Steel connection beam-beam eccentricity

[Mike1998]

Hello everyone,

I have a beam to beam simple connection (beams 1-2). As you can see in the attached image, there is an eccentricity, because of the distance between the bolts and beam 1. Should the connection and beam 2 be designed to take the bending moment which arises from this eccentricity? Is this modelled as a beam on 2 supports with a bending moment at the first support, as shown in the image?

Thank you.

 

[HTURKAK]

Dear Mike,

This is bad detail. The column should extent soffit to the level and normally the connection should be to the column. If this detail inevitable, use beam to beam simple shear connection with double notched beam to avoid eccentricity.

 

[Mike1998]

Dear Hturkak,

Thank you for your response. The detail was not designed by me. I just have to check if the connection and the beam provide sufficient resistance. But I think that in the detail you are showing we also have a (small) eccentricity and also a bending moment in the connection and the supported beam, right?

 

[HTURKAK]

...........But I think that in the detail you are showing we also have a (small) eccentricity and also a bending moment in the connection and the supported beam, right?

Yes...but negligible.

 

[molibden]

You can say it is a bad detail, but in the end it is easier to manufacture and easier to assemble. In my area everybody wants this and will do like this. Ofcourse it has limitations and it's not for every situation.
You should design it for extra moment.

 

[Tomfh]

Yes the detail has to carry the moment from the eccentricity, same as a bolted side tab does.

 

[JoshPlumSE]

For true simple shear connections you design EITHER the connection or the member supporting it for the moment due to the eccentricity. I prefer designing the connection for the eccentricity personally.

In a case like yours I wonder if it's really a simple shear connection. What I mean is that a simple shear connection must have the ability to rotate / deform in a way that is consistent with a simple shear connection. This is sometimes referred to as a check on the "rotational ductility" of the connection. I don't know that your connection has sufficient rotational ductility to be considered a simple shear connection. If not, then I would design BOTH the connection and the member for these moments due to eccentricity.

 

[r13]

Using the conventional simple shear connection, you will have a small moment that is likely can be ignored. For the connection in your detail, it is a beam fixed on a rotational support, the analysis is not straight forward. You need to find and define the rotational stiffness of the support beam in order to get correct results.

 

[Tomfh]

Using the conventional simple shear connection, you will have a small moment that is likely can be ignored

The moment can make a real difference when the eccentricity is comparable to the bolt lever arm, e.g. two closely spaced bolts.

 

[r13]

The reason we say a connection is conventional is because that is a standard detail been used for a long time, with lengthy track records, so there are ways and plenty examples to handle every design concerns related to shear connection. However, the OP's detail, to my knowledge' is a novel concept but a standard connection, so uncertainty is high, and design could be difficult. For one, it does not eliminate moment, but induce higher torsion, and makes the magnitude of the moment in the incoming beam uncertain, because the rotation nature of the supporting beam it connects to.

 

[human909]

In the world of engineering we regularly make 'assumptions' that are a fair bit away from reality. That is fine as long as you know you are making them and as long as you know when you need to stop making such assumptions and start digging deeper. Quite a few on the response here make me start scratching my head in confusion....

You can say it is a bad detail, but in the end it is easier to manufacture and easier to assemble. In my area everybody wants this and will do like this. Of course it has limitations and it's not for every situation.
You should design it for extra moment.

Alot of this. From my observations many engineers are obsessed with removing eccentricity with no regard for ensuring economical fabrication and erection. On the other hand there is a reason why eccentricity is avoided and if detailers are given free reign you end up with some very ugly eccentric scenarios.

For true simple shear connections you design EITHER the connection or the member supporting it for the moment due to the eccentricity.

Maybe I'm misinterpreting things. But this sounds odd. Moment due to eccentricity acts on both the connection AND the member. You can't just design for one and forget about the other.

You typical beam doesn't have a whole lot of torsion resistance on its own so eccentric loads applied to them can be pretty brutal and result in significant torsion. However in many circumstances the primary beam is restrained from twisting with other members. Often it is all good to ignore the eccentricities, until it isn't....

 

[Tomfh]

You can't just design for one and forget about the other.

