Barn - concrete opinions?

[moeh1]

When we poured mine, I wanted a floor good enought o roll a creeper over. We set a center board and the outside forms for level height and screeded the first side, then filled the second side and pulled center sections as we went. A little fill in work and we were good. Later we bull floated all the way around from the sides. I am only about ~1/4 inch off in a 25 by 40 and I'm happy. I also went 4000 PSI with fiber and rebar instead of wire. Good luck, have some friends and some beer for when the concrete sets up and you'll have fun /forums/images/graemlins/grin.gif By the way, I figure if you set anchors and if a few are off, can't you grind them off and still set the hiltis or would it be weakened in the process. I don't know, I never have worked with them.

 

[drm]

Chris,

Let me start by saying a good Super on the job can make the difference between a good smooth running construction experience and a lack of coordination nightmare. Got some good advice from the field and I have hopefully learned from it.

As you know, most metal building designs rely heavily on the foundation for dead weight to counteract the uplift from wind. Without knowing all of the specifics and the building engineer's intent, I must assume that they have sized the piers based on plain concrete design. In other words, they use greatly reduced design values because they are not going to use any reinforcing. I do not have that code here (just moved and most of my stuff is at my old office or in storage) so I can not confirm if this is feasible.

I certainly would not do it this way but some metal building designs push the minimum code requirements. I would hope that the footings bear on a virgin and stable soil, that may be part of the reason for the depth.

As far as the turned down slab edge, I always like this type of design but it requires design (as you have stated) and will require reinforcing. Like you said we are speculating based on what we are used to seeing. I have designed in the Midwest, east and east coast and each area has it’s way of doing things that work in that area of the country.

As far as timing I am in the process of design a building and looking for ideas. Hopefully I can get some good information & if I can help someone else in the process all the better.

 

[RichT]

drm, again many thx for your valuable input. I appreciate the explanation of the hilti failure pattern as it helps me visualize your suggested rebar plan.

My homegrown pier reinf plan is essentially the same as yours except I planned #4 rebar for the verticals circled by #3, mostly because I can do the bends myself. The #4 verticals would then tie in to the #4 slab crisscross pattern, with 4' spacing. Only one question in this matter...would the "90deg hooks" be 90deg bends on the vertical rebar, inward toward the anchor area, so as to intersect the projected failure region, yet not interfere with hilti placement?

No need to apologize for your disclaimer, it just adds a professional touch.

Chris, I don't know what I did to deserve such advice either, but I have found the TBN forum in general to be a class act, in terms of both advisors and advisees. I posted questions along these lines in a couple popular construction/contractor forums, but the advice received here is considerably more useful (as I suspected it would be based on my tractor exchanges). Theres some good folk in this place. I'm an electronics engr by trade so I'm soaking up all this materials and civil engr advice.

With my limited experience I agree with your foundation assessment. I was surprised the plans specified a pier vice monolithic foundation (as were the concrete contractors who bid on finishing the slab).

Chris, just to clarify "screed form or two"...do you mean temporary screed pipes/wood which break up the expanse of the 24x36 into manageable screed sections, then pulled out prior to finish? Can you recommend what sort of pipes/support works best? Good advice on the garage door, thx.

Marty, the risk with the grind plan is, unless I'm way off on measurements, chances are the anchor would take up a portion of the hole needed for the hilti.

I haven't excavated for the piers yet, but I've dug a 20in deep trench for the electrical. At that depth the soil was hard clay and appeared virgin. I do understand that even hard clay will absorb water, contract and expand, so I'm using a 3/4 minus gravel base for cushion.

I did some research on the 5/8" Kwik Bolt II hiltis. A published report shows allowable tension at 3100lbs (with SI), 1600lbs (w/o SI), and 3100lbs shear, all at 4in depth with 4000psi concrete. The Special Investigation (SI) includes verification of drill bit compliance, hole depth, cleanliness, concrete compressive strength, and verification of proper install procedure. Trying to figure out how this compares with standard 10 or 12in 5/8" anchors.

 

[bugstruck]

Rich, With regard to the screed forms, they are generally employeed in slightly larger pours than yours, but not always then either. I'd check with my concrete contractor first as they may prefer rebar grade stakes at certain intervals. When using them they typically spot a wheelbarrow load or two at the grade stakes and then use them as screed points while placing the balance of the infill concrete. If I were pouring your slab that's how I'd do it. The screed forms do give you some added control for level if you choose to go that way. Again, check with you concrete contractor for his recommendation on # of screed forms and location. Typically for a pour like yours, I'd use temporary wood screed forms which would just be 2 x 4's attached to 1 x 2 or 1 x 3 wood stakes. The top of the 2 x 4 would be level and straight with the top of the pour of course. Three sections (4 max) would work best for your pour. Generally, as you place the concrete you hop scotch or skip over the adjacent pour area and then return to it later. Gives the concrete a little time to set against the earlier poured screed form and is less likely to sag when it is removed. You do however, want to overshoot a little concrete (2 - 3") to the opposite side of the screed form as you are placing the concrete, This mitigates the sag when you later pull it out. One of the problems with this "screed form" method is that they slow you down slightly as you have to jump the concrete truck and men around a little. If they are not pumping, but wheelbarrowing, the concrete you can't run the wheelbarrow over the screed forms either.

