Semirigid Base Plates 1

[Italo01]

Hello,

I usually use AISC's Design Guide 1 to design base plates. This design guide doesn't consider the case of stiffened base plates nor consider the possility of semirigid connection (At least the version that i have). Recently, i started using Idea Statica's stiffness analysis module and saw that a lot of unstiffened base plates calculated to support bending moments fall into semirigid connection category, according to Eurocode, and the stiffness diagram is nonlinear.

My question is: Do you guys usually compute the stiffness of the base plates and input in the model to increase accuracy?

Since the Stiffness diagram is nonlinear, due to the nonlinear behaviour of the concrete, the secant stiffness at 2/3 M[sub]j,Rd[/sub] defined in Eurocode is overly conservative and any software which doesn't have the capability to consider the nonlinear stiffness will give excessive deflections.

 

[driftLimiter]

My thought is that most people don't usually use a software capable of calculating the baseplate stiffness.

And for most projects I question whether that additional level of effort would provide a proportional amount of benefit.

 

[bones206]

And for most projects I question whether that additional level of effort would provide a proportional amount of benefit.

Ironically, the only time I explicitly consider partial fixity at column bases is when I’m trying to limit drift of a moment frame. I’ve found that a modest amount of base stiffness can significantly reduce drift, which can result in a more economical frame.

 

[driftLimiter]

yes that is ironic (well its actually fitting but point taken). I can definitely see cost savings there. Still in my experience its seldom done because of the available tools. When I have the same problem I design the base connection for full fixity. Certainly for large moment frames your approach could help strike the balance and get a more economical way than going fully fixed or pinned.