One More Bridge To Cross--But Quick

Bv R. T. Titus Director oI Trade Extension, West Coast Lumbermen's Association

Ponton bridges have been used to cross streams for thousands of years. There are instances of their use in the writings of Homer, around 9OO B.C. The Persian kings, Cyrus, Darius and Xerxes, used them. The Persian Army Engineers under 9yto. built the first ponton bridge of actual record in 537 .B.C. Xerxes made l.ris famous crossing of the Hellespont some 50 years later, using one row of 36O boats, and a parallel line of 314. The flooring was "made of tree trunks sawn into planks and cut the width of the bridge."

Compared with other types of bridges, say the storied bridge of San Luis Rey with its knotted fibrous cables swaying high in the air, the ponton bridge is about as basically complicated as tossing a log in a stream. Out of these two early designs alone have evolved the most modern bridges in the world. One haq become a great suspension span over the Golden Gate, the other has become the tool of hard-hitting armies in the field.

With militarv use as old as the ponton bridge itself, each war has seen its higher development. Today is a far cry from Xerxes. It is even somewhat removed from the time when the Union Army was stopped at the Rappahannock, had to wait 20 days while a ponton bridge was secured and assembled-a delay that cost them a resounding defeat in the Battle of Fredericksburg. fn this war, ponton bridges are on the spot and ready when needed. Blasted out bridges are sometimes crossed by superimposing prefabricated structures on the wreckage, whereas pontons come in handy for cross-country work, and fighting in primitive areas and islands.

The pontons are lined up, frequently kicked into place with little outboard motors, anchored, and the decking 'quickly laid down, and the crossing effected. as heavy mobile guns, troops, and supplies roll forward. The same bridge can cross as many streams as necessary. The Allies don't burn their briiges, they take 'em with them.

To match the exacting demands of this up-to-date military precision, the behind the lines story reaches back to the lumber mills and to the forest itself. Douglas Fir ponton lumber has a specification and science of its own.

With a deviation of straightness of grain that cannot

World Wcr II Ponton Bridge exceed one in 15" in slope, the first job is to find areas where Douglas Firs coul'd yield a high percentage of suitable logs. There is no precise rule to locate such an area. The lumber companies didit by noting where their straight-grained logs came from. Only those who found a good .supply of them undertook to produce ponton lumber. In some cases, special devices at the sawmill's headrig were used to check for straightness of grain.

For texture, a simple test may be given the doubtful pieces to save needless further handling and processing. By sticking a small knife blade in flatways, at right angle to the grain, and prying upward, the direction of the fibers could be determined.

After the mill has discovered a green candidate tor a ponton balk (stringer) or chess (plank,) the next concern is to reduce its water content to l9/o or less without developing more than four moderate surface checks "not more than 12" deep, 12" long, an'd well distributed." Specifications like this, on top of hungry war demands, made air drying too risky and too slow. Kiln drying at proper humidity is also slow and delicate. The manufacturers turned to chemical seasoning. In a nutshell, the purpose of chemical seasoning is to keep the surface from checking rvhile the interior is dried by established methods. When rvood dries, it shrinks. It follows that if the surface of a timber gets too small to go around its green interior, severe strains are set up that result in twisting, checking and downright splits. By use of certain chemicals uniformity of drying is secured through retaining some moisture in the exterior, while the interior is drying out. While the chemical does not in itself speed the drying process, it makes possible the operation of kilns on a faster drying schedule than would be possible on untreated lumber.

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