Civil Engineering Reference
In-Depth Information
chapter to the problem of
external
costs and benefits, which affect the city,
the region, or society as a whole.
Even for the seemingly straightforward question “How much will it
cost to build?” the answer is, during early estimation, far from certain. Con-
struction costing is difficult. There is site work with excavation, backfill,
and the driving of piles; tons of concrete for the deck and arches plus steel
for reinforcement; and various kinds of labor needs, not to mention the
construction equipment, from concrete mixers to truck-trailers, cranes, and
crawler tractors. And there is the problem of labor and equipment productiv-
ity: just how much can they accomplish per hour in the river mud and in
the fog that often envelops the downtown? The answers are, furthermore,
subject to price changes after the estimate is made, and not just because of
overall inflation, but also because costs will fluctuate item by item, depend-
ing on supply and demand.
For a shortcut, we can just estimate costs of a bridge by looking at
the state's average construction costs per square foot of deck area. These are
quite reliable for short highway overpasses and can give a reasonable range
with which to start, but are less reliable for predicting the unique specifics
at a longer river crossing. Having to make an early case on whether the
bridge makes sense or not, the initial estimator has the further problem that
the engineering and environmental studies, which are still ongoing, may
conclude that the structure has to be different than first expected. They
may show that rocks meant to support the feet of the arches are less reli-
able than expected, requiring deeper drilling, hence greater cost. Or seismic
study can reveal a greater-than-expected chance of earthquakes, forcing a
redesign that makes the bridge more resilient during shaking but also makes
it more expensive.
Maintenance cost estimates offer a particular opportunity to make the
bridge seem cheaper than it is. The authorities making the decision can
easily be lulled into letting that happen. They are under pressure to solve
downtown traffic problems at the river's edge, but also under pressure to
reduce city and state borrowing. They want to borrow just enough to solve
the problem. When built, the bridge will be new; deferred maintenance
won't be noticed for years. Under pressure for results now, the authori-
ties may not want to be bothered with the price a decade from now of
repaving the deck that weathered too quickly or fixing a pier that was not
built strongly enough to withstand minor boat collisions. By constructing
the bridge with less durable materials now, it is possible to shift costs to
those who will later be responsible for maintenance. To prevent such tricks,
infrastructure decision makers must remember that the correct desideratum
is the
life-cycle cost,
which includes the original project cost plus all costs
to accrue over the years of service.
