×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Flexible Moment Connection

Flexible Moment Connection

Flexible Moment Connection

(OP)
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.

RE: Flexible Moment Connection

That doesn't look like a FMC. It looks like a WT shear connection with a erection seat. You could assume that the connection does not transfer any moment.

RE: Flexible Moment Connection

I disagree. This connection will transmit moment to the column as a force couple exists between the 4-bolt connection and the seat connection.

It is a relatively weak moment connection with the ultimate transfer capability defined by the lateral shear of the bolts or deformation in the web at the 4-bolt connection.

RE: Flexible Moment Connection

(OP)
Ron,
That is essentially how I determine its capacity to resist the seismic moment. This connection is being used as a FMC. So, it is assumed that this will yield under gravity loading. I am basically trying to justify that it will in fact yield under gravity loads. If it's too rigid, the columns will be underdesigned as they are only designed for the moment due to seismic loading only.

My thinking is that the top of the flat portion of the wing tee (that portion welded to the column) will yield first as it wants to pull away from the column (notice there is no weld along the top). I am looking for a simplified way to analyze that. Thoughts?

RE: Flexible Moment Connection

I'm getting an allowable moment of 97 inK for the W14 and 73 inK for the W12 (bending in the 3/8" plate). The failure mechanism will be shear in the bolts with a maximum moment transfer after a great deflection (considering the overstrength factor) SWAG of around W14 - 228 inK, W12 - 171 inK.

RE: Flexible Moment Connection

(OP)
Teguci,
Once the 3/8" plate yields, how can it transmit any more moment? You say that the maximum moment is 228k-in (W14) and 171k-in (W12). How did you calculate these values?

RE: Flexible Moment Connection

Allowable is based on Fy/Omega x Z, Ultimate for the plate is Fu x Z, where Fu can be 80 ksi.

RE: Flexible Moment Connection

(OP)
Are you assuming the plate is simply supported for bending? And are you assuming the entire plate is in bending or just a portion? Using 36ksi yield, I am calculating an allowable moment of 39 k-in (w12).

RE: Flexible Moment Connection

fixed at the weld and no bending (infinite stiffness) over the 3/4" distance between the 3/8" stem plate and welds. or for half the plate, fixed - no rotation over 2.125" distance

RE: Flexible Moment Connection

(OP)
Thanks. That explains our differences. I assumed a pinned connection at the weld.

RE: Flexible Moment Connection

This is not a moment connection. AWS/AISC does not permit a one-sided fillet weld in prying. Which is the condition if a tension force is applied to the WT web connection. The beam web connection can develop only shear. As shown the seat and beam web connection cannot share the shear load. In order to load the seat, there must be yielding of the beam web connection. And if the seat is in bearing, the web bolts may not be carrying any load.

Typically a flexible moment connection will provide angles at the top and bottom flange. Or flange plates (with controlled weld lengths) at the top and bottom. These connections provide flexibility due to the ductility of the connection material. Rotation of the heal of the angle or elongation of the flange plates.

Best professor I ever had, had a simple guide. "If it looks wrong, it probably is"

http://www.FerrellEngineering.com

RE: Flexible Moment Connection

The one premise that we are all overlooking is that the physical condition will prevail without regard to how we model the connection. No, it is not a "moment connection". Will it still transmit moment to the column? Of course it will...until the components fail...not just yield.

Backcalculate the moment that will be transmitted until component failure occurs and make sure the column can handle it.

We can assume the idealized conditions of pinned or fixed all we want....the actual restraint will dictate how the members respond.

RE: Flexible Moment Connection

So does a clip angle connection or a shear tab. Nearly any shear connection lacks the theoretical flexibility we rely on for a pinned analysis. This does not allow us to reverse engineer a shear connection to find a little more fixity. This is OK for a doctorate research project. But, there is also valid research for the intended applications of AISC recommended shear connections.

www.FerrellEngineering.com

RE: Flexible Moment Connection

(OP)
Ron,
Yielding of the connection will cause rotation of the beam end. Once it rotates sufficiently it acts as a simply supported beam and therefore will not transmit any more moment.

RE: Flexible Moment Connection

In the case of the detail provided, yielding of the WT flange will result in bending of the flange. Bending of the flange will result in stress concentrations at the root of the fillet welds. These stresses will result in a crack, propagation of which significantly reduces the tensile and shear strength of the weld.

www.FerrellEngineering.com

RE: Flexible Moment Connection

I'm not an expert in this subject but I'm think I agree with connectegr. Assuming the connection is only located in a few areas my advice would be to remove the seat and then add plates or angles as needed to get the design you need. At any rate by removing the seat you would know three things that would help make your problem simpler.

1) You would know that the connection is not transfering any moment, and all shear is going through the tee part of the connection.

