Bolt pretensioning / Concrete Splitting

[slickdeals]

Folks,
I was wondering if pretensioning an anchor bolt in a concrete pedestal can cause a splitting failure originating at the bottom of the bolt or am I worrying too much?

 

[ishvaaag]

Normally the recommendations of fabricators and codes if followed should prevent that. If one gets into risky details some of this could happen. I once read an articla at ACI structural Journal testing a number of embedments on some mat. Most in that sound concrete and thick mat and around 20 mm diameters failed in steel tearout where the embedments were 200 mm or so. So sound concretes with sound practices and recommended levels of stress should bar these problems.

 

[racookpe1978]

You have a couple of different failures modes possible: BUT - if (big "if" there!) - you follow manufactor's guidelines and stay within stress limits for your flavor of hardened concrete, you will avoid them.

Example: Bolt (or embed) is properly sized, but the concrete is green, not capable of giving you full strength. The concrete will fail, but probably at someodd combination of size and shape: It might be a combination of the bolt pulling out of the hole, the cement/epoxy pulling out of the hole, and he concrete separating from itself.

If the concrete isn't strong enough, you'll get a conical shaped pullout. Avoid this by going deep enough with a big enough bolt/embed diameter so the 45 degree cones develope enough surface area to not separate. Bolts too close together will cause overlapping concrete cones = Same result, concrete fails.

Bolts can pull out of the hole if there isn't enough surface area for the epoxy/mechanical fastener to "grip" the walls of the hole. Fix that by going with a deeper hole deeper, getting a bigger diameter bolt, adding more bolts to your fixture.

 

[Ron]

Why do you want to pre-tension an anchor bolt to anything above "snug tight"?

 

[rowingengineer]

We pretension bolts for wind towers to ensure fatigue isn't a problem, we treat the pretension like a pull out force on the bolts. AKA so as long as you ultimate pull out force is higher than the pretension loading and you have designed your bolt for this ultimate pull out force, than the pretension is not going to fail your connection.

When in doubt, just take the next small step.

 

[conceng]

When pretensioning, do you have a leveling nut on the bottom side of the flange? If this is the case, the bolt tension only exists between the nuts and does not transmit to the bottom of the anchor bolt.