How long of a clearspan could a movable bridge have with only late-medieval materials and craftsmanship?

Question

Let us say we have a large, late medieval era trade city that is also a port, sited on an estuary. Due to the city's narrow, winding streets, caravans are not allowed within the walls: cargo must be portaged through the city on foot or the backs of small pack animals such as donkeys. Furthermore, not only are there roads and seagoing ships going in and out, there are tradeships that ply further upriver, carrying loads far in excess of what even a caravan can.

However, the volume of road trade is overwhelming the portage system and leading to caravans camping for days, even weeks, just to get from one side of town to the other. Worse yet, there are no bridges within town, just ferryboats, as the estuary itself cannot be bridged without the invention of caissons.

This near-gridlock has led city leaders to consider the seemingly unthinkable -- bridging the river upstream as part of a bypass road project, as it is much shallower and somewhat narrower there. However, there is a complication: this bridge is being built without the benefit of high banks to elevate it, yet must clear the masts of tradeships 20m high while providing at least 30m of clear channel width and preferably as much as is feasible with the materials available.

How could our medieval bridgebuilders build a movable bridge that can handle this?

Answer 1

I'm not sure they would actually need to build such a substantial bridge. Upstream traffic is likely to be towed barges, or similar; these can pass under a fairly low bridge. Large, deep-draught tall-masted ships are much less use in a confined river! Thus, the city effectively becomes a transhipment point for ocean-going ships to riverine traffic. They'd build a low bridge and have a lot of lighters shuttling around.

But, okay, a bridge is impressive and you want one :-)

Basically, two options - build up or build movable. Building up is not architecturally impossible. The Pont du Nimes, built by the Romans, has a lower tier of 22m arches across a river - with a lot of work to sink foundations into the estuary bed, a span rising 25-30m above water level would seem achievable. You would not need an incredibly wide central span, if you accepted that ships would be slowly taken through individually. This approach normally relies on steep banks - but if you have a lot of labour to play with, and you probably do, then build mounds at each side and run the bridge between them. (Bonus points for style - these are basically castle mottes. So turn them into part of the city walls.)

Movable bridges are more complex, but consider again the point that you only need a narrow ship channel. You could thus build a sturdy low bridge with a single gap of about 10m, and use a simple drawbridge.

Answer 2

Build a floating bridge, basically a series of double anchored rafts (or in this case small ships) with conventional bridges between them, some of which are drawbridges just wide enough to let shipping through. It would be an expensive structure to build, operate and maintain but on the other hand you can charge a toll to both the caravans crossing the bridge and the ships passing through it, so it might be quite profitable.

Answer 3

If the Romans could manage their aqueducts and some truly impressive bridges in the their day…and London seems to have had similar problem to your scenario coming up with several solutions over several centuries.

I don't think your civilization should have too much engineering difficulty with their bridge, apart from the near epic scale.

https://en.wikipedia.org/wiki/London_Bridge#.22Old.22_.28medieval.29_London_Bridge