800 tonnes of steel

Big, blue and capable of managing up to six ‘erections’ in one night, it’s small wonder that the ‘Big Blue’ overhead launching gantry – the show-stopping construction equipment used on the NZTA’s Newmarket Connection Viaduct Replacement Project – has become such a conversation piece around Newmarket.

Since being commissioned for use in February, Big Blue has rapidly propelled itself southward, helping to build the new southbound half of the replacement Newmarket Viaduct, segment by segment, alongside the existing motorway bridge. Importantly, it has allowed the project delivery team (NGA Newmarket) to keep all six lanes of this vital link in the state highway network safely open, despite the shoulder-rubbing proximity between old and new.

However, while Newmarket marvels at the speed with which its new bridge is being built, most Aucklanders will have only seen Big Blue’s imposing 800 tonne, 140 metre long frame from the motorway, not knowing really what it is or how it works. So with the help of a few images taken whilst it was busy above Broadway earlier this year, here’s a quick 101 …

The gantry builds out either side of each bridge column in a balanced cantilever sequence, generally by winching into place the 68–80 tonne bridge segments from below. Before the gantry can be moved into position to commence this sequence, a platform for its front legs has to be established, therefore the segments directly above the bridge columns (known as the Pier Head) and the segments either side (P+1 and P–1) are installed in advance by conventional crane.

Once these are in place, and supported by the steel bracing system known as out-of-balance props, the gantry can launch itself forward, setting down its front legs to begin building out the span above the column equally on either side.

Once it has completed a full span (up to 10 segments either side of the column), the gantry launches itself forward to the next column to repeat the sequence. The next span then creates a virtually complete columnto- column link on one side, interrupted only by a 300 mm gap that is then closed with an in-situ concrete pour, while the gantry continues its journey along the alignment to the next column.

The bridge segments are individually designed for their exact position in the chain, match-cast against their neighbouring segment at the project’s East Tamaki pre-cast yard. When needed, they are then transported to the site.

Use of the gantry allows the project team to erect the segments without closing any lanes on the adjoining motorway, meaning the operation can be carried out day or night. Only when building above – and therefore requiring closures to – major arterial routes, such as Newmarket’s busy Broadway thoroughfare, do the team have to carry this work out at night. But when they do, it can look truly spectacular.

Video footage of the gantry in operation is available on the project’s website at www.nzta. govt.nz/newmarketconnection