Spun Concrete Pole Design

A spun concrete pole is a hollow circular section that tapers upward, typically at about 1.5 %. Where the loads act at several different heights, one section check is therefore not enough to find the governing section: a demand that varies along the pole has to be reconciled with manufacturing pieces that are constant.

StructuralMind's spun pole design treats the two reinforcement layers separately and splits the work into two stages. The first answers "what is required" along the whole pole. The second answers "is the reinforcement you used enough". The automatic design searches the candidate diameters for the smallest that covers each station, and reports any station it cannot cover. You can define as many longitudinal and transverse zones as you like, so the layout can be kept simple to fabricate.

Two independent reinforcement layers#

The two layers have independent boundaries: a transverse zone need not line up with any longitudinal zone. That flexibility matches how poles are actually made, but it has a consequence — what works at any given height is the pair of the longitudinal steel covering that height and the stirrup covering it, and that pair is specific to your design preference.

With a double wrap, the effective spacing is half the stated pitch. Producing a loose pitch by wrapping it twice is easy in the shop. A closed hoop has no wrap, only spacing.

Two stages#

Stage A — what is required#

A full section design runs at stations along the pole, and at every height a load acts on. Each station is sized with its own six-component force set ($N$, $V_2$, $V_3$, $M_{22}$, $M_{33}$, $T$) under full PMM + shear + torsion interaction. The output is a continuous envelope of required longitudinal steel and required stirrup/spiral that varies from station to station.

Why is the design station-based? On a tapered pole the governing section usually does not coincide with the point of maximum internal force. The moment grows downward while the section grows downward as well, and because the two compete, designing only where the internal force peaks can miss the governing section. There is a second, practical reason: the taper means the reinforcement used at the base is curtailed towards the top, and the heights where it is curtailed have to be designed too.

Stage A's envelope cannot be manufactured as it stands. You cannot change the reinforcement continuously from one station to the next. Stage A describes the ideal recipe, and the buildable automatic suggestion is derived from it.

Stage B — is the reinforcement you used enough#

The automatic suggestion takes the governing result from Stage A and lays one longitudinal zone and one transverse zone over the whole pole. Depending on how the loads sit along the pole, that layout may not be the most economical or the most practical one to build. Stage B lets you define independent longitudinal and transverse zones, so you can follow the required envelope and optimise the bars and stirrups of the pole you intend to produce, visually.

Within a zone the bar count and diameter, the stirrup/spiral diameter, the stirrup/spiral spacing, the stirrup shape (spiral or closed hoop) and the single/double wrap option all stay constant. At each station the zones covering that height are found and the real demand/capacity ratio is computed under the same full interaction. A longitudinal zone's result is set by the most critical station in the range it covers, and transverse zones are handled the same way. Whether the bars physically fit the section is also checked.

Letting the tool lay out the zones#

Instead of building the zones by hand you can have them computed. The problem is this: cover a required-reinforcement envelope that decreases with height using at most the number of constant steps you allow, without dropping below it anywhere. That problem has an exact solution; it is not found by trial and error but by computing the lightest layout directly for every zone count.

The answer is not a single recommendation but a curve. Steel drops as zones are added, yet the gain flattens quickly. Where you stop depends on what labour costs against steel in your own shop, so you make that call by reading the curve.

Two more things enter the calculation:

• Every candidate diameter on one chart. A thicker bar quantises more coarsely: meeting the same area with fewer bars means extra steel. A thinner bar looks cheaper but adds labour and may not fit the section. The diameter curves cross one another, so which diameter wins cannot be read by eye.
• Cutting from stock bars. Bars are cut from 12 m lengths and the offcut is scrap, so a layout with a lower theoretical takeoff can cost more once the offcut is counted. Zone boundaries are therefore also searched for cut lengths that sit better in a stock bar. Where the pole is longer than the stock length a splice becomes unavoidable and the lap allowance of every splice is included.

The recommendation points at the layout that buys the least steel. Where several layouts buy the same steel, the one with the fewest zones is chosen: the same money, less labour.

The calculation rests on three assumptions. Bars are taken as equally spaced around the section. The takeoff covers longitudinal reinforcement only, stirrups excluded. The clear spacing between bars is reported for every zone but is not checked against a code limit; whether the congestion is acceptable is your decision.

Why is the check done in two stages?#

The interaction physics of axial load, bending moment, shear and torsion is the same in both stages. The difference is between what Stage A designs and what Stage B verifies.

1. The optimum values at the stations point to a different reinforcement at every station, so they are not suitable for manufacture. Using that interaction physics, Stage A gives every station its own ideal bars and stirrup. In the shop, one or more zones of constant reinforcement are used along the pole, and that constant reinforcement has to satisfy the most critical station in the range its zone covers.
2. The longitudinal and transverse zones may differ in number and in their start and end heights. Stage A never evaluated the constant bar-and-stirrup pair you placed, because at each station it builds its own optimum pair. Stage A may say "11.3 bars of Ø13.7 required" while you place 12 of Ø14. Stage A reasons in terms of area; whether discrete bars actually fit inside the wall thickness and cover is only visible in Stage B.

Stage B is not a re-derivation, it is a verification gate. Its real value shows when you intervene by hand: splitting a zone elsewhere, reducing a count, moving a boundary or switching to a double wrap. Only Stage B can tell you whether the change still passes.

TEDAŞ safety factor#

TEDAŞ-MLZ/99-34 clause 2.12 defines the safety factor as a ratio of two forces: the tip force that breaks the pole divided by the nominal tip force printed on it. It is neither a material factor nor a load factor. The specification has that ratio demonstrated by a bending test; the calculation produces the same ratio numerically.

