Digging around online the other day I came across an Auckland Council Archives webpage with a series of old plans and charts on early Auckland harbour bridge and tunnel concepts. I do recommend having a poke around for fans of this sort of thing.

The one plan that really caught my eye was the one below, a 1930s concept for a suspension bridge from Devonport to Parnell.  Click this link for the full size version. 

ACC 015 - 7853-004, proposed bridge layout 1930s

Look at that: tall towers with graceful parabolas of cable holding up a long spanning slender deck. Boy I do love a good suspension bridge! And check the engineers stamp in the bottom right corner. None other than J.J. Bradfield himself, designer of both the Sydney Harbour Bridge and the connected City Circle underground rail system.

With 175 feet (54m) clearance above the water this would have had more room under it than our existing bridge, however it would have ended up on the wrong side of the modern container port. If they had built this perhaps the port would have been forced to move out by now.

The four lanes and two tram tracks sounds like just what we need today, if you could squeeze in a footpath and cycleway. Having said that, you can presume the trams would have been ripped out and the bridge converted to a six lanes all for traffic like so many bridges from that era.

I also wonder what would have become of the motorway system without the harbour bridge to Northcote Point, presumably spaghetti junction would have ended up further east and Parnell would be sliced in half by a motorway. Would we lament the swathe carved through Devonport and Bayswater the way we lament the long gone villas of Newton gully and the lost beaches at St Marys Bay, or would we simply forget like we have the City of Cork Beach? Perhaps the Eastern Motorway would have been built, perhaps instead of the Southern?

In any case, the shape of our city would have been considerably different. A reminder that new transport infrastructure shapes our future city as much as it responds to our existing one.

…and doesn’t it make our actual bridge look like an ugly bugger in comparison?

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  1. What a fabulous design. I love a good suspension bridge as well (and a good cable-stayed bridge). I often think that bridges in New Zealand are designed to be as “unobtrusive” as possible, with the end result that they contribute nothing to their surrounds. If we built a little bigger, and with a little more vision our bridges could stand on their own as features of our landscape – as they often do in other places (with spectacular results).

    1. I think in reality Kerr St to the west would have been severed and the bridge connected to the intersection with Church St and Vauxhall Rd. So turn left to suburban Devonport, right to the town centre, and straight ahead on the ‘highway’ to Takapuna.

      At the time Lake Rd didn’t exist and Vauxhall Rd/Old Lake Rd was the main road up to Takapuna and beyond, so pretty sure they were aiming at linking to that primarily.

  2. and doesn’t it make our actual bridge look like an ugly bugger in comparison?

    Not really, I like the coat-hanger too.

  3. A Golden Gate style suspension bridge (albeit smaller) would have been an Auckland icon. The trend these days seems to be a cable-stayed construction rather than a suspension type with Calatrava being the master of the art.

    The present harbour bridge is not pretty but the additions are remarkable in that they were feasible at all.

    Pedant alert: the suspension bridge curve is a catenary rather than a parabola; the curve is described by a hyperbolic cosine function.

    1. Actually this one would have been about 300m longer than the Golden Gate from anchorage to anchorage, although the main span would have been shorter.
      I’m not as fond of cable stay bridges, the straight lines aren’t as graceful as a suspension bridge and they tend to have shorter main spans and more viaducts on the approaches. Also I find Calatrava has overdone it so much that any new bridge immediately looks like a pastiche of the style.

      Super pedant alert! Actually its not a catenary, a cable suspended between two points will form a catenary under its own weight, but once you add suspender cables and a bridge deck of significant mass acting on the cable the curve is parabolic. Er, apparently.

      1. Thought experiment:
        Since each vertical load on the cable is uniform and the distribution of these vertical loads is also uniform (since the pitch is uniform) the vertical loads on the cable can be considered analogous to an additional mass per unit length ergo still a catenary (at least in the limit as the suspender pitch approaches zero). So, from a gross standpoint still a catenary but up close a series of catenary curves with discontinuities at the points where the suspenders are attached.

        Interesting fact: The shape of early structural arches was determined by using a suspended chain as it was realised that the shape of the curve in a chain resulting from interplay of tensile forces was the same as the ideal shape (ie only compression forces) of an arch that would resist uniform vertical loadings above it…at least I found it interesting.

        Puente del Alamillo in Seville impressed me when I saw it in 92.

        1. The load from the deck is uniform in the horizontal dimension, not per unit length of cable. If the cable was weightless it would be parabolic in shape. The actual shape will lie in between a parabola and a catenary.

        2. Quite right, Matthew…the load will be greater at the ends per unit length but the hypothetical weightless cable will be a series of straight lines that approximate a curve…but I am going to play the railway card and point out that the overhead power wires are known as the catenary, not the parabolary!

        3. Well you guys seem to know what you are talking about, I don’t and was only quoting a fairly dodgy looking website I found: http://www.carondelet.pvt.k12.ca.us/Family/Math/03210/page5.htm.

          But if I am reading this right…. MFD would be correct in the case of a self-supporting suspension bridge (which do exist but are less common) where the weight of the deck and hangers are the only force acting on the main cables. However in the case of a conventional suspension bridge where the main cable is also acted on by tensile force from the cable anchorages at either end, the cable is parabolic?

          This is probably the geekiest discussion thread on the blog in a long time (and that is saying something!). 😉

        4. Matthew is right. In short spans most of the weight is in the deck which is uniform across the span and the cable less significant so you get close to a parabola. As the span increases the cable weight becomes more significant. A pre channel tunnel feasibility of a super suspension bridge across the English Channel showed massive cables were required to support 4.5km spans which would have been a catenary. Goodness only knows how the authors of that study thought they could keep it still in the wind.

        5. “so you get close to a parabola”

          I have been mulling this one over and have concluded that the fact that once the droppers are in tension the main cable cannot be described by a single mathematical function since the nodes where the droppers are connected are discontinuities. The shape can therefore be considered a series of catenaries punctuated by the points of attachment of the droppers.

  4. A fun exercise that may have been undertaken by two engineers whose office windows overlooked the construction of a new suspension bridge:
    They traced a cartesian coordinate system on the window and watched as the shape the cable made changed from a catenery y=cosh(x) to a parabola y=x^2 as the load of the bridge structure was added to the cable.

    1. I notice that the project secretary is Moses…quite fitting since they are parting the sea for a passage through the seafloor! 🙂

  5. The parabolic suspension form bears a striking resemblance to Aucklands iconic volcano Rangitoto.. What an icon had it been built! Can’t believe the designers or the current harbour bridge missed this opportunity.. Can we get a new bridge that does this.. Perhaps even more gracefully?

    1. That’s interesting, I wonder if they could be described with the same function? In both cases it’s basically gravity acting on mass…

      I’ve been wondering about a modern application of a suspension bridge In Auckland, stay tuned for a concept.

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