Barn foundation tall wall - vertical rebar??

[bebster]

I'm getting ready to pour a foundation for my barn (which here on Cape Cod we need a full foundation for code). But the wall will be 9'8" high to allow room for the tractor (of course!). The long wall of the barn is 54' and has backfill on one side, so it needs some vertical rebar for strength.

Problem is I have 2 different engineers telling me where to put the rebar. The attached drawing shows the 2 alternatives. One says put it on the inside of the wall (2" from the inside edge), and the other says to put it near the outside edge. Anyone have any experience here? I'd love to hear a reason why one is better than another. Thanks!

 

[Egon]

The wall has the base anchored with earth forces acting on the outside of the wall. This would place the outside wall of concrete in tension so that is where the vertical rebar should be placed. If the wall was solidly supported at the top and bottom then the inside wall would be in tension and that is where the rebar goes.

First case would be a cantilevered beam, the second a simple beam.

For the cost of rebar why not put it on both sides. I'm surprised the code doesn't specify this.

Egon

 

[Inspector507]

One wall @54' long........I'd do both sides. Maybe tie them together.
Just my opinion though, what do I know?? I do electric /forums/images/graemlins/grin.gif

 

[Junkman]

I agree with Egon on this...... use it in both places. When they poured the footing did they put a "key-way" into it? A key-way is where they set a 2 x 4 into the center of the footing at the top. After the footing sets up, the 2x4 is removed. When the wall is poured, the concrete fills in the depression on the footing and this keeps the wall from moving off the footing. I would suggest that you install a footing drain at the lowest point of the footing before backfilling. It is also a good idea to put a drain around the perimeter of the footing on the inside. Both of these should be exited at the lowest point on the property so they will always be draining any water away from the building. If you install 2" Styrofoam board to the outside of the foundation before you backfill, it will help to keep the building warm and will eliminate any rocks from pushing on the out side of the foundation. All of these suggestions won't cost much if you do them yourself...
Is the foundation contractor going to vibrate the concrete during the pour? This makes for a denser wall, but is not always required in residential building. If there are any places in the wall that you might want to have a pipe go through, let the contractor know. They can put a over sized piece of PVC pipe into the form before pouring and this will give you a hole for later on. I put a few into the walls of my foundation for water lines etc. Easier to do it before hand than to be drilling them later on, especially if there is re-bar in the wall. Plan ahead for every possibility. It is cheap when you are building, but expensive to retrofit later on...

 

[Inspector507]

Junkman,
In his drawing I see a keyway and rebar stubbing up through. That should work to hold it in place.

Jerry

 

[Junkman]

I hadn't scrolled down far enough to notice that detail in the drawing, since the post was referring to the re-bar in the wall. I have since noticed that the engineers don't agree on the size of the re-bar. I don't agree with the thickness of the footing. I think that it should be at least 18" in hight. I am not an engineer, but I do remember my days working in construction of office buildings. The footings were always at least 50% as high as they were wide. When I built my home, the engineer specified 3' wide footing and 18" thick for the footing wall. My cellar walls are 12" thick and 10' high. They also have re-bar placed in the center of them at 18" intervals in both directions. The concrete is 4000 # mix. I don't know if it is overbuilt, but I have no cracks in any of the walls of the foundation or the house. I believe that no matter which way he goes, that it will be fine. Even the 12" footing would be OK in all probability. In the overall scheme of things, it doesn't cost very much more to go the extra mile in comparison to the total cost of the whole building. As it has been said here on the forums many times in the past.... "We don't have any problems spending other peoples money!"... /forums/images/graemlins/grin.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/wink.gif

 

[cinckid]

I took a quick look at your foundation design and I would like to suggest you hire an registered engineer to do a structural design. The size of the toe and heel look entirely too small to conteract the overturning moments in a wall with approximately 8 feet of backfill on the exterior. With the size and cost of the barn you are proposing to build, the additional engineering cost will be small. I am a registered civil engineer in Ohio and if you have questions, I would be happly to respond. Also, the interior and exterior rebar is a must.

 

[yooperdave]

I do not agree with either one /forums/images/graemlins/blush.gif!

You basically have a choice of 2 systems.

