Should I build over this???...

heehaw · Jun 26, 2003

i'll throw this out there, in the hopes someone will know what i am talking about, because i don't....i had a friend that moved to texas, was having a house built, and he called me and said they where putting cables in the concrete, and tightening them to stress or prestress the concrete or something like that??? supposedly the theory was the dirt could wash out from under the slab, and it would still stay together. like i said i don't know anything about it, but if it is actually something like i was told, it sounds like a perfect solution to your problem, but it could be one of those urban legends?? anyone out there know anything about this type of concrete pouring??
heehaw

tc35dforme · Jun 26, 2003

As you may realize from playing with concrete...it is excellent in compressive strength ( crushing ) but really bad in shear / twist / or tensile strength. What the tensioning cables do is keep the slab in compression not only vertically, but also in the horizontal plane, making it less likely to fail or crack.

Those premade slabs you see being used in parking garages are tension or pre-stressed panels. It allows the plank to span a gap, which would normally weaken it due to tension loads ( pulling apart as it sags ). Rebar with its deformed surface, kind of serves the same purpose, but with much less strength. The cables are pulled under hydraulic rams to really pre-load the slab.

A tension or prestreesed slab would stay together on top of unstable base materials, but you still need a secure footing or the slab ( and building ) will slide down the hill, or sink in the mud, or disappear into the sinkhole....

For a shed, I'd look at a better location.

RobS · Jun 26, 2003

If you want an interesting look at using cables to stress concrete do a search for Frank Lloyd Wrights "Falling Water" home. They did some restoration work using cables. (sorry, no time right now to dig up the link myself /forums/images/graemlins/tongue.gif )

MarkV · Jun 26, 2003

Yep the other guys explained the use of cables. I will bet that in the area your buddy is building the ground tends to dry and crack to extremes in the summer. If you remember some of Birds posts about Texas soil he has mentioned people watering their house to keep the soil around the footings from drying and opening up which causes the house to shift.

MarkV

BB_TX · Jun 26, 2003

MarkV-You are correct. Bird was certainly not exagerating. If we go a month or so with no rain during a hot summer, the back part of my yard that I don't water will open up in a spider web of cracks 2" to 3" wide. You have to watch walking across it or you will step in one and twist your ankle. /forums/images/graemlins/frown.gif Mowing with my lawn tractor is a real experience. Will rattle your teeth. /forums/images/graemlins/tongue.gif

RobS · Jun 26, 2003

OK, so it is possible to build over things you shouldn't. In this case a stream complete with waterfall. Unfortunately, things can go wrong and end up costing a lot of money to correct (much later).

This is only slightly on-topic but it's just such a fascinating place Fallingwater /forums/images/graemlins/smile.gif

yooperdave · Jun 28, 2003

Run like the wind! /forums/images/graemlins/blush.gif

Sounds like you might have a nice parking area for awhile though. /forums/images/graemlins/cool.gif

Yooper Dave

Anonymous Poster · Jun 28, 2003

Don't give up yet. By using reinforced concrete in a grid system as previously mentioned combined with pilings you should be able to build in this location. I built an Autozone that "floated" over a swamp. We use pilings 20' apart in both directions and poured a heavily reinforced slab on top of the pilings. It is still there as far as I know. This obviously is not an inexpensive way to build a barn.

yooperdave · Jun 28, 2003

Deep foundations with caissons or pilings along with grade beams and structural slabs can force the issue if the site is worth it.

Get some engineering drawings and specifications, along with a few bids to do this, and I think the extra $$,$$$.$$ will give you some quick incentive to try something else.

Good luck.

Yooper Dave

EddieWalker · Jun 29, 2003

These are some of the best replies to a question I've read so far. I love the use of piles for a shed. /forums/images/graemlins/grin.gif

You can build anything in anyplace if you want to spend enough money to do it!