Bolted moment connection

[MALKOBANI]

You designed a beam carrying a UDL as fixed end connections. The shop drawings show the flanges of the beam bolted with a clip angle and welded to the column. (Typical right?). Now given that the holes are larger than the bolts, the ends will rotate if slip is not prevented. You compared the possible rotation of the ends to the rotation of a simply supported beam,( WL^3/24EI), and found that the rotation due to slip is larger. Means the beam is really a simply supported not a fixed- fixed one. What would you do?

 

[rb1957]

just to paraphrase your post, you have oversize holes in the flanges such that the beam can rotate (develop slope at the ends) more than the would want to if it was simply supported.

first, your rotation assumes the beam is free to deflect. on installation you would have a shear load path (friction), so i think your rotation is conservative, but we don't want to bet on friction.

could you use oversize bolts ? presumably you have enough flange width for the oversize holes that exist.

or you could bush the holes back to their BP dim'n.

or you could add more fasteners with the proper size holes, and the original fasteners would be somewhat redundant.

 

[MALKOBANI]

To facilitate constructin, Hole diameter= bolt diameter + 2mm.

 

[rb1957]

ok, i thought the oversize holes were a manufacturing problem you were trying to work around. 2mm clearance on diameter isn't much (0.04" on radius).

i visualise that you're considering that the top flange can displace 1mm and the bottom flange displaces 1mm in the opposite direction, thus creating your slope. i think this is highly conservative.

how about calculating the couple required to represnet hte end moment, and see if it is reasonable for this to be carried in friction (between to clamped surafaces) ?

do you have more than one bolt row on a flange ? multiple rows of fasteners would create another loadpath for the moment.

 

[MALKOBANI]

rb1957:

If the rotation needed for a simply supported beam is less than what a possible slip will provide, then if the beam was designed as fixed-fixed,(for less positive moment), then this is not conservative. Or am I missing something here?

My question is general. Clearly, the fabricator would provide a number of bolts to satisfay the required end moments. The problom is with the clearance and the possible slip.

 

[rb1957]

i don't think you've got a problem. but you can assume things differently, then the logical conclusion is a simply supported beam, which would logically imply stregthening the middle of the beam. you may also have a problem with deflections.

 

[dik]

I'm not sure that I understand the situation, but it appears that the connection has been designed as fixed ended and the fabricator is proposing to use clip angles to develop this.

The clip angles will not give you the stiffness of a fixed end condition unless they are temporary and it is intended that the connection be welded. For fixed ended conditions, I usually use a welded end plate type of connection. with slip critical bolts. Occasionally it is necessary to use steel shims to take up the slack required for placing the member.

Dik

 

[seattlemike]

Even if there was no slop in the bolted connections, this beam you describe is in no way fixed, unless you had really massive angles with a really puny beam (atypical). If you actually did an analysis for fixity, this how much of the beam capacity you would develop with that connection, I think it's safe to say that you would be lucky to be in the 10% range, not the 100% range.

But to make it a better simple connection, can you just put in a simple shear tab (bolted or welded)?

Mike

 

[Tomfh]

Sounds like a simple connection to me.

 

[GoodDayGoodNight]

If you want to make it be a fix connection, how about jack the mid of the beam up, then fully weld the beam flange to the clip angle. (note: make sure the clip itself is strong enough)

 

[sdz]

Connections should be detailed to behave substantially as assumed in design and designed as they are detailed.

If it was designed as a fixed end then it should be detailed as a fixed end. The fabricator should change their detail. Is it too late for that?

 

[connect2]

ran into this lately where we only specified the moment and shear at the connection but designed and showed a fully fixed eight bolt end plate moment connection on plans for a steel frame carrying two 100 ton cement bins. The steel fabricators engineers came back with a connection that satisfied the specified moments and shears. Drift or serviceability were not shown directly on our design drawings but implied in our specification and covered in our our eight bolt end plate connection detail. In any case the fabricator had to go with our original design. We analysed it as a fully fixed moment connection so it was built as one. agree with sdz.

 

[WillisV]

Slightly off subject - the top and bottom clip angle connection you described is generally used in Partially Restrained design so at to develop a portion of the beam moment but not all (typically in conjunction with a web angle to carry the shear load). The flange bolts are designed as slip-critical and therefore the tolerences you are concerned about are not critical.

 

[JLNJ]

I agree with Willis above in that you seem to have a classic partially restrained connection, magically knowing to release the gravity moment yet resist the reversing wind moments. You may not be able to spec an angle stiff enough to acheive your "fixed" end condition.

I'm not sure that the code even requires the bolts to be slip critical (i.e. is the load reversal "significant"?).

 

[WillisV]

JLNJ - as a side note, though there are some methods of PR design that do prescribe to the "simply supported under gravity moment" methodology you aluded to, there are also published rigorious analysis methods using the true moment-rotation curves of the PR connections that rely a bit less on "magic" and a bit more on "engineering."

 

[rb1957]

i think JLNJ's point was why would the beam act S.S. for some loading and fixed for others ... that sounds magical to me too. obvoiusly its is a simplification; i'd have thought it was just as simple to assume 50% fixity (ilo 100% for a fully fixed joint) ?