California Department of Transportation
 

Technical Discussion

Scope

Development of a new reusable type of permanent foundation which will enable sign crews to quickly remove and replace small wood posts has been identified as an effective means to reduce exposure of maintenance crews to traffic hazards.

Test Facilities

All fabrication and testing was conducted by Caltrans personnel from either the Office of Research or Materials Engineering and Testing Services at the Transportation Laboratory located in Sacramento, California. All the equipment and fabricated test fixtures for the prototype testing were located at this facility. The knockdown tests were conducted at the Caltrans Dynamic Test Facility located at the California Highway Patrol Academy in West Sacramento, California.

Test Equipment

Knockdown Image #1 Knockdown Image #2
Knockdown Image #3 Knockdown Image #4

Click on the Thumbnail to View a Larger Image of the Knockdown Series

Full scale crash test were not required or economically feasible during this prototype design stage or for the subsequent testing of the final design. It was necessary to conduct a series of sign post knockdown tests to determine if the footing size was adequate to prevent movement and to check the performance of the selected plastic material used for wedge construction. This was accomplished by hitting the posts approximately 18 inches above grade with a heavy steel tube attached to a vehicle. An 11770kg truck was fitted with a 203.2mm x 152.4mm x 8mm thick wall structural steel tube 3.0 meters long. This tube was cantilevered 1.57 meters out on the drivers side and was able to pivot about the end of the pivot point to prevent damage to the tube or vehicle during a severe impact. On October 29, 1996 a number of different post configurations were hit using this vehicle mounted tube. A speed trap and a set of tape switches was used during the knockdown test to accurately determine the vehicle speed at impact.

Prototype Construction

The prototype footings were 24" in diameter by 48" tall and, even with the cavity, each footing still weighs approximately 1600lbs. This size was needed in order to resist the forces due to wind loading on the face of The. Sign panel as well as the various soil and climate conditions that exist throughout the state. These aspects governed not only the diameter of the footing and it's depth in the soil, but also the depth of the wood within the concrete. If the concrete footing was not tall enough (shallow embedment in the soil) or large enough in diameter, then the entire footing would not have enough soil surface area to resist the wind loading and would result in a sign/footing that is leaning. The cavity extended 42" deep for both the 4" x 4" and the 4" x 6" models. If the wood embedment depth was too shallow, the resulting moment arm induced in the wood might cause premature sign post failures. If the wood embedment depth or the concrete footing was too high, then the footing would be larger, heavier, and more expensive than necessary. Further, a 1" tube runs between the bottom of the cavity and the bottom of the footing to allow for drainage and to allow an escape path for any trapped air during installation. Two 14" rebar hoops were placed in the fresh concrete at approximately 2" and 6" from the top surface to help keep the footing from cracking during impact. 2" ferrule loop inserts were also placed in the fresh concrete during finishing as a means for hoisting the footing for transport and installation.

Wedge Characteristics

Wedge Material

The wedges material chosen for this project is manufactured by Collins & Aikman and sold under the brand name of ER3. The wedge is made from recycled carpet fibers that are formed into blocks in an extrusion process and sold for industrial flooring purposes. This product was selected due to its durability and ability to be shaped and cut using ordinary hand and power tools. Also, it does not readily absorb moisture and is environmentally benign in this application. Two different tapered profiles were used in the prototype wedge design. One had a taper of 1.5 inch over 18" of length (4.8 degrees) while the other had a taper of 2.5" over 12" of length (11.8 degrees). These profiles were selected to determine what type of profile would provide an adequate clamping force between the concrete and the sign posts while keeping the overall length of the wedges as short as possible. The taper profile also dictates how much of the wedge will protrude above the top surface of the footing if a dimensionally large sign post is used. The wedges were positioned on the downstream and shoulder sides of the post because it was believe that the force of the impact during a "hit" would compress the wedges and aid in their removal; however, later the wedge location was changed to the outside and upstream location to prevent the wedges from being bent over during an impact. Further, the wedges are not interchangeable because one has a special cutout so that it does not contact the other interior tapered wall of the concrete footing. As a means of extracting the wedge during a sign post replacement procedure, a 1" hole was drilled through the top portion of all the wedges so the sign crews could use a pry bar and a block of wood for leverage to remove the wedges.

Wedge Design