Wooden Support Posts

Wooden support posts are available in shaped sizes, as engineered products, and as timber posts. The shaped sizes are described by their nominal dimensions, such as 4 in X 4 in (100 mm X 100 mm). This is their size prior to the surfacing required to provide smooth and straight posts. Their actual size is typically less than the nominal size. A 4-in X 4-in (100-mm X 100-mm) post will therefore have an actual size of 3.5 in X 3.5 in (90 mm X 90 mm). The engineered products are made from laminated or pressure-glued wood and nonwood recycled products. Timber posts are round in shape.

All wooden posts are of breakaway design, with the intended fracture of the post near the base and less than 4 in (100 mm) above the ground. The post features that influence fracture include the size of the post, effective cross-sectional area, embedment depth, type of soil, and the species of wood. The majority of wood post tests have been conducted using grade 2 southern yellow pine posts.

Shaped Wood Posts. The most common size wood post used for single sign installations is the 3.5-in X 3.5-in (90-mm X 90-mm) support. This support should be buried directly in strong soil to a depth of at least 36 in (920 mm). The cross-sectional area of this post is sufficiently small that drilled holes are not needed to provide a weakened section.

A 3.5-in X 6-in (90-mm X 140-mm) post installed in strong soil will provide acceptable performance upon impact without reducing the cross-section. Tests have shown, however, that the 3.5-in X 6-in (90-mm X 140-mm), when installed in loose or

sandy soil, is unacceptable when impacted by a small vehicle. Single wood posts of that size installed in weak soil should be modified with 1.5-in-diameter (40-mm) holes to be crashworthy. The holes should be centered at 4 and 18 in (100 and 460 mm) above the ground line and perpendicular to the roadway centerline.

Table 7.3 provides the modifications required for acceptable performance of various shaped wood post sizes. The holes in each case are drilled at 4 and 18 in (100 and 460 mm) above the ground line and perpendicular to the roadway centerline.

Typical details for installation of shaped wood posts by direct burial and concrete methods are presented in Fig. 7.12. The 6-in X 6-in (140-mm X 140-mm) post should be set in unreinforced concrete to help ensure that the post fractures upon impact. To make it easier to remove a broken stub, a post can be wrapped with 0.5-in-thick (13-mm) Styrofoam prior to filling with concrete.

90 50 0ІА HOLES

2750

CROSS SECTION 90×90 OR 90X140

STYROFOAM

910

Some states have used larger shaped wooden posts, such as 6 in X 8 in (140 mm X 215 mm), with appropriately sized holes to reduce the cross-section area. These holes provide a weak section that appears acceptable, but the increased mass of these posts and lack of testing result in unpredictable impact performance. Shaped wooden posts larger than those that have been crash-tested should not be used. If larger posts are required, then a multiple-post configuration, slip base design, or other alternatives should be used.

Engineered Wood Posts. A number of relatively new products have been developed for use as sign supports. These include engineered wood product posts made from recycled plastics and wood chips, and laminated veneer lumber posts. The Microllam laminated posts in 8 in X 8 in (200 mm X 200 mm) and in 15 in X 8 in (380 mm X 200 mm) have been accepted for use. These posts, manufactured by the Trus Joist MacMillan Corporation, have a wall thickness of 1.25 in (32 mm) and mitered 45° corners. The post is placed in predrilled holes and backfilled. The posts require four 1-in-diameter (25-mm) holes drilled on the two sides parallel to the direction of travel. Two of the holes are at 3 in (76 mm), and the other two holes are at 18 in (457 mm) above ground height. A saw cut parallel to the ground that connects each set of holes is required.

Timber Poles. The majority of wooden sign-support systems consist of square or rectangular shapes. However, round timber poles, up to 7.5 in (190 mm) in diameter of southern pine, grade 2, have been accepted for use by the FHWA [25, 26]. The acceptable sizes and required holes to provide acceptable breakaway performance are presented in Table 7.4.