Anchor Bolts Need to be Pretensioned.?

[krus1972]

Hello,

My question is do anchor bolts have to be pretensioned before a moment is applied over the base plate?

A little more info on my project:

I am working on the structural support of a new stair that is being placed in an existing building. The stair is very fancy and very very heavy.

To make a long story short I am saw-cutting the existing slab to make way for a new footing for a column to support the required loads from the stair.

The column will be installed first and then the footing will follow. The footing and the finish slab will be completed as one unit. The anchor bolts will be furnished and will be in the base plate before the footing/slab is placed. The Base Plate as well as the anchor bolt nuts will not be exposed after the footing/slab unit concrete is fully cured. In other words the base plate with anchor bolts will be incased in the concrete. The concrete will be fully vibrated so the base plate will have fullt bearing once the concrete is cured. The anchor bolts will be tack welded to the base plate to insure they do not lift up during the placing of the concrete.

When everything is cured and ready to go moment will be applied over top placing some of the anchor bolts in tension from the moment. There is really no problem from the design stand point here accept that the anchor bolts will never have any pretension in them before the applied moment is applied over the base plate. Is this going to be a problem? If so, how so?


Thank you all,

Jeff

 

[apetr26542]

What anchor rods are you using, A36/A307?

I do not believe a pretension is required, never heard of it for anchor bolts. Usually just the anchor bolts are furnished, then a leveling plate is set on a bed of non shrink grout, then the base plate with attached column is erected and bolted down. The non shrink grout bed gives full bearing, the leveling plate allows for plumbness. This requires the footing be cast prior to the slab, with the base plate usually 8" below the slab to conceal the base plate and anchor rod bolts.

How do you attach the structural member to the base plate?

 

[krus1972]

apetr26542,

Thanks for youtr post.

Everything is taken care of. The Base Plate is welded to the column all aorund. The weld has been design to accept the bending. All of the other design issues are taken care of.

The reason why I am asking about anchor bolt pretensioning is because everything you mentione is correct. In normal or "every day" anchor bolt construction as we know it, the anchor bolts are tighten by the steel erector. This holds the column from tipping and gives chance for the rest of the building frame to be erected.

In the "every day" anchor bolt construction, the anchor bolt is tighten before all of the service loads are placed on the column. Since the anchor bolts are tighten they are pretensioned before the loads are placed.

Since in many anchor bolt application the anchor bolts are tighten (pretensioned to a certain degree) before the loads are applied leads me to this puzzling question. Although many applications do not have moment over top of the base plate my application does have moment - placing some anchor bolts into tension. That is why I am asking if pretensioning is required and if so how much?.

Thank you all,

Jeffrey Krus P.E.

 

[apetr26542]

In my opinion, I would say no. This sounds like that what you are refering to as anchor bolts could just as well be embeds, headed studs etc.

Per the specification of structural joints Using ASTM A325 or ASTM A490 bolts, pre tension is required for joints subject to significant load reversal, fatigue loading, tensile fatigue or A490 bolts subject to tension or shear and tension with or with out fatigue.

Now, A325 does not apply to anchor rods, these fall under A36, A307 or F1554. F1554 has three grades, 36, 55 and 105 (tensile strengths in ksi) .

 

[krus1972]

apetr26542,

Actually these will be A325 Fy=92KSI Anchor Bolts. Due to the moment A36 or A307 would require many more anchor bolts and would not be a practical design.

From your previous post the issue is do A325 ANCHOR bolts need to be pretensioned for fatigue Live Load. There is no load reversal here. There is Dead Load moment that will always be there. When people use the stair the Live Load will add to the dead load moment increasing the total moment (tension) applied to the anchor bolts - (the L.L moment is the same direction as D.L.).

Could you point me to the area in the ASD spec that talks about this issue?

I have to be sure that all of the "t's" are crossed and "i's" are dotted. I will have to seal the calculations and this is the only issue I am having a hard time justifing by referencing the code(s)

Thank you for your help,

Jeffrey Krus P.E.

 

[apetr26542]

The A325 bolts, are these greater than 8" long, if so, you will need to go to the F1554 spec.

Anyway, the specification is RCSC Specification for Structural Joints Using ASTM A325 or A490 Bolts.

http://www.boltcouncil.org/download/RCSC Specification 6-23-2000.pdf

Page 25 / 26 as well as other information in the document about varying degrees of loading and when or not necessary to pretension.

