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Archive for the ‘Bridges’ Category

A Bridge Design Problem

Fort de Roovere, 1751. From Wikipedia commons

Let’s consider a design problem.  You have an earthen fort that is part of a defensive line that goes back to the 17th century.  It has fallen into ruins over the years, but it’s historically significant and is in the process of being restored.  The owners want to be able to access it from the front so they can incorporate it into their system of parks and greenways, but there’s no way over the moat.  The obvious solution is to build a bridge, but it’s going to spoil the history of the site.  What do you do?

The site is Fort de Roovere in the Netherlands.  When the question was posed to RO&AD Architects, they came up with a solution that impressed the hell out of me, the jaded bridge engineer.  It’s been called a ‘trench bridge’ and a lot of sites on the internet have taken to referring to it as the ‘Moses Bridge’.  I have to say, this is some impressive outside the box thinking.

The bridge is that thin yellow line just where the moat turns to the right. (Photo copyright RO&AD Architects)

The bridge is built out of wood, and waterproofed with foil.  A stair case was cut into either side of the moat and a bridge was built below water level.  The bridge has railings on either side that keep it, and any pedestrians, dry.  The top of the railing is at water level so it isn’t visible.  From any significant distance only the heads and upper bodies of pedestrians using the bridge are visible.  The foil is meant to waterproof the bridge, and the wood used is specially treated to keep out fungal decay.

Photo by Flickr user.

I would love to see the design plans for this thing.  The wood of the railings would have to be pretty impressive on order to hold back all that hydrostatic pressure, and there’s no visible cross-bracing so each side stands alone.  I also have to assume they have some way of controlling the water level.  In the pictures there is very little distance between the water level and the top of the bridge rails so it seems like any decent rain could raise the water level enough for it to pour over the top and flood the bridge.  I assume the moat has some way of controlling the water level to keep that from happening.  Let’s just hope it’s not stagnant enough to develop a hungry population of mosquitoes.

Apparently the design was inspired by a respect for the original builders of the fort.  The design team felt it would be disrespectful to put in a high level bridge that would blatantly violate the original intent of the fort.  I think they did a great job conceptually, but the attempt to respect the original fort backfired.  The innovative nature of the bridge steals all the attention away from the fort itself.  If you do an internet search for Fort de Roovere 95% of all the hits you get are about the bridge.

The pictures below are all copyright RO&AD Architects, and I got them from the Daily Mail.

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Over the weekend the local news stations were carrying a story about a bridge failure in Kentucky.  In light of that story, it seems like a good time to share some research I’ve been doing into bridge failure statistics.

It’s tough to find any statistics on bridge failures.  Surprisingly, there’s no national database on bridge failures.  The best I was able to find is an database that New York State DOT keeps on bridge failures.  It basically includes entries for whatever they have heard about either in the media or what was volunteered by other state DOTs.  It’s nowhere near comprehensive on the national level, but presumably it would be pretty accurate for New York since they would be well informed about their own state.

Before I share the statistics, there are a couple of things to keep in mind:

  1. This includes all bridges, not just the big ones that end up in the national media.  Most bridges are small structures  and not of interest to anyone outside the immediate area they are located in.
  2. Failure doesn’t necessarily mean going down in a spectacular cloud of dust.  A structure failure is anything that keeps it from being used in the way it was intended.  And now for the statistics…….

The biggest number of failures by far are caused by bridge hydraulics.  This means anything related to water and includes things like bridge scour, being clogged by ice or debris, approach road wash-outs, and just being pushed over by water.  Hydraulics is my specialty area and I’ll cover some of those in more depth in later posts.  The second greatest is collision.  I’m not entirely clear about this label, but I assume it means collisions by boats as well as collisions by trucks.  The overload and earthquake labels seem self-explanatory.  I assume fire refers to wooden bridges, but I do know of at least once instance  where a tanker truck blew up under an interstate bridge and melted the beams so badly the entire structure had to be replaced.  (It does occasionally happen outside of Michael Bey movies.)

I’m going to do a more in-depth analysis of the statistics in my next post so you folks who got your fill here can skip it.

 

Source: Wikipedia

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