If the wall is laterally braced at the top, then you have a typical basement wall - the vertical bars should be near the inside face. There would be no overturning moment, so why is the footing so wide? Are you assuming 1,000 psf soil bearing pressure with a whole lot of load?

If the wall is not laterally braced at the top, then you have a cantilevered retaining wall, so the vertical bars should be located at the outside face of the wall. In this case, the footing does not appear to be large enough and detailed properly to resist the overturning moment.

Retaining walls are usually much more expensive, so can you tie in the top of the wall?

Good luck with your project.

Yooper Dave, (P.E.)

 

[tomnky]

When I had a six foot retaining wall designed and poured We used 5/8's grade 60 rebar which was a double run (inside and outside) laced9"X12" and of course pilasters as well as a keyway ..........It has been there for 13 years with no problems whatever ...also backfilled it with 57 stone to reduce pressure on the wall.

The footer was much thicker than your drawing (mine was 24 inches thick) and much wider, seems like it was four feet wide or more , poured with 4,000 pound mix.

 

[bebster]

Based on the great feedback from all of you, I’ve redrawn and attached the revised concrete plan for the 54’ foundation west Wall. I’ve also hired TWO structural engineers, but they have given me different designs (essentially the two I originally posted, with rebar on the inside vs outside edge of the Wall). So I’ve decided to tap TBN expertise to help sort it out.

I’ll synthesize what I’ve concluded so far and hopefully can get additional feedback so I can finalize the design and move the project forward. To keep this post focused, I’ll address only the structural issues for this long Wall (and save discussion of drainage, pouring technique, concrete mix, conduit pipes, etc. for a later post). Basically I am combining both systems suggested, with an emphasis on one.

WALL IS SUPPORTED AT TOP AND BOTTOM
The Wall is indeed supported at the top and bottom, so “simple beam” analysis would indicate that the inside of the wall would be in tension and so rebar should go on the inside (I’m not a structural engineer, but picking up the jargon thanks to my friends here). The inside rebar system is essentially #5 rebar cage 18” on center.

BOTTOM SUPPORT -- GOOD
The support at the bottom is provided by both the 2x4 keyway and the rebar hooks placed in the footing and extending at least 30” up into the Wall. This should be sufficient to bond to the Wall and anchor its bottom. Eventually the slab will be added which will further anchor the bottom, but not until well after its backfilled.

TOP SUPPORT -- GOOD AT BOTH ENDS
The support at the Wall top is provided by the floor system, which is14” TJI joists with tongue and groove 3/4” plywood subfloor. This system should provide excellent support at either end of the Wall, where the 30’ end foundation walls are perpendicular to this Wall and serve effectively as pilasters. This floor system is anchored to these end foundation walls so that the floor provides that lateral support as well.

TOP SUPPORT -- POSSIBLY WEAKER IN CENTER
In the center of the 54’ Wall, the floor system is 27’ away from the end walls. It seems that this might be a point of weakness in the support of the Wall top, especially since the opposite wall is wood framed for garage doors and not backfilled. Forces from the sandy backfill might not be that strong to cause the floor to be pushed back, but then again it might be good to provide some insurance for that. The cost of the insurance rebar will be less than 5% of the total wall cost, so that seem like it would be worth it.

With top support weaker in the middle of its length, the Wall at its middle might behave like a “cantilevered beam” with the outside in tension. So I’ve added vertical rebar on the outside as well to counter a possible force that would want to push the wall in from the top if the floor system doesn’t resist it fully. The vertical rebar is #7 and is wired to the rebar hook coming up from the footing. Horizontal #5 rebar placed just to the inside of the #7 vertical bar provides additional support.

FOOTING
The footing was originally 12’ x 24”, considered sufficient for the soil conditions, was elongated by 12” to 36” wide to provide additional support for any moment forces that might arise at the middle of the wall. The depth was not altered since the soil support was adequate. A piece of horizontal rebar runs across the width of the footing to provide additional support.

OPEN QUESTIONS
Do you think this is a good enough design? Would it be improved if we were to tie the inside and outside rebars together somehow? Does the grade of rebar matter and if so what grade should I specify? Any other structural suggestions? Many thanks again!