Calculating defelction of concrete

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Discussion Overview

The discussion revolves around the requirements and considerations for constructing a concrete foundation capable of supporting a large machine with a strict deflection limit of 20 microns. Participants explore various types of concrete, specifications, and engineering practices relevant to ensuring the foundation meets these requirements.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants inquire about the types of concrete suitable for a foundation that must support heavy machinery with minimal deflection.
  • One participant emphasizes the need for professional consultation with a civil engineer due to the complexity and critical nature of the foundation.
  • Concerns are raised regarding the feasibility of achieving less than 20 microns of deflection in such a large structure, with some questioning the realism of this expectation.
  • Participants discuss the specifications for the concrete, specifically mentioning the European standard C30/37 and the importance of proper reinforcement with rebar.
  • There are mentions of alternative measurement systems, such as laser trackers, and their relative accuracy compared to coordinate measuring machines (CMMs).
  • One participant details the weights and dimensions of the machine components, raising questions about the necessary rebar specifications to prevent deflection under load.
  • Another participant suggests that nitrogen spraying during the concrete pour may help manage heat build-up, although this remains unverified.
  • Concerns are expressed about the mixture of metric and imperial units in the specifications, which could complicate the engineering process.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of achieving the specified deflection limit, with no consensus on whether it is technically possible. There is general agreement on the necessity of consulting a foundation engineer, but varying opinions on the specifics of concrete types and reinforcement methods.

Contextual Notes

Participants note the importance of adhering to specific engineering standards and the potential complications arising from using mixed measurement systems. There is also uncertainty regarding the exact rebar requirements and the implications of the foundation's design on deflection.

Ranger Mike
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I have a large machine that must be installed on concrete foundation. It can have no more than 20 micron deflection. Are there different types of concrete?
The specs i have from the manufacturer recommend Foundation block reinforced concrete C30/37.
this is 1500 mm deep foundation that is 7730 mm long and 6000 mm wide.
i have a 34,000 pound granite plate on the concrete and 30,000 pound part resting on it. The gantry machine surrounds the part and plate and weighs less than 30,000 per side
Any tips of links that will assist are much appreciated.
 
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Ranger Mike said:
I have a large machine that must be installed on concrete foundation. It can have no more than 20 micron deflection. Are there different types of concrete?

Most assuredly, yes.

The specs i have from the manufacturer recommend Foundation block reinforced concrete C30/37.
this is 1500 mm deep foundation that is 7730 mm long and 6000 mm wide.
i have a 34,000 pound granite plate on the concrete and 30,000 pound part resting on it. The gantry machine surrounds the part and plate and weighs less than 30,000 per side
Any tips of links that will assist are much appreciated.

It's not clear where your foundation is located or if it's built yet. Either way, you should talk to a civil engineer about this foundation. It's not something you're going to mix up in a wheelbarrow using a couple bags of Sak-crete and pour yourself. It's also probably not going to set overnite either, so you'll want to have the setting monitored to make sure the concrete has reached its maximum strength. The concrete probably should be tested before installing your machine.

If your foundation is a solid block, your pour is going to be almost 91 cu. yards of material, and a single cement mixing truck might be able to carry about 8 yards of wet cement. You can expect to pay in the range of $120-$200/yd. of cement delivered, so I would call around to several suppliers and get estimates for a job this size. IDK how much arranging and installing the necessary re-bar will run you.
 
Ranger Mike said:
I have a large machine that must be installed on concrete foundation. It can have no more than 20 micron deflection...
this is 1500 mm deep foundation that is 7730 mm long and 6000 mm wide.

Sounds like a BIG coordinate measuring machine... Is it realistic to expect less than 20 microns of deflection out of such a large structure?
 
Mech_Engineer said:
Sounds like a BIG coordinate measuring machine... Is it realistic to expect less than 20 microns of deflection out of such a large structure?

