Reinforcement of Block Foundation for Heavy Machine 1

Machine foundation design code is available in every country.

In your case I suggest an old CP 2012. It is good engineering practice. The control depends on the vibration control of the machine. A steam or gas turbine say has vibratory monitoring equipment to trip the machine once the vibration exceeds certain threshold. This is the way the manufacturer's way to protect its product, and the warranty, from abuse and unwarranted operations as parts are manufactured to tight tolerance. On a rough set say a reciprocating diesel set, which is nowhere balanced to that of a turbine (best balance is a hard disk) you have more freedom in the foundation design.

The mission of holding down bolt is to attach the machine to a lump of concrete. If that lump of concrete deliminated or cracked you lose the benefit of the attached mass and you machine will vibrate differently, almost certainly to larger magnitudes. Therefore making your foundation as homogeneous as possible is a design requirement.

All machine holding down bolts are beefy steel rods much larger than the largest rebar, say 40mm, and must be soundly anchored. Thus the design needs to ensure the HD bolts are properly embedded and have enough reinforcement for dissipating the stresses into the block. There is no specific guideline but more based on good engineering practice established over the years.

The British code of practice recommends a 3D cage, think it is 16mm bars at 600mm centres, as it is the minimum reinforcement for holding the block as a rigid body. In any machine foundation design that lump of concrete is assumed rigid when coupled to the machine. You can't afford have the lump of concrete flexing in any vibration control.

As a rule of thumb the lump of concrete is based on the mass ratio between the foundation to the machine. ACSE recommends 3 to 5 times foundation weight to a steam turbine. On gas turbines a mass ratio not below 2 generally give trouble free service. On a diesel set the mass is heavier and comparable to steam turbines. Other machines you have to check with the vendors.

Machine foundations never fail structurally. They always cracked and the the machines vibrated more than normal resulting premature and more frequent outages for repair and parts replacement. On any vibratory foundation the cracks grow with time and are the worst enemy for the designer because once it fails there is very little can be done except a total replacement. Equally when a machine vendor is asked to come back mre frequently to do maintenance he does not always know that you have given him a bad foundation!

Lastly when you analyse a machine foundation you are supposed to include the machine, the foundation and the soil or piling in the analysis. The very least you should include is to ensure the operating frequencies of the set doesn't get too close to the natural frequencies of the foundation/soil interaction. The CP2012 is more or less a hand calculation but you can do it with a spreadsheet or computerize the procedures. Equally you can carry out the analysis with any structural software by requesting natural frequencies and mode shapes.

For solid mass concrete foundation, you can use 2-3 times of concrete cover distance as thickness in calculation of minimum T/S steel.
You need to follow ACI App. D to determine whether cage is required around the anchor bolts.

Thank you guys,
i m following ACI App. D. I will look for cage requirement and reinforcement on app D