Solid Wood Products 2683 



as large as 12 by 12 inches and 16 or more feet in length for temporary street 

 support during underground construction in cities, e.g., during subway con- 

 struction in Washington, D.C. In aggregate, significant volumes are sold for 

 such construction uses. 



HIGHWAY POSTS 



Reid and McKeever (1978) reported that there are about 3.8 million miles of 

 streets and highways in the United States, and that since 1960 the total mileage 

 has been increasing at an average of about 20,000 miles annually. They found 

 that approximately 200 million board feet of lumber and 500 million square feet 

 of plywood (ys-inch basis) are used annually for highway construction, together 

 with about 325 million board feet of timbers, falsework, piling, fence posts, and 

 guardrail posts. Of these several categories of wood, guardrail posts appear to be 

 the product most suitable for manufacture from pine-site hardwoods. Steel posts 

 are the major competitor for this market, and they dominate it — for reasons not 

 entirely clear. 



Impact strength of wood and steel posts. — The widely used semi-rigid 

 strong-post guardrail system protects vehicle occupants by transmitting the 

 energy of a vehicle hitting guardrails through the supporting posts to the earth. 

 Infrequently run full-scale crash tests involve a 4,000-pound vehicle that is 

 crashed at 60 miles per hour into a barrier, at an approach angle of 25 degrees. 

 Gatchell and Michie (1974) evaluated the dynamic performance of wood and 

 steel guardrail posts, through use of a pendulum dynamometer. They found that 

 for strong-post guarding systems, red oak 6- by 6-inch posts and southern pine 6- 

 by 8-inch posts provide impact-strength characteristics that are superior to the 

 W6 X 8.5 steel posts commonly used. For use in weak post systems (in which 

 the post is intended to break), southern pine 6- by 6-inch posts were equal in 

 impact strength to S3 x 5.7 steel posts. They concluded that in wood-guardrail- 

 post specifications knot-associated grain distortion in the middle third of the 

 tension face should not exceed one-third the width of the tension face. Knots in 

 the outer third of the length of wood guardrail posts have no effect on post impact 

 strength. (See also table 22-26, last column, for impact strengths of round and 

 sawn posts of hickory, oak, and Virginia pine.) 



Machine driving of pressure-treated wood posts. — If pressure-treated 

 wood posts are hand set in augered holes, they require more than twice the 

 installation time of machine-driven steel posts. Gatchell (1967ab) was success- 

 ful in machine driving Osmose-treated 7-inch-diameter southern pine posts and 

 6- by 6-inch and 6- by 8-inch creosoted red oak posts in rocky and in gravelly 

 substrates. Driving rates ranged from 15 posts per hour on the rocky sites to 30 

 posts per hour in gravel. The longest average time for driving was 1 .7 minutes 

 for the 6- by 8-inch red oak posts driven on the rocky site, the shortest was 0.4 

 minutes for the 7-inch pine posts on a gravel site. Gatchell concluded that on 

 sites free of large rocks or other obstructions, guardrail contractors can expect to 

 install from 2,500 to 3,500 linear feet (12.5-foot post spacing) of guardrail line 



