Designing Anchor rods: pullout and yielding

[mikesg]

While reading AISC Design guide 1 - Base Plate and Anchor Rod Design (2ed) I came up with the anchor pullout failure. There are tables 3.1 and 3.2 that show anchor strength (steel) and anchor rod pullout (concrete) capacities for anchor rods with heavy hex nuts. Comparison shows that for rod diameters 1" and more it is the pullout strength that controls /without taking account for the 40% overstrength in case of uncracked concrete/. Also, in this Steel interchange article

http://www.aisc.org/MSCTemplate.cfm?Section=Steel_Interchange2&FaqID=2079

it is recommended that there should not be concrete bond for upper half of the rod.

Now - the question.

According to textbooks I have studied from one can choose an anchor rod that works by concrete bond and is embedded 35d in concrete and headed rods were an alternative when deep anchorage is not possible. I somehow miss to see that case in ACI318-05 appendix D. Does it mean that in fact it is recommended not to rely on the concrete bond for anchors at all?
For enhanced ductility we should design the rod to yield before concrete pullout or breakout faliure modes occur. Does it mean (reading the tables mentioned above) that we should not use anchors with heavy nuts for diameters 1" and above? Can the pullout strenght be increased - for example by increasing Abrg participating in the formula, if yes - what are the required calculations for the "thing" that we replace the heavy nut with?
I am sure I miss to see something quite obvious and will be grateful if someone helps me to better understand how anchor connections work.

Many thanks

Mike

 

[WillisV]

Pullout strength can be increased by using a hardened washer on the end of the anchor rod over the heavy hex bolt to increase Abrg. See Table 7-15 of the 13th ed. steel manual (Table 7-1 of 3rd Ed.) for typical dimensions of F436 washers.

As an example, for a F436 circular washer on a 1" bolt, the nominal outside diameter is 2". As this washer will rest on the hex nut, only the diameter of the anchor rod need be removed from this dimension for determining the bearing area. So for a 2" circle with a 1" hole in the center, you can calculate an area Abrg of 2.36in^2 to use in your pullout calcs.

 

[mikesg]

Many thanks for your reply.
What about the other issue, with the long anchor rod relying on the friction with concrete. Is this something that is now considered outdated or incorrect?

Thanks

Mike

 

[mikesg]

It seems I found explanation in my country's codes and Eurocode 3.
Our codes dictate that anchorage is determined either by the anchoring device (nut, washer...) or by bondm but not by mixing two of them. A short statement that I had overlloked.
EC3 states that hooked anchors can be used for steels with yield strength not exceeding 300MPa if sufficient anchorage length is provided. If headed anchors are used bond should not be taken into account.

Mike