| TTI Research Supervisor: Sofokli Cakalli Associate Research Scientist Texas A&M Transportation Institute Texas A&M University System TAMU 3135, College Station, Texas, 77843-3135 (512) 998-6039 [email protected] |
Pooled Fund Technical Representative: |
For decades, state departments of transportation (DOTs) and other government transportation agencies have used breakaway frangible cast aluminum transformer bases (designated TB1- 17, TB2-17, and TB3-17) produced by Akron Foundry. These bases were accepted by the Federal Highway Administration (FHWA) for use on the national highway system based on pendulum testing.
These bases are widely used in combination with at least two major manufacturers of light poles that DOTs often choose for lighting systems. Demand for the base is high and because there is only one manufacturer, leads times are very long and supply is low. Currently contractors are stating that they are being told that there is a 40-to-60-week lead time on getting the Akron breakaway bases. As a result, there are delays with federally funded infrastructure projects that involve lighting and traffic signal systems.
The objective of this project is to develop a non-proprietary breakaway frangible cast aluminum transformer base for luminaire and traffic signal head supports that satisfies requirements of the American Association of State Highway and Transportation Officials (AASHTO) Manual for Assessing Safety Hardware (MASH) (1). The base will be designed for use with the various support configurations identified by DOTs and Pooled Fund members.
Development of a new MASH compliant, non-proprietary breakaway cast aluminum transformer base will allow new manufacturers to enter the market, thereby increasing supply and reducing infrastructure delays. The new base will provide state DOTs with a MASH compliant design option for meeting their infrastructure needs.
The TTI research team will provide a final report that will include the results of the development, testing, and evaluation of the new breakaway cast aluminum transformer base.
The work plan for accomplishing the objective of this project consists of five tasks described below.
Task 1 – Literature Review and Survey
The TTI research team will review previous and ongoing research projects to determine luminaire pole configurations that have met MASH requirements with various types of breakaway bases. The TTI research team will use these results to select a pole configuration for eventual crash testing with the new breakaway cast aluminum transformer base. The literature search will also be used to determine material properties (i.e., grade of cast aluminum) used for the current cast aluminum transformer bases.
The TTI research team will distribute a survey instrument to the Roadside Safety Pooled Fund members to identify the types of breakaway cast aluminum transformer base designs being used and their relative frequency of use for lighting and other types of infrastructure. Information regarding common pole configurations used in conjunction with those bases will also be requested. The survey will also be used to identify any issues with the existing bases or desired modifications that can be considered in the design process.
The TTI research team will use the survey results to select a design moment capacity for the new base. For example, the current Akron Foundry base configurations (TB1-17, TB2-17, and TB3- 17) all have a different rate moment capacity that is used to select an appropriate base configuration for the anticipated service (wind) loads. The TTI research team will use the most commonly used base configuration among the Pooled Fund members to determine the desired design characteristics for the new base developed under this project.
Task 2 – Design of Non-Proprietary Cast Aluminum Transformer Base
The TTI research team will identify and review relevant patents related to the current Akron Foundry transformer bases to evaluate if there is any related remaining proprietary technology. Since these bases were accepted by FHWA in the 1980s and have remained essentially unchanged since that time, it is unlikely there is any remaining patent protection on the existing designs given that patents have a 20-year life.
The TTI research team will work in cooperation with PM Castings Ltd. to design and cast prototypes of a new transformer base to satisfy the moment capacity identified in Task 1. If there is no remaining proprietary technology associated with the current Akron Foundry base designs, one acceptable option would be to reverse engineer the base with the appropriate moment capacity. If this option is followed, an existing base will be obtained and scanned to obtain detailed part geometry, and material properties identified in the literature will be used. Any modifications desired by the member state DOTs, such as a set bolt hole pattern rather than slotted bolt holes, will be considered.
If the current base designs still retain some form of proprietary technology, or if it is a preference of the state members, the TTI research team will work with PM Castings Ltd. to design a new prototype base that has a similar moment capacity. There is a significant expense associated with fabricating molds for the casting process. Therefore, the number of possible design iterations will be limited. This will make developing a new design that satisfies both moment capacity and impact performance challenging, but PM Castings Ltd. has some design ideas that can be considered and the TTI research team will draw upon all available information to meet the project objective.
