Pre-Tensioned Anchor Design with Appendix D 1

[JAX91]

I am designing some anchors for a piece of equipment using ACI Appendix D. The vendor requires a pre-tension load. Placing the pre-tension on the anchors will place tension into the concrete as well. The pre-tension is much greater than any load I will be placing on the anchors. When checking the pre-tension loads on the concrete with the factored loads from ACI, what load factor should I use on the pre-tension load?

 

[Gumpmaster]

Check out this quarter's AISC Engineering Journal. It has an article on designing pre-tensioned anchors.

 

[JAX91]

I guess I should have started by asking if concrete breakout needs to be considered with pre-tension loads?

 

[amec2004]

>> if concrete breakout needs to be considered with pre-tension loads

Yes. You should consider that.

Below is from PIP STE05121-2006 Anchor Bolt Design Guide Page 23

10.4 Concrete Failure
In certain situations, the use of high-strength anchors in concrete with high pretension forces may exceed the ultimate capacity of the concrete by prematurely breaking out the concrete in the typical failure pyramid. Whether this situation can occur depends on the depth of the anchor and on other factors, such as edge conditions and arrangement of the base plate. To ensure that premature concrete failure does not occur, [red]pretensioned anchors shall be designed so that the breakout strength of the anchor in tension is greater than the maximum pretension force applied to the anchor.[/red] In the case of a stiff base plate covering the concrete failure pyramid, the stresses induced by external uplift on the concrete are offset by the clamping force and the gravity loads. For this case, the breakout strength needs only to be designed for the amount that the external uplift exceeds the gravity plus pretensioning force loads.


From API 686 Recommended Practices for Machinery Installation and Installation Design-2009 Annex B Table B.1, it uses pretension stress 30ksi, almost 70% of ASTM F1554 Grade 36, A307, or A36 anchor rod material capacity, phi*futa=0.75*58ksi=43.5ksi, to work out the pretension load. If high strength bolt is used, it may go to even higher pretension load.


Anchor bolts used for equipment supports should be preloaded to the equipment manufacturer's recommendations when specified. This is especially true for bolts anchoring rotating or vibrating equipment.

Anchor bolts of ASTM F1554 Grade 36, A307, or A36 material (i.e., regular carbon steel bolts) should have only a nominal preload applied. It is recommended that they be tightened to a snug tight condition. Snug tight is defined as tightness attained by a few impacts of an impact wrench or the full effort of a man using an ordinary spud wrench. When bolts are anchoring equipment or are subject to possible loosening during operation, a locking device should be provided. Acceptable locking devices include double nuts or jam nuts, interrupted threads, and tack welds (for weldable materials only).

The three basic methods used for applying a preload to a high-strength anchor bolt— using hydraulic tensioners, torquing to a specified level, and using turn-of-the-nut method.

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[JAX91]

When I'm designing for the breakout of the concrete, what load factor is most applicable? The pretension is a fairly controlled load case, so I would assume it could be treated similar to a dead load. Straight dead load uses a 1.4 load factor. Or would a 1.2 load factor be adequate?

I have a pump manufacturer specifying a pretension of 32 kip per anchor on four anchors, which seems absurdly high. The pump only has a 75 HP motor. Normally the foundation would not be much, but I will have to increase the foundation size and do special supplementary reinforcement just to accommodate the breakout from the specified pretension load. I want to make sure I use as small of a load factor as possible to minimize the increase in foundation thickness and special reinforcement.

 

[amec2004]

>> Straight dead load uses a 1.4 load factor. Or would a 1.2 load factor be adequate?

I would say it's left to your own judgment. How confident you are on the accuracy of pretension load ? Will it go for 100% instead of 70% ? In the 100% case it would be 1/0.7=1.43

75HP is nothing. Normally <500HP is small pump and no dynamic analysis is required.

* Use mass ratio=3 for concrete foundation.

*Use anchor reinforcement such as hairpin to resist concrete breakout as per ACI 318-08 D.5.2.9.
Supplementary reinforcement approach won't help that much.

anchor bolt design crane beam design

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