What you say makes sense.
A structural engineer friend of mine spent several years designing
concrete bridges and similar structures. He told me that, in theory,
smaller reinforcing elements (assuming the same total cross sectional
area of reinforcement) make a stronger finished structure if uniformly
distributed throughout the concrete. Steel mesh in that context lies
somewhere between rebar and steel fibers.
The problem is that it is very difficult to uniformly distribute enough
steel mesh to equal the same cross sectional area as you would get with
rebar, and it is even more difficult to do so with steel fibers long
enough to work properly. As a result, steel mesh is generally used only
in slabs lying on the ground where the concrete is not structural. Most
steel fibers commonly used in concrete construction are 1/2 to 3/4 inch
long and are only used to minimize microcracks ... the codes in most
cases do not allow them as a structural substitute for rebar because
they are too short.
I've heard of techniques to mix lots of long strand carbon fibers with
the concrete before (or while) placing it and I'm confident that would
be extremely strong ... but expensive and (to borrow from another
thread) probably overkill for a tower base.
In general practice you want to use really thick (strong) reinforcement
to hold together the large chunks of any concrete structure and
progressively smaller (but of greater quantity) pieces of reinforcement
for the more localized parts of the structure (such as edges and
surfaces).
Just remember that concrete is extremely strong in compression but
almost hopelessly unreliable in tension, so the rebar needs to be
located where it will resist tension forces. For a concrete beam, for
example, having a single piece of strong rebar running down the center
of the beam is poor use of the rebar. It would be better to have two
smaller pieces of rebar running lengthwise the beam each some distance
from the center ... as far as possible from the center that still allows
the concrete outside the rebar to grip it without fracturing. That way
a flexing influence on the beam has the least mechanical leverage on the
rebar that is in tension. Tower bases are typically spec'd to have a
rebar cage with the majority of the steel toward the outside.
73,
Dave AB7E
K1TTT wrote:
> Actually it is in my old rohn catalog also. With caveat that it have the
> same steel cross section area as the specified rebar. I can see where it
> might be a nice substitute for the large top and bottom areas of a self
> supporting pad base.
>
>
> David Robbins K1TTT
> e-mail: mailto:k1ttt@arrl.net
> web: http://www.k1ttt.net
> AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
>
>
>
>> -----Original Message-----
>> From: k7lxc@aol.com [mailto:k7lxc@aol.com]
>> Sent: Monday, January 18, 2010 21:09
>> To: towertalk@contesting.com
>> Subject: [TowerTalk] Wire mesh
>>
>> Howdy, TowerTalkians --
>>
>> Regarding using mesh for a rebar cage, it has several problems.
>> First,
>> it's not in any tower base spec I've ever seen. Second, how would you
>> stiffen it up to have it stand up? Last, it's WAY too small. Most rebar
>> is 1/2"
>> (#4) or bigger. Mesh is just mesh.
>>
>> So quite meshing around and use the real stuff.
>>
>> Cheers,
>> Steve K7LXC
>> TOWER TECH
>> _______________________________________________
>>
>>
>>
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>
>
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