Tall floodlight masts, 30–42 m: design, foundations and erection

Which masts of our range fall into the 30–42 m band

Three ranges work in this band. PMK masts with a mobile crown come in the standard range at 30, 35 and 40 m, PMV masts with 360° lighting at 30 and 34 m, and PMO masts with an inclined crown for sports facilities start at 25 m and reach 55 m in the catalogue; to order they can be of any height.

The standard ranges have no 42 m height. Such a mast is calculated and made to an individual brief, in the PMO range. The PM (12–20 m) and PMT (16–20 m) ranges do not reach this band.

How the ranges differ in purpose and light angle is described in the article Which floodlight mast to choose, and an example of the PMV-30 mast with its embedded part is in Floodlight mast, 30 m. This article is about something else: what changes when a mast gets taller than thirty metres.

Sections and joints: what is added with height

For transport the shaft is made of sections up to 12 m long. For a 30 m mast that is at least three sections, for 42 m at least four. Each joint is a separate detail that is calculated and made, not just the place where one tube ends and the next begins.

Sections are joined telescopically or with flanges. A telescopic joint holds by wedging: the sections enter one another and on the ground are drawn together with jacks or a winch to the specified overlap, which is checked against a mark on the section. High flagpoles are assembled the same way; see the article on assembling a tall flagpole.

In a flange joint the bolts are high-strength and tightened to a controlled torque, and no gaps are left between the flanges. The welds attaching the flanges are made with full penetration and checked by non-destructive testing, most often ultrasonic. Which joint to use is decided by calculation, and the place under the crown and the inclined section are checked separately.

Section length is also tied to galvanizing: the bath in our works measures 13,000 × 1,600 × 3,200 mm, so a 12 m section fits in it. The cross-section of the bottom section together with its flange must also fit in the bath; this is taken into account when the base diameter and the size of the base flange are chosen.

Wind, moment and displacement of the top

The moment at the base grows faster than the height. In the PM range at a wind pressure of 500 Pa and almost the same permissible equipment area (2.53 m² for 12 m and 2.67 m² for 20 m) the height grows 1.7 times, while the bending moment at the base goes from 63.21 to 153.63 kN·m, that is 2.4 times. Above thirty metres the gap only widens.

On sports masts of 35–42 m with dozens of floodlights the moments at the base are measured in thousands of kN·m. In known design solutions for such masts it is roughly 2000 to 2700 kN·m, 13–18 times more than for a PM-20. The comparison is rough, because the layouts and equipment differ, but the order of magnitude is clear.

PM range, the “Structures for outdoor lighting” catalogue (2023): pressure 500 Pa, Caer = 1, terrain type III, equipment mass 180 kg. Q is shear, M is the moment at the base. For 30–42 m masts the loads come from a calculation for your wind region and set of floodlights.

Dynamics, ice and displacement of the top

The dynamic action of wind is checked separately. A tall flexible mast oscillates in gusts, so the loads code requires a separate dynamic calculation of the fluctuating component when the structure is flexible enough. Ice adds weight and wind area, so the calculation is also run for wind combined with ice.

The second question is displacement of the top. Floodlights are aimed at the site at a set angle, and sway of the top in wind changes the illuminance. The permissible sway is set by the lighting brief or the floodlight maker. Values from the tables for framed buildings are not transferred automatically to a flexible mast: the article on limiting deflections under DSTU B V.1.2-3:2006 explains what the standard does not cover.

The crown: fixed, mobile or inclined

At 30–42 m the choice of crown affects the structure more than at 12–20 m. A fixed crown sits immovably on the top section, and the equipment is serviced from the mast. On a PMV the service platform carries up to 16 floodlights around its perimeter. The mobile crown of a PMK is lowered by a winch: floodlights are changed and aimed on the ground, and no special equipment is needed.

With a mobile crown the weight of the crown with floodlights determines the winch and rope, and a travel of 30–40 m means a long rope and a long lowering, during which wind swings the crown. The crown moves, while the lightning rod and down conductor must remain reliably connected: see the article on the floodlight mast with a lightning rod.

The inclined crown of PMO masts tilts the top section away from the vertical towards the field, and the floodlights stand along the inclined plane. The weight of the crown acts eccentrically, which adds moment in the inclined section and at its flange. In a known example of such a mast about 42 m high, the top section leans 15° from the vertical.

The inclined crown is assembled with the floodlights on the ground, lifted by crane separately from the shaft and seated on the flange of the top section. So that seating does not need fitting at height, guide pins are provided on the flange.

Ladder, platforms and access to the equipment

On PMV and PMO masts the equipment is reached by a ladder with a cage, with rest platforms on the way up; on the PMO a permanent fall-arrest system may replace the cage. The lower part of the ladder has a hatch that locks with a padlock, and the inspection hatch of the shaft closes with a security bolt. Whether a ladder is fitted on a mast of any series is agreed with the customer when the order is placed, and a mast without a ladder is serviced from an aerial lift.

