Maximum slenderness koef. for sway frame

[SnapSpace]

Hi!
Usualy we use monogram for determing value of K,wich will be multiplied with columns lenght. l=k x lo.
For non-sway frames maximum teoretical value is k=1,00, but for sway frame its k=infinite!
Anyway I found that people are always talking about k,max=2 for sway frames. Even so, this value would be applied only to a column thats fully restraind at the bottom and compleatly free at the top, wich clearly, in a typical frame constuction is never a case, because you always have columns that are conected to beams/slabs!
So my question is wich would be a maximum vaule of K for sway frames?

 

[Lion06]

The connections to the slabs/beams only affect k if they are moment connections.
That beind said, many people don't design sway (moment) frames with fixed bases. That is typically a last resort to keep column sizes under control or to control drift. Therefore, theoreticaly, the bases re pinned and the intermediate stories are restrained by girders/beams. This could easily result in k>2.
This sort of goes away with the new Direct Analysis Method.

 

[SnapSpace]

I dont get why would someone model a base column as pinned connection, expecialy in a sway frame?

 

[JAE]

Usually the base of columns on the foundation are designed as pinned (a conservative approach) because you don't know the following:

1. The moment/rotation relationship in the base connection due to flexibility in the base plate.
2. The moment/rotation relationship in the base connection due to the flexibility in the anchor bolts.
3. The moment/rotation relationship in the base connection due to the flexibility in the footing on the soil.

These are many times very difficult to quantify and put in an analysis model, so engineers tend to assume they are pinned, allow the full rotation to occur, and design the frame above with rigid connections between columns and beams.

 

[SnapSpace]

O.k. I hear you!
But what if I can model the footing beneath the column and also define subgrade reaction of the ground?

 

[hokie66]

I have designed many industrial buildings with fixed (or usually partially fixed) column bases. The column to footing connection can be modeled--big columns require heavily stiffened bases and the anchor bolt elongation is the main thing which detracts from fixity. However, preventing rotation of a footing on soil is very problematic, and I have normally used bored piers or precast concrete piling to provide a more rigid base. Base fixity, as you apparently know, makes a big difference, probably in the order of 20% in maximum moment, but more importantly in stiffness. Buildings with cranes in particular, in my opinion, should have fixed or at least partially fixed base columns. For lateral loads due to the cranes, the base can usually be assumed fixed even if the true fixity is partial, because the loads are transient in nature.

 

[csd72]

We used to model this as pinned for strength and then a separate moment for serviceability to calculate what fixity we need to bring the deflections down. We would then spread the bolts far enough apart to get the fixity for serviceability.

I havent done one in a while, but that is how I would still do it.

 

[SnapSpace]

Can the slenderness koef. for sway RC frames be greater then 2 if you are using linear analysis and you want to add moments because of the buckling effect?
Im talking reinforced concerete structures here!