Task 3 – Dynamic Impact Testing
As mentioned, the new base will need to satisfy requirements for both impact performance and moment capacity. The TTI research team will evaluate these requirements through testing prior to conducting the more expensive full-scale crash testing. Following this approach, it may be possible to modify the design if one of the design requirements is not satisfied by the initial prototype.
The TTI research team will perform the dynamic impact testing using a gravitational pendulum. The pendulum is designed to replicate the weight, bumper height, and frontal crush stiffness of a small passenger car. The TTI research team will adjust the impact speed of the pendulum to match the energy level of MASH Test 3-60. This low-speed impact with a small passenger car is considered the most critical test for the evaluation of breakaway luminaire poles.
A suitable size pole will be mounted to the new transformer base prototype. The transformer base will be anchored to a steel reaction plate in the pendulum test facility. Acceleration time history of the impact pendulum will be measured. The TTI research team will use the acceleration data to determine if the new base fractures and releases at a force level that satisfies MASH occupant risk criteria for Test 3-60. Note that the pendulum does not assess potential for secondary vehicle contact from the released pole.
A total of six pendulum tests have been budgeted for this task, allowing for three tests of two different design variations. Multiple tests of the same design are desired to understand the variability in the breakaway characteristics of the base.
Task 4 – Static Load Testing
If the prototype base satisfies the MASH occupant impact criteria in the dynamic impact tests, the TTI research team will perform quasi-static load tests to determine the moment capacity of the new base. This data is needed for use in wind load design calculations to determine suitable pole configurations for the new base.
A pole will be attached to the base, and the base will be secured to a load frame in a horizontal (cantilever) position. A vertical load will be applied to the horizontal pole at a location corresponding to its center of mass. The load and deflection of the post will be measured with respect to time. The pole will be loaded until the base fails. The maximum force and location of the applied force (i.e., moment arm) will be used to calculate the ultimate moment capacity of the base.
A total of six static load tests have been budgeted for this task, allowing for three tests of two different design variations. The experimental procedure for determination of a design moment capacity calls for averaging the results of a minimum of three tests.
Task 5 – Full-Scale Crash Testing
The TTI research team will use the literature review and design capacity of the breakaway transformer base to select a luminaire pole configuration for full-scale crash testing following MASH TL-3 criteria for breakaway support structures. The TTI research team will use a pole configuration that is commonly used by the member state DOTs and has previously satisfied MASH requirements for a comparable base type. Given the range of luminaire pole properties that satisfy MASH criteria is not fully known, this will focus the evaluation on the breakaway performance of the new base rather than the pole. The pole configuration selected will also satisfy the AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals based on the capacity of the new base.
The MASH TL-3 test matrix for support structures consists of three tests: Test 3-60 (low-speed impact with passenger car), Test 3-61 (high-speed impact with passenger car), and Test 3-62 (high-speed test with pickup truck). The full MASH matrix has been budgeted for this project. Successful performance of the full test matrix will enable an eligibility request to be submitted to the Federal Highway Administration (FHWA).
The most critical test for the new base will be Test 3-60. This low-speed test evaluates the breakaway characteristics of the base and subsequent occupant compartment deformation associated with secondary contact of the released pole with the windshield and hood of the impacting vehicle. The TTI research team will perform this test first in the matrix, followed by the high-speed tests.
Task 6 – Deliverables
The TTI research team will document the research effort in a final report. The report will document all aspects of the research including literature review, survey, design and development of the new base, and full-scale crash testing. The report will include recommended design details for the new base. The TTI research team will submit a draft final report to the technical representative for review and comment. The TTI research team will address any comments received in a revised final report.
An eligibility request for the new breakaway transformer base will be prepared and submitted to FHWA. The submittal package will include a eligibility request form, report, drawings, test summary pages, before and after photographs for each test, and video for each test.