[TowerTalk] galvaniizing heat treated steel

[Jim Lux]

On 12/28/12 10:30 AM, Grant Saviers wrote:
>
>> In the ham antenna situation, I have no idea if fatigue is an issue.
>> Wind loads are repetitive, but generally quite small, so they may not
>> get up to where fatigue is an issue.
> I think the rule of thumb is that fatigue life is unlimited if the
> cyclic stress doesn't exceed 25% or so of the tensile stress.  At 50%
> stress, life is about 10,000 cycles from a reference I found, which
> could happen in a mast.  One time high stress substantially reduces the
> fatigue life.  So designing with a big safety factor is a good idea.

Yeah.. consider the usual gusty wind.  You get a gust every few minutes, 
and you could get 1000 cycles in a weekend.


>>
>> Yes, a lot of folks invest in strong steel for their masts, and it
>> works just fine, but from an engineering standpoint, one wonders if
>> it's worth the extra expense.  Putting up cold rolled 1020 at half the
>> ultimate yield might have survived just as well.  Or, alternately,
>> spending a few hundred bucks to not worry about it might be worth it.
> Previous posts have emphasized the benefit of increased diameter for a
> mast.  Generally, going thicker than 0.25" wall is not a good investment
> and has excess weight.  A 2.5" od mast with 0.25" wall has more than 2x
> resistance to bending and increase in strength with a 29% weight
> increase vs 2.0"od and .25" wall .  A 2.0" od by .375" wall only
> increases strength by 27% with 50% more weight.
>
> Also, heat treating and 4130 costs money.  Normalized 4130 has a yield
> strength of 63,000psi and is tough to machine, yet is available heat
> treated to more than 3x that, so if cost is no object or weight/diameter
> is a constrained, then heat treating 4130 is a good choice.  OTOH, A513
> steel has a yield of 72,000psi and is about 40% the cost of normalized
> 4130, so for me it is my choice for high strength DOM (drawn over
> mandrel) tube.
>>
>> It's not like people instrument their masts to measure the actual loads.
> I think the mast calculators are pretty decent estimates of loading.
> There are also numerous web applets for calculating stresses and
> deflections in beams with almost any load configuration that a mast will
> have.  It is instructive to see how much a mast can bend without damage.

I've done a fair number of cheesy portable schemes, so I've seen a lot 
of "bending with and without catastrophic failure" with all sorts of 
materials.. PVC pipe, aluminum tubing, wood, steel EMT, fence rail,

I think I'd look to categorizing the possible failure modes and their 
consequences/repair cost with an eye to minimizing "brittle" systems 
that fail hard and disastrously.  Is a slightly bent mast better or 
worse than one that snaps off?  What's the distribution of wind speed 
events.  Here in Southern California, you get lots of 10-30 mi/hr wind, 
but once a year or so thare's 50-70 mi/hr events, but never a 120 mi/hr 
event.  If you lived in Key West, I think the distribution of speeds is 
somewhat different.  But around here, designing to survive a 50 mi/hr 
event, as long as the failure was soft and easily fixable would probably 
be acceptable. If you were building for a remote site and servicing is a 
pain, then designing for 90-100 mi/hr might be a better strategy.  (and 
of course, your insurance company or local building code enforcers would 
probably want to weigh in, as well)