Adding Banners and an Antenna to an Existing 35-ft Pole
A 35-ft galvanized steel pole supports a 4-ft davit arm and a 55-lb LED luminaire. The pole was designed for a base moment of 12,000 ft-lb at 90 mph, Exposure C. The owner wants to add two 2-ft by 4-ft vertical banners, one on each side, centered 24 ft above grade, plus a 15-lb small-cell antenna with 1.5 sq ft of projected area at 32 ft. The existing luminaire and arm produce about 6,500 ft-lb at the base under the same wind. The pole is 12 years old; anchor bolts are 1-in diameter, four per base, with 24-in embedment in dense sand. Inspectors note surface rust at the base plate and a 1/16-in gap under the grout. The banner supplier says the banners are wind-rated and the pole has extra capacity because the original luminaire was replaced with a lighter LED.
1. Estimate the added wind moment from the banners and antenna at 90 mph, and determine whether the pole remains within its 12,000 ft-lb design moment. State assumptions.
Read the worked answer
At 90 mph, q = 0.00256 V² = 0.00256 × 8,100 ≈ 20.7 psf. Banner area: 2 ft × 4 ft = 8 ft² each, 16 ft² total. Using a flat-plate drag coefficient of 1.2, force ≈ 20.7 × 16 × 1.2 ≈ 398 lb at 24 ft, giving about 9,550 ft-lb. The antenna projected area is 1.5 ft²; 20.7 × 1.5 × 1.2 ≈ 37 lb at 32 ft, or about 1,190 ft-lb. Added moment ≈ 10,740 ft-lb. Adding the existing 6,500 ft-lb gives about 17,240 ft-lb, well above the 12,000 ft-lb design moment. Even with Cd = 1.0 and ignoring the antenna, the banners alone add about 7,960 ft-lb, and the total is about 14,460 ft-lb. The pole does not have the claimed spare capacity.
2. How should the rust, grout gap, and anchor bolts affect your recommendation?
Read the worked answer
The rust and grout gap are not cosmetic. A 1/16-in gap under the grout can indicate grout deterioration, poor bearing, or water intrusion, and surface rust at the base plate may mean coating damage and ongoing corrosion. The anchor bolts must be inspected for section loss, tightness, and embedment; 24-in embedment in dense sand may be adequate for the original load, but the added moment increases tension and shear. A structural engineer should verify the base plate, anchor bolts, and foundation for the new moment, including uplift and shear, and require repair of the grout and coating. Do not rely on the banner supplier’s wind-rated statement; it addresses the banner, not the pole-foundation system.
3. What final recommendation should go to the owner?
Read the worked answer
Reject the installation as proposed. Options are to reduce the banner area, remove one or both banners, relocate the antenna, or add a new pole designed for the combined load. If banners are required, obtain a stamped structural calculation for the actual geometry and wind exposure, repair the base grout and corrosion, and torque and inspect the anchor bolts. After repair, verify the final configuration and keep the load within the pole’s rated moment with an appropriate margin. The owner should not accept a field judgement based only on the lighter LED luminaire.