The connection detail I posted is being used as a FMC. How would you go about calculating the capcity of this connection to transmit gravity load moments from the beam? I am trying to justify that this connection is flexible enough that it will not transmit more moment than the moment due to seismic loading.
Tek-Tips is the largest IT community on the Internet today!
Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!
-
Congratulations TugboatEng on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
Flexible Moment Connection
- Thread starter CJJS
- Start date
- Thread starter
- #21
Thanks for the reference. I would agree with teguci that returning the fillet at the top of the plate would eliminate this problem although it will stiffen the plate. Since the entire plate would bend, the return would be needed on the bottom as well, correct?
I do not believe that this is a very flexible connection. Seismic moment will add to gravity moment. The two bolts at the bottom of the beam will fail in shear because they were designed only for the seismic moment.
A flexible connection should yield at the factored seismic moment so that it cannot carry any more than its design value.
BA
A flexible connection should yield at the factored seismic moment so that it cannot carry any more than its design value.
BA
connectegr
Structural
Weld returns will not prevent the concentrated stress at the root of the weld.
I agree with Connectegr, you are playing with fire when you design a one sided fillet welded joint where the stresses/forces are in tension perpendicular to the weld axis at the root of the fillet weld. Your WT side fillets..., and short returns t&b will not fix this. This area of the weld is prone to almost every possible weld defect; poor fit-up or root opening, lack of fusion and lack of penetration, potential root cracking, etc.; all weakening features along the axis of the weld, and you are trying to apply tensile forces/stresses perpendicular to this axis; whereas shear stresses in the weld and parallel to the axis of the weld are much less susceptible to these same weld quality issues.
A moment on your detail will do two things which are detrimental, and additive. It will flex the flange of the WT in such a way as to cause a bending tensile stress (prying?) across the side fillet welds at their roots, and the magnitude of these tensile stresses will be a max. at the terminations of the side fillet welds. Another location which is prone to weld defects, their terminations. The sum of these two conditions just tends to unzip that fillet weld from the ends of the welds. You could help mitigate the above problems by welding all around on the WT and the seat angle, but this has some drawbacks too. Now the WT has a hard spot at its web and the wall of the HSS will flex w.r.t. the weld root at this point... the same problem as above? Welding around the top corners on the seat angle will as likely as not leave a notch in the top corner of the horiz. leg of the seat angle, a stress raiser right where you don’t want it.
As Ron suggested, 30MAY12, 20:16, however we model it, the joint will take some moment until it starts to yield or fails, how much is another question. And, you must guard against those WT side fillets just unzipping, from the top down. The successful ‘flexible moment connections’ so far, have provided a well defined ‘fuse mechanism’ or yield mechanism and location for this action, away from the welds and pretty much eliminated a weld root or weld hard spot as being a potential failure starter.
A moment on your detail will do two things which are detrimental, and additive. It will flex the flange of the WT in such a way as to cause a bending tensile stress (prying?) across the side fillet welds at their roots, and the magnitude of these tensile stresses will be a max. at the terminations of the side fillet welds. Another location which is prone to weld defects, their terminations. The sum of these two conditions just tends to unzip that fillet weld from the ends of the welds. You could help mitigate the above problems by welding all around on the WT and the seat angle, but this has some drawbacks too. Now the WT has a hard spot at its web and the wall of the HSS will flex w.r.t. the weld root at this point... the same problem as above? Welding around the top corners on the seat angle will as likely as not leave a notch in the top corner of the horiz. leg of the seat angle, a stress raiser right where you don’t want it.
As Ron suggested, 30MAY12, 20:16, however we model it, the joint will take some moment until it starts to yield or fails, how much is another question. And, you must guard against those WT side fillets just unzipping, from the top down. The successful ‘flexible moment connections’ so far, have provided a well defined ‘fuse mechanism’ or yield mechanism and location for this action, away from the welds and pretty much eliminated a weld root or weld hard spot as being a potential failure starter.
You'd be hard pressed to fabricate and install a more ideal pinned connection.
If anybody gave me that and told me it was a moment connection, I'd toss it directly into File 13 and they'd spend the rest of the year convincing me they didn't really have a MS in computer science.
What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.
If anybody gave me that and told me it was a moment connection, I'd toss it directly into File 13 and they'd spend the rest of the year convincing me they didn't really have a MS in computer science.
What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.
- Thread starter
- #26
Well, it seems that AISC must allow prying on the one sided fillet welds of simple sheer connections (at least at the top half of the connection),since the connections must be flexible enough to provide the necessary end rotation. Rotation will cause bending in the TEE and therefore will cause prying on the fillet weld. I would agree, though, that this isn't as critical since the entire length of the weld is not affected. Still, there is some prying in these circumstances.
Connect- Just for my own clarification here. When you are referring to prying at the root of the weld or when you say the WT should be very stiff. You are referring to the prying action how Teguci has showed correct? Meaning if you put a stiffener in the web of the WT this would be a very stiff WT? Or does the WT need to be stiff about its major axis?
Bending of the WT about its major axis is really not the concern it is more the tension at teh top of the WT associated with the bending, correct?
EIT
Bending of the WT about its major axis is really not the concern it is more the tension at teh top of the WT associated with the bending, correct?
EIT
abusementpark
Structural
How do you know if the WT flange is stiff enough? What if the WT is welded all around?
How would you design a WT connection to take axial load?
How would you design a WT connection to take axial load?
connectegr
Structural
Firstly, you should not use a WT connection for axial load.
If you did design the flange for stiffness, you could design for weak axis bending. This would result in a very thick flange. The forces in the weld would be shear and tension, with no rotation at the root of the weld.
Bending at the top is associated with flexibility of a shear connection. Beam end rotation results in a very small degree of rotation. And even less in composite construction. Research has found that these connections are OK with only a weld return at the top. Return should be 2 to 4 times the weld size, but no more than 4 times. Research has also found that welding too much is also detrimental. If a crack is initiated, propagation of the crack with less applied stress.
If you did design the flange for stiffness, you could design for weak axis bending. This would result in a very thick flange. The forces in the weld would be shear and tension, with no rotation at the root of the weld.
Bending at the top is associated with flexibility of a shear connection. Beam end rotation results in a very small degree of rotation. And even less in composite construction. Research has found that these connections are OK with only a weld return at the top. Return should be 2 to 4 times the weld size, but no more than 4 times. Research has also found that welding too much is also detrimental. If a crack is initiated, propagation of the crack with less applied stress.
abusementpark
Structural
So a WT connection has no axial load capacity? Even if welded all around? Why is welding too much detrimental?
- Status
Similar threads
- Question
- Question
