fatigue design of A307 bolts

[RoseAtSGH]

I'm in search for design guidelines (i.e. allowable stresses) for A307 bolts subjected to cyclic loading (on the order of millions of cycles).

We are designing a retrofit of a large base plate anchored to a slab, with pretenstioned bolts. The drawings spec'd A325, but the fabricator supplied A307 and they were installed. Now, 25 years later, we discoved this snafu and have discovered that the bolts have yielded some and are causing problems. The bolts cannot be removed and we have been asked to design a retrofit to reduce the stresses in the bolts. Unfortutely, we cannot replace the bolts. We have inspected the bolts using ultrasound and have determined that they are uncracked and are usable, so long as we limit the stresses. Unfortunately, almost all bolt fatigue research has focused on A325 and A490 bolts.

So my question is, what should I limit my max bolt stress and stress range to avoid fatigue problems (on nominal or root thread area).

Thanks

Brian Rose
Structural Engineer
Simpson, Gumpertz and Heger

 

[JAE]

Per AISC Appendix K3.4, the net tensile area Af shall be used for fatigue checks where:

Af = pi / 4 x (db - 0.9743/n)^2

db - nominal diameter of bolt, inches
n - threads per inch

 

[Watermelon]

Somebody please correct me if I am wrong, but ASTM A325 is a high-strength hex bolt spec. Anchor bolts are not made to A325, so the original installer didn't really make a mistake. It would have been impossible for him to obtain A325 anchor bolts.

 

[RoseAtSGH]

Watermelon, you are correct about the ASTM specs. They cover hex bolts up to 1.5" diameter. The original construction drawings and specs list 2.5" diameter A325 threaded rod, with nuts at both ends. One end is tightened against the base plate and the other end is tightened against a channel and the channel end is encased in about 5 feet of concrete. I agree with you that the original designers made a mistake. But it wasn't ethically correct for the fabricator to make a substitition without notifying owner and engineer.

Regardless, the owner is stuck with mild steel instead of high strength steel, and we need a remedy. So back to my original question - any specs on max stress & stress range for A307 bolts or threaded rods made of similiar material?

 

[Watermelon]

You're right, the fabricator should have called first.

I am unaware of any codes or standards dealing specifically with your problem. And I can't point you to any research papers.

If it was my problem, I'd use .4Fy maximum allowable stress on the unthreaded body and a maximum allowable stress range of not more than 10ksi.

But I think you need better advice than what I can give.

 

[evelrod]

I see a basic problem in what you are labeling 'A-307'.
Basically, mild steel, ungraded threaded rod stock.
The key word is 'ungraded' and difficult to estimate allowable stress. I concur with watermelon, you need better advice than I can give.

Rod

 

[Metalguy]

If your anchor bolts are in the "rigid connection" category, you may not have to be too concerned with fatigue cracking IF you can get the preload up past the max. service loads. The idea is that if the preload is high enough, there won't be much in the way of cyclic stresses. Apparently you are able to reduce the service stress on them; if so your solution may be easy.