THE obvious solution to the predicament of sinking bridges in our state seems to have been missed amidst all the confusion.
One has only to look as far as the Bremerton Floating Pier to see that a concrete floating bridge filled with styrofoam would be unsinkable and that it could be built for half the cost of the structure lost in the Thanksgiving weekend storm.
Floating concrete structures are an economical solution to bridging Lake Washington - if, as recent experience has shown, these structures can be ``idiot-proofed.''
The probable cause of failure in both the Hood Canal Floating Bridge in 1979 and the Interstate 90 floating bridge recently appears to be open hatches, through which water entered the pontoon compartments, causing the pontoons to lose buoyancy.
Just as a prudent mariner would not take a sailboat out with a hole in its side, a floating bridge or any other floating vessel should not be allowed to lie afloat with gaping holes at the waterline.
Ironically, hatches on floating bridges are intended as safety features. They are supposed to ensure that pontoon compartments remain watertight, by providing access for inspection and maintenance work. Yet these hatches have caused floating concrete bridges to become vulnerable to sinking.
These hatches can be eliminated if positive buoyancy is assured under all damage conditions. This design objective is achieved by filling the pontoon compartments with expanded polystyrene, commonly known as styrofoam.
Styrofoam inhibits water from entering the pontoon compartments, ensuring that the pontoons remain buoyant even under severe damage conditions.
Concrete is the best construction material for floating bridges for two reasons.
-- First, concrete is inherently resistant to corrosion, and it protects the steel reinforcement embedded in it from rusting. A steel structure would require regular maintenance of the paint or protective coating.
The choice of construction materials, however, has little bearing on safety, since a steel vessel with hatches at the waterline can sink just as easily as a concrete one.
-- Secondly, concrete results in a massive structure desirable for damping wave motion, so commuters don't get seasick crossing the bridge in stormy weather. A steel structure, because it would be much lighter, would require enormous amounts of concrete as permanent ballast to keep it from bobbing around in a storm.
Floating structures are tuned just as a stringed instrument is. By adjusting the weight of the structure and the tension in the mooring lines, the desired dynamic response can be achieved.
The goal is to tighten the mooring lines just enough to limit structure motions to imperceptible levels, yet letting the structure remain as flexible as possible so that wave energy can either bounce off or pass through it, rather than taking the foolhardy approach of attempting to resist the effects of wave motion by brute force.
Or, in plain English, the more we let the floating bridge move, the lesser the force required to keep it in place.
It is not just a simple matter of the wind and waves pushing the bridge in one direction and the mooring lines resisting that force in the other direction. The question is, did the flotilla of tugboats in Lake Washington help or exacerbate the situation? The answer can probably be found with a bit of heavy-duty number-crunching.
A gut reaction: Let us thank the ``Northwest Wind Goddess'' for sparing the new I-90 bridge.
A ``concrete-encased styrofoam'' floating bridge would probably cost about half as much as the state Department of Transportation design, because the additional cost of styrofoam is more than offset by reduction in the cost of forming concrete, by eliminating hatches and other hardware, and by reduction in reinforcing-steel requirements.
Similar floating concrete structures range in cost from $80 to $200 per square foot of deck surface. Based upon a unit cost of $150 a square foot, the cost of replacing the floating section of the old I-90 bridge in its entirety, with 75-foot-wide pontoons totaling 5,800 feet in length, would be around $65 million, much less than the $100 million estimated by DOT.
Several precast-concrete manufacturers in this area can fabricate such pontoons. A number of these companies have built concrete-encased styrofoam pontoons for floating breakwaters and ferry terminals.
The point is, existing technology can save the taxpayer a bundle. After all, the Puget Sound area is the floating-concrete capital of the world - even though we may have egg on our face at this time.
Arun Bhalaik of Seattle is a consulting engineer specializing in marine structures.