigger wheels weigh more = greater unsprung mass" but I hadn't even thought about RPM. Told you I was was asleep for that bit !

 

 

I was awake but I still didn't/don't understand it !

[Bomag]

For most bridges bending forces are more critical than shear. Therefore the position and weight through the axles is more important than the total load.  A small 50T hopper in the centre of a bridge is going to be at least as critical as a 100T tank wagon. Hammer blow is much less of an issue with most bridges than with the track. Kettles are simply not fast enough for dynamic loads to be critical.

 

Edited June 15, 2020 by Bomag

[Mark Saunders]

The bogies fitted to wagons affects the running of trains!

 

Many years ago BR decided that all freight bogies should be fitted with primary suspension but EWS started using three piece cast bogies and as such when fully loaded at 102 tonnes , RA 10 these have speed restrictions on a number of bridges.

 

 

[kevinlms]

Another factor that needs to be considered, is that the weight on each axle of a locomotive (steam in particular), are not all the same. Pony/bogie wheels usually less than driving wheels.

Might not be a huge difference, but the civil engineer will want to know! I suspect the unevenness was a reason why some loco classes were banned from certain bridges and so lines. Most 4-6-4Ts were notorious and so banned from the very lines they were especially built for. The LT&SR 4-6-4Ts come to mind, as not being allowed to enter Fenchurch St and the approaches - so useless for their intended purpose!

 

I do remember reading somewhere, that for a Stanier 8F, the heaviest loaded axle was actually a tender axle, but it doesn't matter so much, as these axles have no hammerblow.

[LMS2968]

51 minutes ago, kevinlms said:

I do remember reading somewhere, that for a Stanier 8F, the heaviest loaded axle was actually a tender axle, but it doesn't matter so much, as these axles have no hammerblow.

I wish you could have told the Severn Valley Railway that 1n 1969! for some years we were not allowed more than 2,500 gallons of water and 4.5 tons of coal in the tender.

[G-BOAF]

Thanks for all the info. Most illuminating in showing the complexities of forces.