Could probably use a laser tracker instead of a CMM. However, 20 micron deviation over 7.7m does seem rather tight.
 
acutally a big CMM with 4 meter x 5 meter x 2 meter travel
spec is 6.9 + L/350 per ISO 10360-2/4 with SP25 scan probe
laser not that accurate
 
Ranger Mike said:
acutally a big CMM with 4 meter x 5 meter x 2 meter travel
spec is 6.9 + L/350 per ISO 10360-2/4 with SP25 scan probe
laser not that accurate

Well yes, a CMM is more accurate compared to a laser tracker.
 
i been selling cmms since 1980...lol
 
Ranger Mike said:
i been selling cmms since 1980...lol

That is a long time. For my previous project at work, I had to research various 3d measurement system. CMMs were excluded because they aren't very mobile, which was one of the requirements.

Just out of curiosity, what is the accuracy of the CMM you will use to measure the deflection?
 
the customer is responsible for the foundation..we recommend Euro spec C 30/37 for the concrete and if it is met the cmm will be good to go.
A laser plane scanner is probably best to find the deflection i would think.

FYI- we use the ISO standard 10360 to measure the machines uncertainty. Basically a series of bi directional measurements of gage step bar that is up to 2000 mm long and is certified by calibration lab. there are about 105 measurements taken in three directions and the procedure is repeated three times. The calculations are made and the accuracy or degree of uncertainty is determined for the volume.
 
  • #10
Weight of complete machine is approx.. 17000 kg * 2.204623 lbs/kg = 37500 lbs.

- weight of the bridge/portal (dynamic mass) is approx.. 4000 kg * 2.204623 lbs/kg = 8820 lbs

- weight of the right rail is approx.. 7000 kg * 2.204623 lbs/kg = 15450 lbs

- weight of the left rail is approx.. 6000 kg * 2.204623 lbs/kg = 13230 lbs

Granite Table

- 3500 x 4500 x 355.6 mm

- Grade AA, 0-Ledge Black Granite Surface Plate with (50) ¼-20 threaded inserts

- 33,643 lbs

- (3) pedestals and leveling jacks 14” x 14” and thickness specified by customer depending upon working height needed.

- <image002.jpg>
Foundation

- Size of foundation and tub as specified on the attached drawing, R20.70.40.50.20.

- Foam lining per the attached drawings, though substitutions are allowed; i.e. an Owen Corning product of similar characteristics which is locally available.

- Foundation concrete type could be Portland Cement, Class A with modifications to meet the European Specification C30/37, as noted in the drawing, R20.70.40.50.20

- Tub concrete type could be Portland Cement, Class A with modifications to meet the European Specification C20/25, as noted in the drawing, R20.70.40.50.20

- Foundation should be reinforced with rebar as appropriate to meet the requirements noted above. For example, Mk606 bars on 12” O.C. spacing, in all directions may be appropriate and adequate to meet current standards and local codes.

I am having difficulty with what the rebar Mk606 specs are? When going for a quotation for the foundation contractor i need to have this foundation not deflect when i put a 33,000 part on the granite plate.
How much rebar do i need to prevent deflection over 20 micron when the part is loaded on to the plate?

do i need nitrogen sprayed while pouring concrete to reduce heat build up?
 
Last edited:
  • #11
There's no drawing attached to your post.

Again, for such a critical function as this foundation serves, and the small tolerances allowed, soliciting advice thru an anonymous forum is not recommended. You need to talk to an experienced foundation engineer, one who can review your drawings, inspect the equipment to be installed, and evaluate the site where the installation is to take place. I don't think you can just turnkey this process without some first-hand knowledge of what's going on and where this machine is supposed to go.

It appears that the manufacturer of the machine has specified a lot of European standards to follow, which standards may not be familiar to engineers practicing outside of Europe, but it's not clear if this machine is being installed in Europe. There seems to be a mixture of metric items and USCS items being used, which could present complications if not carefully managed.

I'm not familiar enough with concrete construction to know if it is even technically feasible to engineer a foundation which won't deflect 20 microns under load. I suspect anchoring a steel foundation in concrete might be an alternative, but that's why you should consult with a foundation engineer.
 

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