Minimum Pretension of Bolts 1

[Sostratus]

Two picking points will be used to lift a precast column during construction. At each lifting eye, the baseplate is fastened to the column with four 1-1/4" A325 bolts. These bolts will be attached to a coupler and 1-1/4" B5S "stress gradient" pigtail embedded in the concrete. The connection will be slip critical, so the bolts are required to be pretensioned to at least 71 k, which is 70% of the tensile strength of the bolt. The pigtail has an ultimate strength of 102 k, and the Williams catalog lists a Safe Working Load of 51 k (F.S.=2).

My questions are:

If the 1-1/4" A325 bolts are pretensioned to the recommended 71 k, how much of this force is transferred to the pigtails? Will the safety factor be 102/71 = 1.44, or is that conservative?

During lifting, the clamping force will be reduced by the axial tension in the bolt, but there will be sufficient clamping force to resist shear. If the pretension could be reduced to say 50 k, the pigtails would not be exceeding the manufacturer recommended safe working load of 51 k. However, 50 k is only about 50% of the tensile strength. Why is 70% specified by AISC/ASTM as the minimum bolt pretension?

Thanks!

 

[ishvaaag]

High strength bolts, irrespective of their ultimate behaviour and use, use to be pretensioned because they are usually thought to be put to use mainly (or only) in slip critical conenctions at service level.

The inference is that whatever else, for non-slip critical behaviour at service level can be (and should be) done with other cheaper bolts.

In spite of this it is clear that High strength bolts could be used in the ways other bolts are used, and I bemuse there is a bunch of people doing so.

Respect your question, lacking a sketch, and wanting to meet the code, I only have to suggest maybe adding more of the elements that go overstressed may be the solution.

Some high level of prestress is thought to be generally benefitial to resistance to slippage and so (irrespective of particular cases) is so demanded, in part surely as an inbuilt test to the quality of the HS bolt once installed.

 

[Sostratus]

Thank you for your response ishvaaag, but I'm afraid there is not sufficient space to add more bolts/pigtails. So there are three choices:

(a) reduce the pretension force in the bolt to 51 k so the pigtail is not over the manufacturer's safe working load

(b) decrease the bolt strength/size so that the 70% of the tensile strength is less than 51 k

(c) not change anything, since there is a safety factor of 102/71=1.44 and a load factor was already used to account for impact.

AISC does not allow (a), (b) might be difficult when buying couplers and (c) would be ideal.

But to get a better sense of the issue, I wanted to know how much of the 71 k pretension force is transferred to the pigtail from the bolt. Does the coupler contribute to resisting the bolt pretension force, or is the whole 71 k transferred to the pigtail?

 

[ishvaaag]

I think that the coupler as an embedded element like the pigtail itself might be counted on bond to diminish the load required from the pigtails. Examine the contribution of the surface of the coupler at its diameter and length, say, as proportional to the straight part of some hook anchor by AISC manual to see if this is enough. If the coupler is externally deformed for better bond the contribution would be even greater.

Of course the contrary way is the safe one, not counting the coupler length, and loss of bond throug shearing action at the bond surface upon application of the transfer load may be argued. But these considerations are not made in ordinary length of embedments, at which, as they sustain tension, of course they are also as well a factor; so we may argue it is as sound proceed in the same way for this detail.

To be just, these considerations are truly really made in prestressed concrete to which the length of introduction of the load is paramount, and here we have pretension precisely to be transferred.

But examining the question more closely, in the prestressed concrete case the stresses need be transferred directly from the steel to the concrete at say some average maximum bond strength, whereas in our case we put the bolt within the coupler, start to pretension against the flat surface and then, the inner part of the coupler is demanded to take in shear a force to be transmitted to anything that resists, be it a pigtail beyond, some bond outside or even support in the plate itself. The coupler is tightened to the plate by the pretension, and this support at the plate will control the relative displacement of the external surface of the coupler respect concrete, and then the degradation of bond. We are then to most effects more in a case like a welded pigtail, far more than in one of prestress transfer. There is tension, as in any anchor, but the outer surface of the coupler remains quite static as in passive anchors. See this otherwise: trough the coupler you are "welding" the pigtail to the plate. Externally, at the outfit, the behaviour is passive. Then AISC's estimate of anchor forces seems acceptable.

Any loss of bond is tied to the analysis of the coupler having pretension, support at the plate, bond with concrete and pigtail tension. If the bond with concrete is acceptable, one can count on it for design.