moment vs shear base connections 1

[M1Can]

I am looking for some clarification on moment vs shear base connections. I typically used to treat base connections as shear and would design the base plate as such. On a more recent project we designed the base plate as a moment connection.

My question is if I design a base plate as either moment or shear how do I know it actually will or will not transfers moment

ie If I assume it is a shear connection, design it as a shear connection, is it still possible that will transfer some moment

Thanks

M1 Engineering

http://www.m1engineering.ca

 

[youngstructural]

Your connection will attract load based on stiffness. Whether or not it is able to handle the loads it attracts will govern whether or not it fails.

Let me make this simple and as clear as I can:

A true shear base connection, where you are acheiving the shear capacity with bolts only, is very rare. The reality is that when you have only two bolts (often not permitted, but this depends on the jurisdiction), you still have the ability for the flange to push against the base plate, which pushes against the concrete onto which this is seated, and in turn you can pull up on the anchor bolts provided. Thus you have a Tension-Compression force couple, or a Moment. A TRUE shear connection at the base uses some physical block-out to transfer shear, and the bolts are typically loose fitting in their shrouds, being present for uplift only. These connections are not cheap, and are typically used only where really necessary.

Okay, so if the typical "shear base connection" is actually not a shear connection and attracts loads, why don't we get failures? Because the structure has been designed to compensate for full rotation at the base (ie: modelled as a pin) and thus is stiffer than required. If the base attracts some load (as it will) this just results in a minor moment redistribution, and is conservative. As the base tried to rotate (ie: under the moment load it is capable of resisting), the frame must rotate similarly. Since it is relatively stiff, this causes resistance to the rotation and as a result much of the load is concentrated right where it should be, ie: somewhere other than the base place (normally the knee).

There is a serious caution to be mindful of: IF you need four bolts to transfer your shear, and you have a tall structure, you probably need to consider the stiffness of the connection in your design. When not designing specifically to make the base connection stiff (ie: No stiffeners, bolts are all within the depth of the webs, etc), this will typically be 10 to 20% of EI. You can enter an appropriate stiffness into your model and ensure that you provide sufficient strength against pullout for the bolts you specify.

Sorry for the length of the post, but I hope that helps!

Cheers,

YS

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!