2684 Chapter 22 



per 8-hour shift — about equal to installation time when steel posts are used. 

 Whether sloped or flat, tops of wood posts will not be damaged by the driving 

 machine. For machine driving, posts with blunt bottoms are preferable to those 

 with slopes. Wooden post driving time is proportional to cross-sectional area — 

 the greater the cross-sectional area, the longer the driving time. Gatchell found 

 that wood posts can be driven on any site where steel posts can be driven; they 

 also can be driven on some sites where steel cannot, and where an auger cannot 

 be used. 



Damage to wood and steel posts during driving. — The effectiveness of a 

 strong-post guardrail is in part determined by below-ground damage the post 

 may have suffered during machine driving. Gatchell and Lucas ( 197 1 ) compared 

 damage to flat-bottom, 7-inch-diameter, 6-foot-long southern pine posts treated 

 with water-borne salt preservative, with that suffered by 6B 8.5 steel posts with 

 6-inch web, 4-inch flange, measuring 5 feet 9 inches long and weighing 8.5 

 pounds per lineal foot. All posts were driven in a 65-year-old one-lane gravel 

 road providing a variety of sites adverse to machine driving. Two commercial 

 post drivers were used, one with hammer weight of 950 pounds, the other with 

 800- and 1,200-pound hammers. On both machines, hammer initial free-fall 

 height to the top of a 6-foot post was 8 feet. All posts were driven to 4-foot 

 penetration. Gatchell and Lucas found that both wood and steel posts driven 

 under adverse conditions perform well, but wood resists damage better than 

 steel. Cones of compressed top-layer material beneath flat-bottomed wood posts 

 reduce the energy required for insertion and protect the bottom of the posts. The 

 presence of knots within 3 to 12 inches of the base of a wood post further 

 minimizes any damage that might occur. Results suggested that wood performs 

 better than steel when harder rocks are found in the road base; if rocks resist 

 breaking and do not have wedge-shaped edges, the wood posts displace them 

 without suffering damage. 



Comparison of installed costs of wood versus steel guardrail posts. — 

 Gatchell (1967b) found that the installed cost of machine-driven wood posts was 

 significantly less than that of driven steel, as follows: 



Purchase 

 Post description price Setting cost Total 



Dollars per post 



Driven treated wood 2.75-2.79 0.35-0.70 3. 10-3.49 



Hand-set treated wood 2.75-2.79 1 .87-2.35 4.62-5. 14 



Driven galvanized steel 5.25-5.85 .35- .52 5.60-6.37 



Strength of roundwood compared to sawn posts of three species. — Doyle 

 and Wilkinson (1969) evaluated the bending-strength of three Appalachian 

 species. Specimens of round and sawn hickory (mostly shagbark and mocker- 

 nut), oak (mostly scarlet, northern red, and black), and Virginia pine, each type 

 in three sizes, were tested under static and impact bending in green condition, 

 pressure-treated with creosote, and dried (but not treated). A total of 1,020 

 specimens were tested: 480 round and sawn posts of run-of-the-mill quality, and 

 540 matching small clear specimens cut from the posts that were static tested. 

 They found (table 22-26) that round posts had higher strength, stiffness, and 



