ACI 318 Appendix D Question

[ron9876]

Section D.5.2.9 indicates that you can use reinforcing steel to resist tension loads when the loads exceed the concrete capacity. However in the commentary it says that the largest bars tested were #5's and that reinforcement should consist of stirrups, ties and hairpins.

So what about a bearing plate on the top of a concrete column with hooked vertical reinforcement under the bearing plate. The reinforcement is located closer than 0.6hef to the anchor bolts. Has anyone seen any literature on this? Thanks.

 

[JedClampett]

I'm not clear what you're describing. Is there anchor bolts and a bearing plate? Why do you ahve both. How does the load get from the anchor bolts to the hooked reinforcing? Could you supply a sketch?

 

[ron9876]

I have a steel bearing plate on top of a concrete column with a significant uplift. There are anchor bolts that extend down into the column. The vertical reinforcing steel in the column hooks at the top of the column. So the uplift load is transferred into the concrete by the anchor bolts and is then resisted by the column reinforcement. The hooked reinforcement has enough area to resist the uplift. My problem is that they are either #9's or #11's and that is outside the tested data that the code provision was based on.

 

[abusementpark]

I don't think the code says that you can't use bigger bars for anchor reinforcement. They are just noting what type of research has been done. I wouldn't hesitate to use bigger bars. Just make sure you lap the anchors with the reinforcement a distance greater or equal to the hooked development length (ldh).

If you use anchor reinforcement, don't they make you design the anchor reinforcement to take everything and neglect the contribution from concrete breakout? There is some added conservatism in that procedure.

 

[ron9876]

Yes you have to resist all the tension with reinforcement.

 

[southard2]

Wow, I think I might actually have the solution for you. Nelson stud makes an anchor called the D2L. It is I THINK an 80 ksi anchor. You can get a 1/2 diameter or even 5/8" diameter. They call these deformed bar concrete anchors. They weld to the baseplate or embed plate like studs. But because they are extra long and deformed you simply lap them with the vertical rebar in your column and you have esentially have an uniterrupted tension tie with your column. It has been a while since I've used one of these, but the ACI code might require mechanical splices like one should use when using a tie rod foundation for metal building foundations. But at any rate these anchors are long enough to fully develop with your column's vertical steel. In order for your baseplate to pullout the column's vertical reinforcement would essentially have to fail also which is unlikely if your column is designed for the tension forces properly.

And I have to also be honest with you I have not yet had to read the latest version of ACI appendix D so I haven't really been through the new sections yet that discuss the help of reinforcement. To me this is an area where it would be nice if more research were done and we need a whole lot more explanation on how to implement it.

I'd like to hear what others have to say about this proposed idea.

John Southard, M.S., P.E.

http://www.pdhlibrary.com/

 

[structSU10]

Here is a paper on methods to design reinforcement in pier and such that may be useful.

 

[amec2004]

No where in the ACI 318-08 or ACI 318-11 code says you cannot use >#5 bar as anchor reinforcement.

Just use it and make sure you have enough rebar development lengh ld or ldh on both side of failure plane

anchor bolt design crane beam design

http://www.civilbay.com