Either way works and on a 24 x 36 slab. A good concrete contractor should get you a real good job with just the rebar grade stakes if you opt for that. Another advantage of the stakes is that you can slightly elevated those in the interior by say an inch at the middle of the pour. You could do this with the screed forms too but it's not quite as simple. This may be desirable as unlike a residential garage, you can't tilt the entire pour towards the doors for water runoff. Most places won't let you slope towards the center and do a floor drain in a garage anymore because of fuel spills. Being high in the middle forces any water to the outer perimeter of the building. It also mitigates any tendancy for water puddling on small spills or washing which is alway an issue with all but the most exact flat pours. This also helps a little at the exterior perimeter if you are not depressing the pour to receive the metal shell sheathing as covered in an earlier post.

HTH. Sorry I took so long to get back.

 

[drm]

Rich,

Your description of the bent rebar & placement is correct. Also is you assement of locating the anchorbolts if you are off a little. As far at strength of the 5/8" bolt, it depends on material: A307 bolt or A36 threaded rod will be about 6,000 pounds.

 

[caylor]

Drm,
Way to cover yourself with the engineering caveat statement! I know no TBNers are litigous /forums/images/graemlins/grin.gif, but you never know who is reading these posts and what they will do with the opinions expressed here.

A Fellow Engineer (translated-Geek, i.e. Nerd) /forums/images/graemlins/tongue.gif

 

[RichT]

drm, do you think my pier rebar size plan is reasonable or should I go with the larger sizes per your original recommendation?

 

[caylor]

RichT,

Please forgive me answering a question that was not asked directly to me. Based on your 24" diameter piers, I calculate that the minimum reinforcing steel required is 4.52 sq. inches, or 10 #4 bars spaced evenly in a circular pattern.

Drm...I know you started this with Rich, so I'll stay out of it until asked back in. FYI, I used the ACI 318 Code using minimum reinforcing steel for columns as 1% of the gross cross-sectional area of the pier.

As = ((PI*(24)^2)/4)) x 1% = 4.52 sq. inches (10 #4 bars)

I noticed that the barn is in Southern California, so Seismic activity might need to be considered, I have no expertise in this area, so my above comment may not be valid...Drm, what do you think?

 

[drm]

caylor,

Your exactly right, I am not worried about the regular users of a site like this because they are a do it yourself type that would not even consider such actions. It is the others that want everything handed to them for free that I worry about.

 

[RichT]

caylor, your input is appreciated. I spoke to the barn engr today about the various options we've discussed in the forum. Though not required, he liked the idea of rebar cages in the piers and rebar lattice in the slab. He said to keep the rebar about 2" from edges.

We had a long discussion about hiltis vs preset anchors. If I preset the anchors, he suggested I have the barn installer do a pre-inspect to verify placement accuracy prior to the pour. He recommended (as someone did earlier) using threaded anchors with bolt/washer to expand the failure cone and increase pullout resistance. The preset gives me the option of tying the rebar directly to the anchors. After looking at some resource material, he said the preset, threaded anchor/rebar approach should roughly double the pull out resistance vs hilti/no rebar...and improve seismic strength as well (something we think about alot in these parts).

 

[drm]

Rich,

Use of the #4 vertical rebar and #3 hoops is reasonable since technically no reinforcing is required per the barn engineer. My recommendation is what I would put in; but, I would not be bending the rebar but buying it that way. If you are concerned about the amount of steel; the area of a #4 rebar is 0.20 in**2 and a #5 rebar is 0.31 in**2. Therefore, the #5 would provide 50% more steel. So you could use 9 -#4 bars in place of 6 -#5 rebars. But the call is up to you.

I personally would use more steel but I am an engineer and a bit ****. I get questioned about some things like this situation where the rebar technically is not required and say to the contractor or junior engineer to just do it. It just makes me feel better (sleep at night) to get enough steel and concrete to make it "look good" to me. I have done trouble shooting for steel in power plants, precast concrete manufacturers, and general construction including steel, concrete, and wood. It is very difficult to go back and fix concrete reinforcing problems. I have done many and have developed some unique methods for precast but never like fixing them. It just isn't as good as being done right the first time.

