Structures
Design a steel monopole tower
Model a tapered tubular monopole with an antenna platform, analyse it under wind and design the pole, platform and base plate in Struxt3D step by step.
A monopole is a single steel tube standing from its foundation, usually carrying telecom antennas or lights near the top. It takes less ground space than a lattice tower and is quick to erect. Structurally it is the simplest tower there is: a cantilever. The Monopole template in Struxt3D builds a pole in segments, a square antenna platform 1.5 m below the top and four antenna mounts at its corners.
How a monopole carries load
Wind on the pole and on the platform pushes the top sideways. The pole resists as a cantilever fixed at its base: the shear and bending moment are zero at the top and largest at the foundation. The weight of the pole and the antennas adds axial compression, and — once the pole leans under wind — a small extra moment from that weight acting off-centre (a second-order effect that Struxt3D's linear analysis leaves out).
Build the model
- Pick the template: Towers › Monopole.
- Set the size using the parameters below. The number of pole segments is behind More options; more segments give smoother force diagrams.
- Choose the materials. Steel, with groups Pole (circular tubes), Platform and Antenna mounts. Choose the code and grade.
- Set supports and loads. The pole base is always fixed — a cantilever needs it. Set the Wind speed for the site. Switch on Pad footings to get the foundation sized from the base reactions.
- Run the analysis (F5).
The template's parameters
| Parameter | Default | Range | What it does |
|---|---|---|---|
| Height | 30 m | 10–60 m | Height of the pole. |
| Platform width | 3 m | 1.5–5 m | Side of the square antenna platform. |
| Weight per antenna mount | 3 kN | 0–30 kN | Weight of the equipment on each of the four mounts, applied at the mount tops as dead load. |
| Pole segments | 10 | 4–30 | How many pieces the pole is divided into (More options). |
The platform frame is a square ring of beams with four arms back to the pole. Each mount is a 3.5 m post at a platform corner, 1 m below and 2.5 m above the platform.
Loads
- Dead: steel self-weight plus the antenna weights (four times the value you set).
- Wind ±X, ±Y: drag on the pole, platform and mounts, growing with height. The pole is round and symmetric, so the four directions give the same base moment.
The antenna weight is applied, but the wind on the antennas themselves is not: Struxt3D sees only the steel members. Antenna panels often catch more wind than the pole. Add their wind force as point loads at the mount tops (use the Point load tool, then edit the force in its panel), with the area and drag coefficient from the antenna supplier. Struxt3D's drag coefficient of 2.0 is also high for a round tube, so the pole's own wind is on the safe side.
What to check in the results
- Base moment. Show Moment: the diagram grows from the top to the base. The base moment is the number that sizes the pole, base plate and foundation.
- Top movement. Show Displacement under the service combination. Telecom operators limit the tilt and twist at the antennas — often more strictly than strength does.
- Platform. Show Moment on the platform arms: they carry the mount weights back to the pole.
- Reactions: the base moment, shear and axial force in the Reactions table.
A hand check for the base moment from a uniform wind w (kN/m) on the pole plus a point force F at the platform height hp:
Design the pole
Click Run design (F6). The pole is checked as a steel tube in compression and bending, the platform members as beams, and the mounts as columns. Auto-sizing picks the lightest tube that passes the strength checks. The deflection check covers beams and slabs under D + L, not the sway of the pole under wind, so compare the top movement with your own limit.
With steel connections on, the base gets a base plate with anchor bolts designed for the base moment, and the platform joints get their connections. Open the base joint to see the plate, bolts and checks. If footings are on, a pad footing is sized for the moment and the allowable bearing pressure — a monopole footing is usually governed by overturning and bearing at the toe. See Footing design.
Common problems and fixes
| What you see | Why | Fix |
|---|---|---|
| Pole fails at the base | The base moment is too big for the tube. | A larger diameter works better than a thicker wall: bending capacity grows with the diameter squared. |
| Top moves more than your limit | Tall, slender pole. | Larger diameter, or a lower height. |
| Very large footing | Overturning moment with low bearing pressure. | Deeper base, or a piled foundation designed separately. |
Rules of thumb
Starting points only.
- Base diameter about height/30 to height/40 for a telecom monopole.
- Diameter-to-thickness ratio within the code's limits for local buckling (thin tubes lose capacity).
- Top deflection under service wind often limited to about height/100, or by an antenna tilt limit.
The same template is a good first exercise in cantilever behaviour: see How structural analysis works.