Early / Partial Loading on Recently Placed Concrete Foundations 2

[FLCraneBuilder]

A discussion in our office has come up regarding placement of structural steel columns & overhead crane runway beams on recently placed foundations. We recognize that concrete reaches published strength in the 28 day time frame. But if we simply want to erect the steel say 5 to 7 days after the pour, are there any downsides?
The current example is 8" tube columns with 18x18 base plates. When the crane is operational, the axial column load will be 26 kips. But, the column load due to weight of cranes & structure is only 6 kips.
Note also the anchors will be drilled & epoxy set in the new concrete

For many years we have erected crane systems quite soon after foundations are poured; with no apparent problems...but, someone has brought up the concern, and we want to understand this a better.

We have thought about "high-early" concrete... but we're not sure if this really resolves the potential problems.

If any of you have thoughts or experience on this topic, we would greatly appreciate your insight.
Regards;

 

[dik]

You have to watch out that the anchor rods are sufficiently anchored... after 5 to 7 days, your concrete should be at approx 60% of it's 28 day strength... closer to 70% for 7 day strength... normally not an issue.

Compression would not normally be an issue.

Dik

 

[ElliottJames]

Drilled and epoxied anchors into new concrete? Not cast-in-place?

 

[dik]

Agreed... and the comments are the same for CIP anchor rods or glued...

Dik

 

[WARose]

I'm not a fan of loading up concrete prior to it's reaching 28 days strength. The downsides are excessive cracking and higher deflections. I've used "high early" concrete in many instances where schedule was a concern.

 

[JAE]

You simply need to evaluate the loads to be imposed on the concrete prior to reaching the 28 day strength.

If you allow steel erection to occur, say, at 7 days, then determine your current f'c value from 7 day breaks (or 5 day or 3 day - whatever) and then simply check the footing shear, moments, bearing, anchors, etc. for the anticipated loads between that early point in time and the 28 day mark.

This takes more work for you but if you already have your system analyzed, modeled, etc. then usually it is no trick to quickly check the lighter loads at the lower f'c.



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[RPMG]

This is very common. It's also common to specify 7-day strength for the concrete mix design.

For the anchors, you have to get a Hilti judgement. Hilti will either tell you not to torque the anchors (tighten the nut) or to expect time-of-install strength. Other manufacturer's such as Simpson and Powers will not give a judgement.

 

[FLCraneBuilder]

Thank You for the quick responses!
Note... We ALWAYS epoxy set our own anchors because the cast-in-place guys cant get them right... No matter how hard they try... Crane runways must be set to very tight straightness tolerances

 

[wannabeSE]

ACI 318-14 section 17.1.2 requires 21 day cure before installing adhesive anchors. Dewalt (Powers Fastener) has done some test and different adhesives perform differently when installed in early age concrete.

http://www.powers.com/pdfs/recent_u...e-adhesive_anchor_installation.pdf?1510627789

Otherwise, I agree with the others. But, the bottom line comes from the FlCraneEngineer responsible for the design.

 

[dik]

RPMG:

I normally spec 28 day strength for legal/spec reasons... and sometimes stipulate a minimum concrete strength for things such as form stripping, anchor rods, etc.

Dik

 

[RPMG]

wannabe: Awesome reference. I've asked a Powers' salesman and a tech rep about this, and they acted like I was the first person to ask. Anyway, the code allows manufacturers to make exceptions. If Hilti gives a judgement, it meets code.

dik: 28-day is the standard, absolutely. But you can specify 7-day, and the mix designer will meet it.