[Towertalk] structural assumptions// rope--bolts

[Tom Rauch]

> > *  Bolts specified for racecar use are NOT necessarily stronger than
> > or appropriate for tower use.  They MAY be; it depends on what they
> > were used for.  You need to know the specs to know.
> >    My personal recommendation is to go with Rohn galvanized hardware
> > everywhere.  Zero doubts.  Zero rust.  Worth the price.

I've had original equipment Rohn bolts rust. 

I also doubt you'll find any reasonable quality hardened or stainless 
bolts weaker than the Rohn bolts. Not that it matters, because even 
the Rohn bolt is considerably harder than the material in the tower 
legs at the joints.

> Higher strength bolts can also be stiffer, and can actually take more
> of the load as a result.  I would imagine that the average tower isn't
> a "ragged edge of material science" design, but, the designer does
> choose bolts according to their stiffness and stretch under load.

This sure isn't rocket science. Any hardened bolt of reasonable 
quality will work in a cross-bolted tower joint, the only major 
consideration is rust or corrosion. Flange-joint towers are another 
matter.

The bolts in a cross-bolted tower joint only serve as "pins". They 
are not under stretch or compression, they are only under shear 
forces. Long before the tightening load stretches a hardened bolt of 
almost any grade, you would crush and weaken the joint.

Critical fasteners in cars are a different matter entirely, and can't 
be compared to cross-bolted joint tower bolts. Many fasteners in cars 
MUST be stretched because the bolt is in tension only under part of 
the load cycle, and relaxed in others. Others have wide operating 
tension ranges from temperature changes. (This is worlds apart from a 
fastener that primarily under shear.) You **must** select a bolt 
that, when tight, pre-loads with tension greater than the zero at the 
minimum load condition and has know amounts of stretch, otherwise the 
fastener will quickly work-loose or fail. (Many Caddy V-8 owners 
learned this when GM used the wrong headbolt design, and anyone with 
a race car knows it.) Virtually all automotive fasteners stay tight 
because of stretch, where the bolt or stud actually deforms and 
elongates. That's true even with lug nuts.

In cross-bolted towers, the legs are considerably weaker than even 
fairly poor bolts. The holes will elongate long before even cheap 
poorly made bolts will shear. In an application like this, all you 
have to do is satisfy a minimum shear strength and hardness in the 
bolt. Stretch is unimportant and undesirable, since all of the 
materials around the bolt are much weaker than the bolt and will give 
first. 

The only real worry is if the bolts look ugly from surface rust after 
time, which Rohn bolts sometimes do.73, Tom W8JI
W8JI@contesting.com