Partial fixity

[MALKOBANI]

Some structural software accept specifying partial fixity for connections. What does partial fixity really mean?
Doest mean that the connection can resist some moment without allowing rotation or it allows some rotation before resisting moment or neither?

 

[ash060]

It means that you can assign a rotational stiffness to the joint. The units would be in something like kip-in/radian and moment would develop in the connection, but not the full wl^2/12 moment like a true fixed connection. And the amount of moment would be dependent on the stiffness you assigned to the connection.

 

[prsconsultant]

Partially restrained connections resist moment and also allow rotation. Below is the definition of PR connections per AISC specifications.

Three basic types of construction and associated design assumptions are permissible under the respective conditions stated herein, and each will govern in
a specific manner the size of members and the types and strength of their connections:

Type 1, commonly designated as "rigid-frame" (continuous frame), assumes that beam-to-column connections have sufficient rigidity to hold virtually unchanged the original angles between intersecting members.

Type 2, commonly designated as "simple framing" (unrestrained, freeended),assumes that, insofar as gravity loading is concerned, ends of beams and girders are connected for shear only and are free to rotate under gravity load.

Type 3, commonly designated as "semi-rigid framing" (partially restrained), assumes that the connections of beams and girders possess a dependable and known moment capacity intermediate in degree between the rigidity of Type 1 and the flexibility of Type 2.

 

[csd72]

Think of the connections as being coil springs which will provide a level of restraint depending on the rotational deflection.

You really should get a text on this before you start analysing for partial fixity, it is a complex subject.

csd

 

[rb1957]

i think of it as being the real world, being somewhere between the two ideal states (fixed and pinned)

 

[JLNJ]

You might also use partial fixity to define the base of a "fixed base" column.

 

[MALKOBANI]

Thanks all for the replies.

Is there any reference that provides modification factors or capacity reduction factors for properly designed typical connections as moments connections?. What I mean is this:

I assumed that a connection is fully fixed and designed it accordingly. Because this connection is not really fully fixed, and if tested, it would provided less strength than the theoratical fully fixed. What is the strength reduction factor for this connection.

 

[rb1957]

i'd spin it a little differently ... design it as tho' it's fully fixed (this should maximise the stresses at the end. then test it, if it survives, isn't everyone happy ?

if you want to be conservative, design the ends to the fully fixed, and the beam to be pinned. if you design the beam as tho' the ends are fully fixed, and fine tune the cross-section accordingly, i think you'll be understrength in the mid-span ('cause the fully fixed end moment won't fully develop and the relief that this gives the mid-span won't develop and the section will see higher loads than you anticipated.

if you want to be precise, measure the rotation of the joint when you test it. this'll tell you how fixed the end is, and so you can to more detailled calcs

 

[J1D]

MALKOBANI, it seems a bit mixing up between strength and stiffness. Here is my view about what happens if you design (or analyze) a connection as fully fixed but it is actually partially fixed (say 80% rigid):

1. there must be some rotation occurred at the connection (due to bending of column flange, beam end plate, bolt elongation, etc) to make it Partially rigid;

2. the rotation should in lots of cases reduce the force (bending moment) at the connection;

3. the rotation reduces the rigidity of the connection instead of strength. For instance, there is a cantilever beam with connection to a column. Under the load, the connection rotates a little amount (full to partial), but the connection strength is not reduced. The strength is determined by the elements making up the connection.

 

[csd72]

J1D,

any reduction in base stiffness for a portal frame means higher moment at the eaves. It also means a higher maximum moment in the column (also at the eaves).

csd

 

[ash060]

There is some good information in the PCI Handbook 6th Edition. Check chapter 3.6

 

[J1D]

csd72,
You're right, if we look at a member or a frame the connection stiffness reduction (and reduction of the forces there) is not necessary benefitial to others. Such as a beam forming a rigid frame with columns. The bending moment at mid span is much higher when the connections become "soft".