Sunday, August 18, 2013

How Can a Flat Bridge Be "Extradosed"?

The bridge over the Quinnipiac River may be more expensive  and less efficient than necessary.

This is certainly not my opinion because I am not an engineer or bridge designer.  This is the stated stand of the Structural Engineering Forum of India.

Before we get into that, what exactly is an "extradosed" bridge? Just because the state Department of Transportation calls the structure that in press releases, does not excuse publications that print the press releases from explaining.

To summarize, extradose is the outside measure of an arch. The inside curve is called, what else, the "intradose." Architecture is filled with esoteric names for things. Know what "ogee" means?

Architecturally, a Gothic arch with a peak is made out of two ogees. (right)

 "Ogive" describes the  shape of the nose of a rocket or a bullet. (below)


 There are dozens of words for different curves. The Sears-Hack Body, for instance, is a an aerodynamic shape that produces the least drag.

If you're sitting around with nothing to do, search "names of curves" on Google. 

Now, the bridge over the Quinnipiac does not seem to have much extradose, or extrados.

Extradose and iontradose
 In fact, it is a cable-stayed extradosed design. This looks superficially like a suspension bridge, but is fundamentally and physically different.

Suspension bridges use much larger towers to carry a thick "caternary" of cable. Smaller cables connected to the large one are connected to the bridge deck, holding it up. 

A cable-stayed bridge uses cables from shorter towers to connect directly to the bridge deck.  The cables also perform different work. Since they meet the deck at a low angle, they tend to add to the longitudinal strength of the bridge. 

Cable-stayed extradosed bridges have a box-girder bridge deck, but are thinner than  plain box-girder construction as typified by the old  Q bridge that is now being torn down.

The idea of cable-staying a bridge dates to the 19th century. Now cable-stayed exterdosed bridges are used because they require shorter towers, have less of a footprint than box-girder bridges, and they look kind of cool.

Lots of cable-stayed extradosed bridges have been built in Europe and Asia. Which brings us back to the Structural Engineering Forum of India. India has many of its own cable-stayed extradosed bridges.

Writing in October of 2012, Dr. Narayanan said:

Extradosed bridges are relatively expensive and material inefficient. Almost any span that could be bridged by an extradosed bridge could be spanned more inexpensively with a continuous girder, or more efficiently (but at even greater cost) with a cable-stayed. In most cases the spans are short enough that the use of cables at all is an aesthetic rather than engineering-necessitated choice. This does not imply that is a "bad" choice, since in some cases the difference in cost and efficiency is small, and the extradosed type is a very elegant form. 

This is one person's views, although Subramanian is the author of several books on the subjects of concrete, bridge design, and related topics. He probably knows as much as the people who designed and are carrying out the Quinnipiac project at glacial speed. 

So, there you have it.  Seems like whoever runs bridge construction in Connecticut could have saved money and perhaps decades, by building a new,  bigger  box-girder bridge than by selecting a nonsensically named "extradosed" design. 

Just food for thought as you negotiate the ever-changing lanes and soaring ramps of the Pearl Harbor Memorial Bridge.

 Let's hope it is finished while veterans of Pearl Harbor are still alive. 

Extrados(e) and intrados(e) turn up all over the place, including the design of airplane wings. 

1 comment:

  1. is there any Intradosed bridge as well ????
    what does it mean by extradosed bridge ?
    Does it mean that a Bridge deck having convex curve between Pylons is called an extradosed Bridge?