big concrete pier

[structuralnerd]

I am designing a very large concrete pier - 4' deep by 7' wide by 7' wide that has a small column load at the center and is bearing on soil. Does anyone know of any good reading material of steel reinforcement guidelines to go by? I originally reinforced it based off of standard pier guidelines (aka 1/2% steel)and concrete column tie guidelines, but the contractor is not happy about how much reinforcement that is. I would appreciate any help you could give me. Thank you.

 

[csd72]

Will it work as a plain concrete section?

You can treat it like this even if it has some reinforcement.

csd

 

[JStephen]

Maybe check it as a footing rather than as a pier. It will still have quite a bit of rebar, but different directions and detailing.

There are also provisions in ACI-318 for unreinforced concrete- you might look into that, figuring that if the design is adequate with zero reinforcing, then you can put whatever you want in it- not sure if that's kosher or not.

 

[MarcbSE]

You could design a portion below the column as your actual pier with some nominal dimensions and appropriate reinforcing, and then consider the rest as structural plain concrete.

 

[jike]

With a small column load, it probably will be sufficient as unreinforced concrete. You can run the numbers. I would add #6's at 12" oc around the perimeter.

 

[BigH]

Just a question. Why is the footing so thick compared to its size - the pier depth is more than 50% of the dimensions. I think that it would probably be thick enough to almost, as one said, be designed a plain concrete footing. Secondly, you probably could do a tapered footing with, say a 3 ft by 3 ft square at the top and then go to th 7 by 7 at say 2 ft from the bottom - will save you some concrete. Thirdly, and I haven't done an structural steel design for a long long time, but isn't one supposed to design the needed steel on the forces, shears and moments that are induced by the column in the footing rather than just use a codal provision of 1/2% steel?

 

[structuralnerd]

BigH - Reinforcement needed in the pier to resist the moment is very small. Generally, it's the temp. and shrinkage that is worrisome...hence in a pedestal/pier...the general rule of thumb I was taught is 1/2% of steel. I cannot do a tapered footing in this case because the contractor/architect wants to trench out the footing.

 

[Qshake]

First off, the contractor is not typically the engineer or an engineer and his opinion, while his right, means nothing to the design engineer.

Second, As I recall ACI, it is 1/8 square inch per foot of concrete surface for temperature but in no case less than #blanks as 18".



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[hokie66]

In a 7' x 7' x 4' deep footing bearing on soil, why would you be concerned about temperature and shrinkage? What restrains the footing to cause cracking? The main type of cracking which could be a problem is plastic settlement cracking at the top reinforcing (if used) due to inadequate compaction. This can be addressed by revibration after initial consolidation.

 

[JStructsteel]

If the column load is small, just design a smaller pier, or am I missing something?

 

[structuralnerd]

The architect in this project is also the contractor, and they love to do trench footings, meaning they take a big bucket and dig out a continuous footing under the walls and basically a big hole under the columns. No forming necessary. That's the appeal for my client. The geotech report has said that trench footings are acceptable in this area because of cohesive soils. I have never used a trench footing under a steel column before - always a spread footing with a pier if needed. Generally just a couple feet wide, poured with the concrete wall. To get the needed bearing pressure, some of the big piers/footings range from 4' wide to 8' wide. I did the analysis and it passes as an unreinforced footing. My question is... is should I put some reinforcement in there just to combat a little temp and shrinkage or not? If so, is there a guideline as to how much and where I should place it? Does the standard .0018Ag hold true here? I've been getting some conflicting answers, so maybe it's based on some engineering judgement?...

 

[MarcbSE]

I think most of the previous posts actually are leading in the same direction. Your last post actually clarifies the situation quite a bit. In this situation you can design the concrete mass as a footing rather than a pier. You've already stated that it is adequate as a structural plain concrete footing, so technically no reinforcing is required. However, I dare say most engineers would still put at least the minimum steel for t&s in the footing. There is nothing wrong with that.

 

[civilperson]

Designed as plain concrete, any amount of steel is OK. The footings are not visible so a little cracking due to shrinkage and temperature change should not bother anything. Use shrinkage compensating cement if you are worried. Also place with construction joints every thirty feet or so and alternate the placement days. That way the initial placement will have a chance to shrink before placing the adjacent concrete.

 

[BigH]

Shouldn't be that much temperature change if located below frost depth.

 

[DRC1]

I may be the lone desenting voice but I would put the code required T&S bars in. A. rebar is cheap in respect to the massive amount of money they are wasting on concrete. B. This type of construction, can be done in a manner that may not have the best quality control. If you get a little settlement or rotation, the bar can carry the unanticipated stress better than plain concrete. c. if I was the engineer, I would make sure the concrete met code, esp. in this scenario.
Note: Why is it 7x7? One should be able to excavate a much smaller pier.

 

[lkjh345]

Structuralnerd:

We do this kind of footing all the time, where the footings are trench footings down to frost depth under the walls of the building, and then widen out at columns. It is pretty much standard in the midwest. Very rarely are shallow footings formed around here. Cheaper for the contractor to waste the concrete than to spend time forming them up.

Personally, for a 7'x7' footing, I would design the footing as a 7'x7'x1'-0" thick footing. Figure out area of steel required, put that in the bottom of the column footing, run the trench beam reinforcement trough the footing, and call it good. The extra 3 feet of concrete at the column footing is just backfill for the excavation.

 

[shin25]

ACI-318-02, art 22.2.2 states the following as a limitation-

"Members that are continuously supported by soil or supported be other structural members capable of providing continuous vertical support."

Given small magnitude of the load compared to the size of footing, I believe the above requirement is satisfied and hence the footing can be unreinforced.

But as DRC1 has mentioned, the concrete may not be of very best quality, so providing #4 rebars at 12" on center near top and bottom should be sufficient.

 

[JKW05]

It may be of interest to note that ACI 350 (Environmental Structures), Section 712.2.1 allows shrinkage and temperature reinforcing for sections that are at least 24" to be based on a 12 inch concrete layer at each face. While ACI 318 does not have a similar provision, it seems that it may be reasonable to consider that application here.