Quote:
Originally Posted by rrskylar
Why steel, there is a 50 year old bridge in Winnipeg being replaced BECAUSE it was all steel construction.
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That bridge was probably designed for a 50 year lifespan. Also there are many other factors as to why it needs replacing but I haven't researched this specific bridge.
Because concrete has no tensile strength, it only works under compression. A beam under bending has to overcome both tensile and compressive forces. In order to span the distance of the supports, you have to use steel unless you make more supports, and make concrete arches that support the load by (almost) pure compression. You can use reinforcing bar to take the tensile forces on a concrete beam, but when you span that sort of distance, it's no longer economical. The amount of concrete plus the increased labor results in an a significant increase in cost. In addition to this, concrete structures also require a certain amount of maintenance due to our climate. Concrete also has more uncertainty, as the material can vary due to how it's mixed, conditions when pouring and curring, and the accuracy of the dimension (of the forms). Steel is made and tested in a factory, concrete is in this case is not. Steel also has better cyclical loading properties than concrete ( if you jump on concrete a bunch, it'll fail sooner than if you jumped on a steel beam a whole bunch)
This bridge isn't going to be a signature bridge, therefore the architects and engineers designed it based on economics and, most likely, 100 year life span. Sure a concrete arch bridge would have been nice, but it wasn't worth it. Most people won't see much of that bridge anyway unless they are driving on it (you can't see it from other bridges or from downtown very well)
Essentially the answer to your question: It's cheaper, faster, and more reliable.
In other news, rumor is that FIAT is moving into 2nd avenue lofts beside the lobby.
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