Challenge accepted

22 August 2016


Bernadette Ballantyne talks to Liebherr’s Christoph Eiwan about some of the challenges posed to tower crane standards bodies and manufacturers by a move to cycle-based classification.

According to Christoph Eiwan, a member of ISO technical committees SC3 and SC7, and head of the structural analysis department for top slewing tower cranes at Liebherr, the most topical issue at the forefront of standards discussion today is related to the design of rope drives. “Design of rope drives is still carried out according to the existing FEM1001, even though we have moved to EN13001 regarding the steel structure, so it is a very mixed situation regarding standards and what is applicable,” he says noting that the sub-project group working on section 3.2 of the EN13001 standard is introducing the cycle based approach with regard to the rope drive.

The most significant change in the new section 3.2 is that by moving to the limit state concept the previous admissible stress concept is replaced by a more complex calculation which takes into account many more factors.

Under the previous system of calculation for the rope drive one simple factor known as the Fat P factor was used to take into account the effects of wear, static strength, fatigue strength and whether or not it is multi-layered. The new cycle-based approach shows three different methods of verification; first a static verification, a fatigue verification and a verification regarding multilayer reeving. “All three provide a different level of safety than the previous standard but you do not see this because in the past you had only one figure. Now you have three different figures so it depends if your crane drive is affected more by static strength or by the fatigue or if your crane drive consists of a multi-layer drum and it is a question of wear. So there is not a simple yes or no answer, there are many parameters which influence your calculation. For some it is favourable for others it is unfavourable.”

What this means is that the level of safety in the limit state concept from a cumulative point of view is higher than it was before, potentially presenting a challenge to manufacturers. “If we have utilised the previous level of safety to the limit and now you are changing to the new concept, then all your previous calculations and designs are utilising the components above the previous limits. As a result the mobile business and tower crane business is afraid that it is losing in terms of competition to other manufacturers because there are so many different interpretations of standards and therefore it is not clear whether we are all working within the same level.”

In some cases arguments are being made to keep the existing time-cycle approach in place, however the general move across to cycle-based classification has been agreed at an international level. The best that can be hoped for are some modifications as the standards are updated, to keep them compatible with the earlier method.

“At my company we are testing to take the new approach, but in parallel we are always calculating to the old method to learn what the influence is [of the new method] and what may be the possible changes due to the new approach,” says Eiwan. “As there are so many parameters that have to be taken into account that have a range, today we are trying to learn what the influence is of each of the values of each of the parameters.

From our point of view we are in a testing phase. We are trying to learn and adapt perfectly to our products and this is the best we can do right now.”

For tower cranes then, what changes could the new standards require? Eiwan says that jibs on tower cranes may be slightly lighter than before and that for future cranes the tower section might require higher strength.

Existing towers, some of which have been in the market for many years, may face higher requirements. “For these tower sections the stiffness is well defined and we have no chance to improve the strength so we have to use it, but as the requirements are increasing the result is a reduced height of hook. One method to solve the problem is to reinforce the base to get the same hook height as before,” says Eiwan. Another solution is that users may move up a category. Eiwan likens the situation to that of New York, where the change to regulations means that cranes must be designed to withstand greater wind speed. “With the higher wind loads the existing cranes and tower sections are not appropriate any longer and thereby the standard recommendation would be to replace all of the tower sections but of course no one can afford this.”

Potential solutions include the creation of reinforced sections or a change to the clamping distance for climbing tower cranes. How the industry will actually respond is yet to be seen.

Back in Europe the change from time-based classification to cycle based classification continues to roll on and although the general approach has been agreed, there remain many details to discuss before the standards, and the industry can reflect the new system.

Christoph Eiwan