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First, a neat image from Wikipedia

lattice cell tower with triangular cross-section

Here the cell tower has three vertical (or maybe slightly inclined to the inside) thicker pipes which are located at angles of a triangle and those three thicker pipes are interconnected with numerous thinner pipes and this yields a lattice tower with triangular horizontal cross-section.

Clearly you need at least three vertical pipes to make this happen. The design above with triangular cross-section is very popular.

However sometimes cell towers are built with square cross-section - they use four vertical pipes located at angles of a square instead of three pipes. It looks like waste of steel and design complication - what can a tower with square cross-section do what one with triangular cross-section cannot?

I could imagine there're was a difference when a tower had notable load concentrated in one direction. Yet I'm asking about cell towers that carry rather lightweight equipment and mostly have to withstand wind loads.

Why are square cross-section cell towers used instead of triangular cross-section towers?

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    $\begingroup$ Don't underestimate wind load, especially when the tower is covered with ice and the wind crossection is signifcantly more than just the bare metal pieces. $\endgroup$ – Olin Lathrop Mar 2 '15 at 14:47
  • $\begingroup$ @OlinLathrop I know that wind load can be a serious issue but it's equally probable from any side so my guess is a tower with triangular cross-section can deal with it just as well as one with square cross-section. $\endgroup$ – sharptooth Mar 2 '15 at 14:55
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    $\begingroup$ Actually that is one issue of triagular crossection. The difference in strength by direction varies more with a triangle. The triangle has to be built with the weakest direction being able to handle the maximum design load, so is more over-built in the stronger direction than crossections with more sides. $\endgroup$ – Olin Lathrop Mar 2 '15 at 14:59
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You are correct that a triangle is the most efficient basic shape. Once the overall design has some constraints, other factors may hold more sway over the final design.

Here are some of the design considerations that may make a square cross section preferable to a triangular one:

  • Limited ground area - For a given ground area, more steel and thus more strength will be available if 4 legs are used instead of 3. The use of 3 legs may require the distance between each leg to exceed the available area on the ground.
  • Ease of connecting accessories - Notice how the lower transmitter on your picture is greatly skewed compared to the bar that it is attached to. Adding another side to the tower may make these connections more square. This will also affect the antennae at the top of the tower. The anticipated uses of the tower will help to determine this.
  • Ease of making cross-brace connections - Each of the cross braces need to be connected to the main legs of the tower. Typically there are multiple braces connecting at the same location. Making the tower cross section be square will help to make these connections at 90deg, which may be easier to construct or fabricate.
  • Shape of material used for legs - Not all towers use round tubes for the vertical legs. With a round tube, a connection to the leg is equally difficult in any direction. If "L" or square shapes are used, connections are much easier if everything is square.
  • Proprietary designs - Some manufactures may only build one shape or the other. They are used to the design that they typically make.
  • Steel weight - Your argument for triangular cross sections being the most efficient is basically a steel weight consideration. The actual weight of material may not be that large of a factor in the final cost. Adding 25% to the weight of steel will not increase the final tower cost by 25%.
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