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RE: Steel Fibers Replacing Structural Reinforcing?

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Perhaps this is a bit off the topic, but having designed many rack supported buildings (including some very heavily loaded ones over 120' tall), I have to ask: what are they storing which generates a load which would require such a slab? Or is the soil really pathological?

For posts on roughly 8' centers both ways, and decent dirt, using even rudimentary slab-plate/soil-halfspace methods, your design would be good for at least 180 kips per post working load or 2,800 psf. Even at 120' high, this is 23 pcf gross volume. I don't know of any rack supported building with that sort of density. The pallet itself is usually this value, and they're typically less than 1/4 of the total building volume (the rest is mainly air). My personal record is less than 35 pcf of reserved pallet volume (the actual pallet was 45-50 pcf including the volume of the skid), and that one was only 85 ft tall for around 10-12 pcf of building volume.

Either your're sitting on a world record holder for density (by a very wide margin), or someone's not computing the loads properly. I'd ask them to recompute the design load and see if they forgot to divide by 2 somewhere, before I'd embark on such a monster slab for an ASRS building.

Peter S. Higgins, SE

"A Miraftab" <seaosd(--nospam--at)hotmail.com> wrote:

> 
> 
> 
>Professor Bayasi in San Diego State University has done lots of research
>about SFRC (Steel Fiber Reinforced Concrete) and has valuable
>publications for that.
> 
>as far as design, it is really simple. (like a rectangular section of
>concrete with tensile capacity bellow N.A and compression at top)
>tensile strength value you can get is a function of the type of fiber
>you use (length and shape of fiber are really important) 
> 
>In general:
>the common percentage of fiber is btwn .5% to 2% and what they are
>providing in your case is about 1.5% (which is in range)
>it improves impact ,toughness,tensile strength,cracking behavior and
>shear resistance of concrete. (advantages)
>needing more cement in mix (more shrinkage), being usually more
>expensive (for equivalent design), and its workability are the major
>disadvantages of that.
>my main concern in your case is adequacy of using only fiber instead of
>#7 @12. based on my rough calc I get half of moment strength. so I think
>their design should be something like 1.5% fiber in addition to #7
>@24"o/c (or equivalent)
> 
>other concern would be where you have top reinforcing. because if you
>use SFRC, after you pour concrete most of fibers go to the bot of slab
>and you will get more tensile strength at bot. So I would probably like
>to keep up to 70% of top reinforcements.
> 
>AM
> 
>
>-----Original Message-----
>From: Meyer, Jonathan [mailto:jmeyer(--nospam--at)webbersmith.com] 
>Sent: Thursday, June 17, 2004 3:52 AM
>To: seaint(--nospam--at)seaint.org
>Subject: Steel Fibers Replacing Structural Reinforcing?
>
>
>
>I am currently designing an industrial facility a portion of which is a
>rack supported building. We designed the mat foundation to support this
>building and came up with an 18" thick slab with two layers of #7 bars
>@12 OCEW. One of the concrete contractors has asked a steel fiber
>manufacturer to value engineer the reinforcing to determine if they can
>do it with steel fibers instead of the reinforcing specified. There are
>also slab on grade and elevated composite slabs in this facility. That's
>the background, here is my question - Has anyone replaced structural
>reinforcing with steel fibers and are there any standards related to
>such a replacement? 
>
>The steel fiber representative called me and told me that they would
>have to increase the slab thickness to 19" and use 60# of fiber per
>cubic yard. He invited me to use their software to "verify" the design.
>I guess I am a bit skeptical about using someone else's software that
>has a vested interest in the outcome of the design to produce a design.
>I told the steel fiber manufacturer that they would have to take full
>responsibility for the design and produce sealed stamped plans if they
>wanted to change my designs to steel fiber for the mat slab of the rack
>supported building. As far as the slabs on grade are concerned, I feel
>that if the client wants to use steel fibers in those areas that would
>be fine as long as they understand any limitations of the fibers.
>
>Any input would be greatly appreciated. Thanks for you assistance.
>
>Jonathan Meyer
>
>Webber Smith Associates
>
>

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