How do you know if your column base connection is a moment connection? 4

[reverbz]

Hey guys,

How do you confirm your column base connection is a moment connection? With the anchors obviously it's just ensuring they can handle the loads but for your baseplate how do you know if it's enough thickness to be considered fixed?

Thank you!

 

[Tomfh]

For strength you need to ensure the plate is thick enough not to yield. Yield line methods, finite element methods, etc.

For stiffness you need to ensure the connection’s stiffness is comparable to the member stiffness.

 

[human909]

The above answer is great and straight to the true point.

Though for a more practical answer and easier to implement advice, it might be better to consult relevant connection guidelines with recipes and ready made formulas for what is "sufficiently thick".

Unless you have stiffening gussets (which are expensive) your plate will likely be thicker than you expected. As a rough and likely conservative starting point; don't be surprised if you plate thickness is required to be 10% the depth of the section.

 

[soheyla_sa]

And Also I suggest to pay attension to place of anchors. in moment connection anchors are in outer side but in simple connection mostly we put them near center of plate.

 

[HTURKAK]

I would like to add my points to the above valuable, answers;
- The base plate thk. is very important factor . The rule of thumb for me is to compare the flange thk. of the column with base plate thk. The resistance
of the column is limited by its slenderness and so , the minimum stiffness of the column base depends relative slenderness of the column.
- The rotational stiffness of the footing is another factor. A small size spread footing may not satisfy rigid conn. assumption even the column to base plate and to footing is rigid enough .
- The following doc. ( could be a reference at Eurozone ) worth to look :

 

[soheyla_sa]

I would like to add my points to the above valuable, answers;
- The base plate thk. is very important factor . The rule of thumb for me is to compare the flange thk. of the column with base plate thk. The resistance
of the column is limited by its slenderness and so , the minimum stiffness of the column base depends relative slenderness of the column.
- The rotational stiffness of the footing is another factor. A small size spread footing may not satisfy rigid conn. assumption even the column to base plate and to footing is rigid enough .
- The following doc. ( could be a reference at Eurozone ) worth to look :

thanks for shring the document. Its wonderful

 

[cliff234]

Also pay attention to the centroid of the reaction when investigating the “worst case” load combinations. Is there significant tension in the anchor rods? Is there any “tension” in the soil or foundation elements? The answers to both of these questions can be quickly determined, and if the answer to both is “no”, there is a strong probability that the foundation can be considered fixed (although it might otherwise still be considered fixed, but you just need to investigate further).

 

[reverbz]

@human909 Yes that is what I would be interested in. I'm sure he's correct that it's checking that the plate is thick enough to not yield, but can someone point me in the right direction for how to check that?

 

[dik]

Unless you have stiffening gussets (which are expensive)

It's generally less costly to thicken the plate than add stiffening gussets.

When designing connections for metal fabricators, I add the note for the EOR to confirm the weld size, or to provide design loads.

 

[lexpatrie]

Generally speaking they aren't for the more practical weld all around connections, and the more common procedures intend for the plate to yield or deform.

The older AISC commentaries will discuss this in the k factor portion of the code, say circa 1986, 1989.

 

[Howlyn2]

@reverbz the connection is either pinned or fixed based on the designer's intent (yours I presume). The demand and resistances can be calculated using hand calculations, software, or following design guides. Is your questions more directed at if a plate is thick enough should I change it to a fixed connection? AISC Design Guide #: 29 - Vertical Bracing Connections Analysis and Design discusses design philosophy in section 1.2. The Lower Bound Theorem states pending satisfaction of any admissible internal force field (distribution of internal forces due to applied load) external load in equilibrium with the internal force field is less than or at most equal to the connection's capacity. The rest of the frame would have been designed for the residual forces and then the base plate's limit states that match it's design intent will have been satisfied regardless of thickness. They go on to give an example about column design loads, how they vary, and the applicability of the Lower Bound Theorem.

 

[icebloom]

I think there are hand methods with which you can do the calcs, but if you have access to FEA software or commercial software like idea statica you can do a stiffness analysis in it which gives you the joint spring stiffness.

Thick baseplates can provide a moment connection, but as others have said stiffeners will give much more stiffness enabling a moment connection to form. These are much more rigid than baseplates bending about the minor axis.