Base Plate Pratical Assumption (Fixed or Pinned)-Urgent Reply Needed

[allimuthug]

I have a Base plate 25 mm thick Supporting HE300A Column . The bolts are kept Outside the Flange of Column ,
I don't know weather the Base plate has to be designed as Fixed or Pinned Base plate.
I was told that if the base plate is thin it would rotate and wont carry any moment further . But before It rotates it would
carry some moment to base plate and the anchor bolt .

If i want a really pinned base plate the bolts must be kept inside the Column section , It leads to very short spacind between anchor bolts 150 mm (5 times diamter).
I don't want to design fixed base plate as my foundation would become big and un conservative as well as i cannot keep the bolts inside. If i have to keep the bolts outside and design as pinned i have to prove by calculation as per american code .
Can anybody help me in this regards.
Kindly help me and spare time for Answering this
Please find the pic of base plate attached , Also there are values given in base plate if it is fixed.


Link

 

[allimuthug]

Also if i was to keep my anchor bolt inside the Column what is minimum centre to centre between anchor bolt as per ACI 318 .

 

[Guastavino]

Standard answer is that it's somewhere in between fixed and pinned. I traditionally assume pinned for wide flange columns on square base plates with (4) anchor bolts, similar to what you had shown. Reason being is that the plate has some flexibility and if I don't design the foundation for moment, then there is more flexibility there.

As per the minimum center to center per ACI, look at D.8 in appendix D of ACI 318 (I have 2008 one on my desk). It gives minimums based on splitting, etc.

The metal building folks use 4" spacing patterns all the time for base plates, but I don't know anyone outside of their world that does them like that. Not saying you can't, but I just don't see it done very often.

The other thing to keep in mind is that usually columns are encased in concrete which is surrounded by a concrete slab above. All of that introduces fixity that we don't typically account for, right or wrong. Ultimately it's a judgment call on your end. I'll say this though, I would be much more inclined to consider that pinned than fixed. For it to be fixed I'd have to check the base plate thickness, anchor bolts, etc. extensively.

Not sure if you're aware of it, but RISA has a great program called "RISA Base" for base plate design. It's not too expensive and would likely be very helpful for what you're trying to accomplish. I don't work for RISA or get any kickbacks so that's why I'm recommending it. I'm sure other software providers have similar stuff too.

See what others think too. If there is one thing I've learned on this board is that there are a lot of good engineers to learn from. There isn't always one right answer.

 

[structSU10]

PCA has a method you could use to see how much fixity you have in the baseplate, I think it has been posted on here before so a search may find it. The calculation may lead to a moment so small it can be neglected, or you could design the assembly based on the fixity found.

 

[allimuthug]

I worked out the moment as per PCA METHOD I got a considerable reduction in moment . Still how i would design the base plate fixed or pinned if i have significant moment.

 

[jayrod12]

If you've used the PCA method then design for the fixity you determined.

 

[allimuthug]

I have Risa Base plate let me check in that also . But i want everybody to post their valuable answers .Which would help me a lot.

 

[racookpe1978]

Now, what you have not answered yet - as you ask for more and more "designs" to your problem.

Do YOU want the bottom of the column fixed or pinned? What is better (more conservative), or cheaper, or safer for YOUR design? The baseplate design process requires assumptions and approximations and heavily uses nominal lengths, strengths, and resistances.

EVERY welded baseplate bolted into concrete is actually somewhere in the grey zone between an ideal pin and an ideal moment-restraint. ONLY a pure, friction-free, "perfect" pinned column base is going to behave like a pinned bottom - and even then - ONLY around the direction of the pin's axis. NO moment-resisting column base is going to be entirely fixed either. There will ALWAYS be some bending and flexure as the stress is transmitted through the member's fibers and plates.

 

[KootK]

This is easier than it seems. Base fixity has little to do with what's going on at the bottom of the column. It's got much more to do with what's going on at the top of the column.

Is the top of the column braced against lateral translation directly or indirectly?

Is the top of the column free of substantial moments applied from beams that are moment connected to the column?

If the answers to these two questions are both "yes", then you can probably consider the base plate to be pinned.

Regardless of the fixity that you assume at the base plate connection, don't be fooled into thinking that grouping the anchor bolts close together makes the connection less fixed. All that will do is increase the prying tension on the bolts and cause them to fail earlier in the load history of the column.

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.