BUILDING A HARPER SUSPENSION BRIDGE
These bridges were built for out of the way places at home and abroad. The commission involved knowledge of the span required, the nature of the banks and the flood height. Plans were then drawn up and the manufacture undertaken. Towers were normally delivered complete but could be sent out as components. Louis Harper includes the following instruction:
We have some detail about the process from two sources. First a remarkable series of photographs from the Narva bridge in Estonia. Second, a promotional booklet produced by Turner, Hoare and Co, Bombay. The photographs were by Arthur Hoare and very likely the text too. Click on the images on the left for magnification.
The abutments have been prepared and the towers raised and fixed, supported either by guys or scaffolding. Narva, Estonia, on the left, Baroda, India on the right.
The platform cables are attached first to the anchorages and allowed to sag, often into the stream. At this stage the cross bearers are attached to the cables, using special hook-bolts.
The main cables have been strung over the saddles, but not fully tensioned. The finials are in place. Beams have been positioned in mid span above the main cables and below the deck cables, connected to each other by a block and tackle.
To form the arch the hangers have to be connected at the middle first. The hangers have first been hooked over and fixed to the main cables at marked points that will result in a horizontal 2 feet interval when the bridge is fully tensioned. On the beam the platform cables are lowered onto the preformed hooks at the lower end of the hangers, and the hook closed.
Now is the time for tensioning the bridge at all 8 points to ensure that it has the correct camber (1 foot rise for every 50 feet span).
The timber for the bridge is sourced locally and here the longitudinal decking has been fitted to the crossbearers.
The guard mesh and skirting boards at either side has been fitted.
The bridge complete, designed to bear a load of 160lb/sq ft.
The bridge at Baroda (right) was the largest known Harper bridge – 300 feet span, 8 feet wide.