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HuckleberryFinn (Structural) (OP)
14 Apr 12 12:18
Morning Folks,

Apologies if this has been covered in another thread but the quick search I ran did not reveal any useful information.

Problem: We are evaluating the existing seismic anchorage of a large silo structure in a poor seismic condition in the US (SDC is an "E"). The Silo is approximately 200 feet tall with a diameter of approximately 75 feet. It has been unloaded and out of use for several years but is adjacent to several other large silos that are storing highly volatile material [Boom!]. The existing anchorage is a series of large diameter j bolts embedded several feet into a concrete pile cap. I am in the process of working through the problem but under Appendix D I think I have slim to no chance of getting the j bolt anchors to work. If I idealize the bolts as rebar (not appropriate as they are smooth rod) they are approximately 1/3 of the length required to develop the tensile capacity of an equivalent diameter bar.

I am leaning (quite heavily) toward informing our client to remove the existing silo skirt and retrofit with a large quantity of epoxy bolts. Has anyone come across any white papers on evaluating the tensile capacities of non-standard hooks on rebar OR large diameter J-bolt anchorage? Trying to make sure we have done our due diligence before ordering a costly repair.

Thanks Folks,

-Huck

 
JedClampett (Structural)
14 Apr 12 13:43
I think you can find a wealth of literature in recommending that J-bolts not be used for anchorage, in particular seismic anchorage. The stresses at the corner tend to straighten out the bend and greatly reduce the calculated capacity. So even if you come up with capacities, I would recommend that they be replaced.
Appendix D of ACI 318-08 has a capacity of hooked anchors in paragraph D.5.3.5, but without knowing the details, I still suspect it's pretty small. There's also a reference in ACI on hooked anchors.
Personlly, I don't think J bolts are as bad as everyone thinks, but I'm in the minority and the code doesn't help.
BAretired (Structural)
14 Apr 12 14:06
I agree with JedClampett that the existing J-bolts should be replaced.  Following a tornado in our area in 1987, I witnessed numerous J-bolts which had pulled out of the foundation, leaving perfectly smooth round holes in the concrete.  They are not a reliable anchor bolt and should not be used where significant tension forces are required.

BA

dhengr (Structural)
14 Apr 12 15:40
I would 'lean more heavily' toward, not removing the existing silo skirt, but rather retrofitting the existing silo skirt and adding what might be needed to bring things up to code.  But, I can't see your exact details from here.  A good sketch would help a lot.  J bolts do tend to straighten out and pull out as BA suggests, but as JedC suggests they are worth something in pull out and they should be taken advantage of, rather than removed.  You will likely do more damage and expend more work effort in trying to remove and replace the existing skirt than you would in designing an add-on skirt/brackets to add some hold down capacity, along with the existing.

I can't point you at a specific study or article, although they are out there.  But, the up-shot of what I remember reading on the matter was that the J bolts are worth more than their total length times the bond strength, but not as much as we had assumed by adding a larger bearing strength against the conc. at the large radius bent end.  They just tended to crush a bit of conc., in bearing, in the area of the bend, start to straighten in that area, and then continue this process as they pulled out.  I would look to ACI and appendix D testing and reports to gain some insight on the newer J bolt thinking.  You may also want to study the use of epoxied A.B's. in EQ situations.
 
amec2004 (Aeronautics)
15 Apr 12 0:16

Hooked rods have limited pullout strength -->     Not allowed

While hooked rods may be less expensive when compared to headed rods or threaded rods with a nut, it is recommended that hooked rods not be used whenever there is a calculated tension force on the rods. Why?

Hooked rods have limited pullout strength, due to the bearing mechanism and tendency to slip under tension loads—especially when the oil from cutting threads remains on the rod.

From http://www.modernsteel.com/steelinterchange_details.php?id=62

Neither the Specification nor the Manual stipulates what type of anchor rod is to be used on a particular project. I am not aware of a building code that makes such a stipulation either. The Manual simply recommends that hooked anchor rods should not be used for applications involving tensile loads. See page 14-10 in the 13th Edition Steel Construction Manual for the actual discussion.
Instead, the Manual recommends that headed or threaded and nutted anchor rods be used for embedments subjected to tension. The usual material specifications for anchor rods remain applicable. ASTM F1554 provides all you need, covering three strength grades, as well as the requirements for the headed, threaded and nutted, and hooked configurations. No special head size or nut size requirements are required, as the common head or nut dimensions are sufficient. If you have not yet seen ASTM F1554, you should give it a look. It is the only ASTM specification that covers all aspects and requirements applicable to anchor rods. It is included in Selected ASTM Standards for Structural Steel Fabrication, available from AISC at www.aisc.org/bookstore.
ACI 318 Appendix D provides methods of determining available pullout capacity for both types. This subject is discussed in Section 2.5 of AISC's Design Guide 1, Base Plate and Anchor Rod Design, Second Edition. Table 2.2 of the design guide lists anchor rod materials; see also Table 2-5 in the 13th Edition Manual.
Kurt Gustafson, S.E., P.E.
American Institute of Steel Construction

anchor bolt design crane beam design
http://www.civilbay.com

hokie66 (Structural)
15 Apr 12 2:38
amec2004,

I think your are missing some quotation marks, unless you are Kurt Gustafson.  You didn't need both the link and the direct quotation.
HuckleberryFinn (Structural) (OP)
15 Apr 12 11:11
Thanks for the responses all!

Unfortunately, further investigation has revealed that the silo itself is significantly under-designed per current code provisions. I'm not sure yet where we will be going from here in terms of replacement of the silo or retrofit. I think that the plant will probably have to to shut down operations in either case due to proximity to the in-use silo structures near it. I'll keep you folks informed of the final outcome.

Thanks Again all,

-Huck

 

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