Small Concrete Pads and Acceptable Levelness Tolerances

[milwaukeebob]

Hello folks,

I'm an power systems electrical engineer and have a concrete related question. Not sure if this is best posted here in the Civil or in the Structural forum.

I'm working on a new project that involves the installation of two (2) small dry-type transformers rated 1500kVA. Each one weighs approximately 11,000 lbs. Both of the transformers have their own concrete pad with approximate dimensions of 8'-8"W x 13'6"L. Last week my colleagues and I discovered the transformer pads are not even close to being level - at least what I would think to be acceptable levelness tolerances. One of the pads and associated transformers slopes down south-to-north by 2.5" in the length and over 1" east-to-west in the width. In general, we believe the pads are moving/sinking but that's not what my question is related to.

Is there an ACI standard or other industry standard that defines the maximum allowed slope for a concrete pad of this type? I was able to find some industry related company specifications that indicate level is "within 1" in all directions". That still seems like a lot to me for such a small pad but, then again I'm electrical engineer who just wants my xfmr to sit perfectly level.

I appreciate any and all guidance in this matter. Thanks

MB

 

[SlideRuleEra]

MB - About 1990, ACI adopted a method of specifying concrete slab surfaces, called "F-Numbers". There are two values to be considered, "flat" (F_F) and "level" (F_L)... they are not the same thing. The slab you reference is certainly not level, but it may or may not be flat.

Read this summary of the F-Number system. Typical values for floors are suggested... higher numbers are not necessarily "better" because of excessive construction cost. The goal is to pick the optimum value for a given application... and that is up to the engineer.

I worked in the power industry (generation), for your project I would likely go with, say, F_F35 / F_L25.

 

[TigerGuy]

How much tilt can the transformers handle? Most of the legitimate contractors I have seen could construct the described pad with less than 1/4-1/2 inch of total slope.

 

[milwaukeebob]

SlideRuleEra, thanks for your reply. I sincerely appreciate the info and the technical reference you provided. Good stuff.
TigerGuy, great question. Some electrical equipment OEMs specify a maximum slope/angle their gear can be placed on. Prior to posting my question, I did look in the OEM manual and there wasn't anything listed. My next step is to contact the OEM directly and discuss. Also I thoroughly agree with your comment about a "legitimate contractor" capabilities. I'll end by saying we've had challenges on this job and they don't stop at the concrete work.

 

[SwinnyGG]

When we place transformer pads, we typically specify them very slightly domed - I'm talking 1/2"-3/8" from the high point in the center to all edges, and edges no more than 1/4" from dead level or plumb. On pads that small, this is very easy to achieve.

2.5"-3" is way beyond acceptable. I'd also be surprised if these are settling without cracks - usually if they move, they will crack, even on pads this small.

 

[milwaukeebob]

Thanks Swinny. I don't have any experience with domed pads but I understand what you're saying and in many applications would be ideal. I whole-heartedly agree the much tighter tolerances you mentioned are very easy to achieve and 2.5" is unacceptable. I've seen alot in my 30 years in the industry but it seems like the most shocking things have been in the last 5+ years. The industry is changing and not necessarily for the good. Thanks for you post and guidance.