Slinging steel structures and poles: schemes, sling angles, lifting points and common mistakes

Why lifting points are shown in KMD

An element is lifted by places that can carry its weight, and these places are better not chosen on site. If the drawing is silent, people take whatever is handy: a flange, a rib, the first hole they find. A flange or rib not designed for lifting can be crushed or crack, and the zinc at the grip point suffers in any case.

The occupational safety rules require large steel structures to be slung by special devices, slinging nodes or marked places, depending on the position of the centre of gravity and the mass. The marked places come from the design: lugs, marks on the sheet, a scheme. The lift on site is made by our partner construction company under its own works execution project, but the input data for it comes from KMD: the mass of the mark, the centre of gravity, the places designed for lifting.

So slinging concerns both the designer and the fabricator. The designer checks that the element will stand the lift in every position: lying, in motion, when turned upright. The fabricator makes the lugs and reinforcement and knows in advance that protrusions count toward the galvanizing envelope.

Types of slings and what suits galvanized steel

All slings, spreader beams and grabs are removable load-handling devices. Each carries a tag with a number, the test date and the rated capacity, and the slinger selects them by the mass of the load, the number of legs and the angle (the rules call a leg a «vitka»). The safety factors below are taken from the occupational safety rules.

The factors are given under NPAOP 0.00-1.80-18, section VIII, chapter 22. A sling is selected by its passport and tag, not by this table; for textile and wire rope slings the rules also have separate rejection criteria (cuts, broken wires, damage at the end fittings).

Sling angle and the force in a leg

A load of mass Q hangs on n legs. If the legs are evenly tensioned and each deviates from the vertical by the angle β, the force in one leg is F = Q / (n · cos β). It is elementary mechanics: the vertical component of the leg force must carry 1/n of the weight, and the more the leg leans, the harder it has to pull along itself.

There is also a horizontal component. The legs pull the lifting points towards each other, and a compression H = (Q / n) · tan β arises in the element between them. A massive block does not care, but for a long beam or truss with little out-of-plane stiffness it is this compression that threatens loss of stability.

An example. A pole section of 4.0 t including zinc hangs on a two-leg sling, n = 2. The load is the same, only the angle changes.

Q = 4.0 t, n = 2, equal tension in the legs; 1 tf ≈ 9.81 kN. The arithmetic is illustrative: the lift is calculated and planned by whoever carries it out, and the sling is selected by its passport.

What the rules say about angle and factor

The rules determine the permitted working load of a sling at a design angle of 45° between the leg and the vertical (that is 90° between the legs). An additional load is permitted for 60° (120° between the legs). For a sling with three or more legs carrying the load, not more than three are counted in the calculation, because how the weight is shared among four legs of a rigid load is not known in advance. The slinger selects a sling so that the angle of the legs to the vertical does not exceed that stated in the passport or on the tag.

Passports use both ways of stating the angle: to the vertical and between the legs. Mix them up and the angle is wrongly doubled or halved.

For our example at 45° a leg carries 2.83 tf. A rope must have a breaking force of at least 6 × 2.83 ≈ 17.0 tf (about 166 kN), a chain 4 × 2.83 ≈ 11.3 tf (111 kN), synthetic webbing 7 × 2.83 ≈ 19.8 tf (194 kN). The rated capacity in the sling passport already contains these factors; the figures are given to show the order of magnitude.

Lifting points and the centre of gravity

A sling is attached so that the element hangs level and does not turn over, and for this the centre of gravity has to be known; it is rarely in the middle. In a pole with a massive base flange at one end the centre of gravity is shifted towards the flange, in a tapered multi-sided pole towards the thicker end, and brackets, arms and cross-arms pull it sideways.

Lifting by one point, when a pole is brought from horizontal to vertical: the point is above the centre of gravity. In our recommendations for erecting poles and gantries the sling is put on not less than 500 mm above the centre of gravity; below it the element would turn over. A long pole bends under its own weight when turned upright, which the calculation checks, and if needed it is temporarily reinforced.

Lifting lying down, by two points: for a straight uniform element the smallest bending from self-weight comes when the distance from a point to the end is about 0.207 of the length. For a 12 m beam that is roughly 2.5 m from each end. Real elements have uneven mass, so the points are moved towards the centre of gravity and the final position comes from the calculation, not from this formula.

The centre of gravity and the mass of the mark (with lugs, flanges and zinc) are given on the KMD sheet. The mass in the specification is counted for black steel, the zinc adds to it separately, and slings are selected for the full mass; more about it in the article on the KMD sheet and specification.

Long elements and spreader beams

Long loads are slung in at least two places, and for loading and unloading the rules require a spreader beam and two slings. A single sling on a long pole or beam causes bending in a section nobody calculated, and the element swings and slides along the sling.

A spreader beam is a beam or truss on the hook with slings running from it to the element vertically. Then β = 0, there is no compression between the points, and the force in a leg is Q / n. For long thin-walled poles and trusses with little out-of-plane stiffness it is usually the only way to lift the element without loading it sideways.

A spreader beam is calculated as a beam, has a passport, is tested and is inspected monthly; slings are inspected every 10 days. If the centre of gravity is shifted, a beam with a balancer is used, and for large dimensions a spatial one. A galvanized element is held by the beam on soft slings or on grabs with soft liners.

What is slung and how

The schemes below are typical, and the final scheme of each lift comes from the works execution project. The table shows what is usually watched in KMD.

Lifting lugs and holes: what to show in KMD

A lug is a plate with a hole welded to the element. The drawing should give its position from the datum, the thickness and shape of the plate, the hole diameter, the weld leg and the direction of load. The lug works in the plane of the plate: a side force bends it and opens the weld. So it is turned along the sling line, and the direction is shown on the scheme with an arrow.

It also helps to state the mass the lugs are designed for: «designed for lifting an element of not more than N t by the scheme on the sheet». Then erectors will not lift an assembled pair of marks by them.

The wall where the lug is welded is checked separately by the designer. In a thin-walled tube or multi-sided pole without reinforcement it may be crushed, and then a doubler plate or rib is put under the lug. Lug welds are inspected, and the extent of inspection is set by the project.

Flange bolt holes are not lifting points. They are designed for a different load, have clearance for the bolt, and a sling through such a hole is a typical mistake, covered below.

For galvanizing a lug enlarges the envelope, so its protrusion goes into the bath check: see size limits of items for press, bath and oven. After galvanizing the hole becomes smaller by the coating thickness, and the diameter for the shackle is taken with an allowance for it; the article on design for galvanizing gives, as a guide for flat surfaces, an allowance of at least 1 mm. Lugs also serve for hanging in the bath, but the hanging points are agreed with the galvanizing plant.

If a lug is cut off after erection, bare metal is left at the cut. Areas up to 10 cm², and up to 0.5 % of the surface in total, are repaired with zinc-rich paint or by spraying, as described in the article on zinc coating thickness. So the project states at once which lug stays and which is removed.

Slings, zinc and sharp corners

The threat comes from both sides here. A hard sling scratches the zinc, and a soft one is cut by a sharp edge. Hence a few simple rules.

• a steel rope only in a rubber-fabric sleeve, a bare one is not allowed;
• pads are put under sharp edges (flange ends, cuts, ribs, section corners), and the rules require this;
• the sling is put on the main body of the element, without knots or loops, and is not hooked onto protrusions not meant for lifting;
• a pad that protects the zinc coating is put at the sling contact;
• elements are stored on wooden supports, and between bundles in a stack pads of at least 40 mm thickness are laid so that the sling can be pulled out;
• scratches from slings are repaired like chips: areas up to 10 cm², up to 0.5 % of the surface in total, with zinc-rich paint or by spraying.

How to store galvanized elements so that white rust does not appear is covered in the article How to store galvanized structures and what white rust is.

Lifting and removing the slings

• before the lift the mass is checked against the tags and the list, the tags of the slings are checked, and so is that the load is not fixed, jammed or frozen down;
• the load is raised by 200–300 mm and the slinging and the brake are checked, and only then the lift continues;
• the load is not held or turned by hand; for turning there are guide hooks and tag lines;
• it is not hung from one horn of a two-horn hook and slings are not adjusted by blows;
• a jammed sling is not freed with the crane;
• the slings are removed only after the element is firmly fixed. For a pole that means all nuts on the foundation fully tightened, as written in our articles on erecting a switchyard gantry and erecting multi-sided poles.

Supports are laid under the element beforehand so that the sling comes out without a jerk and without tearing the coating. Acceptance of the pole after the lift, including the check of verticality before the slings are removed, is described in the article on acceptance checks of a multi-sided pole.

Common mistakes

• slinging by a flange or bolt holes without calculation: the flange is designed for wind, not for lifting;
• one sling on a long pole or beam: the element sags and slides;
• short sling legs and a large angle between them: the force in the legs grows and the element is compressed between the points;
• a choker hitch on a tapered pole without a stop: during the lift the loop slides to the thin end;
• a bare rope or chain on zinc and no pads on sharp corners;
• twisted legs, knots in webbing, hooking onto one horn of a hook;
• selecting a sling by the mass of black steel: the mass of zinc and fittings is not counted;
• removing the slings before the final fixing on the foundation.

What to state in KMD and in the request

• the mass of each mark: black steel and with zinc separately;
• the position of the centre of gravity and, for non-uniform elements (with arms, with a heavy flange), how it was calculated;
• the position, dimensions and design mass of the lugs, and what is done with them after erection;
• the slinging scheme: leg lengths, angles, the spreader beam, the direction of load on the lugs;
• whether elements are lifted one at a time or in a bundle, and the mass of the bundle;
• the requirements for protecting the coating: soft slings, pads on edges, the place of storage.

We fabricate steel structures to the customer’s drawings in our workshop and prepare KM and KMD drawings ourselves: KM and KMD design, steel fabrication. Erection, including the lift, is done by our partner construction company, and the scheme of each lift comes from its works execution project. What else to pass to the fabricator with KMD is covered in the article on the package of documents for the fabricator. We do not put the design mass on the lugs in the KMD by default, but we do so when it is needed.

Standards and validity

Requirements for slings, slingers and loading and unloading work are contained in NPAOP 0.00-1.80-18 «Occupational safety rules for the operation of load-lifting cranes, lifting devices and related equipment» (order of the Ministry of Social Policy of 19.01.2018 No. 62, registered with the Ministry of Justice on 27.02.2018 under No. 244/31696, in force from 10.04.2018, no amendments in the current edition). From it we took the design angle of 45° and 60° and the factors 6, 4, 7 (section VIII, chapter 22), the duties of the slinger (section VI, chapter 6, cl. 17, 19, 20), the requirements for loading and unloading (section VII, chapter 3, cl. 4, 7, 18–20, 22, 25) and the inspection intervals of slings and spreader beams (section VI, chapter 3, cl. 39).

The design of steel structures and the recommendation to divide them into dispatch elements with regard to transport and the lifting equipment of the erectors come from DBN V.2.6-198:2014 «Steel structures. Design standards» (in force, edition with Amendment No. 1 of 2022, cl. 5.2.1). Hot-dip galvanizing of steel structures is covered by DSTU EN ISO 1461:2024.

The slings, spreader beams and grabs themselves come with the manufacturer’s passport; which current DSTU corresponds to each type of sling is checked before it is cited in a works execution project. The formulas and figures in this article are elementary mechanics and the requirements of the rules; the scheme and calculation of a particular lift are made by the organisation that carries it out.