Paul Nordegen,  CSIR... working out the bridge formula is not rocket science.

Here’s a question regarding the application of the bridge formula from Otto Wood, Applications Engineer at Hulamin Extrusions: 

“I don't understand the technicalities behind overloading fully. Just yesterday I was informed of another ‘clever abuse’ of the system where vehicles could carry 34 tons legally if you look at the axle ratings, but if you used the bridge formula, they could only legally carry 25 tons. As far as I understand the bridge formula is difficult to work with and is rarely used by law enforcement officers. Therefore vehicles are overloaded by about nine tons, but all looks legal.” 

This seems to be a common problem and Paul Collings asked the CSIR’s Paul Nordengen to provide an answer. “Working out the bridge formula isn’t rocket science using non-linear parabolas or complex algorithms. It’s a simple straight-line graph based on an equation that calculates the permissible load between one or more adjacent axles and/or axle units on the vehicle. It may occur that while the overall load complies with the axle restrictions from steer axle to rear trailer axle, its mass over a particular group of axles/axle units may exceed the load intensity limit controlled by the bridge formula – two adjacent tandem or tridem axle units, for example.” 

Nordengen explains why the bridge formula exists: “Bridges in South Africa have been designed and built over many years in accordance with a number of different (mainly) British and South African loading codes and some cannot safely carry as much load as others. They have a weaker ‘bending moment’ or ‘shear force’ capacity. The spacing between loadcarrying axles has an effect on the maximum bending moment and shear force generated by a particular vehicle on a particular bridge. If the span of the bridge is say five metres, it may suffer a greater downward force as a short vehicle, such as a tipper, passes over it. A 22 m interlink, on the other hand, will have much of its overall mass off this 5 m bridge at any given moment as it passes over it.” The bridge formula is thus designed to limit the load intensity of a vehicle, rather than individual axle and axle unit loads. 

So what’s the formula then? 

Permissible mass (in tonnes) between any two axles or axle units [P] = 2.1 x Length [L] + 18 Good luck!