It’s common to do this. Design the member to take the eccentricity, or design the connection to take it.

 

[JoshPlumSE]

Maybe I'm misinterpreting things. But this sounds odd. Moment due to eccentricity acts on both the connection AND the member. You can't just design for one and forget about the other.

Yup, it's a little counter-intuitive. But, that's what is generally done. See page 10-132 of the AISC 15th edition manual Eccentric Effect of Extended Gauges - Alternative Treatment of Eccentric Moment. There are also previous eng-tips threads that have discussed the topic.

The idea is related to the concept of rotational ductility. Let's say there is slip in the bolt holes, does the moment develop? Let's say the supporting beam is incapable of resisting torsion, so it rotates freely when a moment is applied. Does any moment develop then?

It doesn't really satisfy statics, but that's the long standing industry practice. You're free to do design BOTH to take the moment if you'd like to, obviously.

 

[r13]

Let's say there is slip in the bolt holes, does the moment develop? Let's say the supporting beam is incapable of resisting torsion, so it rotates freely when a moment is applied. Does any moment develop then?

In this case, the slip of the bolts has less to do with the develop of moment in the connection, as compared to connection through bolting to the web. The magnitude of the moment depends on the ductility of the bolt to deform (in tension), rigidity of the end plate, and depends on the rotational /capability/capacity of the support beam, which is not so easy to quantify.

There is a moment on this connection, which results in torsion on the support beam, both need to be checked/designed.

 

[JoshPlumSE]

There is a moment on this connection, which results in torsion on the support beam, both need to be checked/designed.

Retired13: I would have you would have encountered this discussion before. Perhaps, it's that you disagree with AISC and the current industry practice. Or, do you not acknowledge that this is the way most of the industry (at least in the US) treats these connecctions?

This of this another way:
You can model a beam as fixed - fixed beam and you have moment at the end of the beam that you have to design for, right?
Now, you all the end of that beam to rotate a good amount. I.e. you soften up the support so that it is no longer fixed - fixed. Does the same moment develop? Nope.... The more flexible it is, the more rotation that is allowed, the more the moment tends towards zero. That was my point about what happens when bolt slip occurs.

Don't believe me? Create an FEM model where you attempt to replicate this behavior.

 

[r13]

Now, you all the end of that beam to rotate a good amount. I.e. you soften up the support so that it is no longer fixed - fixed. Does the same moment develop? Nope.... The more flexible it is, the more rotation that is allowed, the more the moment tends towards zero. That was my point about what happens when bolt slip occurs.

This exactly I wanted to point out. The support beam is liking a rotational spring, thus the moment of the incoming beam is depending on the stiffness of the spring - the more stiff, the closer to fixed; the more flexible, the closer to pinned, at a point the fixity can be ignored due to large rotation. The above is well understood, but the stiffness of the support is difficult to pin point. So, IMO, unless result from a well/correctly modelled analysis indicates otherwise, the incoming beam to the end plate connection shall be conservatively designed for the moment, and the support beam and its end connections shall be checked for the torsion caused by the moment, and eccentricity of the shear force. I think the sketch below represents my thinking, right or wrong.

 

[r13]

I guess this is the AISC and industry practice you addressed. I avoid this type of connection to the best extent I could.

 

[JoshPlumSE]

Retired13 -

I feel better now. I thought there was a significant disagreement between us. But, it sounds like were on the same page.... or reasonably close.

I'd say that AISC's method (of ignoring the moment in either the beam or the connection) is based on an assumed rotational ductility of the connection. So, that method is dependent on the connection conforming to the standards of what AISC considers a simple shear connection.

Because the detail the OP posted doesn't (IMO) seem to fully satisfy AISC's guidelines for a simple shear connection, it probably makes sense (for THIS case) to design BOTH the connection and the member for that moment. Now, this could probably be relaxed a bit, if the OP can do some calcs to demonstrate the connection meets the rotational ductility requirements for a simple shear connection.

 

[molibden]

This paper deals with this particular connection type. Basically it's a semi-rigid connection, borderline rigid.

I use this type of connection for moment connections on continuous secondary beams. Sometimes I have a cantilever and no space for splices and this connection does the job.