>Hi Stan,
>
>My point was that you can take the calculations done by a professional
>engineer and modify them to your situation -- specifically mounting
>an antenna on a longer mast than assumed by the engineer (as all hams
>do). This information will provide you far more insight than just simply
>putting it up and hoping for the best. Obviously, these calcs are all
>static load calcs and do not take into account the dynamics than can
>occur under windy conditions (as someone else pointed out). If you want
>to do dynamic modeling, then you're probably talking a level of complexity
>far beyond what a professional engineer is going to do and will need
>a supercomputer to run it.
Hi Bruce,
Here is an example of what I mean when I say I haven't seen the calculations
I think we need. Assume a Rohn 25 tower with the rotator mounted in the
normal place, about 3 feet below the top guys. assume a 20 foot mast
sticking out the top of the tower about 17 feet. Assume 3 stacked Yagis,
say a 10/15 duo bander on top, a 4 element 20 in the middle, and a 2 element
shorty 40 on the bottom. It is no real trick to calculate the stresses on
the mast to determine if it will fold over at the top of the tower or not.
What I want to know is whether the tower itself will bend where the rotator
is mounted using the top guy point as the axis of bending. I have NEVER
seen any engineering calculations addressing that potential problem so I
have nothing I can modify with my own numbers. By the way, I have never
seen a tower fail in that mode either, so maybe it is not a problem. Common
sense tells me there has to be a lot of stress at that point, however. How
much is there and how much will it take?
>The engineering calcs for towers furnished by manufacturers and required
>by building departments are not "rocket science." There just a series
>of fundamental engineering calculations. What a professional brings to
>the calculation is judgment -- not calculational ability. In this case
>I already have the professional engineer's set of calculations, so I
>can "see" how he applied his judgment in certain situations. I'm not
>advocating that hams do their owns engineering calcs to submit to a
>building department. The fact is that most hams erect towers and mount
>antennas on them in a way that violates the configuration used by the
>engineer for his calculations. If that's the case, why is it better to
>just put up whatever one wants and hope it stays up than it is to "get
>a handle on" the ability of the tower and antennas to stay up by
>doing some calculations?
>
>73 de Bruce, WA7BNM (bhorn@netcom.com)
I think you are right in that it is always better to do some calculations
where you can even if your model is not perfect. We are really talking
about two separate problems here. The first one is satisfying the
City/County Building Department and the second one is trying to determine if
the thing will actually stay up or not. I frankly do not have enough
confidence in City/County Engineers to trust that what they accept as "good
design" will actually stay up. All you need to do is study a few
City/County regulations to know the people who write them are often not very
smart about towers. They could make errors either direction. The bottom
line is that once you get the permit in your hand, you are only half way
there. Now you need to find out if your design is REALLY SAFE or not.
Stan W7NI@teleport.com
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