top and bottom reinforcement of footing

[hiluxxx03]

i have a problem for my cutting list of the footing reinforcement... the footing is 2500mm x 3200mm x 600mm... the bottom reinforcement is 16mm @ 200mm o.c. bothways while the top reinforcement is 10mm @ 200m o.c. bothways... if i bend 90degrees and use e=12db, the top and bottom bars wont meet... there is a space of 65mm... i'm worried about the installation... should i overlap the top and bottom reinforcement? how much would be my required lap length? and may i know the theory behind? i'm new to the field and did not study well during college but i'm trying to make-up now for my loss... please help me and thank you... by the way, i used 75mm concrete covering for all sides of the footing... i have doubt for my top bars concrete cover... hope you would mentor me guys...

 

[ishvaaag]

This is a small footing (well, respect what one could build sometimes) and normally your setup would work without problems. However, it has been customary (where I live and work, Spain) to make one of the reinforcements in the lateral faces long enough to allow for the overlap you are referring to, mostly to complete the cage, cheaper in your case by extending bars from the top 10 mm mesh to be vertical.

Reinforced concrete is a generous kind structure, that makes up many times for deficient designs that have forgotten to properly consider some subject at design time. It is also a customary procedure to build structures that with the refined systems that have appeared along time has delivered designers, constructors, owners and users of many of the accidents, hassles, costs and troubles other older methods of constructions had. In correspondence, designers usually are not in the need to go for extreme economies like cutting your vertical faces rebar short and then stay wondering if one has been in error by making just a small saving.

Respect the cover for the top reinforcement, if not in direct contact with the soil, it may be bigger than usually recommended. Review the recommendations for cover in your standing code and apply them for the exposure class or conditions you have.

 

[hokie66]

It is helpful for construction purposes to be able to tie the legs of the layers together, although not required from a technical standpoint. You will have to provide some chairs/standees in the interior of the footing.

 

[hiluxxx03]

thank you for the replies guys... this make me laugh at myself... i guess i need to work on my common sense with regards to construction actual procedure and read structural codes... anyway, better be a dumb for a day, than to become to ignorant the whole life... that's why i won't stop asking for things i do not know and comprehend... this is site is a worthwhile community...

 

[dik]

Bars that size don't require hooks and can be readily placed... if from a constructability purpose, add the extra few mm (in) to allow them to be tied...

Why the added top bars?

Dik

 

[hokie66]

dik's question is the best of all. Why the top bars, indeed?

 

[IDS]

Why the added top bars?

It is standard practice in some areas of construction to provide reinforcement to all concrete faces. It is a requirement of the Australian Bridge Code (AS 5100) for instance.

An alternative to using U bars in the top and bottom face is use straight bars (providing there is sufficient development length for the bottom face) and small diameter U bars in the side face.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[dik]

It may be a standard practice in some areas... but, may not likely be an intelligent engineering application... or have any engineering basis whatsoever!

I'm just reviewing a centre pier foundation for a tank that had a 10'x10' spread footing... the footing was 12" thick and had 25M@12 T&B ea way... I don't know why the footing was that thin and the reinforcing was so massive. Even the 2' wide x 12" thick strip footings with a 10" wall on them had reinforcing both top and bottom.

I replaced it with 20M@12 Bot only, and still had 3x the reinforcing I needed. The 4'x4' pier was left (a 2'x2' would have been lots); I just didn't know where to stop the changes.

Dik

 

[hokie66]

Top bars are often the cause of cracking in thick footings, as they lead to settlement cracking if revibration does not occur. If the footing is not subject to uplift or overturning, there is no basis for top bars to be used in footings of moderate size.

 

[asixth]

Top bars for crack control.

You can extend the length of the cog or place stands.

For a 600thk footing with 10mm bars top I would think that the bars would require some stands to keep the reinforcement in place. Nothing wrong with the design and I'm sure your specifications would state that the reinforcement is required to be securley fixed in place.

I would provide top and side face reo for crack control.

 

[dik]

The plan area is small, and the top is in compression... not much chance for cracking!

The U bars in the ends, and tops of walls, may aide in placing rebar, but for deep footings, contractors, in these environs, usually use 'Z' bars tied to the bottom reinforcing for spacing with diagonal wires to prevent 'tipping'.

If the bars in the edge serve any purpose... in 40+ years, I've not seen a horizontal crack in a footing... but, most of the time these are buried... I've removed these from standard details for tops of walls, etc...

