RUBB

This month we have two questions dealing with conventional racking, the first looks at what’s involved when moving existing racking to a new site The second looks at what fixings can be used to secure racking when original fixings need to be replaced.

Moving existing Racking to a new site

Q. In our current location we have a quantity of Dexion type pallet racking which with some modification we will be looking to move to a new location. The current design is secured to a solid concrete floor with anchor fasteners.

Test drilling at the new premises revealed that the construction is approximately 50mm of asphalt on top of 150mm of MOT type 2 material over layer of rail ballast 1m thick. The floor was laid 40 years ago and a structural engineer has advised that the floor loading capability would be one tonne per square foot.

The racking installation would be runs of 4 or 5 bays lengthways located back to back with another similar run. Each bay is around 4m high, 2.7m wide with a total loading of about 8 tonnes per bay.

What would you propose as being the best solution? If the cost of providing the footings is too high then the move may become impractical. I estimate that there will around 80 individual feet to secure.

A. We will try and help as much as we can however you have a tricky one here.

The type of warehouse floor you describe was cheap to put down, tolerant to subsequent settlement in bad ground conditions and was immanently suitable for block stacking which was its likely purpose many years ago. The fact that you have a metre of railway type ballast under the slab probably means that the ground was heavily made up at the time and may well have had a previous industrial use. The possibility of different specifications under local areas of the floor cannot be ignored.

The floor loading you have been quoted may be conservative however there is no way of knowing this and obviously comes no where near your requirement of having four tonnes come through a typical racking base plate that might measure something like 80mm x 120mm. Long term punching failure of the rack base plate through the asphalt is likely to be the risk causing sinking of the rack that may not be uniform over the warehouse area. Verticality problems for racking can then result which can be dangerous. Old asphalt can sometimes be surprisingly tough however we have no idea of the original specification and trying to predict a performance is next to impossible.

If you were desperate then you could provide your racking with spreader plates under each footplate however even if you used 12 inch x 12 inch this would still only get you down to four times the recommended bearing pressure for the slab. Large foot plates can also cause problems for truck tyres which tend to run over the edges causing rapid tyre wear. Some people would provide a channel section between front and back uprights acting as a ground beam to spread the load however this also causes obstruction problems for operations.

Having done all this you also need to fix the racking to the floor and obviously cannot use traditional expanding bolt type fixings. Usually a ‘metal spike’ solution hammered through the footplate is favoured though some trial and error is required to identify the right length and diameter. Uplift resistance of such fixings is always questionable even if the horizontal shear requirements are met.

All in all it is possible to say that it can be done. We believe that where this has been done in the past there have been very few options short of digging up and relaying the floor, and it is questionable if the effort is really worth the outcome.

On the positive side you only have 80 foot plates, your rack is not very high and I am assuming that it is not very narrow which might also be a problem.

I hope the above comments help you to make a decision.

What Safety lock to use with established racking?

Q. I am looking for some advice on what I can use as a safety lock equivalent on Dexion Mk1 racking. I have tried several types of safety lock but none fit. Am I allowed to use 6mm roofing bolts? I have spoken to a variety of suppliers but cannot get a definite answer.

A. In general terms I would suggest that bolts in place of locking pins should only be contemplated if there is a very good reason for doing so and if the supplier permits such use. The locking pin is there to act as a shear pin and provides a limited amount of uplift resistance against an accidental over-lift of a pallet underneath the beam. Once that force is exceeded then the pin will shear or pop out and allow the beam to come out of the upright before forces sufficient to destroy the upright are generated. This generally allows a single beam knockout to take place rather than a full upright failure which can then sometimes be followed by a progressive collapse along the run of racking.

The use of a bolt instead of the locking pin holds the beam tightly into the face of the upright and prevents the connector from disengaging in such accidental circumstances thus turning what might have been a minor knockout into what can turn out to be a major collapse. Manufacturers do not recommend non-standard beam locks are used unless there are exceptional circumstances (the front beam of a live-storage lane for instance).

Dexion Mark 1material is an older type product  and while I am sure that you have looked after it very well and it is in very good condition perhaps it is approaching the time that some replacement policy might be considered as obtaining spares for something of this age will become progressively more difficult.

SEMA is delighted to be working with WLN on the storage Question and Answer Column which is published in WLN on a monthly basis. On the WLN website is a list of previously published columns which we hope you find useful.

For more information, please go to www.sema.org.uk

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