Steel Reinforcement per ACI 207.2R for Machine Foundation

[Abdel22]

Hello all,

anyone familiar with ACI 207.2R steel reinforcement for thick block foundation

I need some clarification ACI 207.2R . their example about 6' thick slab for plant, they come out with # 11 @ 6 in. o.c in each face.

is it only at face or through out the block?

i know for machine block foundation reinforcement have to be in all directions.

i thought 6 in. is too tight.
anyone have a reference about reinforcement layout for heavy machine foundation.

Thank you

 

[r13]

For mass concrete block, the face cools much rapidly than internal - the internal continue to expand, as the outer surface starts to shrink, so it is necessary to provide reinforcement near the surface to minimize the thermal crack. Yes, #11 @ 6" is quite tight, is the example was for water retention structure? You can use bundled bar at farther spacing though, if permitted, I remember 3 bars bundle was the maximum.

 

[Abdel22]

That example was for 6 ft thick power-plant base slab.

I m designing 5 ft block for grinding machine. i m using that example as reference but got me confuse.

is the steel only at face not inside the block.

 

[r13]

Yes. What formula you were using to determine the reinforcement? I remember ACI 318 & 350 has formula for reinforcement for crack width control.

 

[Abdel22]

i m using Gergerly-Lutz crack width formula for the stress in reinforcement fs and another formula to get area of reinforcing bar from ACI 207.2R

just i didnt picture concrete block with reinforcement only near all faces with nothing inside.


thank you

 

[r13]

As shrinkage starts on the concrete surface, the reinforcement you are calculating is temperature and shrinkage steel to minimize the width of those surface cracks, and prevent the cracks propagate deeper into the concrete block. The reinforcement inside of the block can be considered as a waste, since the surface reinforcement is much more effective in doing the job.

 

[saikee119]

Just curious

Isn't ACI 207.2R is for mass concrete?

Foundations for Dynamic equipment should be per ACI 351.3R?

I have in the past come across design of a few machine foundations based on mass concrete design but that was in Dark Ages.

Any machine block design I would have thought functionally must be

(1) Able to withstand a thermal gradient during operation when the operating face can be significantly hotter than the buried section. This isn't always covered by the code but always a design requirement.

(2) Have enough reinforcement to guarantee the foundation can be dynamically coupled to the machine to vibration as one entity and not separated by concrete lumps attached by rebar.

(3) Be able to grip all the holding down bolts tight under all loading conditions.

One can treat a machine foundation as a piece of mass concrete but that isn't an international standard.

 

[Abdel22]

saikee119

Yes, that s right ACI351.3R for foundation for dynamic equipment.
i m using ACI 351.3R but for foundation thicker than 4 ft ACI 351.3R suggested to use ACI207.2R for minimum reinforcement.

this my first time using ACI207.2R , so i was confused about reinforcement layout for block foundation.

does anyone have a typical example of block foundation steel reinforcement?

thank you

 

[saikee119]

Abdel22 (Civil/Environmental)(OP),

For a machine block design the primary consideration is to control its vibration. The best way to control vibration is by coupling the machine to a mass much heavier or several times more than the machine itself.

For this reason you may end up with a big lump of concrete and it seldom fails structurally so for economy many designs could be based on minmum reinforcement overall plus additional bars at the stragetic locations.

In general you need to check the previous machine foundations for your equipment as different machines can have different balance grades producing different dynamic forces in operation.

If you foundation is for a small machine the vibration characteristic could be modelled as rigid body movements in 6 modes by analysing the effect of the soil. A very good and practical guide can be found by Bristish CP 2012. Above that you may have to do a 3D finite element, 2D plate or a comination of plate and frame to model the foundation sitting on elastic springs for the soil.

From my own experience I always recommend 3D cage as described by CP2012 as it does provide a continuity to pass the holding down bolt stresses into the block and justify the rigid body coupling concept. The 3D cage is additional to the surface reinforcement at all sides. Your block should finished with additional trimmers at corners, holes and cutout where stress changes suddenly to control cracks.

A non-performing machine foundation never collapses structurally. It just cracks and fails to damp down the vibration resulting more than normal outage for maintenance and parts replacements. Many machine vendors install vibration sensors to monitor the performance of their products. You could void the warranty if the foundation fails to control the vibration within a set threshold. Such threshold typically sound an alrm and then ultimately trip the equipment.

 

[SlideRuleEra]

Does anyone have a typical example of block foundation steel reinforcement?

http://www.SlideRuleEra.net

 

[saikee119]

Some machine foundation drawings

A GT foundation

Another GT foundation

CP2012 Fig 4

 

[Abdel22]

SlideRuleEra

so the steel reinforcement only near faces no intermediate layer needed.

my block foundation is 23' Long 15'Width 5'Thick

Thanks everyone for valuable information.

 

[saikee119]

For balance grade like a gas turbine I have not seen a problematic foundation with foundation/machine mass ratio higher than 2 unless the foundation is piled or vibration mat or springs are used.

Also the specification I used to enforce mandates rebar content not less than 80kg/m3 for a machine foundation. For minimum reinforcement you should have about 30kg/m3. Above 100kg/m3 the design is congested with reinforcement. If you design the foundation to take say 15 to 20 degree C thermal gradient, between the top and bottom face, your design will be around 70kg/m3. This is just good engineering practice. For a machine foundation it is the availability of the equipment that matters most. If you lose the equipment you lose production and income stream so a robust design goes a long way to ensure trouble free service.

 

[r13]

Abdel,

You have to ask yourself why you need to provide temperature and shrinkage steel, and at what location the cracks are most likely to occur, if the free of load block is left unreinforced. Also think about why the code name it "temperature-shrinkage steel", not "temperature-expansion steel".

 

[Abdel22]

Thank you for valuable information.