Dik

 

[Ron]

In my world, top bars are almost always required because of uplift and overturning requirements. I usually cut the brain damage on the contractor and specify the controlling case for both conditions....keeps from having screw-ups in the field with top being on bottom and bottom being on top!

 

[hokie66]

Top bars for crack control in a spread footing which supports a single column are totally unnecessary, and I believe no code requires them. As I said before, the main thing they do is promote plastic settlement cracking, so the aim of controlling cracking is defeated. For uplift and overturning, of course they are required.

 

[IDS]

Top bars for crack control in a spread footing which supports a single column are totally unnecessary, and I believe no code requires them

There is at least one code that requires them, AS 5100:

2.8 CRACKING
...
(b) For members with a thickness greater than 150 mm, each face of the member shall be
reinforced with not less than 500 mm2 per metre for each of two directions at right
angles to each other. The layers shall be placed as close to each surface as cover and
detailing permit.
Reinforcement provided for structural reasons and located within 80 mm of the face
can be considered as contributing towards this requirement.

Of course top steel will often be required for bridge column footings for overturning, but the code requires it whether there is any design tensile stress in the top face or not; and as I said before it has been standard practice on every construction project I have ever worked on.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

Bridge Code...OK. I don't work on bridges, but lots of buildings and industrial structures, where if they are not required for flexure, we don't use top bars.

 

[rowingengineer]

I am a supporter of Hokie's position.

Doug, In reference to the start of the clause you are referencing, generally these clause would state something like "where crack width is considered detrimental to the appearance of the structure". I have never seen top reinforcement to a bored pier yet, and I have seen 1200mm bored piers for bridge construction work.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[IDS]

Doug, In reference to the start of the clause you are referencing, generally these clause would state something like "where crack width is considered detrimental to the appearance of the structure". I have never seen top reinforcement to a bored pier yet, and I have seen 1200mm bored piers for bridge construction work.

But AS 5100 doesn't say anything like that. The clause applies to external faces, so it wouldn't apply to the top face of a bored pier. I'm not saying that top reinforcement should always be used everywhere, but the implication of earlier responses seemed to be that if it wasn't required for overturning or uplift then there was never any reason to provide it. The fact is that it is standard practice in heavy construction work, and at least one code specfically requires it.

Where it is not normal practice and the codes don't require it of course there is no problem with not providing it.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

"Heavy construction work" perhaps needs defining. It is not standard practice in high rise buildings founded on pad footings on rock. It is not standard practice in coal preparation plants. I can see that most bridge footings will involve flexure, and there should be top bars, but I can't see the reason for a minimum requirement as quoted in AS5100. But then, I can't see a reason for a lot of code requirements. Committees do funny things.

 

[dik]

...and I'm not sure about the rationale for using it...

Dik

 

[rowingengineer]

Doug,
I am bored and working a holiday so forgive the uncouth behavior, but I have copied the part from AS5100-2004 and it states:

2.8 CRACKING
The cracking of beams or slabs under service conditions shall be controlled in accordance
with Clause 8.6 or Clause 9.4, as appropriate.
Where considered necessary for durability requirements, e.g., for exposure
classifications B2 or more severe, or where crack width is considered detrimental to the appearance of the structure, consideration shall be given to limiting the steel stresses near the tension face to values less than those given in this Standard. In addition, in such conditions consideration shall be given to the detailing of the structure to minimize cracking due to restraint and shrinkage.

All concrete members shall be provided with a minimum of reinforcement as follows:
(a) For members with a thickness of 150 mm or less, a single layer of reinforcement of
not less than 500 mm per metre for each of two directions, at right angles to each
other.
Reinforcement provided for structural reasons can be considered as contributing
towards this requirement.
(b) For members with a thickness greater than 150 mm, each face of the member shall be
reinforced with not less than 500 mm per metre for each of two directions at right
angles to each other. The layers shall be placed as close to each surface as cover and
detailing permit.
Reinforcement provided for structural reasons and located within 80 mm of the face
can be considered as contributing towards this requirement.
Reinforcement shall be provided in two directions at right angles to each other and with a
spacing that is less than or equal to 300 mm.

The requirements of this Clause in relation to the quantity of reinforcement in a particular
direction may be relaxed if the member is unrestrained against longitudinal movement in that direction and the effects of differential temperature and humidity are minimal.

From the underlined areas it would appear permissible by this code to leave out top reinforcement. Interpretation of the clause is interesting because it appears that the "all concrete members" statement is separate to the above wordings, but it could be argued either way, but I am sure the end of the clause is fairly clear that unless you have a need for the reo, you don't need it.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."