Design a steel lattice tower

Lattice towers carry antennas, power lines, lights and wind instruments. They are tall, light and governed almost entirely by wind. The Lattice tower template in Struxt3D builds a square tower that tapers from the base to the top, with four legs, X-bracing in every face, horizontals at every panel level and plan bracing to keep the square shape.

#How a lattice tower carries load

A tower is a vertical cantilever. Wind on the members creates a shear that grows down the tower and an overturning moment that grows even faster. The four legs carry the moment as a couple: the windward legs are pulled up (tension), the leeward legs pushed down (compression). The X-braces in each face carry the shear between levels, like the web of a truss.

The taper puts the legs far apart at the base, where the moment is largest, and keeps the top compact where antennas are mounted. Plan bracing (the diagonals across the square at every fourth level and at the top) stops the tower twisting and holds the faces square.

#Build the model

1. Pick the template: Towers › Lattice tower.
2. Set the size using the parameters below.
3. Choose the materials. Steel, with groups Legs, Bracing and Horizontals. Pick the code first: it sets the wind rules and steel grades.
4. Set supports and loads. Choose Fixed or Pinned bases. Lattice tower legs are often treated as pinned at their base plates; the template's horizontals are framed members, so the tower stays stable either way. Set the Wind speed for your site — this is the load that matters. Switch on Pad footings to size a footing under each leg.
5. Run the analysis (F5).

#The template's parameters

Parameter	Default	Range	What it does
Height	30 m	10–80 m	Total height of the tower.
Panels	10	4–30	Number of bracing panels up the height.
Base width	6 m	2–15 m	Leg spacing at the ground.
Top width	2 m	1–6 m	Leg spacing at the top (never more than the base width).

#Loads

• Dead: self-weight of the steel. The template has no antenna loads; add them with the Point load tool at the top joints if you need them (the tool adds 10 kN, edit the value in the panel).
• Wind ±X and ±Y: drag on every member. Struxt3D takes the wind's component across each member, a force coefficient of 2.0 on the section width and the code's pressure at the member's height. Because the tower is square and symmetric, ±X and ±Y give the same leg forces in mirror image.
• Seismic: optional. For a light steel tower wind almost always governs.

There is no live load on a tower, so the combinations reduce to dead load with each wind direction. See Wind loads for how the pressure is worked out for each code.

#What to check in the results

1. Leg forces. Show Axial with the Envelope. The largest compression and tension are in the bottom panel. Compare the two: compression = self-weight share + overturning; tension = overturning − self-weight share.
2. Bracing. Pick a wind case and show Axial: in each face one diagonal of each X is in tension and the other in compression. The bottom panels carry the most shear.
3. Top movement. Show Displacement under the service combination. For towers carrying dishes the twist and tilt at the top are often the real limit, not strength.
4. Reactions. The Reactions table gives the uplift and compression at each leg base — the numbers your foundation designer needs.

Quick check of the leg force from the overturning moment M at the base:

#Design the tower

Click Run design (F6). Legs are checked as steel columns (compression with flexural buckling over the panel length, plus bending from the framed horizontals); the X-braces as truss members in tension and compression; the horizontals as beams. Auto-sizing gives each group the lightest passing section — the same leg section all the way up, set by the bottom panel.

With steel connections on, the bases get base plates with anchors (designed for the uplift as well as compression) and the brace ends get gusset plates. Show Steel connections to see them, and open a base joint for its checks and sketch. See Steel connection design.

#Common problems and fixes

What you see	Why	Fix
Bottom legs fail in buckling	Long panels at the base.	More panels, or a wider base to reduce the leg force.
Large tip deflection	Narrow base for the height.	Base width about 1/6 to 1/10 of the height.
Base uplift is large	Light tower, big overturning.	Wider base; foundations heavy enough to hold the uplift.

#Rules of thumb

For a single-pole alternative see the monopole tutorial.