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Section 3: Breakaway Light Poles

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Frangibility Requirement

The FHWA adopted Section 7 of the 1985 AASHTO publication entitled “Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals” for implementation beginning in July 1990. This section requires more stringent breakaway characteristics. TxDOT’s previous design of roadway illumination assemblies (based on a 1975 AASHTO specification) does not meet these requirements.

Breakaway light poles rely on frangible transformer bases to provide the breakaway feature. The 1985 AASHTO specification provides for a maximum change in momentum of a 1800 pound car to break the pole away. The 1975 AASHTO specification provided for a similar change in momentum except that the test vehicle weighed 2,250 pounds.

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Structural Requirement

While providing the necessary frangibility, the base must also be structurally adequate to support the illumination pole for design wind speeds. Transformer bases meeting the ’85 AASHTO specification may not adequately support some 50 foot steel poles, notably those designed to meet the ’75 AASHTO specification.

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Replacement Guidelines

Lighting systems built under the previous requirements should be maintained with ’75 AASHTO standard poles and transformer bases currently in stock.

Replacement transformer bases for lighting systems built under the ’85 AASHTO requirements should be specified to be the same model number as the bases being replaced. This model number is required to be incised on the base.

Two variables of pole construction affect structural compatibility with the ’85 AASHTO transformer bases top: bolt circle and base plate thickness. No tolerances for these variables have been established. For this reason, TxDOT does not recommend the direct replacement of ’75 AASHTO standard transformer bases with ’85 AASHTO standard transformer bases for 50 foot steel poles.

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New Installation Guidelines

Current breakaway requirements can be met on new installations by using the new transformer base and pole designed for that base, both approved by the FHWA to meet ’85 AASHTO standards.

TxDOT typically uses breakaway poles wherever possible, even when not required. For these types of new installations, where breakaway poles are used but not required (outside the clear zone or behind barriers), transformer base poles meeting the ’75 AASHTO guidelines may be used when currently in stock but should not be considered breakaway.

Do not use shims or washers to level the ’85 AASHTO breakaway base, because this may change the breakaway characteristics of the base. Ensure that the top of the concrete foundation is level.

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Relocated Poles

Relocated light poles should be upgraded to the ’85 AASHTO breakaway requirements and should be of a design that has been tested with the ’85 AASHTO base.

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Poles Placed by Maintenance Forces

New light poles placed by maintenance forces should meet all safety related guidelines set forth in Chapter 6, Section 7, Pole Placement Guidelines, of this manual and the ’85 AASHTO breakaway requirements. This includes breakaway poles installed by maintenance forces at new locations.

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Guidelines Apply to Cities

Cities installing light poles or maintaining lighting systems on state right-of-way must abide by these same guidelines.

Districts should keep city maintenance personnel informed of the different system requirements and the allowable uses of the various transformer bases.

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Review of Shop Drawings

See Review and Approval of Shop Drawings of this chapter for discussion of review of shop drawings for breakaway light poles.

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Identifying Transformer Bases

The old and new transformer bases can be distinguished from each other by height and model number. The old bases are 20 inches tall, while the new ones are 17 inches tall. All new base model numbers have “-17” as the last three digits.

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Striking Height

Tests have shown that breakaway luminaire supports do not operate properly when the vehicle strikes the pole too high above the ground. Breakaway poles should, therefore, not be placed in areas where they are likely to be struck more than 28 inches above the top of the foundation. Limiting the negative side slopes to 1:6 between roadway and luminaire supports should ensure acceptable striking height.

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Anchor Bolts

Torque anchor bolts to manufacturer’s recommendation. This will help to ensure proper operation of the pole and breakaway device.

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