I still don’t understand why “tram loading” - to some extent spread by the track - is worse than a convoy of buses/HGVs
Good question.
One CAF Urbos 3 tram weighs 56 tonnes empty. Assuming a 50% loading factor, for 125 industry standard passengers who weigh 75kg each, we get a total weight of 66 tonnes. This is spread across 12 axles, so the axle load is 5.5 tonnes.
Alternatively, we can carry 125 passengers using one 20-tonne Volvo Enviro400XLB (the one with three axles that carries 100 pax) and one 18-tonne Volvo Enviro400 (two axles, 85 pax). If each bus is carrying half of the passengers then the total weight on the bridge is 48 tonnes. This is spread across 5 axles, for an average axle load of 9.6 tonnes.
In terms of an idealised uniform weight distribution on the bridge, the tram weight is carried over about 40 metres of carriageway (thus, 1.6 tonnes/metre), whereas the two buses in a traffic jam are carried over about 26 metres (1.8 tonnes/metre).
If the tram was at maximum capacity (75 tonnes, 250 passengers) you would need three (84 tonnes) of the Enviro400XLB buses, which in a traffic jam would take up almost exactly the same length of road as the tram. The axle loadings for tram and bus would become 6.2 tonnes/axle and 9.3 tonnes/axle respectively, and the distributed weights 1.7 tonnes/metre and 2.0 tonnes/metre respectively.
The elephant in the room would be the addition of rails and OHLE. South Bridge is 350 metres long between the High Street and South College Street. Tram rails alone are about 58kg/metre, and you need double track, so call it 81 tonnes of rails.
Overhead line equipment is mainly poles and foundations, and works out at about 100kg/metre/track, so altogether weighs about 75 tonnes.
But you need to mount those rails on a substructure, which is probably going to be made of reinforced concrete. 350 metres at about 7 metres wide and say 30cm deep is going to weigh nearly 1800 tonnes, and that is a lot even compared with the current loading from cars, buses and trucks. You'd need to have 40 44-tonne artics to deliver that sort of load. Does South Bridge even have an existing weight limit for goods vehicles?
But, you are digging up tarmac and replacing it with a tram track. Tarmac (as the surface, plus the binder course plus the granular base) is about 2.4 tonnes per cubic metre. If you remove 350 metres worth over the width of a double tram track, the equivalent amount of material removed is about 1600 tonnes.
So what we have is a net increase in weight of about 350-400 tonnes in terms of infrastructure to run trams instead of buses, but a net decrease in vehicle weight of about 10 tonnes for every 250 people carried in each direction.
There are probably further decreases in weight because trams would likely displace parked vehicles to some extent too.
(Back of the envelope stuff here, so might be wrong)
posts
