SPEAKING on the opening day of the two-day workshop preceding International Heavy Haul Association (IHHA) conference in Rio de Janeiro, Dr Allan Zarembski, professor of practice and director of railway engineering and safety at the University of Delaware, said a trend over the past decade has been towards the introduction of standards regulating dynamic instead of static loading of wagons.
“The load environment includes vertical loads, lateral loads, and longitudinal loads, and in many cases, we get all three at the same time,” Zarembski said. “And the proper behaviour of the track structure is to be able to support and dissipate all three loads continuously.”
Vertical load is the primary standard used when laying track, Zarembski said, which can range from an axleload of 18 tonnes for a passenger railway to over 40 tonnes for a heavy haul freight railway.
“But that's static, and the railroad world is dynamic,” he said. These dynamic loads can be increased by a range of factors including track geometry imperfections, wheel or rail surface imperfections, the unsprung mass of the vehicle, and stiffness transitions.
“We have measured in the field dynamic loads of a factor of four or greater, so that means that we've actually measured dynamic loads of the order of 550kN in the real world,” Zarembski said. “And the effect of those dynamic loads is so significant that one of the interesting developments in the last decade has been moving towards controlling dynamic loads.
“This is what we call performance standards. A design standard says I maintain my track geometry imperfections to a certain size because hopefully that will control my dynamic loads. My performance standard says that I want to have a load dynamic load no greater than 400kN, and if it's more than 400kN I do something about it. Personally, I like performance standards. We're not there yet, but we're moving in that direction.”
Zarembski said that most railways in the United States now use various types of measurement systems such as wheel impact load detectors (Wild) to measure dynamic loads. The sector is also moving towards restricting dynamic loads, with the Association of American Railroads (AAR) setting a limit of 400kN.
“That means that if a railway measures a load of 420kN, they can stop the train, pull the wagon out, send the wagon to a shop, replace the wheels and, most importantly, send the bill to the wagon owner and the wagon owner has to pay.”
Zarembski says railways in Europe have also been working to introduce similar limits but are facing the issue of finding ways to ensure that the wagon owner can be made to pay for the repairs required to a damaged wheel to ensure it remains within dynamic loading standards.
“They can bring the wheels to a shop, they can change out the wheels, but then they're left stuck with the bill,” he said. “If it's a French vehicle or a German vehicle, they send the bill to the vehicle manufacturers and say: ‘I didn't authorise you to change my wheels.’”
