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GRF cable tray for container port gantry cranes

News International-French

17 Jun 2014

The cable tray is constructed from infused, then cut-to-size glass fibre reinforced plastic (GRP) panels moulded using Scott Bader Crystic products.

The Transnet National Port, in Cape Town, South Africa is a busy port, with a rapidly growing volume of container ships using its wharfs. The Port Authority commissioned local civil construction engineers to expand the existing quayside facilities to enable the port to handle more cargo shipping. To meet quayside safety standards, an alternative to contain the power cables and rail system for the massive gantry cranes had to be rapidly designed and installed along a stretch of over 1200m (3936 ft), as part of the port’s upgrade. A new, composite power cable tray system was specified as one of the best solutions to overcome a major safety issue.

It was constructed from infused, then cut-to-size glass fibre reinforced plastic (GRP) panels moulded using Crystic 783 LVLR isophthalic polyester infusion resin, stepped GRP ‘feet’ and levelling shims, pultruded GRP rods and Crestomer 1152PA structural adhesive. The safer, flush fitting 3m (~10ft) wide GRP cable tray system now in operation, encases the rails and power cables which run beneath the gantry cranes. The new cable tray must withstand daily exposure to seawater and strong sunlight, as well as be tough enough and strong enough to cope with containers weighing up to 36 metric tonnes (~80,000lbs) placed, and occasionally even dropped, onto the cable tray area during loading and unloading.

At an early stage in the project, the civil engineers advised the Port Authority of the need to improve personnel safety, since the originally installed rail mounted gantry crane system presented a serious potential trip hazard along the quayside. The Port Authority quickly agreed that this hazard had to be eliminated. A rigorous evaluation and testing program was subsequently conducted by the civil engineers to evaluate all possible alternative cable tray designs and material options, which had to be retrofitted to the existing gantry crane rail system. The GRP panel system design finally approved was developed by Aerontec, a well-established local supplier and distributor of composite materials based in Cape Town, with extensive technical expertise in this type of construction project.

Aerontec’s GRP power cable tray panel design was selected in preference to concrete or other construction materials as the composite system was the lightest, most cost effective and the fastest system to install, yet it still provided all the physical properties needed in such a demanding outdoor environment; any additions to the quayside had to be within the structural engineering weight constraint. This was achieved by the GRP composite panel cable tray option, calculated to have a combined weight of only 226 metric tonnes (~498,000 lbs), which was a quarter of the weight of the initially proposed steel reinforced concrete option and so well within the maximum weight constraint limit.

The innovative GRP cable tray design combines 32mm (1.5 inches) thick glass reinforced rectangular panel sections, which were initially moulded as 3 metre x 6 metre GRP ‘master slabs’ using a full resin transfer moulding (RTM) process. The next step was to apply a UV-stabilized iso-NPG (neopentyl glycol) topcoat plus a special ‘non slip’ coating to the upper surface of the GRP slabs. The laminate specification for each GRP slab was a combination of an isophthalic polyester resin, such as Crystic 783 LVLR, reinforced with 1080 g/sq.m fibreglass woven rovings; to assist resin flow during infusion an adhesive coated open weave mesh was also used.

After post curing, the GRP master slabs were then water-jet cut into the different panel sizes needed for this new, custom built cable tray system and drainage holes were drilled through each of the GRP panel sections. The infusion, coating, water jet cutting, drilling and assembly of the GRP panel sections was carried out by MMS Technology, located in Centurion, near Pretoria; a total of 1161 individual cut-to-size and numbered (for easy installation) GRP panels were supplied for this project. As well as needing to provide a 300 mm (~12 inch) channel between the panel sections for the gantry crane rollers to pass between on the front rail, a number of obstacles on the quayside, such as the large cast iron mooring bollards, maintenance access covers and fire hydrants, also had to be accommodated at regular intervals, requiring a variety of different panel sections at various points along the quay.

A critical aspect of this novel GRP panel design was its ability to overcome the trip hazard issue. The design challenge was how to cope with the differences in the height of the original quayside, which varied by up to 80mm (3.15 inches) either way along the entire 1200m length of the quay where the gantry crane rails ran. To overcome this height variation and create a safe ‘trip free’ flush fitted cable tray surface virtually level with the crane rails, during installation each panel had to be raised to the exact height needed using GRP shims. The shims were first placed and then bonded into place under the stepped solid GRP ‘feet’ already attached to the panel sections. As well as the panel sections, off site, MMS Technology also fabricated and attached the GRP feet. Each stepped foot was cored in the centre and then a vertical pultruded GRP rod was inserted and bonded to the underside of the cut-to-size GRP panels.

Aerontec specified Crystic Crestomer 1152PA throughout this project due to its toughness, high strength and its proven use in other construction and marine applications to bond GRP together; the Crestomer range was specifically developed by Scott Bader for structurally bonding GRP substrates, where the bond strength obtained from the urethane acrylate chemistry used for Crestomers provides a bond line far stronger than the GRP substrates. On site, Crestomer 1152PA was used to bond the GRP shims into place in order to make the finished cable tray sections solid and level. It was also used to bond stainless steel bolts into the concrete jetty deck. These steel bolts were used to secure each panel in place and to prevent them from moving.

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