Decorative lighting poles with brackets: how they are designed and what to look at

How a decorative pole differs from an ordinary one

Such a pole has a shaft a few metres high, with a bracket curved in an arc on top and with one or two arms. An arm is the place where the luminaire is fixed. In the examples considered the shaft is 4 or 5 m: the height of parks, avenues and pedestrian zones, where the luminaire should hang closer to people.

A decorative bracket differs from an ordinary straight one in shape. The arc is longer than a straight overhang, it carries extra strips and half-rings, and the weight of the luminaire acts as bending in a curved tube. So the main place in the calculation becomes the transition from bracket to shaft, not the shaft itself.

The shaft for such a pole is taken from the park series, the polygonal PMO or the conical PKO, described in the article Park lighting poles. The general construction of a pole and the types of ordinary KO and KP brackets are covered in The lamp post and Brackets, crossarms and heads.

Single-arm and double-arm brackets

A single-arm bracket carries one luminaire on one side of the shaft axis. A double-arm bracket is symmetrical: two luminaires hang on both sides, and if they are identical their static moments on the top of the shaft cancel each other. What remains is bending from wind, and for a single-arm bracket a constant twisting of the shaft top is added.

The table gives the parameters of the two brackets considered. Both are made of Ø48×3 mm tube in C245 steel and sit on 4 m and 5 m shafts.

What the bracket is made of and how

The base is two Ø48×3 mm tubes: an arc about 2.07 m long, bent to a 300 mm radius, and a vertical leg of about 0.93 m. They are joined by plates that pass through oval 20×30 mm holes in the tube walls and by cross strips 4 mm thick. A separate 4×40 mm strip bent along the arc is purely a decorative element.

At the end of the arc there is a separate fitting for the luminaire, and at the bottom of the bracket there is a smaller-diameter spigot that goes into the top of the shaft. All shop welds are made by semi-automatic welding, and all of them are closed, except those specified. This matters for moisture: water gets into an open gap, while a closed volume needs an outlet for gases during galvanising, so a Ø8 mm process hole is provided at the end of the tube. It also serves for hanging the tag with the mark.

The joint to the shaft

The joint works in a more complicated way than the drawing suggests. A spigot about 0.3 m long goes into the top of the shaft, and above it there is a disc 85 mm in diameter. Bending from the weight of the luminaire and from wind passes from the arc through the leg to this spigot, and from there to the thin wall of the shaft.

Several things matter for the joint. The spigot must seat in the shaft without play, because play under wind loosens the bracket and fatigues the weld. The position of the bracket is fixed after alignment, not left held by friction alone. The cable to the luminaire passes through the joint, so its hole must not cut the wires, and the cable must not bend sharply where it leaves the tube.

The wall of the shaft near the spigot is looked at separately. In park poles it is 3 mm, and where the bracket and spigot add load it is checked for local strength and stability, not only for the overall moment at the base.

Loads: weight, wind, vibration

The luminaire weight is taken from its data sheet. At an offset of 0.75 m a luminaire of 15 kg (the limit per arm in our tables) gives a static moment of about 110 N·m; on a double-arm bracket it is symmetrical, on a single-arm one it acts as a twist on the top of the shaft. The value is fairly small, but it acts constantly and adds to the wind moment.

The wind load on a luminaire is made up of the side-view area, the pressure and coefficients. For the example from the pole selection calculator (a luminaire of 700 × 350 mm, area 0.245 m²) and a pressure of 500 Pa the result is about 122 N before the coefficients are applied. At a height of 5 m that gives roughly 0.6 kN·m at the base of the pole from one luminaire, and two on a double-arm bracket double this moment. The bracket tubes and decorative strips add their own wind area, which is also counted.

Vibration is looked at separately. An arc with a luminaire at its end behaves like a spring: a gust sets it swinging, and the oscillations repeat thousands of times a year. So the welds at the foot of the leg and at the spigot are designed and made with a margin for fatigue. Extra load on the bracket, such as flower baskets, flags or cameras, changes both the weight and the area, so it is mentioned before the order.

How a pole with a decorative bracket is checked

The check goes along one chain, from the bottom up and from the top down. First the input data is gathered: the bracket type, the offset, the mass and area of the luminaire from its data sheet, the number of arms, the wind region. Then the bracket itself is checked for bending in the arc and leg and for weld strength, then the spigot and the wall of the shaft top, then the shaft and the moment at the base, the flange and the anchors.

To select the series and height by luminaire area and mass, the pole selection calculator is convenient. It gives a preliminary selection, and the final check is made for your project. Its tables stop at 15 kg per arm and an offset up to 1.5 m; three luminaires on a bracket or a non-standard arc are calculated separately.

For a decorative pole the deflection of the top matters not only for strength: when the top sways or is tilted, the luminaire no longer points where it was aimed. For the foundation and anchors, see Foundations for lighting poles and Flanged or direct-embedded poles.

Galvanising and painting of decorative elements

Protection of the bracket starts with hot-dip galvanising at a thickness of not less than 80 µm, then primer and paint are applied if the customer wants. Zinc keeps the metal from corroding, and the paint sets the colour; a pole with zinc alone has a grey matt look. Colour painting over zinc is done as a separate service: powder coating.

For galvanising a tubular structure needs holes. In closed volumes pressure builds up on immersion in the melt, so the ends of tubes and the places where plates overlap have holes for gases to escape and zinc to drain. The rules and hole sizes are covered in the article Designing structures for hot-dip galvanising. The size of the bracket is no limit: the bath in our works measures 13,000 × 1,600 × 3,200 mm.

Stresses remain in the metal after bending and welding, and in the melt (about 450 °C) part of them is released, so the part can distort. That is why the bracket is made symmetrical in its joints and hung so that the arc does not sag under its own weight. Strips of 4 mm on a 3 mm tube give no sharp difference in thickness, and such a difference, for example a strip on a massive plate, is avoided.

After galvanising the welds and fixing points are inspected. Where powder coating will follow, the zinc surface is prepared, otherwise the coating peels. What affects powder coating is covered in the article on powder coating cost factors.

What to send with an order

For us to offer a pole with a decorative bracket, send:

• the shaft height and the way it is fixed: on a foundation or in the ground;
• the bracket type: single- or double-arm, offset and height;
• the luminaire data sheet: mass, side-view area, diameter of the seat;
• the town or region for the wind load;
• extra elements on the pole, if any;
• the colour, if the pole is to be painted over the zinc;
• a drawing or sketch, if the shape of the bracket is set by the customer: we also make to customer drawings.

We prepare KM and KMD drawings as the service KM and KMD design and carry out fabrication at our steel fabrication works. The general list of data is in the article How to order poles and masts.

Standards and validity

The bracket and the pole are calculated under DBN V.2.6-198:2014 “Steel structures. Design standards” (in force, with amendment No. 1), and the wind load is determined under DBN V.1.2-2:2006 “Loads and actions. Design standards” (according to the Ministry register in force, with amendments No. 1 and No. 2).

The article does not replace the calculation of the pole for your project. Check the current edition of each document before citing it in a project.