Now that I am off my soap box, what you proposed should work if you and the building engineer are comfortable. Heck, I don't even know the size of the building so my “recommendation” is just based on a 24” diameter pier size.

Sorry I didn't reply sooner got pulled away.

 

[drm]

caylor,

Correct, the minimum required area for a column design is 1% of the gross cross sectional area of concrete. The cross sectional area of a #4 rebar is 0.20 in**2. The area of a #6 rebar is 0.44 in**2. That is why I stated 10 -#6 bars.

For foundation piers it is common to use 1/2 of this amount of steel or 2.26 in**2. So 8 - #5 bars would be appropriate if you take the liberty to reduce the amount of steel. I am not familiar with construction practices in Calif. Since I have never worked there; but, I know seismic requirements are more stringent period. I have done work on east coast is some low to moderate seismic areas. And yes, the local seismic design requirements do need to be considered. I am assuming the building engineer has already done this.

 

[Brad_Blazer]

100 tons of tensile strength in a short slug of concrete attached to a building with a few 5/8" anchors? What am I missing? When I think of a "column" I think of a tall element subject to seismic bending/buckling loads, not a buried pier with an aspect ratio of less than 1:2.

 

[keeney]

I have to agree with Brad - the piers are sounding way over-built. I'd put the steel (money) into the floor slab instead.

If you are still worried about failure in tension, either use a longer anchor bolt, or hook the end of a #4 over the ear of the anchor bolt and put another short bend on the other end down near the bottom.

The 5/8" anchor bolt is plenty. Even at the conservatively estimated 6000 lb capacity, such a bolt could pull the entire pier out of the ground! 2'x2'x5' x 150 lbs/cu-ft is only 3000 lbs.

(neglecting friction, wedging, suction, or otherwise lifting the surrounding soil).

- Rick

 

[RichT]

Since I started this thread, to all those who have contributed, my sincere thanks. The exchanges here have substantially improved my ability to understand and evaluate foundation design tradeoffs for my tractor shelter/barn project.

I recognize that some folks tend to offer "commercial grade" solutions. Such input is valuable for understanding the boundary conditions on a design. In my business (electrical engr) I tend toward the same thing. The customer/advisee can chose to take a more conservative route within the design constraints as cost, schedule, or other factors may dictate.

The quality of commentary in this forum continues to impress me. We should all be thankful for such a resource.
rich

 

[drm]

Brad,

To be blunt two things:
1). Concrete Design
2). Building Code MINIMUM design requirements.

Minimum amounts of steel are dictated in the building codes to ensure that the concrete acts as reinforced concrete if it is designed with reinforcing. In an earlier post I touched on plain concrete design (designing the concrete without any reinforcing). I also stated that even though the minimum amount of reinforcing dictated by the building code is 1% it is common in areas that I have worked to use ½% steel in foundations. The reinforcing steel not only acts to take tensile forces (in any reinforced concrete element) but to ensure the integrity of the element. In this particular case the later is its primary purpose.

Heck I have seen buildings built on posts (wood poles) set in soil with no foundation, concrete, etc. I have also been the one who is called in after a building, roof, wall, etc. collapses to determine what happened and why. Most of the time is because someone did not think meeting the Building Code minimum requirements was necessary. The majority of these are structures that were not engineered or were engineered and built wrong.

My reply may seem hostile but I do not think you have understood the exchange I am trying to make. I am not dictating anything. Only giving and opinion and some code requirements. BTW, I do not take any of this criticism personally and I hope you do not either. I have worked on and around construction sites for over 20 years and have heard it all.

In closing, I have repeatedly stated that the design is totally up to the owner and engineer of record. I am only trying to answer the questions raised with enough information so that the poster of this message can make an intelligent decision.

 

[keeney]

Part of the misunderstanding here is that the pier is being made 24" in diameter all the way up not due to the load it will be bearing, but for convenience. The base or footing might be specified at 24" because its easier to just say make it 24" to handle the worst case instead of figuring out what kind of soil its actually sitting on.

Another way to think of it - its a 24" footing with an 8" dia column. The extra concrete is just being dumped in the hole as a convenience so that the position doesn't have to be so accurate.

The only other reason to make it so big would be as wind-load ballast - just raw weight to keep the thing in the ground.

I do agree that if you really did need a 24" diameter column to support the load, then you would really want all the steel that goes with it.

In this case, however, I suspect that the other poster is right. Very few pole-barns will have loads in the 100's of tons range that would actually necessitate a 24" diameter column. He's not building Trump tower

- Rick

 

[drm]

Rick,

I do understand. Look at Rich's last post and he does also.