Reinforcing heavy equipment foundations 5

[johnp]

I am designing foundations for FAG Mills and Ball Mills. I need guidance on use of nominal reinforcing steel.

Most of the guidelines relate to beams for building construction, or columns up to 1 m square, and to concrete slabs. If I apply those to raft foundations 2 to 3 m thick the numbers are way outside normal comfort zones, and don't make much sense.

The equipment plinths may be 3 m wide x 1 m thick x 5 m high. The vendors say most of the 2,000 ton loads will be vertical with comparatively small dynamic side loads.

Any guidance on the subject would be appreciated.

 

[BigInch]

What do you mean by "comfort zone" and what minimum ratio does your design code specify?

Going the Big Inch!

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[DaveVikingPE]

You might want to take a look at the US ARmy technical manual TM 5-809-12 "Concrete Floor Slabs on Grade Subjected to Heavy Loads." It's free and avaiable at

http://www.usace.army.mil/publications/armytm/tm5-809-12/.

 

[SlideRuleEra]

johnp - That's what #14 and #18 rebar are for. In principle, just design the foundation the same way as smaller one, make sure that you meet the minimum percentage rebar, and use at least one mat top & bottom. Keep all the bars the same size. If the rebar spacing gets too close (which I doubt for the thickness of 2 to 3 meters) go with two mats, top and bottom (one mat stacked on top of the other).

Contractors, hate the big bars - they have to perform their own (means & methods) structural design just to support the the top mat(s). Often they propose sizable WF columns to support the top rebar - these are completely encased within the foundation.

http://www.SlideRuleEra.net

 

[johnp]

Thanks for your comments.

1 - For this size of foundations minimal steel ratios are not applicable. The "comfort zone" is the limited range of values over which rule-of-thumb reinforcement ratios provide realistic guidelines.

Usually the minimum steel requirement is intended to limit unsightly cracks in beams and slabs in building construction. When taken out of context, and extrapolated to 10 times normal values these guidlines break down. Hence the motivation for this thread.

2 - Thanks for the US Guidelines for concrete floor slabs. When dealing with floor loads of 3,000 tons floor slab guidelines are way outside the range of normal recommended practice.

3 - Thanks for the recommendation to use maximum size bars, and planing how to support these as a top mat 2 m above the floor mat. Effectively we need to construct a second floor for this top reinforcing.

The Codes say nothing about minimum percent reinforcing for such applications. The forword to most codes will specifically exclude such unusual structures. However, in today's mining engineering there is nothing unusual about FAG and Ball Mills, except their size.

I would rather put reinforcing where it is required by design, than to fill the concrete void with steel that I don't understand, using rules-of-thumb taken way out of their normal context.

That is why I'm seeking better understanding of such structures, of which there are many forms - even in rafts to multistorey buildings.

 

[BigInch]

I've done many very large foundations for petrochemical plant towers, foundations for mixing reactor support structures several stories high, mat foundations for a row of very tall pellet silos, compressor foundations, nuclear plant equipment foundations, a great number of which have been 2 meters or more in thickness and the mentality at all the many different engineering companies I worked for was that the code made no distiction between "applicable" or "non applicable" in regard to minimum reinforcement and we put it in each and every one.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[civilperson]

The ACI minimums apply to this size footing as well as larger elements. The minimums are listed and a exception is given for excess strength in 10.5.3, (4/3 of requirement for flexural strength). The 5000 ton loads will be supported by many square feet of soil and the using shear to set the minimum depth, solve for the area of steel needed for flexure. Compare 4/3 of this amount with the various minimums, 200/fy X bd & 0.0018hd. This size flexural member also has other requirements: 10.6.7, (side face reinforcing) and 10.3.5 ,(maximum reinforcement).

 

[BigInch]

Civilperson thanks for listing the ACI requirements. I have not designed a foundation in a very long time and I did not know if the minimums had changed or not. I see they are still exactly the same as they always have been.

Going the Big Inch!

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[civilperson]

An alternative to using reinforced concrete is to use "Structural Plain Concrete" as specified in chapter 22 of ACI 318. The soil supported foundations and plinths of sizes listed would be suitable for this design methodology. Reinforcing less than the minimums earlier discussed puts the design into this category.

 

[johnp]

Thanks for your input. If I were in a hurry I'd just throw steel at it and get on with the job - but now that I've got time to think I can afford to ask - WHY? Am I putting in reinforcement just to keep the steel fixers busy? Or would it be some underfined liability insurance issue?

If I have a spot footing 4 m square x 2 m deep it will carry 800 ton without any reinforcing at all. To keep shuttering simple we usually cast the full box - providing more concete than is strictly required.

The subsoil could settle a bit but there would be no distress to the concrete. The side of the block could be trimmed back at 45 degrees. But the extra side wedges gives the concrete additional confined compression strength.

Top steel in footings is only needed if there is the possibility of serious stress reversal due to uplift.

If I put two of these units together to form a 4 m x 8 m x 2 m combined footing they would still act as a mass concrete block requiring no reinforcing. However, if I spread the two 800 ton loads by 200 more millimeters, we would get a 4 m x 8.2 m x 2 m block with designer reinforcement, minimum steel ratios, and side face steel. This smells of overkill - but may be justified in some cases.

 

[BigInch]

Maybe you should have a structural PE take a look at that design.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[johnp]

Thanks mate - I is one

 

[civilperson]

The soil pressure under the 800 tons will exceed 10 ksf. Check allowable bearing with your geotech. The 4 x 8.2 x 2 meter dimensions could be design as Structural Plain Concrete, (ACI Chapter 22).

 

[SlideRuleEra]

Unreinforced concrete is fine for static loads - a ball mill and other rotating equipment have to potential to put dynamic loads on the foundation - this may be fine, too... until the day "something" goes wrong with the equipment. That is the day the reinforcing steel can save the equipment and the maybe even the foundation - it happens.

As usual, weigh the risks and make a decision - there is no "right" or "wrong", just probabilities and consequences.

http://www.SlideRuleEra.net

 

[BigInch]

...and when some laywer finds out there is not even "minimum steel" in a dynamic fdtn. They don't have a real good concept of para. 22, but do understand very well the concept of minimum steel... and I just bet there will be no shortage of Xpert engineers to agree with him.

Oh well, for once, my hands are clean.



BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[csd72]

Have you tried to meet the criteria for unreinforced concrete?

If you can make it work as unreinforced, then I think the code allows you to put some reinforcement in it for serviceability only.

According to the ACI, just because it has reinforcement doesnt necessarily mean that it is not unreinforced!

 

[tgmcg]

Gentlemen:

I'm a machinery engineer with over 30 years experence, and would like to offer my 2 cents worth of advice.

For dynamic equipment PLEASE use reinforcing steel, and plenty of it. More than code minimums. Post-tensioning should also be considered for crticial service equipment foundations....that's what we're using to repair our existing 7000HP recip compressor foundations, on the advice of a leading consultant in this field. The CE's who designed the original foundation didn't use enough reinforcing steel.

An alternative solution is to lop off the top 36-48" of the block and replace with a heavy duty structural steel baseplate, to be subsequently filled with concrete.

The cost of having to go in 10-20 years from now and replace machinery foundations in an operating plant can be astronomical. It's not uncommon to have to rebuild a complete new compressor station due to foundation damage...were talking many millions of $$ and lost revenue. Generally speaking, I do NOT like using profiled reinforced concrete foundations as they always seem to fail through under-design and incur huge costs to repair.