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High stack, high risk?

16 Feb 2009

"We are seeing additional strain from increased static loads from stacked containers (stacking higher)," Julian Johanson-Brown, Halcrow

How high, how wide, how safe and, ultimately, how efficient? Felicity Landon reports on moves towards higher density stacking solutions

Years ago, in a port which perhaps should remain nameless, container port guru Robin McLeod recalls seeing empties stacked 12-high using a building-site tower crane which lifted a worker - with his foot in the lifting hook and holding on to the lifting wire - on to the top of the selected pile to attach two hooks at opposite corners, before riding down again with the container. That, of course, is an extreme example of high stacking and low safety. But where is the balance to be found? With space at a premium, higher-density stacking seems the obvious answer. But quite apart from a series of safety issues, loss of efficiency - i.e. the loss of fast, random access to boxes - is clearly too high a price to pay.

"The higher you stack, the more unpaid household moves to have to do," says Jan van Beemen, Rotterdam-based terminal planning consultant for Royal Haskoning. "The ideal would be to stack all containers one-high, so that you can access each container without moving any other container. Yards in the 60s were like that. The only disadvantage is that such yards require excessive space."

As Mr van Beemen points out, on containerships, laden containers are stacked up to nine-high below decks, in between the cell guides.

"In heavy seas the load on the bottom container can be considerable. Containers are designed to withstand such loads. As there are no dynamic forces in the stack on the terminal, stacking height is not a technical problem, as far as the container itself is concerned.

"If the base is horizontal and of sufficient strength, I see no technical reason why we cannot stack laden containers the same height on container terminals as on vessels."

The big question of high stacking, however, is the operational practicality, he says. In this respect, high stacking is less of a problem for export containers than for import containers. "You therefore cannot compare stacking heights between transhipment and gateway (import) terminals."

High density stacking raises a number of issues. First, there is the additional strain on the yard pavement. "As terminals increase their throughput and yard densities, we are seeing additional strain from increased static loads from stacked containers (stacking higher) and the increased number of axle passes on the connecting roadways in yards that are fed by tractor trailer units or yard hustlers," says Julian Johanson-Brown, market sector director - containers for Halcrow.

"We are involved in numerous studies and due diligence commissions looking at the design and capacity of existing yard pavements around the world, including asphalt, block paving and concrete support plinths."

Clearly, the higher the stacks, the more ground pressure - but also, the increased weight of the machine or crane required places further demands on ground strength and stability.

Stability is another issue. There are empty handlers on the market that can stack up to 11-high, but high stacked empties are extremely vulnerable in high winds, points out Mr van Beemen.

Robin McLeod adds: "In reliably calm conditions, even slightly misaligned containers will remain safely stacked to any height - try it with children's bricks - but with any wind, even a double stack can be vulnerable. For example, wind can topple containers on the sheltered side of a block of containers through vortices and suction. Ports subject to typhoons, for example, have to stop work upon the intermediate warning, flatten the stacks, lash down the windward sides, secure container tops to each other. This alone must limit the basic stack height."

Kalmar's Jari Pirhonen says: "The stacking height may increase the effective wind force to the top containers and the wind may also move lightweight loaded containers, not just empties."

Then there are environmental issues, in particular visual intrusion, with at least one ongoing container port proposal having to accept a stack height limitation of 25 metres - equivalent to nine nine-foot high containers.

High stack, high risk?

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