2) You would know that you only have to design your moment plates for live loads in the structure whether they be floor live loads, roof live loads, or seismic live loads. All dead loads are all ready being transfered in simple shear. This helps you in not overstressing the columns. Hopefully your live moments are less than your seismic load moments.

3) You don't need to analyze the tee connection anymore as the moment will be handled by the added on plates.

Assuming you don't have a lot of P-delta issues in your frame analysis you could superimpose the dead load case onto the other cases and see if your columns work as if the connection were traditional moment connection. If that didn't work than you could size the plates or angles just right so that they would qualify as a flexible moment connection.

If by removing dead load moments your column overstressing problem is solved than you could probably get away with just removing the bolts from the seated connection and then replacing them. It would be like releasing the dead load moments from each end one at a time. Friction might prevent this though.

Living in Florida I don't have to deal with the seismic requirements thank God. I honestly don't know how you can make ends meet dealing with all the seismic provisions but that is another story. For me wind is enough. And I'm still not yet comfortable with the whole flexible moment connection idea to be honest. You would really have to do it just right and have really good inspectors in the field. In my practice I either assume its a moment connection or simple shear. Of course this doesn't help you situation out much.

I agree with other comments above also that we never really have a pure simple shear connection unless of course we one bolted everything. I always try to be conservative with my columns. This was a fun question to think about and I'm glad its not my problem. I've gotten to a point though where I'm tired of trying to do physics gymnastics trying to justify things I'm not really sure about. Hence my advice above. Do something that makes this problem simpler and lets you sleep at night.

John Southard, M.S., P.E.
http://www.pdhlibrary.com/

RE: Flexible Moment Connection

Is the column a W Shape? One detail indicates the angles connected to the web. The other detail indicates the angles connected to the flanges.

Or is the column a HSS?

BA

RE: Flexible Moment Connection

ConnectEgr - I tried finding the "no moment stress through fillet weld" statement in both AISC and AWS but couldn't find it (J2.2b.(2) seems to suggest that end returns aren't necessary?). Makes sense though. Is the attached diagram appropriate for what you are referring too?

In this case, the prying action of the plate should be avoided with the use of a returned fillet weld along the top (length = 2 W) similar to single angle connections (J2.2b). This will add to the bending stiffness of the connection but should remove the fillet weld as the failure mechanism.

RE: Flexible Moment Connection

(OP)
This is an HSS column.

This detail has been used in storage platform design often in seismic design categories C or less (R=3). The lateral loads are relatively small. I have reviewed calculations on these mezzanines from several different engineers. It seems that all of the engineers have designed the mezzanines similarly. The beams are designed as simply supported. The columns are designed for axial loads due to gravity loads and moments due to seismic loads. The connection is designed to take the moment due to seismic only; end moments from the beam are never considered. Usually, engineers check the shear capcity of the bolts and/or bolt bearing acting over the moment arm (top of seat connection to center of beam) as the moment capacity. If the connection is capable of transmitting moment due to seismic, logic tells me that it must also be able to transmit moment due to gravity loads. I understand the theory behind the flexible moment connection design, and the key to that working correctly is that the connections yield under gravity load, thus my whole concern of determining when the connection will yield.

Generally these platforms are designed for 125 psf LL, however, they rarely see anything close to this loading. I have never heard of a platform failing, although I'm am sure there are failures out there that we never hear about.

connectegr,
You stated that "AWS/AISC does not permit a one-sided fillet weld in prying." Can you point me to the refrence?

RE: Flexible Moment Connection

See One-Sided Fillet Welds in AISC Chapter 8. The detail shown is a "T" joint and shows where cracks initiate at the root of the weld. A similar condition occurs due to prying. In some cases the prying condition can be worst, since initial the stress in concentrated at the top of the weld and not evenly distributed. If the flange of the WT is extremely stiff, then it can be argued that tension only is in the welds and no rotating is allowed at the root of the weld.

www.FerrellEngineering.com

RE: Flexible Moment Connection

(OP)
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?

RE: Flexible Moment Connection

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

RE: Flexible Moment Connection

Weld returns will not prevent the concentrated stress at the root of the weld.

www.FerrellEngineering.com

RE: Flexible Moment Connection

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.

RE: Flexible Moment Connection

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.

RE: Flexible Moment Connection

(OP)
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.

RE: Flexible Moment Connection

That's correct.

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

RE: Flexible Moment Connection

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
www.HowToEngineer.com

RE: Flexible Moment Connection

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?

RE: Flexible Moment Connection

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.

www.FerrellEngineering.com

RE: Flexible Moment Connection

So a WT connection has no axial load capacity? Even if welded all around? Why is welding too much detrimental?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources