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)
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