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Posted by D. Sherman [24.32.202.83] on Friday, January 22, 2010 at 18:29:43 :
In Reply to: Re: OT - SF Bay Bridge posted by Nick [64.13.117.14] on Friday, January 22, 2010 at 18:04:49 :
Bridges are designed with at least a 5X safety factor, which means in your design calculations you assume, for example, that steel rated for 100,000 PSI tensile strength is only good for 20,000 PSI. Also, modern bridge codes require that any single element be able to fail without the bridge falling down. This is why they don't build truss bridges any more. Most trusses will fail totally if certain key elements fail.
So, it's unlikely that a modern bridge will fall down completely. What's more likely to happen is that over the course of a few years, metal fatigue, corrosion, poor quality materials, poor weld technique, etc will cause cracks to start appearing, which will be picked up in routine inspections. Then comes the very expensive on-going inspect-and-repair work that will continue for the life of the bridge.
It seems to me that with the slowdown in the US economy, particularly in construction, and particularly in the traditional steel towns, caltrans should have been able to find a US fabricator that would build these bridge sections very competitively. One hitch might be that they wanted sections so big that they could not be shipped overland. A few years ago they brought the center section of the new Tacoma Narrows bridge through Wallace, coming from some fabricator in Ohio or Michigan, I think, and it was at the absolute limit of what could be hauled overland, somewhere between 500,000 and a million pounds as I recall. So perhaps to use a US contractor, caltrans would have had to design it with smaller sections which would mean more high-priced American labor to assemble the pieces.
Better for the State of California save as much money as possible on their construction projects so that they'll have the money available to pay unemployment and welfare benefits to all the out of work California construction workers.
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