|
Posted by Chuck on July 3, 2007, 4:09 am
>>
>> > Rebar in thin slabs has several fatal flaws. The standard practice is
>> > far from the recommended practice - typically rebar is placed without
>> > any attempt at raising it above the bottom of the slab, then when the
>> > concrete is placed, the workers pull up on the rebar in an attempt to
>> > position it in the middle of the slab. This is not a good practice.
>> > Other times it is raised up on brick pieces - also not a good
>> > practice.
>>
>> > My major objection to rebar in a thin slab is that it can't do what
>> > people think it is doing when they spec it and install it. Rebar does
>> > essentially no work when place in the middle of the slab - the neutral
>> > axis. Rebar is meant to take the tension that concrete can't. That
>> > requires the rebar to be near the tension side of the slab/beam, not
>> > in the center. Placed in the center of the slab rebar only does work
>> > if the slab has already failed. In other words, rebar in a thin slab
>> > is planning on having the slab fail. That's backwards.
>>
>> > Slab failures can be entirely prevented by proper sub-grade and site
>> > preparation. That preparation is a better way to spend the money than
>> > on rebar.
>>
>> I was always understood that when you are using 'Ultimate Strength
>> Design"
>> the concrete has already cracked and the rebar is yielding. Isn't this
>> considered as concrete failure????
>
> Yes. That's why it's called Ultimate Strength Design. If the 4" slab
> experiences that ultimate load, you have problems.
>
>> The plastic neutral axis is not the
>> middle of the slab and is just be low the top of the slab ( ' a'/.85 of
>> the
>> compression block ) and the location of rebar 'd' is located at mid
>> depth.
>
> That's the theoretical axis based on theoretically perfect rebar
> placement. I can tell you that is theoretically impossible to achieve
> in a 4" slab.
>
>> You are saying that it doesn't work even though compression equals
>> tension
>> and the moment capacity exceeds the factored moment required.
>> This is the same way you would design a steel composite slab considering
>> compression block in the slab portion to take the compression and the
>> steel
>> beam to take the tension. Are you saying this process is incorrect?
>
> Sigh. No, Chuck, I am not reinventing structural engineering. How
> thick are the composite slabs you're designing? 4"? Didn't think
> so. How accurately do you think rebar in a very thin slab can be?
> Design location +/- 25% in depth on a thin slab would not be unusual
> at all.
>
> A 4" slab with rebar is an exercise in futility. It's an insurance
> policy the owner pays for to hopefully take care of improper sub-grade
> preparation. Oh, and the owner owns the insurance company. Lose-lose
> situation.
>
>> Even so, the slab on grade rebar is minimum at mid depth primarily
>> temperature reinforcing to minimize cracking.
>
> Minimize and prevent are not the same. You won't eliminate cracking,
> you can only control the location and the severity. The first by
> control joints, the second by sub-grade preparation. If you believe
> the main purpose of the rebar is to control thermal expansion,
> wouldn't welded wire mesh make more sense? The loads are small, so
> the steel section can be far smaller than a #3 bar. The WWM also has
> a more closely spaced grid of steel which would be another benefit.
> Spending the time and money on rebar installation does not make
> structural or financial sense on such thin slabs. WWM makes more
> sense than rebar, but it has its own issues and won't replace adequate
> preparations.
>
>>The slab on grade needs to be
>> placed on a sub base of gravel with vapor barriers if required.
>
> Exactly. If you're designing the slab to be a beam, use rebar. If
> you're designing a slab on grade, take care of the grade first.
>
Well Mr. R. You seem to be very intelligent when it comes to engineering.
I never use 4 in slabs myself the thread started out with the use of 4 inch
slab.
It is used very commonly with residential construction. I commonly use 5 to
6 inch slab on grade and at least 6 inch for composite sections. And, it is
the geotech engineer to inform us on the correct soil preperation prior to
pouring a slab on grade. If you remember I suggestion control joints in the
4 inch slab at the beginning. With this in mind why do you consider
yourself the moderator of the alt.architecture news group for the fun of it?
Chuck...
____________________________________________________________
Charles I. Dinsmore, PE SE RA, M.ASCE ~ ci.dinsmore@gmail.com
| 
Yahoo! 
Google 
Windows Live 
del.icio.us 
digg 
Netscape 
XML Sitemap