Bending in anchor rods. 2

[3doorsdwn]

I have a situation where I have a column base that (among other loads) has a tension and shear case. To transfer the shear to the top of my pedestal I was using bending in the anchor bolts (i.e. assuming the grout has cracked and no friction [due to tension]). The thing about it is: my anchor bolt has a larger diameter than the grout thickness, so I'm wondering if this may be overkill. (But it's what I have always done.) [After I figure the bending, I check the combined tension and bending in the bolts (together with the shear).]

I found an older thread on this but that was with the base plates sitting on leveling nuts.

http://www.eng-tips.com/viewthread.cfm?qid=192488&page=312

The link [in that thread] to an AISC discussion on this subject doesn't work.

Thoughts?

 

[JAE]

Here's a few links:

http://www.modernsteel.com/steelinterchange_details.php?id=96

http://www.modernsteel.com/steelinterchange_details.php?id=140

http://www.modernsteel.com/steelinterchange_details.php?id=162

http://www.modernsteel.com/steelinterchange_details.php?id=212

 

[3doorsdwn]

Thanks JAE.

 

[Lion06]

I haven't had a chance to check out those links, but App. D is pretty clear that if you have a grout bed that you take a 0.8 hit on the steel capacity. That's all I've ever done.

 

[BAretired]

JAE's references seem to be saying you should not take shear on anchor rods. That seems to be contrary to standard practice.

BA

 

[WillisV]

Look in AISC Design Guide 1. 2 Methods for low to moderate shear loads recommended: if you do not mind some slipping of the baseplate to engage the rods then you can take all of the shear in just 2 of the anchor rods with the 0.8 factor Lion06 mentions applied to the steel, if you want to engage all 4 rods (assuming a 4 corner rod pattern) you need to use welded plate washers and then check the rods in combined bending/shear/tension as you have been doing. No 0.8 necessary when doing this.


For higher shears there are several alternate methods including lugs (which I do not like for constructability issues), embedding the column base, or using angles welded to the side of the baseplate and post-install anchored to the concrete to take the shear.

 

[ToadJones]

I'd say 75% of all the large industrial projects I have worked on had shear lugs in one form or another. Most firms I have worked with seem to set a nominal shear load that they will go without lugs on; say 5 or 10 kips.
I have had some problems in the past with "cross" or "T" shaped lugs....forming the grout pocket isn't fun and is screwed up half the time.
I have seen and used some details with a piece of HSS welded to the bottom of the baseplate. The block-out in the form work is easier to make and with a properly placed grout hole in the baseplate they usually go pretty well.

 

[ToadJones]

Does this 0.8 factor apply to only the anchor rods or does it apply in a shear lug design also?
Been a while since I had to design a lug.

 

[Den32]

Agree with Toad and Willis.

If column base has more than 5-10 kips, a shear lug is specified (typical for braced frames).

Contractors may complain that it is 'hard' to do, or they don't know how to do it, but it is done everyday (more common in some regions than others). They can either hang forms or cast in some foam block that can be chipped out later.

Download AISC Design Guide 1 from AISC website which has step-by-step calc instructions and guidelines.

 

[Lion06]

We have a different detail for braced frames to take out the shear. I've never used a shear lug. We provide embed plates with rebar designed for shear friction for the gusset to be welded to.

 

[ToadJones]

this might sound harsh, but if you have a contractor that can't form for a lug, you shouldn't have him on the team.