A student of mine is taking an English course where she was asked to present a paper on a controversial engineering topic. I’m not sure the rift between conventional and ABC bridge construction is vast but it is a subject that could use a bit of clarification. Try to find a paper that gives the advantages and disadvantages of each system, not easy, even on the “Google”.
The Federal Highway Administration is pushing ABC as the construction technique of the future, writing things like “conventional construction methods involve on site activities that are time consuming and weather dependent.” True, but I could as easily write, ABC projects ALWAYS cost more than conventional construction.
So to be fair lets go through some of the advantages and disadvantages of each system.
1) SPEED -Winner ABC
Gotta give ABC it’s props, it is a much faster system than conventional construction. If speed is your thing and you just have to do it in a weekend, than build an accelerated bridge.
2) Cost – Winner – Conventional Bridge Construction
I don’t care what the spin is, conventional construction is always (yes always) cheaper. Now the arguments I hear for ABC bridges being faster and cheaper than conventional are a) wait until everyone is used to the system then the costs will be competitive and b) when you include users costs then ABC bridges are cheaper.
I have not experienced the phenomenon of ABC systems becoming cheaper than conventional construction. In fact I have often seen increased costs after construction to fix the problems associated with ABC. For example, more overlays through the years because of the amount of deck joints.
As to user costs, I have to say they are the most delight works of fiction. I looked at a cost breakdown between a conventional bridge and an ABC bridge where the user costs (subtracted from the first cost) makes the ABC bridge the winner. If you want the ABC bridge to win, just make the user costs a bazillion dollars. Then the ABC bridge will always win, but how do you “prove” user costs?
Also, why is it fair that I help pay a higher first cost for a bridge 300 miles from my house, shouldn’t the benefit of a new bridge be subsided a little by the neighborhood using it?
Conventional bridge – first cost $6,000,000
ABC Bridge $8,000,000 but wait minus a bazillion in user costs = A Free Bridge! Winner ABC.
But you still have to pay the $8,000,000 first cost and you will never see a dime of the bazillion dollars in user costs. Do I have to go house to house to get it?
3) Safety – Winner ABC
I guess the shortened construction period would protect workers from exposure to long periods of crazy drivers. If you are next to crazy drivers during construction….
4) Longevity – Winner Conventional Construction
One of the drawbacks of ABC construction is typically the increase in joints throughout the structure. Joints lead to problems because water gets in them and does bad things. Conventional construction typically, usually, most often, minimizes joints and locks the structure together as one big mass. This makes it extremely strong and long lasting. I would imagine ABC bridges will get there eventually but I still think a conventional constructed bridge will last 20% longer than an ABC bridge.
(When I say this, I have to point out that lateral slide bridges are really just conventional bridges pushed into place. So these types of structures will probably last as long as conventional construction)
5) Overall – Winner – it’s a tie!
Okay, I get it, both systems have advantages and disadvantages. Of course there is a time for conventional construction and of course ABC bridges are great tools to replace structures quickly. Honestly I like ABC, I grew up in a precast plant, just use it wisely. Engineers are not stupid, we know when to use what technology, just calm down on all the cheerleading.
We get it.