Section 2: Breakaway Light Poles
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.
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.
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.
Whenever a pole is knocked down, the transformer
base should be replaced even if it is still intact.
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 breakaway base, because
this may change the breakaway characteristics of the base. Ensure
that the top of the concrete foundation is level.
Relocated Poles
When relocating breakaway roadway illumination poles,
Item 610 requires a new transformer base to be installed. The old
breakaway base should be destroyed to prevent reuse.
Poles Placed by Maintenance Forces
New light poles placed by maintenance forces should meet all safety related guidelines set forth in
, “Pole Placement Guidelines,” of this manual and the ’85 AASHTO breakaway requirements. This includes breakaway poles installed by maintenance forces at new locations.
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.
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.
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.
Anchor Bolts
Torque anchor bolts to manufacturer’s recommendation. This
will help to ensure proper operation of the pole and breakaway device.