Lattice Tower Question

[marvin]

Given the same materials and side dimension what has greater strength a triangular or square cross-section. Any benefit to either other than lower material count on 3 sided?

thanks,

Marv

[Stonebrain]

I think,comparing two towers of the same uniform weight,

posed on the ground one square and one triangular(same height),the

triangular will fall over at a lower horizontal force at the top.

A pentangular will come out even better.

Ok,this doesn't mean anything about strength.I think,but this

is a pure intuif speculation,that if you compare material against

strength(infact 'strength' is a bit of a simplification of several

parameters) the triangular might come out best in a theoretical way.

But why did they build pyramids with a square base and not with a triangular

one?Well the answer is easy.A square is an easy to handle geometrical form.

If ever you try to design a triangular tower you'll know what I'm talking

about.You will end up with a bad headage.

If you're square minded like most of us buildf a square one.If you're

nuts,like me,try a triangular one.

cheers,

stonebrain

[Nando]

Using the same materials for both ( triangular and square), the square wins by a greater than a 1/3 greater strength.

Nando

[stephent]

Moot point if it's commercial and within it's rated load/wind load bearing capacity's.

If making your own, make a good copy of a commercial tower...3 or 4 legged.

Either one will work....it's not the most/best, it's the engineering and design/workmanship.

I've used both for towers....pick one or build one that has the correct ratings for intended use, add a good safety margin to what you think it will have to support or endure. (buying or building)

[Stonebrain]

Nando, I'm impressed by the great precision of your statement.

You must have done some sophisticated math to come there...

cheers,

stonebrain

[Shadow]

I think the jury is still out on this one.Wincharger, among others used the 3 legged towers for most if not all of there electric wind turbines. That may have been more to do with lighter weight of the wind generator compared to often very heavy water pumpers.I dont recall ever seeing a water pumping windmill mounted on a 3 legged tower.

             I have been told by two different, what I would consider reputable sources, that the three legged towers are more stable in high winds.4 legged towers tend to twist, then buckle under pressure. A 3 legged tower is more stable no matter which direction the wind hits it.Round is stronger than square, 3 legged towers are closer to round. But we may need a structural engineer to decide this one.

[woodnmetalguy]

Aermotor made both 3 and 4 legged towers for water pumping.  Lots of the 3-legged ones around here, including the one in my yard, which I still use for pumping water.  The platform is about 40 feet up and the mill is 10 feet in diameter.

[Shadow]

Cool! Around here is mostly all Challenge towers. So I guess 3 legged ones were used for water.I have a Challenge dealers catalogue from 1941, they just associate all the 3 legged ones with wind generators. 6 1/2-10 foot. A 60 foot tower, 3 legs was $100.00,4 legs was $147.00. But there was also different leg spread dimensions for different weights.They list a 22 foot diameter water pumping unit with 27 slats on the wheel, unit weighs 5300 lbs! That is alot of weight on top of a 60 foot tower!

[harrie]

Talking about towers, Im wondering if one could beef up a triangle light weight TV tower, by welding a scedule 40 1 inch black Iron pipe to the inside of each of the three corners? There seems to be alot of these around now, sence satilite took over!

[Scotth]

Marvin,

 I'm using a three legged TV tower,30' tall with another 6' of pipe up top to hold my 11'mill.It's on a tilt base set in about 600 lbs of concrete,guyed on each corner in two places.I raise it with a hand winch and 12' gin pole mounted to the base.It is rock-solid,raises and lowers with virtually no deflection,and has stood up to some frightening winds without a problem.It's the heaviest grade of TV tower you can get and the best part-it was free.I have another 10' to add when I get around to moving the guy wires farther away from the base.

Scotth

[bj]

     Marvin--Beeing a welder, I seem to remember something from

school about a triangle being the strongest structure.  What I

can't remember, however, is in what dimension the load was.  I

think they were talking about compressive load, which kind of

fits here.

    On a more practical note though, I have a five foot genny

on a cheap (thin) triangle tower on top of my shop.  There is 12

feet unsupported by anything.  Not a good plan, but is just an

experiment.  Two years now, and it is still up.  Bent, about

4 inches off plumb, but still up.  The bend came from winds

that were strong enough to cost me 14,000 for new shingles for

my house.

    Logic tells me that more steel, more strength.  But, I think

shape has a lot to do with it.

    So, I guess is what I am saying is investigate. Kind of like

what you are doing.

    bj

[Titantornado]

You're correct.  Triangle is strongest form in geometry.  But a "square" four leg tower, with proper diaginal bracing (which forms triangles) makes it just as strong and stronger than a triangle three leg tower.  Look at a crane's boom, and you'll see a four legs with a lot of triangles making up it's structure.

[Stonebrain]

Shadow,

Sorry to drag down your reputable sources,but they're aside on some points.

1)stability

   The more legs there are, the more its like a cirle,

   Less there are legs,the force needed to make it fall is bigger

   (depends on the direction of the force too)

2)strength

  if ever you do metall work,you should know that square tubing is stronger

  (stiffer)then round.If I extrapolate this to triangular I guess triangular

  will be 'stronger' then a square because the more there are angles the more it's

  like a circle.

By the way,comparing triangle to square is a bit like comparing apples to pears.

Do you have to compare a triangle with a square that has the angles on the same

circle or do you compare a circle and a square with the same surface.The results will be diffferent.

cheers,

stonebrain

 

[Old F]

stone brain

For some real fun try rapping your head around

this type of tower. : )http://www.fieldlines.com/story/2003/12/25/172023/12

Old f

 

[thefinis]

Well it all depends. The triangle is the stronger structure but as Rod points out if you use diaginal braces on a square then it is triangles that make a square. Mainly you need the tower strong enough to support itself and the turbine especially while being lifted into place after that most of the side loads are carried by the guy wires. Most of the tower failures i have seen happened while lifting it into place or from incorrect guying or guy wire failure.

For a free standing tower I would go 4 legs with as much distance between legs at the bottom as possible. Windmill tower style as this gives you 4 spots to anchor to the ground.

Finis

[Shadow]

Here's a little experiment learned in grade school,

http://www.abc.net.au/science/experimentals/experiments/episode22_1.htm

  Round is almost ALWAYS stronger than square tubing! Check out your teleposts in your basement, bicycle frames, motorcycle frames, roll cages, Silos, free standing water towers, Square has 4 weak spots, the corners. I do some metal work and have always known  round is stronger than square especially in a vertical poition.

   " Less there are legs,the force needed to make it fall is bigger

   (depends on the direction of the force too)"  I think this is  what I'm try to say.

[redeyecow]

 

 I've got a question that's been bugging me for quite a while, having used many tripods for yanking motors, butchering beef and slinging sick horses.  Once the cental weight is applyed to the legs it becomes extremely stable even with poles that are light for the job. I think it's because the force is verticaly compressive. Anyway what about a  chain or cable tightened to a central anchor? Does this create long triangles which include the cable, the leg ,and the ground making the structure far stronger than just anchoring the legs?  Keith

[Stonebrain]

Shadow,

Think you're right on the second point.

Can't believe it's a mistake that all modern

wind mills are on a cilindrical tower and not a square one.

On the first point I meant to say the oposite of what I said:

I think,and I can explain in a dynamic diagram,that a triangular

tower can fall over at lower horizontal force than a square one,

with the conditions that both towers have their feet one circle with

the same diameter,have the same height,the same uniform weight.

But this difference is small and has no practical importance,

as the feet normally are well anchored in the ground.

Thanks for pointing me my mistakes.

cheers,

stonebrain

[woodnmetalguy]

Here's a photo of my Aermotor tower looking down from near the top.

-- Dave