Frame end releses

[Greeni]

Hello All,

I have a multi-story steel frame with concrete column foundations (but these are not modeled in SAP). I am getting confused about member end releases.

1) Base columns should be fully released at foundation end because anchor-base plate type connections will act as pins in reality. Should it be a full (100%) release for torsion/moment a that end of column or some other %?

2) Physical moment connections between column-beam and beam-beam of steel would be more desirable by architect as shape of building won't permit use of cross frames to resist lateral wind loads. If frame is unreleased at all ends of beams and columns, I get very large end moments requiring huge sections. What % of release is reasonable to represent a moment connection on field?

3)I also read that even though the structure has moment connections and is modeled as such, it must be stable with releases representing pin connections so that it will stand during construction (before steel crew secures all bolts). This makes sense, and I can just run a second model to check for this stage.

Thanks in advance for your ideas!

 

[zackIX]

1)For the sake of simplicity, I think you should model the base columns as pinned.
To really act as pinned it depends on the final design of that joint. How many anchor rod do you plan to use?
Could you be more specific?

 

[Agent666]

Often if you have a net tension (with a taller building) then when you get to the baseplate design you find you need a thick plate or even an embedded column, so pinned theory sort of goes out the window. It's important to identify this at the start as you can waste a lot of time designing for a pinned behaviour before realising the behaviour will in reality be more like a fixed or semi rigid connection based on the details adopted to deal with the strength requirements.

Though even if the base connection is fixed, foundation flexibility may mean your detail has a degree of flexibility. I like to model any ground beams etc. If this is the case, it can impact on the dynamic characteristics of the building which can give some benefits.

Ultimately we try to model how we think the structure will be behaving in reality based on our knowledge of the materials and details that are going to be used.