[TowerTalk] Real numbers for Rohn BX-64 Re: guying

[Steve Maki]

Jim Lux wrote:

>>>>I'm not quite sure how you so quickly came up with a number
>>>>for base bending moment in the guyed configuration. It seems
>>>>unlikely to me that it can be even 1/4 of the unguyed number,
>>>>unless the guys were left slack.

>>>I looked up a uniformly loaded beam with fixed end on one end and pin
>>>joint support on the other.  the moment is 1/8 w*l^2 compared to 1/2
>>>w*l^2 for fixed/free.  FWIW, the same 1/8 w * l^2 applies for a pin/pin
>>>support.  (because it's really like two half length beams.. and the
>>>moment goes as the square of the length for uniform loading).

>>I can't quite get my head around this. Without knowing how
>>much the top end moves, or the middle bows, how can you
>>know the bending moment? If the top does not move at all,
>>and the tower is perfectly rigid - is there still a base
>>bending moment? I would think there is only horizontal
>>shear force in that case (and straight downward force).

> Yes, there's still a moment.  If it's perfectly rigid, there's no deflection.
> 
> A moment is a just a torque (i.e. a rotational force, as opposed to a 
> translational force, like a shear or compression or tension). In static 
> analysis (which this is), the torque must be resisted by an equal and 
> opposite reaction force from the the support (the ground, in this case).  A 
> pin joint cannot have any moment, because it pivots.

And yet you said just above that the moment would be the same in a 
pin/pin support.

If I had three bathroom scales I'd go do a little experiment, but
I don't. So let me ask you what I'd find if I did this:

Take a 10' three legged tripod, and put a scale under each leg.

Back guy the tripod using a 45º guy angle.

Pull on the top of the tripod horizontally away from the guy.

If the tripod face width at the base is 3', and you pull with
10 pounds of force, what do you read on the three scales?

Steve K8LX