At 35–40 m a climb with tools is work at height, and it is done when floodlights are replaced and aimed. So the choice between a ladder and a mobile crown that is lowered to the ground becomes one of the first questions to the customer at these heights. PMK masts have no ladder.

The ladder with its cage, the platforms and cable trays add weight and wind area to the shaft, and they are entered into the calculation in the same way as the floodlights.

Lightning protection, earthing and signal painting

A mast 30–42 m high is the tallest object on the site, and most likely it is the one lightning will strike. On PMV masts the lightning rod is part of the structure; the protection zone, the down conductor section and the earth electrode are calculated separately. The basics are in the articles Lightning protection for masts and towers and A steel mast as a lightning rod and down conductor.

Near airfields the sections are painted red and white as a signal colour. The requirements for painting and obstruction lights depend on the distance to the airfield, and it is worth finding them out from the aviation supervision authority before ordering, not at acceptance.

Anchors and foundation: what grows with height

The ring of anchors grows with the moment. In the PMV range from 20 to 34 m the number of anchors rises from 16 to 20 and the diameter from M36 to M42; each height has its own embedded foundation part.

There are many anchors in the ring, and the position of each must match the hole in the flange: the bolts are set on two templates and checked before concreting. On a tall mast the flange is not fitted on site, because the error in the anchors becomes an error in the verticality of the whole shaft. Details are in the articles on foundation anchor cages and anchor bolts.

The foundation type is decided by the ground, not by the height. On firm ground a stepped slab is common. In a known example of a polygonal floodlight mast of about 30 m such a slab measures 8 × 8 m, is sunk about 3.5 m and takes about 94 m³ of concrete. For a mast of about 42 m the plan size of the slab stayed the same, and what changed was the ring of anchors (32 M48 bolts for 30 m, 40 M48 bolts for 42 m) and the reinforcement.

On weak made-up and water-saturated ground the slab is replaced by piles. In another example, a polygonal tower of about 32 m, there are nine bored piles 600 mm in diameter and 7 m long under a reinforced concrete grillage of 4.2 × 4.8 m; the moment at the base there is about 275 t·m (roughly 2700 kN·m), and groundwater lies at a depth of 1.15–2.10 m.

These are orders of magnitude, not a recommendation: the type and size of the foundation come only from the design based on the ground survey. We do the foundation design as a separate order for a separate fee, and most often we only supply the foundation loads and the customer designs it. What exactly to pass on is covered in the article Foundations for columns, masts and poles, and the general logic of foundation types in Foundations for masts, towers and line poles.

On tilt: every thousandth of foundation tilt gives 42 mm of displacement at the top of a 42 m mast, and the deflection of the shaft itself is added to it. So the permissible foundation tilt must be stated by whoever sets the aiming accuracy of the floodlights, and written into the brief for the foundation.

Lifting and erecting the mast

A 30–42 m mast is lifted to a works execution plan drawn up by the erection contractor. The plan specifies the truck crane by capacity at the required radius, the slinging scheme, temporary guying and the danger zone. Erection is done for us by our partner construction company.

Slings are placed above the centre of gravity. The load is first lifted 200–300 mm to check the slinging and the crane brakes. Nobody stands under the load or near the winch ropes, and work at height is done under a work permit and with fall-protection equipment.

The wind limit at which the lift is stopped is set by the crane’s data sheet and the works execution plan. The operation most sensitive to wind is lifting the crown with its floodlights.

Coating and maintenance

The masts are protected by hot-dip galvanizing. For the PMV the coating thickness is 80–120 µm, and the design service life of the mast is 40 years.

On a tall mast the joints and anchors need the most attention. Flange bolts and anchor nuts are re-tightened if needed after the first period of service, because the zinc settles. The zinc on the flanges and near the ground, the hatch, the ladder and the earthing bolt are also inspected.

We carry out inspection of a mast already in service, with an assessment of its condition, as a separate service: technical inspection. Signs of wear in metal poles are covered in the article on wear and inspection of metal poles.

What to send for the calculation

To calculate a 30–42 m mast we need:

• the height and the type of crown: fixed, mobile or inclined;
• the number of floodlights, their size and mass (or the data sheet);
• the site location, which sets the wind region and the ice load;
• whether a ladder and service platforms are needed;
• the distance to an airfield, if it is short;
• the ground survey report, if there is one, and who designs the foundation: we or you.

The full list is in the article How to order poles and masts. We prepare KM and KMD drawings as the service KM and KMD design and make the mast at our steel fabrication works.

Standards and validity

Loads and actions on the mast are 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 structures are calculated under DBN V.2.6-198:2014 “Steel structures. Design standards” (in force, with amendment No. 1). The ground and foundations are designed under DBN V.2.1-10:2018 “Bases and foundations of buildings and structures. Basic provisions”. Occupational safety during erection is governed by DBN A.3.2-2:2009 “Occupational safety and industrial safety in construction”.

The article does not replace the calculation of the mast, the foundation design or the works execution plan. Check the current edition of each document before citing it in a project.