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Slab on grade

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Thanks to all that responded,  I sincerely appreciate the advise.

Some additional comments, questions and answers:

Honeydo's:  Yeah, they do get interesting, don't they?  But they
keep us off the streets and out of bars.  The barn we built over
the past 2 years is referred to locally as the Taj Mahal.

Joints:  I will use smooth greased dowels at the cold joint up the
middle.  16" long 1/2" rods @ 12" sound right?  I'll also stop half
the rebar at the saw cut joints.  I plan on getting on the slab the
following morning to cut joints.  Thanks for the reminder on all
those points.  The building will have 42' wide doors, so a 21'
modularity works out fine, and advance planning is no problem.  Of
course, I'd rather go with 42' spacing.  Mark Gilligan questioned,
and Dick Horning responded, with a brief discussion of joint
spacing versus rebar.  Dick - how much more steel would I need for
42' spacing?  How about 105'?  Is Jim Kestner's formula applicable
over a wide range, or only for "reasonable" spacing.  It looks a
little too linear to cover loooong spacing.  Also, shouldn't fs be
60,000 if I'm using Gr 60 steel?

Base:  The base, and sub-base, are what exists.  I can, and am,
moving it around some, but I'm stuck with what I have.  It's clay,
but appears to be non-expansive.  Three years of playing with the
stuff, plus charts showing expansive clay regions of the country,
both indicate that is not a problem.  It's well compacted, and all
the soft spots found during compaction have been removed and
backfilled.  I can add 4" of clean gravel, but my understanding is
that's to prevent capillary action drawing water up through the
slab, not to help the base any.  I'm willing to spend the money for
the gravel if it'll help anything, but would prefer to not waste
it.  Same thing for visqueen - I see it spec'ed a lot, but if
moisture is not a serious problem isn't it a waste of money?  Or
does visqueen reduce the subgrade drag coefficient, and thus the
tendency to crack as it shrinks?  What is an appropriate subgrade
coefficient in the formula from Jim Kestner?

Curing:  The slab will go in before the building, but I can easily
sprinkle the slab for a week after pouring.  I hope to pour it this
winter, so high sun and temperature shouldn't be a problem.  We can
do things like that in Tennessee, doesn't make me miss previous
homes in Pennsylvania and Minnesota at all.

Fibers:  The manufacturers all make their limitations very clear -
I ask only about using them in addition to the steel, not instead. 
They increase the cost of the material and finishing labor about
10%.  Burning off any loose fibers after curing is no problem, and
they might even degrade due to solar radiation by the time we get
the roof on.  But is the $1,000 additional cost worth it?  If it's
the difference between visible surface cracks versus not, than it
is.  But I can add 30% more steel for the same cost, in place. 
Recommendations?  By the way, my costs will vary significantly from
"real" jobs due to way I value my play time.

ACI stuff:  Yeah, I should read all that stuff.  But I don't design
SOG at work, and this is supposed to be fun.  It's a lot more fun
to get out there on a sunny day and play in the mud than it is to
sit in my office and read.

Thickened slab:  Richard Lewis raised a very good point.  I've seen
thickened edges more often than not.  I've also seen the slab
thickened at all the joints, which would seem to defeat the whole
joint.  The only reason I planned to thicken my edge is because the
edge of any slab is weaker than the center.  Adding a little more
steel should do the same.  So I think I'll go with an additional
couple of perimeter bars, and keep the thickness constant.

Usage:  We plan on living at the 100 Aker Wood, with all of our
attendant toys, for the rest of our lives.   Hopefully another 40
years or so.  With that in mind, I'm not going to try and save a
little money now by limiting the areas the heavy equipment can
operate.  For now I know the first bay will always house my main
plane, but the $420 I'd save by using a 4" slab in that one bay
would be likely to backfire some day down the road.

Runway, and other airplane loads:  The loads from a plane in my
category, 3200# or less, are significantly lower than what any
paved runway is designed for.  The runway is grass, and the tires
leave less of an imprint on a wet runway than do the horses or
small dogs, about the same as a large dog or person.  When the
ground is dry (as it has been all summer), there are no marks at
all.  Acceleration is all through the propeller, not drive wheels,
and I rarely brake when I land.  So there are essentially no
horizontal forces on the runway.  None of this is true at even a
normal general aviation airport that might handle light jet
traffic, to say nothing of a commercial airport.  The loads to the
hangar floor will be roughly equivalent to a car.

Fun:  Lots of it.  I've found I am much more conservative in many
of my design assumptions on my own buildings than those for other
clients.  It's easier to explain to the owner the cost/benefit
trade-offs when I'm wearing all the hats.  There is also a much
higher cost of mistake.  On real jobs for which I get paid, if I
make a mistake it can cost me some money, I can lose my license, go
out of business, etc.  All relatively minor compared to having to
live with the mistake.  Joking, or course, but only partially.  On
the steel super-structure for this building I'm using an open-web
trussed three dimensional rigid frame.  Not the most economical way
to do it, but the most fun.  Except I was more nervous designing it
than anything I've designed in the past 30 years.  

Richard Lewis pointed out, correctly, that I have not "failed" if
there is a crack in the slab.  Cracks in concrete are right up
there with death and taxes.  On a "real" job he'd be right, and I
wouldn't worry.  On this job, where my goal is to have a slab with
no extra cracks, I will have failed in that aspect.

One of the neat aspects of building for myself is I get to prove,
to myself, what sort of engineer I am.  I guess that's what makes
me more nervous.  The proof of the pudding is in the tasting, so
I'm out there eating pudding, and loving it.

Phil Hodge