• The nominal tip force comes from reducing the horizontal actions to a point 0.25 m below the top (clause 2.6). Vertical actions and self weight take no part in that reduction and are not scaled: gravity does not grow while the rig pushes.
• The breaking tip force is the load multiple at which the first station reaches its moment capacity, evaluated with characteristic strengths. Breaking means the section reaching its real capacity, so no material factors apply here.

Selecting a product class turns the ratio into a design constraint: longitudinal steel is raised until the target is reached. The TS 500 rules keep running alongside it and the stricter of the two requirements governs. The constraint never reduces reinforcement.

The automatic design finishes 1% above the target. Stopping the instant the target is touched leaves no margin, and a small edit such as shifting a zone boundary or changing a diameter then drops the design below it. The verdict is still given against the raw target; this margin is not a tolerance.

Scope. The ratio covers flexure only. Shear, torsion and the detailing rules stay on the TS 500 design basis and are reported as separate verdicts. Torsion carries its own factor and its own test (clauses 2.10 and 2.11) and is not part of this ratio. Second-order effects are not included. Where a catalogue-free material is defined there is no declared characteristic class to anchor to, so the ratio is not reported.

Bending test simulation#

In the TS EN 12843 clause 5.5.2 bending test the pole is laid down, pushed at the tip, and the load and lateral displacement are measured together. The simulation runs that same test numerically on the reinforcement you applied.

The load is swept up from zero. When the first station reaches the peak of its moment-curvature curve, load control ends and the analysis switches to displacement control, which is how the descending branch gets drawn. The output gives four events: the nominal load, cracking, yield and breaking. Each carries its tip force, its ratio to the nominal force, the lateral displacement and the section height at which it occurs.

Yield is the moment the outermost tension bar reaches its yield strength; it marks the elastic limit, not collapse. Breaking is defined by the outermost compression fibre reaching the crushing strain.

Displacements read off a test report can be entered on the same chart and compared against the prediction. Entered values are overlaid on the curve; they do not change the analysis, so adding or deleting a row never moves the reported events.

The expected strengths belong to this simulation alone. Spun production can push concrete strength above the catalogue value, so entering the values you know from production control gives a more realistic reading of what the test will show. The design itself always uses the ordered class.

The simulation issues no acceptance verdict. Whether the design is adequate is answered by the TS 500 check and the TEDAŞ safety factor. Two separate verdicts on one requirement could contradict each other for a purely modelling reason: the design side works with the stress distribution the code prescribes, while the pushover has to use a material model that carries concrete in tension. These poles are already cracked under their own nominal load, so that behaviour cannot be ignored.

The torsion threshold and stepped transverse steel#

On a tapered pole the transverse steel can tighten and step up in diameter towards the top. The reason is that the threshold below which torsion may be neglected is proportional to the torsional section modulus $S$, and $S$ shrinks as the section shrinks ($S = 2 A_e t_e$ for a hollow section). On a tapered pole the section thins as you go up, so the neglect threshold falls with it.

The torsion diagram depends on how the pole is loaded. With a single torsion applied at the top, the torsion is zero above that load and constant below it. When point loads and eccentric loads that produce torsion are defined at several heights, the torsion diagram can vary in steps with height. Whether torsion enters the design at a given station is decided by comparing the torsion there with the neglect threshold there, and the calculation makes that comparison at every station.

Two things then reduce the stirrup spacing. The first is the maximum spacing the code allows, which depends on the section perimeter and therefore gets smaller as the section thins. The second is the stirrup/spiral area needed for shear and torsion together. When the smallest spiral in the candidate list can no longer provide that area, the next diameter up is used.

At the heights where the torsion is zero, the stirrup spacing is set by the shear and detailing requirements alone. If the result is too restrictive to build, there are two moves: thicken the section, or use a double wrap.

One exception: if the governing check is concrete crushing, more or closer spiral does not help. The crushing limit is a function of the section, so there the section itself has to grow.

The spiral diameter is optimised the same way the longitudinal steel is: from a candidate list, the smallest adequate diameter that satisfies both the shear and the torsion condition. There is no single global spiral-diameter input.

Code clauses applied#

The clauses used in the torsion and transverse-reinforcement checks. The calculation's audit output labels every step with the clause it comes from.

Double wrap#

In a double wrap the spiral is wound twice at the same pitch, one turn in each direction. The calculation applies this by halving the effective spacing. In other words, the transverse steel area per unit length doubles. That effective spacing is used in the capacity calculations and the detailing checks: shear capacity, torsion capacity, the maximum stirrup spacing, the minimum torsion stirrup, and the standard-region maximum spacing. A Ø4/100 double wrap is therefore checked against the maximum spacing at 50 mm, not at 100.

The concrete crushing limit depends on the section dimensions, and more transverse reinforcement does not change it.

Rounding rules#

The stirrup spacing the calculation produces is rounded down to 1 mm: a value of 96.3 mm is reported as 96 mm. Poles are made in a shop, so a 1 mm resolution is buildable, and there is no rounding to 5 or 10 mm.

What is outside the scope of this calculation#

The tip load given in the specifications of electricity distribution utilities, TEDAŞ for example, usually already includes the conductor, wind and ice loads. A pole can therefore be designed from a single tip load and torsion. Additional loads that act on the pole, such as a cross-arm, a transformer or a floodlight platform, and point loads and torsions specific to your own design, can be defined at any height along the pole.

Endpoints#

With auditMode on, check returns the full step-by-step breakdown for the governing station of each layer: the section mesh, the fiber state, the N-M surface and every step of the calculation.