[TowerTalk] BASE REBAR and Lightning

[kr7x at comcast.net]

List:

The "Building Code Requirements for Structural Concrete" ACI-318-05 section
7.7.1 - Cast-in-place concrete (nonprestressed)

states "The following minimum concrete cover shall be provided for reinforcement...

(a) Concrete cast against and permanently exposed to earth          3 in.
(b) Concrete exposed to earth or weather:

            No. 6 through No. 18 Bars                                              2 in.
            No. 5 bar, W31 or D31 Wire, and smaller                       1-1/2 in.

 .... continues .....

This code is used by all building codes in the US, the CBC (California Building Code) and the IBC.

The emphasis is .... the minimum cover shall be.....

The cover requirement is to prevent corrosion and subsequent spalling and failure due to constant exposure to moisture through the microcracks that occure in all concrete. This is a servicablility requirement not a stress design requirement. It becomes a design requirement by providing for durablility.

Regards
Lonberg Design Group, Ltd.
Hank Lonberg, P.E.,S.E.
President

-------------- Original message -------------- 
From: "Ron Chambers" <rwchambers at gmail.com> 

> Minor comment and in very general terms: the part about rebar and its 
> spacing from the edge of the concrete has more to do with structural. 
> Concrete is great is compression and poor in tension, so rebar is added for 
> tension strength. And it is placed typically no more (not *minimum*) than 3" 
> from the edge where the tensile forces would be greatest. 
> 
> On 10/4/07, mecker at peoplepc.com wrote: 
> > 
> > Read that base rebar (which must be electrically tied together) 
> > should be internal min of 4 inches from edge of concrete base. 
> > 
> > Here's my question. (Im learning disabled...etc..etc.) 
> > 
> > What about guys that use a piece of rohn 25 as the base, stick 
> > the end in the 6 inches of gravel, and then fill up the rest 
> > with concrete. There is no 4 inches of insulation with the earth 
> > there, because the gravel has gaps and moisture in it 
> > which connect with the earth anyway. (a chemical fact) 
> > 
> > How important is it to separate the bottom of your base section 
> > and sides of base section rebar in the concrete 4 inches.... or should I 
> > put the base section 
> > on bricks and not stick it in the gravel and keep the rebar the heck away 
> > from the sides? 
> > Im using a chipboard concrete box form (have no choice there cause of 
> > concrete costs..etc...) 
> > 
> > The concrete form will rest on the six inches of gravel, but will 
> > eventually 
> > dissappear (be eaten by bugs..etc..), and whatever touches(or dosent 
> > touch) the bottom of the form will eventually 
> > touch the gravel, hence the earth. 
> > 
> > Am also using grade 8 7/8 size allthread rods on another base, and 
> > was just going to let these rods rest on the bottom of the concrete 
> > form rather than on bricks. (the rods are electrically connected by 1/8 
> > inch 
> > steel 3 inches width) Should I use the bricks here again, insulate from 
> > the earth 
> > again here? 
> > 
> > So I to summarize, should everything be on bricks, and how important 
> > is this rule with reguards to lightning when my tower will surely be 
> > grounded anyway to six rods in a ring, possibly also to a no longer 
> > operational (disconnected)well pump casing down 200 feet. 
> > into water. 
> > 
> > Is this because we want the electricity to go through the wires and not 
> > the base? Have heard about exploding bases. boom! 
> > 
> > (the allthread tiltover rod base was custom made at the Dayton hamfest 
> > and the other base is just a piece of rohn 25G being used as a base.) 
> > 
> > Feedback is welcome. Thanks. KE2BP 
> > MEcker at peoplepc.com 
> > _______________________________________________ 
> > 
> > 
> > 
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