I would consider pretension in concrete to be different, The tensile capaicty of the connection is usually limited to the cone out capacity of the concrete in failure and not usually limited to the material strength of the rod. If you need A325 material strength, you will need more than 8" of embedment, beyond 8", they do not really produce A325.

Hope this helps

 

[apetr26542]

You may want to check out PCA strength design of Anchorage to Concrete. It is a great publication concerning many different factors when designing anchorages to concrete. Although, it does not mention Pre tension or fatigue etc. it is more of a design aid in determining embedments, group action, edge requirements. etc.

 

[UcfSE]

Were you able to embed the anchor rods into the concrete well enough that you can actually use an A325 bolt? It seems that the concrete break out strength would not be high enough to justify using a high strength bolt. In the cases where you need higher steel strength I've used B7 rod instead of A36 or A307. If you pretention anchors you reduce the superimposed capacity in tension of your anchor into the concrete and you also run the risk of it being over tensioned or pulling out due to the pretensioning process. Then you have to replace it after the concrete is set, not good. I would double check the concrete strength before I used such high strength anchor bolts. apetr26542 has a very good point also. Consider using B7 anchor rods instead and just use more. Note also that if you are using the anchor rods to transfer shear to the foundation you may need to use washer plates and weld these to the base plate to prevent the base from slipping in the oversized holes before engaging the bolts. Shear keys or tabs also accomplish this well.

 

[chichuck]

The RCSC standard is applicable to connections between steel members, anchors do not fit this definition. A325 is not an appropriate material for anchors. Properly use A307, A36 or F1554. These would probably not be pretensioned.

Properly designed anchor bolts need to be embedded adequately into the concrete. "Adequately embedded" means that the anchorage will not fail before the anchor bolt material itself fails. This means that the anchorage must be adequate for the full tensile strength of the bolt. If designed this way, the failure at ultimate value will be a ductile one in the bolt, not a brittle one in the concrete. The anchorage can be designed with bonding between the concrete and the bolt, or with an anchorage, either a plate (not preferred) or a bolt head or nut (preferred). The failure is usually governed by a pullot of a concrete cone, unless the bolts are too closely spaced, or too close to an edge of the concrete, then the failure is modified a bit. This covered in ACI 318-02, Appendix D.

I wondered about your statement regarding the tensile loads. This must be one big, honkin' stair to have that size loads. Does the uplift on the anchor bolts come from an uplift on a column or a brace force? Does it come from moment on the column base? Perhaps you've used a rigid baseplate when one is not necessary? Perhaps a different bracing layout would reduce the bolt tension? If the tensile loads you have are indeed correct, I don't recommend using A325 anchor bolts. Use as many, as large anchor bolts as you need and make sure the anchorage is properly designed. My preferred material is F1445, then A36 or A307. With no pretension.

Hope this helps.


chichuck

 

[unclesyd]

The normal recommendation for anchor bolts is "snug tight" as used in the tightening of structural bolting. This is, as stated above, that you have good contact between the base plate and concrete. This is and was the local code requirements.

With all the damage I've seen from the hurricane "Ivan" I have yet to see a properly designed and installed base plate failure. The ones that failed are on light weight construction where the nuts actually pulled through the plate. I didn't get to measure the holes.
The one failure I observed on a large building was from the concrete side, too thin, a little greater than 3".

I have heard of anchor bolt failure on large high buildings, dry boat storage. The available pictures don't show any of the anchor bolts, only the columns being bent.

 

[coloradobridges]

It's worth pointing out that pretension does not reduce the capacity of the connection to resist tensile loads as UcfSE stated. Pretension induces compression into the concrete base that must be "unloaded" before the bolt tension increases. This makes the connection much stiffer as the concrete section is engaged.

It doesn't sound like deflection is much of an issue here so I don't think it matters whether the bolts are pretensioned or not, but they should be at lease snug tight.

 

[UcfSE]

The reduction in strength depends on what aspect of the design you are looking at. I agree that it has a stiffening effect and that the compression must be unloaded before tension takes over. But where does that compression force come from? It comes from the bolt head or hook bearing on the concrete and from the plate bearing on the concrete or grout beneath it. Now the connection as a whole may be stiffer but the the hook or bolt head bearing on the concrete has more load. Hooks typically fail in bearing on the concrete as the bolt experiences tension load. In steel connections, research and experiments have shown that that superimposed tension load on a pretensioned connection does go toward reducing compression forces in the connection as a whole rather than merely increasing the bolt tension. I am aware of no such research for pretensioned concrete connections. Of course I may be wrong but I prefer to err on the safe side. In any case, I agree that pretensioned bolts aren't needed any way.