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Supporting rebar? HockeyFan 02-12-2007
|--> Re: Supporting rebar? Michael Bulatov...02-12-2007
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Posted by Bob Morrison on February 13, 2007, 10:26 am
In a previous post Michael Bulatovich wrote...
> I've never seen the brick used. Is it for work on grade only? Are they
> typical 'concrete bricks' or a special, smaller product?
> --
>

Michael:

Here's a link to a fancy style "dobie". Note that it has tie wires to
hold the rebar in place. Most times the block is just plain.

http://www.forshor.com/accessories/reinforcingbarsupport.html#wired

--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com

Posted by Michael Bulatovich on February 13, 2007, 12:58 pm

> In a previous post Michael Bulatovich wrote...
>> I've never seen the brick used. Is it for work on grade only? Are they
>> typical 'concrete bricks' or a special, smaller product?
>> --
>>
>
> Michael:
>
> Here's a link to a fancy style "dobie". Note that it has tie wires to
> hold the rebar in place. Most times the block is just plain.
>
> http://www.forshor.com/accessories/reinforcingbarsupport.html#wired
>
> --
> Bob Morrison, PE, SE
> R L Morrison Engineering Co
> Structural & Civil Engineering
> Poulsbo WA
> bob at rlmorrisonengr dot com

I've seen those! Thanks, Bob.



Posted by Matt Whiting on February 13, 2007, 6:27 pm
Bob Morrison wrote:
> In a previous post Michael Bulatovich wrote...
>
>>I've never seen the brick used. Is it for work on grade only? Are they
>>typical 'concrete bricks' or a special, smaller product?
>>--
>>
>
>
> Michael:
>
> Here's a link to a fancy style "dobie". Note that it has tie wires to
> hold the rebar in place. Most times the block is just plain.
>
> http://www.forshor.com/accessories/reinforcingbarsupport.html#wired
>

Bob, I've seen these used before and always wondered how much they
weaken the slab. Do they bond pretty well to the concrete? I suppose
if the slab is well supported the lose in bending strength shouldn't be
a big deal, but it does seem as though the dobies would cause a fair
loss in the ability of the slab to resist a positive bending moment in
that area.


Matt

Posted by Bob Morrison on February 13, 2007, 9:13 pm
In a previous post Matt Whiting wrote...
> Bob, I've seen these used before and always wondered how much they
> weaken the slab. Do they bond pretty well to the concrete? I suppose
> if the slab is well supported the lose in bending strength shouldn't be
> a big deal, but it does seem as though the dobies would cause a fair
> loss in the ability of the slab to resist a positive bending moment in
> that area.
>

Not really. The blocks are only a few inches long. The rebar in the area
is anchored on either end of the block by embedment in the slab.

Think of it this way: the rebar works even if there is no concrete at the
middle of a beam as long as it is anchored on the ends. The sections are
assumed to be cracked (no concrete) at ultimate design strength. In
elevated beams the main reason for the bottom concrete is aesthetics and
to protect the rebar from corrosion. A similar thing is true for slabs on
grade.


--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com

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