Column Base Plate to resist uplift force as per AISC DESIGN GUIDE - 01 (2nd Edition) 1

[arefinmahadi]

Dear Sir,

Recently i have gone through the AISC DESIGN GUIDE -01 (2nd Edition) for column base plate and anchor rod design. I have some quarries on it. To simplify i want to mention that i am only working with a column base with uplift force. If u see the JPG file attached below u can find the CASE-01 similar to AISC Design guide example-4.5. In this case it is assumed that only the effective part of the web in between the dashed lines contributing to transfer the bolt force to the plate. Later in Page No. 35 it is also stated that "if column web strength controls consider distributing the forces to the flanges as well as the web". My question is how ? How will i determine the share of web and flange simultaneously? If my beam depth is more than this and the anchors are placed near the center away from the flanges will the force be shared by the flanges.

For Case-02 in the JPEG which length will i consider as b(eff) ?
For Case-03 how will i distribute the bolt force ?
For Case-04 & 05 - which part will be responsible to carry the bolt load to base plate ? What will be the b(eff) for these cases ? How will be the check for welding carried on ?

Initially i am considering all the cases as pinned base and resisting uplift force only. Please help me on these regards.

Thanks in advance.

Arefin

 

[KootK]

I'd check out a copy of this AISC guide: Link. As in that guide, the usual procedure is to use the yield line method. Even at that, accuracy is limited and some conservatism is generally in order.

My question is how ?

You may need more bolts, bolts placed closer to your flanges, or both of those things.

For Case-02 in the JPEG which length will i consider as b(eff) ?

The width from the extreme ends of the yield lines, not counting the overlapping area twice.

For Case-03 how will i distribute the bolt force ?

Short of running an FEM analysis, it's pretty hard to say. Again, I'd recommend the yield line procedure. As long as you can get the capacity that you need in the plastic state, knowing the exact distribution isn't all that important.

For Case-04 & 05 - which part will be responsible to carry the bolt load to base plate ? What will be the b(eff) for these cases ? How will be the check for welding carried on ?

For case four, the load will pretty clearly be going to the flanges. Case five is trickier. The load will still go to the flanges but the distribution will be concentrated around the flange tips in a manner that will be difficult to predict. If you're unable to deal with the uplift of each bolt in, say, 1/4 the flange width, You might consider plate welded to the flange to distribute the tension across the base plate more uniformly.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[arefinmahadi]

Dear Kootk

Thanks a lot for your detailed reply. I was out of internet connection so i have been a bit late answering.

For Case-01 my question was if a bolt is spaced same distance away from both flange and web or placed in a manner or considered to be placed in a manner that both web and flange can share the bolt force how the distribution will be done ?

Case-02 is clear from your answer.

Case-03 as far as i know the whole thing is based on yield line procedure. So i am a bit confused on how to apply. "As long as you can get the capacity that you need in the plastic state, knowing the exact distribution isn't all that important." - this part is not cleat to me.

Case-04 the yield lines do not cover the whole flange width. So should i consider the portion in between the yield lines or the whole flange.

Case-05 Please consider this case that the bolt placed as if the yield lines cover no portion of the flange. Than where the bolt force will be distributed.

I am probably drawing the yield lines wrong probably. Am i ? it would be great help if u clarify.

Thanks in advance

Arefin

 

[Shu Jiang PEng SE]

Several facts you can consider to address the question

1. The uplift load should be transferred to the base plate through both flanges and web uniformly if the bottom surface of the base plate is "glued" to the foundation.

2. The upward restraint of the anchor bolt to flange or web is depend on the anchor bolt distance to the flange or web.

3. Case I, the anchor bolt will restraint web instead of flanges.for the anchor bolts are closer to the web

4. Normally, either the column flanges or web is strong to take care of the uplift forces

5. Case I, The force path is like this: at top of the column, both flanges and web are in tension, the resultant of the tension is equal to the uplift force; at bottom of the column, tension in the web increase and in flanges decrease (it may be close to 0), the resultant of the tension is equal to the uplift force too. But the uplift force is transferred majorly through web to the anchor bolts

6. The principle is applicable to other cases too,

Hope this helps.

 

[KootK]

For Case-01 my question was if a bolt is spaced same distance away from both flange and web or placed in a manner or considered to be placed in a manner that both web and flange can share the bolt force how the distribution will be done ?

Like I said previously, you'll have do to something like a yield line analysis and determine this by calculation. Or, you can conservatively take all of the load to the web AND all of the load to the flange.

Case-03 as far as i know the whole thing is based on yield line procedure. So i am a bit confused on how to apply. "As long as you can get the capacity that you need in the plastic state, knowing the exact distribution isn't all that important." - this part is not cleat to me.

With a generally ductile material like steel, the thing that is important is to have a load path. Any load path. Knowing the exact distribution of load is often necessary. It's also very difficult to estimate accurately as you're discovering.

Case-04 the yield lines do not cover the whole flange width. So should i consider the portion in between the yield lines or the whole flange.

Case-05 Please consider this case that the bolt placed as if the yield lines cover no portion of the flange. Than where the bolt force will be distributed.

Between the yield lines. Taking the whole flange as resisting would not be consistent with the assumptions that went into your yield line analysis.

Case-05 Please consider this case that the bolt placed as if the yield lines cover no portion of the flange. Than where the bolt force will be distributed.

I've already given my answer to this one:

For case four, the load will pretty clearly be going to the flanges. Case five is trickier. The load will still go to the flanges but the distribution will be concentrated around the flange tips in a manner that will be difficult to predict. If you're unable to deal with the uplift of each bolt in, say, 1/4 the flange width, You might consider plate welded to the flange to distribute the tension across the base plate more uniformly.

I am probably drawing the yield lines wrong probably. Am i ? it would be great help if u clarify.

I'm afraid that this is something that you'll have to work out for yourself as I can't spare the time required for such a detailed analysis. You appear to generally be on the right track.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[arefinmahadi]

Thanks a lot for your assistance throughout.


Arefin