Ductile Failure Requirement in Anchor Bolts 2

[azogr]

I have a situation where the metal building supplier has specified the number and size of anchor bolts required and the frame base plates have been built accordingly. The problem I am having is that I can not get the anchor bolt design to be controlled by a ductile failure of the bolts since the quantity and the size are larger than required for the loads. Normally I would just try to lap the bolt with the vertical bars in the pier and hook the top to develop them properly. Unfortunatly the size requirements of the pier are not going to allow me to easily fit all the hooks and the anchor bolts in the top of the pier. Also the pier is too shallow to extend the anchor bolt any further to get the extra lap length.

Does anyone have any suggestions on how to get this problem solved?

Thanks,

 

[dik]

Can you just use headed bolts or anchor rods threaded for an embedded nut? These work better than hooked bars.

Dik

 

[WillisV]

There is an out in the newer codes (IBC 06) whereby you can ignore the ductile failure provision if you provide something like double the required strength. Not near a code right now so I don't remember the exact provision.

 

[JStephen]

See ACI-318 App. D for bolt design information.

If the pier is that small and that shallow, it seems it wouldn't take much steel before you were developing the entire weight of the footing. Or am I missing something?

 

[azogr]

Thank you for the responses. To answer your questions, yes I can use a headed bolt. I usually try to go with them from the start and this is what my analysis is based on. Good suggestion.

I am performing the analysis per ACI-318 App. D and the pier is only 18" tall. If I understand JStephen’s question correctly I can not develop the weight of the footing because I am not extending my anchor bolts down into the footing. I was trying to avoid it because they will be pouring the pedestals separately. I suppose I could if I have no other choice.

I did have a chance to do a search of the IBC ’06 code and I think I found the provision WillisV was referring to. It is in 1908.1.16 and states that ductility can be ignored if the anchors have been designed for more than 2.5 times the load being applied. I'm nearly at three so it looks like this will be my out in this situation.

Thanks again to all!

 

[WillisV]

Yup thats the one.

 

[WillisV]

Note that you can't just provide large anchor bolts and say you meet that provision - the minimum design anchor strength also includes the concrete breakout limit states etc - so what that provision is saying is that for low load levels where the strength of the anchor rod/pier assembly including all applicable limit states is 2.5 times the required strength, then the provision for ductility may be ignored. If you can't meet this provision then I would highly recommend developing the required strength across the breakout plane with hooked bars up into the pier.

 

[vincentpa]

Look at AISC Design Guide 7 for Industrial Buildings. both the First and Second Edition address this. There is a more rational way to deal with with based on a percentage of the yield. Check both editions.

 

[PackerFan]

Also look at RD.4.2.1 in ACI 318-05 App. D, it states in the third paragraph "For anchors exceeding the limitations of D.4.2.2, or for situations where geometric restrictions limit breakout capacity, or both, reinforcement oriented in the direction of load and proportioned to resist the toal load within the breakout prism, and fully anchored on both sides of the breakout planes, my be provided instead of calculation breakout capacity".

If you have a pier with "enough" reinforcement, you do not need to worry about the breakout strength calculation. So I would say for many pier anchorage situations, the rebar in the pier will satisy RD.4.2.1 and you would just calculate steel in tension and pullout in tension and call it a day.