high strength anchor bolts 1

[par060]

I'm designing the foundations for a building. The bulding steel designer has specd A325 anchor bolts to be pretensioned. I believe he needed to do this because he has a case where the bolts need to resist overall uplift and a moment. Pretensioning the bolts I am assuming would preload the baseplate in compression so he could develop the tension to resist the moment.

1. Does this sound reasonable?

2. What pretension value is required for something like this?

3. Does the pretension value get added to the over all tension in the bolt to come up with the required bolt size?


thanks for any insight

 

[UcfSE]

I would be leary of pretensioning anchor bolts that are embedded in concrete, especially high strength bolts. Won't the pretension force transmit that force straight into the concrete? Will the bolts have enough embed and other requirements not to pull out just from pretension? I haven't designed pretension anchor bolts myself but these questions come to mind. I know the AISC manual gives minimum pretension values for bolts but I don't know that applies to embedded anchor bolts.

The effects of pretension are summarized in this link below. The information seems reasonable. It should help answer some questions about how pretensioned connections behave. I would expect one of the more senior members of the forum could give you a better answer than me, I hope my little bit helps some.

http://www.unified-eng.com/scitech/bolt/clamping.html

There's also some good info on weld symbols in the menu to the left side of the above referrenced link.

 

[bridgebuster]

Two things: ASTM F1554 is the preferredanchor bolt material.

Go to the AISC web site. In the "Steel Interchange" questions from September 2003, there was a discussion on pretensioning anchor rods.

 

[JAE]

I agree with UcfSE in that pretensioning anchor bolts is not a standard practice. And using A325 bolts in concrete is especially weird.

 

[par060]

my bad the bolt material is as you say astmF1554

 

[Ron]

I don't see the need for pre-tensioning. The anchor bolts are there to resist lateral shear, uplift (including that caused by overturning), and to resist the tension created from a column moment couple (if it is considered in the design as such).

In short, tension in anchor bolts is normal. As long as the nuts are tightened to prevent a dynamic action on the bolt, it should be sufficient to mobilize the resistance.

 

[par060]

thats correct...but here is the rub.....with the over all uplift where is the compresive force from the couple going to be located...since the baseplate is lifting off the concrete pier the bearing is on the underside of the washers ...the baseplate will rotate in the oversized holes until the top and bottom of the plate bear on the outside of the bolt sets. This puts the bolts in bending

 

[Ron]

That would only happen if:
a. You had a clearance between the base plate and its bearing surface (concrete or grout) that is greater than the edge distance of the bolts (otherwise the compressive side of the couple is placed at the point of contact with the bearing surface)
b. The frame (and/or base plate) had so much flexibility that significant wracking could occur with large lateral deflections (we hope not, since most designs check for this)

If your bolts are even reasonably snug, this will not happen. Further, if you are concerned about such, put nuts above and below the base plate, then back-grout the space.

The premise your are proposing would only occur under very large movements of the column and base plate. This is not the case for most applications.

 

[par060]

Thanks Ron....this appears to be the way things are going to work out...