Tight fit

A tricky job placing a silo in a narrow alley was the winner from a record seven entries in the Crane Project of the Year awards.

Waikato_1.jpgThe winning project involved the removal and replacement of a collapsed 250 cubic metre silo for Fonterra at its Waitoa site. What made this otherwise straightforward job so tricky was the silo’s location – down a narrow alley.

Waikato Cranes says the removal of the old silo was straightforward, but its replacement was much heavier at 14 tonnes, which posed a great challenge.

The company considered a number of options to position the replacement silo.

Option one was to use a large crane, positioned at the northern end of the alley, and place the silo at a 46 metre radius. The problem was, however, there isn’t a mobile crane with sufficient capacity available in the country.

Waikato_2.jpgOption two was to use a large crane positioned at the southern end of the alley and place the silo at a 32 metre radius. But the problem here was there was insufficient room to set up 300 tonne capacity mobile crane in this position.

Option three was to squeeze a 200 tonne crane into the southern end of the alley and place the silo at a 32 metre radius. However, this couldn’t be done as the 200 tonne crane has insufficient capacity to lift 14 tonnes, plus rigging, at a 32 metre radius.

The solution that Waikato Cranes reached involved using two cranes. Its Demag AC200 was positioned in southern end of the alley, as it is the only crane that fits into the space, and a NK250 was positioned down the alley beside the silo foundation – also a tight fit.

Waikato_3.jpgThe team then lifted the silo from transporter with the 200 tonne crane, slewed it over the 18 metre high building into the alley and luffed it down to a 26 metre radius.

Next, the NK250 reached out and was attached to a specially designed, load sharing beam which enabled the NK250 to take 33 percent of the weight. Due to the beam length, the radius for the 200 tonner was reduced by 1.3 metres.

The process was then to luff up with the NK250 and luff down simultaneously with the AC200 while winching with hoist to keep the lifting beam level. The starting radius on the NK250 was nine metres with a safe working load (SWL) of eight tonnes. The load for the NK250 was 33.3 percent of 15 tonnes (five tonnes). The AC200 carried the other 66.6 percent (10 tonnes). Its final radius was 30.7 metres and SWL 12.5 tonnes.

Waikato_6.jpgWaikato Cranes had several other factors to consider which added to the complexity of the job:

A 300 tonne crane couldn’t fit into the southern end of the alley because of the distance between the outriggers and the tail swing.

Boom length – The silo was 16 metres tall and had to be lifted over a 18 metre high building. The rigging added an extra 10.5 metres to the under hook height.  The total under hook height needed to lift the silo over the building was 44.5 metres.

Waikato Cranes’ preference would have been to use a 42.1 metre boom length, giving a 14.1 tonne capacity at a 32 metre radius, but because of the length of the silo and height of the building, it had to use a 46.4 metre boom length giving only 11.9 tonne capacity at a 32 metre radius.

Waikato_8.jpgOutrigger placement and tail swing – The crane had to be placed with just 100mm fore and aft to enable the outriggers to be positioned, and within 50mm of the alley walls to allow for the tail swing.

The judges of the Crane Project of the Year concluded the restrictions placed on this silo lift by the tightness of the site and accessability problems made it a clear winner.

Waikato Cranes managing director Tony Gibson and his team rose to the challenges this lift posed and were delighted to receive the award for a difficult project well executed. 

Contractor Vol.33  No.7  August 2009
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