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[TowerTalk] Rebar and Concrete

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Subject: [TowerTalk] Rebar and Concrete
From: (Ron Chambers)
Date: Fri, 03 Apr 1998 08:49:20 -0600
General Rebar/Concrete Structural design considerations:

Concrete is great in compression but has relatively little strength when in
tension .......simplest case, a concrete pier or pile when just supporting
vertically (key word) a load and to keep it from sinking in the earth is
mostly compression, concrete steps in and handles the load....but other
external forces always enter the equation, such as wind, causing tensile
loads to be placed on the the concrete needs help......rebar is
great in tension, so rebar's #1 purpose is then to add tensile strength to
concrete.....most typically it is placed at the 'edges' of a pier (or at  top
and bottom in slabs), where the greatest tensile stress is placed when
loaded in that mode.....(example, look at the size, location  and density of
rebars in flat slabs and roads)........

Tie rebar to the tower? - Analyzing forces down in the pier, as in the
case of using a tower section in the base, you will see the forces the
tower legs exert are compressive into the concrete, whether vertically
just supporting the tower weight, or laterally when the tower is being
pushed by the wind, again concrete takes the load.......rebar is of no help
there in the center of the pier where the legs are, even if tied to the
tower leg....... rebar is needed out at the 'edge' of the pier adding tensile
strength there to prevent the outside edge of the concrete from
failing/cracking when the same external forces as above are at work
putting tensile stresses along the outside of the pier...

Welding rebar? - Earlier replies explained the practical difficulties with
welding rebar, time consuming, expensive, etc....but most importantly,
when the bars are placed at 90 degrees to each other there is no
structural value gained in welding the bars ....Now, big difference when
you are talking about lapping the bars (that is, when you have a long
continuous run and need to 'connect' one bar to the next bar in-line),
there are very specific design rules for how much overlap is required,
because in this case, you have to 'continue' the tensile strength of the
bar......welding is used here if needed or wanted  but there is a more
common mechanical compression splice that is designed for this purpose
and is less expensive (quicker)......

Grounding? That is 100% electrical....use the reinforcing steel if you will carry current too......I just think the important thing is 
that a
quality, current carrying, grounding conductor be connected from the
tower base to your ground rods or counterpoise system......include the
reinforcing steel in the loop as an 'extra'....won't hurt a thing.....but
doesn't add much IMHO...

Lightning Protection? - if you have ever looked at actual lightning
protection conductors you would be suprised at how light/small they are,
there are typically aluminum, roughly about size 4/0 stranded.....current
carrying capacity of only a  couple hundred amps or so if compared to a
similar conductor being used as a power conductor if free air....


Wish I had said    "....QRper's are the reason contester's have to have
big amps and big antennas..."

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