Sign and Light Standard Foundation Design when Installed on Slopes (2010 WA/13) (405160-22)
Problem Statement
Many steel sign supports and light standards require a concrete foundation. This foundation is usually 2 feet to 3 feet diameter shaft. When these foundations are located on a slope, the down slope edge of the foundations typically extends more than four inches above grade. These foundations are generally used to support a triangular slip base like the one shown in Figure 1. One option is to install the foundation below grade such that the pipe support with slip base connection does not extend more that 4 inches above grade like that shown in Figure 2.
For this project, a new foundation will be designed and detailed along with a structural support with a slip base attachment for smaller signs that incorporate the use of a multi-directional slip base. In addition, a new foundation will be designed and detailed along with a structural support attachment to be used for larger signs. This design will incorporate the use of an omni-directional slip base. For both designs, the structural supports with slip base attachments will extend approximately 4 inches maximum from the down slope grading edge.
Background
Sign supports placed on roadside slopes must not allow impacting vehicles to snag on either the foundation or any substantial remains on the support. Surrounding terrain must be graded to permit vehicles to pass over any non-breakaway portion of the installation that remains in the ground or rigidly attached to the foundation. Figure 3 adopted from the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals (1), illustrates the method used to measure the required 100 mm (4 inches) maximum stub height.
In February 2001, Bligh and Buth (2) reported on a study using various configurations of a bolt-down slip base for small sign supports. These different configurations were tested in an attempt to achieve an anchor system that would accommodate design wind loads, be crashworthy in vehicular collision, and have a high degree of reusability after impact. This information and the full-scale testing data obtained from this study as well as other similar research project involving sign slip base supports, will be used to design the foundations with slip base anchor attachments for this project.
Objective
The objective of this study will be to perform engineering analyses and design a concrete foundation that can be constructed completely below grade similar to that shown in Figure 2 above for a multi-directional sign support slip base attachment for smaller sign supports. This new foundation design will incorporate a new steel stub post that is bolted to the foundation with a multi-directional slip base connection at the ground line for the break-away features for the sign. Foundation details as well as slip base anchoring details to the top of the foundation will be developed for this project. In addition, engineering calculations will be performed for a foundation and sign support attachment to be used for larger signs. TTI will consider the details on the WaDOT Standard Plan J-28.30-01 for this work. The sign support attachment designed for this larger sign will utilize the connection details shown on Standard Plan J-28.30-01.
Benefits
This project will provide two foundation and sign support attachments to be used for small and large sign supports. These designs can be installed on slopes without adverse vehicle interaction with the rigid concrete foundations or sign support attachments.
Products
TTI will provide structural drawings and details of the new foundations and sign support connection in AutoCAD format. A letter report summarizing the two designs and the calculations generated for this project will also be provided. In addition drawings and details for each design will be provided in the format required for inclusion in hardware standards documents of the AASHTO-ATRBA-AGC Task Force 13.
Implementation
As stated above, TTI will provide all the supporting information and written discussion for submitting a request to FHWA for acceptance of the foundation design and sign support connection for use on the National Highway System.
The research will provide information and documentation on design of the foundations and sign support connections developed for this project. Detailed engineering drawings that will facilitate development of standards sheets and specifications will be provided to each participating state.
Drawings provided for Task Force 13 documents will further support implementation of the research.
Work Plan
For this task, the researchers will design two sign support foundations and sign support attachments to the foundations. The first design will be used primarily for smaller signs and will be used to support a multi-directional slip base. The second design will be used for a larger signs. The sign support attachment designed for this larger sign will utilize the connection details shown on Standard Plan J-28.30-01. Engineering calculations and design details will be developed for both designs. AutoCAD Details will be generated for both designs developed for this project.
| TTI Research Supervisor: William F. Williams, P.E. Texas Transportation Institute Texas A&M University System TAMU 3135 College Station, Texas 77843-3135 (979) 862-2297 w-williams@tamu.eduv |
Pooled Fund Technical Representative: Dave Olson Design Policy, Standards and Research Manager Washington Department of Transportation P.O. Box 47329 Olympia, WA 98504-7329 (360)705-7952 Olsonda@wsdot.wa.gov |
____________________
(1.) AASHTO, Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals, ed. Washington, DC American Association of State Highway and Transportation Officials, 2006.
(2.) Roger P. Bligh, C. Eugene Buth, Wanda L. Menges, and Barbara G. Butler. “Evaluation of Design and Retrofit Concepts for Slipbase Sign Support Systems.” Research Report 3911-S, Texas Transportation Institute, College Station, TX, February 2001.
