11.3 Signal Poles, Mast Arms, Foundations, and Head Wiring

7.2 Signal Pole Foundations, Mast Arms, and Head Wiring

Traffic signal poles, mast arms, and span wire assemblies represent major structural investments that must safely withstand significant dead loads (weight of signal heads, signs, cables, and luminaire arms) and dynamic environmental loads (wind velocity pressure, ice accumulation, and gust-induced fatigue). Structural design criteria are governed by the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals.

Field technicians responsible for erecting poles, installing mast arms, aligning signal heads, and pulling cable must understand foundation mechanics, structural torque procedures, MUTCD clearance rules, and proper electrical wiring techniques.

---

Drilled Shaft Foundations and Anchor Assemblies

Traffic signal strain poles and mast arm assemblies are anchored using reinforced concrete drilled shaft foundations (also referred to as caissons). Foundation dimensions—typically 3 feet to 4.5 feet in diameter and 10 feet to 25 feet in depth—are determined by soil shear strength, water table depth, and overturning moment calculations.

Foundation Construction Steps

1. Excavation & Soil Verification: Shafts are augered into undisturbed soil. If groundwater or caving soil is encountered, temporary steel casings or bentonite slurry drilling methods are employed. Technicians inspect soil strata against geotechnical design reports.
2. Rebar Cage Placement: A prefabricated cylindrical reinforcing steel cage (longitudinal rebar tied with spiral or hoop stirrups) is centered in the excavation. Heavy-duty non-conductive rebar chairs (dobies) maintain a minimum concrete cover of 3 inches between the steel cage and raw earth.
3. Anchor Bolt Circle Assembly: Anchor bolts—typically 4 to 6 high-strength steel rods conforming to ASTM F1554 Grade 55 or Grade 105—are secured using top and bottom steel templates to maintain exact bolt circle geometry during concrete placement.
4. Concrete Placement & Curing: Concrete with a minimum 28-day compressive strength of 4,000 PSI is placed using a tremie pipe to prevent concrete segregation or water contamination. Concrete must cure for 7 to 14 days minimum (reaching at least 75% to 80% design strength) before pole erection.

$\text{Anchor Assembly Order: Top Nut} \rightarrow \text{Heavy Washer} \rightarrow \text{Base Plate} \rightarrow \text{Leveling Nut} \rightarrow \text{1-1.5 in Gap} \rightarrow \text{Foundation}$

Leveling Nuts and Base Plate Standoff Gap

A 1-inch to 1.5-inch air standoff gap (equal to the nominal anchor bolt diameter) must be maintained between the top of the concrete foundation and the underside of the pole base plate. This gap permits plumb adjustment via lower leveling nuts. Following pole plumbing, the gap is either left open and protected by a stainless steel rodent screen or filled with non-shrink structural grout fitted with a drain tube to prevent moisture collection beneath the base plate.

---

Pole Erection, Mast Arm Mounting, and Span Wire Assemblies

Once foundation concrete reaches structural strength, structural erection proceeds using heavy crane rigging and calibrated tensioning methods.

Anchor Bolt Tightening and Turn-of-Nut Method

Structural integrity relies on proper clamping force between the anchor bolts and base plate. The Turn-of-Nut Method (per RCSC specifications) ensures proper bolt tension:

1. Leveling nuts are adjusted to bring the pole structure into true vertical plumb.
2. Top nuts are brought to a snug-tight condition using an impact wrench or full effort with an ordinary spud wrench (typically 20% to 30% of total tensioning torque).
3. Match marks are painted on each nut and anchor bolt end.
4. Top nuts are rotated an additional specified rotation (typically 1/3 turn to 1/2 turn beyond snug-tight, depending on bolt length-to-diameter ratio).

$\text{Snug-Tight Condition} \longrightarrow \text{Paint Match Mark} \longrightarrow \text{Rotate Nut } \frac{1}{3} \text{ to } \frac{1}{2} \text{ Turn Past Snug}$

Mast Arm Flange Connections

Mast arms are attached to pole shafts using heavy structural flange plates secured with high-strength ASTM F3125 Grade A325 or A490 structural bolts. Flange bolts are torqued in a crisscross star pattern to prevent uneven gasket or plate compression.

Span Wire Assemblies

For span wire installations (strain pole systems), signal heads are suspended from galvanized steel cables:

• Messenger Cable: High-strength extra-galvanized steel cable (typically 3/8-inch 7-strand Extra High Strength [EHS]) carrying primary structural weight.
• Tether Cable: Lower stabilizing wire (typically 1/4-inch steel) attached to bottom of signal heads to prevent wind swing and twisting.
• Hardware: Strain insulators, heavy-duty drop-forged turnbuckles, thimbles, 3-bolt guy clamps, and preformed guy grips rated for span tension load limits.

---

Signal Head Alignment and MUTCD Standards

Proper physical placement and alignment of signal heads ensure clear visibility for approaching drivers, directly impacting intersection safety and legal compliance.

MUTCD Clearance and Placement Standards (11th Edition)

$\text{Roadway Surface} \xrightarrow{\quad 15.0 \text{ ft Min to } 19.0 \text{ ft Max Clearance }\quad} \text{Bottom of Signal Housing}$

Mounting Hardware and Adjustment

Mast arm signal heads utilize adjustable mounting bracket assemblies (such as Astro-Brackets or terminal-compartment band mounts) featuring serrated tooth elbows. Serrated fittings allow precise vertical and horizontal aiming. Signal heads must be installed perfectly plumb to prevent optical distortion of LED directional lenses.

---

Cable Pulling, Drip Loops, and Field Wiring

Electrical conductors transport phase power from the cabinet terminal facility to individual vehicular heads, pedestrian signals, and push buttons.

Cable Pulling Techniques

Traffic signal cable (typically IMSA Spec 19-1 PVC jacketed or IMSA Spec 20-1 polyethylene jacketed multi-conductor cable, 14 AWG solid or stranded) is pulled into pole shafts and mast arms using fish tapes or braided pulling ropes.

• Wire Pulling Lubricant: Polymer-based approved cable lubricant must be applied continuously to reduce pulling friction coefficient to below 0.5.
• Anti-Chafing Protection: Smooth plastic grommets, chase nipples, or rubber entrance bushings must be fitted in all pole wire exit holes to prevent sharp metal edges from cutting cable jackets.
• Maximum Pulling Tension: Pulling tension must not exceed manufacturer limits ((T = 5 \times N \times \text{Conductor Area}) in lbs) to prevent copper conductor neck-down.

Drip Loops and Terminal Block Connections

Where field cable exits a mast arm or weatherhead to enter a signal housing, a drip loop (a downward U-shaped bend extending 3 to 6 inches below the entrance fitting) must be formed. Gravity forces rainwater to drop off the bottom of the loop rather than following the cable inside the signal housing.

Inside the signal head, conductors are stripped, crimped with insulated ring or spade terminals, and fastened to terminal blocks according to IMSA standard conductor color codes: