Foundations for columns, masts and poles: what a steel structure designer needs from DBN V.2.1-10

Who passes what at the interface

The code divides responsibility simply. The foundation designer chooses the type, depth and size of the foundation from the survey results and the loads supplied. The steel designer is responsible for what stands above the foundation and for the data the foundation is calculated from. An error in that data becomes a foundation that is not calculated for the real pole.

We fabricate poles, masts, columns, steel shell piles, anchor cages and embedded parts and prepare KM and KMD drawings. We do the foundation design as a separate order for a separate fee, but most often we only supply the foundation loads and the customer or their foundation designer designs it. Our part of the interface is the input data and the anchor detail.

Loads at the base plate level

A pole passes more than its weight to the ground. The foundation designer needs three components: axial force, bending moment and shear. For a frame column they come from the roof, cranes and wind; for a mast or tower mostly from wind on the structure and equipment; for a power line pole also from conductors, earth wires and ice. Why the moment usually decides the foundation size is explained in the article on foundations for masts, towers and line poles.

The code requires the foundation to be calculated for two groups of limit states: bearing capacity of the ground and deformations. Each group uses its own combination of loads, so pass both sets, not one “design” figure. Mark the duration too: snow load and floor loads are treated as long-term in the deformation check and as short-term in the bearing capacity check. Loads from moving lifting equipment are short-term in both.

Types of loads and actions are taken from the loads code (DBN V.1.2-2:2006). For line poles the foundation code refers to the rules for electrical installations: loads from conductors, earth wires and line fittings and their combinations are determined under them (see loads on overhead lines). A line pole foundation is checked for all operating cases of the line, and the dynamic effect of wind gusts on the pole structure is taken into account only in the bearing capacity check.

What else the pole foundation designer checks

For line poles, open switchgear, wind turbines and communication antenna structures DBN has a separate section. For deformations the designer checks the average, edge and corner pressures, the pressure on the top of a weak soil layer, settlement and tilt, horizontal forces at the foundation top and rotation of the shaft at ground level. For bearing capacity: strength and stability of the ground, sliding on the base and along a weak layer. The foundation itself is calculated under the reinforced concrete codes: punching of the slab part, bending, cracking and crack width.

By the nature of the load, line poles are divided into suspension, tension and angle poles. Depending on the direction of the load component, foundations either overturn or are pulled out. For pulled-out foundations the reliability factor for importance differs between suspension, tension, angle and special poles. So always state the pole type and the fixing scheme in the input data.

Limiting deformations: tilt, settlement, differential settlement

The deformation check sets the foundation one condition: the calculated combined deformation of the ground and the structure must not exceed the limiting one. Limiting values are set by the design codes of the structure itself, the operating rules of the equipment, or the design brief. If they say nothing, and the structure is not specially designed for forces from uneven settlement, the values from the recommended annex of DBN may be used.

A practical consequence follows: the permissible tilt of a mast, tower or row of poles must come from us or from the equipment maker, not be guessed in the foundation design. The table gives the order of magnitude from the previous edition of the code (2009) for the structures closest to our range. Check the current values against DBN V.2.1-10:2018: they may differ.

For line poles the relative differential settlement is taken over the base L: the distance between the axes of the foundation blocks in the direction of the horizontal loads, and for guyed poles the distance between the axes of the compressed foundation and the anchor. If the ground is made of horizontal layers of even thickness, the code permits the limiting settlements to be raised by 20 %. The values are recommended and apply when the brief sets no others.

What a tilt of 0.002 means

A tilt of 0.002 is two millimetres per metre of height. On a 30 m mast the displacement of the top from foundation tilt alone is 60 mm, and the deflection of the structure itself under wind is added to it. For antennas the tolerance may be tighter than the code value, and it is set by the equipment maker (see communication towers).

For hinged systems and rigid tower-type structures the code permits limiting deformations by strength and crack resistance of the structure not to be set: no forces from uneven settlement arise in them. This does not remove the tilt limit for service, that is, the verticality of the antenna, luminaire or shaft.

Moment, eccentricity and base lift-off

Foundation tilt is calculated with the load eccentricity, that is the moment at base level, the influence of neighbouring foundations and uneven compressibility of the ground. The embedment of the foundation, the stiffness of the superstructure and the fact that tilting itself increases the eccentricity are also taken into account: a pole that has already tilted a little loads the ground even more unevenly. In water-saturated clay soils the tilt from creep of the soil skeleton is added, that is, it grows with time.

The sizes of eccentrically loaded foundations of columns of industrial buildings, including those with overhead cranes, and of tower-type structures are chosen so that the pressure diagram is trapezoidal. A triangular diagram, where an edge of the base lifts off the ground, is permitted only with justification. This is agreed with the foundation designer before the KMD drawings are issued: the lift-off permitted changes the calculated tilt, which comes back to us.

Surveys: what goes into the brief and what the report must contain

The code does not permit designing the ground and foundations without sufficient engineering justification. The survey brief must contain the structural solutions of the object, the intended foundation types and depth, and the loads on them. So the surveyors get this from the foundation designer, and he gets it from us: where each pole stands, what it is and with what loads. A borehole outside the pole’s site says little about the soil under it.

The survey results must give everything the foundation type and depth are chosen from: a section with weak layers, soil properties within the deformed zone, groundwater level with a forecast of its changes (perched water, seasonal fluctuation, man-made changes), aggressiveness of water and soil, and frost depth. The foundation depth is set with the loads, the depth of neighbouring foundations and utilities, the relief, the ground conditions and frost in mind. The standard frost depth is taken as the mean of the annual maxima from observations over at least 10 years.

The code singles out difficult conditions: weak cohesive soils with a deformation modulus below 5 MPa, loess, swelling, frost-heaving and made-up ground, undermined areas, landslides, karst and seismic regions. On frost-heaving soils the foundation is checked for the tangential frost heave forces acting along its side surface. If the site falls into these conditions, say so in the brief for the pole: the height, equipment and permissible tilt may change the choice of foundation type.

Aggressiveness of the environment concerns our parts too: steel anchors and embedded parts in aggressive soil or water need protection, which the design determines. General approaches are in the article on corrosion protection.

The anchor detail: where the code is silent

DBN says nothing about anchor bolts, embedded cages and templates: that is a matter for the pole design and the foundation drawings. Of the general requirements one applies to the detail: the limiting ground deformations are chosen so that connections are not disturbed and the design positions of structures do not change. So tilt or settlement is not absorbed by fitting the flange; it is built into the tolerance for the foundation. The annex on measures against the effect of ground deformation directly mentions devices for levelling structures. For a pole these are levelling nuts under the flange, provided in the detail in advance.

Before the anchor cage is made, the following is agreed with the foundation designer:

• the layout and diameter of the anchors from the drawing of the pole flange;
• the steel grade, embedment length and anchoring method that follow from the foundation calculation;
• the level of the concrete top and the projection of the anchors above it;
• the tolerances on anchor position and the way of checking, which is recorded in the as-built scheme.

Tolerances and checks are covered in the articles on foundation anchor cages and anchor bolts. We make the cage; the foundation size, concrete and reinforcement are a matter for the foundation design.

A new pole on an existing foundation

If a pole is to stand on a ready foundation, anchors that match the layout are not enough. For reconstruction the code requires a survey of the bearing soil layer, the type, structure and condition of the foundations, and an assessment of their bearing capacity and deformability with physical wear taken into account, with the conclusions confirmed by check calculations. The new loads from the pole are compared with those the foundation was calculated for: axial force, moment, shear and permissible tilt.

We carry out surveys of existing steel structures as a separate service: technical inspection. The condition and bearing capacity of the foundation itself are assessed as part of a survey of the ground and foundations.

Our part of the interface and what to send

We fabricate steel structures to the customer’s drawings, prepare KM and KMD drawings, and make poles, masts, anchor cages and embedded parts: KM and KMD design, steel fabrication. For poles in our range the loads at the base are given in the catalogue (for communication towers on the Communication towers page); for custom structures they follow from the KM calculation. Erection is done by our partner construction company.

To agree the interface, send:

• the type and height of the pole, or a drawing;
• the equipment on the pole and the place of construction;
• the tilt requirements from the equipment maker, if any;
• the survey report, if it has been done;
• details of the foundation, if it is ready or designed: drawings, the anchor layout.

Standards and validity

The article relies on DBN V.2.1-10:2018 “Bases and foundations of buildings and structures. Basic provisions” (according to open sources in force since 1 January 2019 and replacing the 2009 edition) and on DBN V.1.2-2:2006 “Loads and actions”. The limiting deformation values in the table are given from the 2009 edition as an order of magnitude. In the current edition they, and the numbering of sections and annexes, must be checked.

The validity and current edition of each document named need checking before it is cited in a project. The article does not replace the design of the ground and foundations and is not a calculation: foundation parameters are determined only by the designer from the survey results.