What is frangible mounting in the civil works?
The planned weak point sits just above the foundation. The civil works keep the foundation low and firm and leave nothing around the break point that could stop it working.
Elevated lights and signs near runways and taxiways are frangible: strong enough for wind and normal use, but built to break away at a designed point if an aircraft strikes them. That point is the frangible coupling, set just above the foundation. Above it the unit separates; below it the base plate, anchor bolts and foundation stay in the ground.
It is a civil interface because the foundation, ground level and cable entry decide whether the coupling can do its job. The civil works keep the foundation top low and at design height, set the duct stub-up below the break point, and add nothing that bridges or stiffens the coupling: no collar of grout or concrete, no built-up soil or turf against the stem, no rigid conduit, earth rod or marker standing beside the unit. The manufacturer’s drawing sets the coupling, the break height and the cable arrangement.
Foundation top, ground level, duct and hardware around the base, so the break point is left free.
Fitting the coupling, torque, connector and slack lead on the fixture itself.
Frequently Asked Questions
Q1What does frangible mounting mean for the civil works?
The civil works must leave the break point free to work. The foundation stays low and firm, nothing solid is built up around the coupling, and the cable and any earth or duct hardware stay below it, so the unit above can break away while the foundation stays in the ground.
Q2Where is the break point in relation to the foundation?
Just above the foundation top, at the frangible coupling. Everything above it is designed to separate or yield in a strike, and everything below it, the base plate, anchor bolts and concrete, is designed to stay.
Q3What civil work can defeat a frangible coupling?
A collar of grout or concrete around the coupling, a foundation top set too high, soil or turf built up against the stem, and rigid conduit, earth rods or markers that rise beside the unit and bridge or stand above the break point.
Q4How should the duct and cable be arranged at the foundation?
The duct stub-up ends below the break point and the lead has slack, usually through a separable connector, so the cable lets go with the unit instead of holding it back or tearing out of the ground. Follow the manufacturer’s drawing for the fixture or sign. Any earth bond to the unit is a flexible lead that can part with it, not a rigid link to an earth rod.
Q5Does ground level matter?
Yes. The foundation top and finished ground are kept at the design level so the break point stays at its designed height. Regrading, resurfacing or heavy turf build-up over time can raise the ground and shorten the stub the standard allows.
Q6What is checked after a strike?
The foundation, anchor bolts, base plate, duct, lead and connector are checked for damage and the frangible coupling is replaced, then the ground level and the unit’s alignment are confirmed before it is returned to service. Work only on a circuit that has been isolated, locked out and proved dead.
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