Double Angle Connection with Torsion

[Valstone]

The EOR has requested that I provide a calculation showing the torsional capacity of a bolted-bolted double angle connection between a W8 beam and a W8 girder. My thought was to resolve the torsion through bolt tension in the bolts on the beam side. My concern now is how I design the angle leg for this torsion. Should I check the angle leg for torsional stresses in addition to the weak axis flexure induced by the bolt force from the torsion OR should I ignore the the torsional stress in the angle leg and design the angle leg for the weak axis flexure only? It would seem that the angle thickness would have to be pretty thick to resist any type of torsional forces. Any input would be greatly appreciated.

Thanks,

Val

 

[BAretired]

There is no bolt tension. Torsion in the beam is transmitted to the girder by bolt shear. Clarify your question.

BA

 

[Valstone]

The beam has torsion and vertical shear. The connection that they are wanting to use is a double angle connection that is bolted to the beam web and bolted to the girder web. How do you suggest resisting the torsion in this type of connection?

 

[BAretired]

If there are four bolts through the girder, each bolt will carry one quarter of the vertical shear. If the torsional moment in the beam is M, each bolt will carry a shear of M/2D (where D is the diagonal distance between diagonally opposing bolts) in addition to the vertical shear. The vector sum of the two shears will give the resultant shear in each bolt. There is no tension in any of the bolts.

BA

 

[hokie66]

I think Valstone is worried about the bolts through the beam web rather than those connecting to the girder.

The only way you can resist a substantial amount of torsion, even if the beam is capable of taking it, is to connect the flanges to create a push-pull couple.

 

[rowingengineer]

I like sketches

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[BAretired]

hokie,

Yes, I should have seen that last night. The beam flanges would have to be coped to clear the girder flanges, so the only part of the beam available to carry torsion is a six inch height of the web.

For any substantial torsion, the beam flanges would have to be connected to the girder flanges. And then the double angles would be carrying only vertical shear.

BA

 

[paddingtongreen]

These can be dangerous. Valstone, I don't know what your responsibility is. First question is can the double coped web handle the load combination and if not, who has the responsibility for reinforcing it.

I've seen jobs where the detailer is required to provide connections like half the maximum UDL, or the full shear capacity of the beam, etc....

If the web is the EOR's responsibility, just use angles that are the next size thicker than the web, with long outstanding legs if two bolt rows are needed. Forget the bolts to the beam web and rely on bearing of the beam web on the angles for the torsion. The angles will then be stronger than the beam.

If the web is your responsibility, you may be into web reinforcing as part of your work.

I have had cases like this because of limited headroom, but then I use an end plate on the beam, connected to a piece of tee that is shop welded to the girder to provide a flush finish.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Valstone]

paddingtongreen - The beam web was actually one of my concerns, but I need to talk to the EOR about the beam web itself. The requirements for this project keep growing exponentially!! I agree that the beam web will not be able to resist much torsion if any at all. I still think that the angle leg on the beam side should be checked for torsional stresses similar to a rectangular bar. I know once I can get the loads to the girder side, it is an easy calculation.

Val

 

[hokie66]

This is a classical example of why the EOR should be responsible for designing all connections.

 

[BAretired]

hokie,

I agree, but the standard practice in my area nowadays is for the EOR to require the fabricator to retain an engineer to design all of the connections, to seal them with his engineering seal and to accept responsibility for their adequacy. I have been retained by fabricators to carry out this service on several occasions. I did not agree with the practice, but because I had a working relationship with the fabricator, I complied, albeit unwillingly.

The fees are really not commensurate with the risk and if I were to continue in practice, I would not perform this service any more.

BA

 

[Valstone]

hokie66 - The problem is that most EOR don't want the responsibility and most don't have a full understanding of connection design based on many conversations and also working in a structural engineering office. This request by the EOR is a great example of why this EOR should not design the connections, because she doesn't fully understand how torsion is resolved at a connection. This type of connection would not be my first choice, but when the EOR demands this type of connection you must oblige. They will find out this week that this type of connection is very weak in torsion.

Val

 

[BAretired]

This type of connection would not be my first choice, but when the EOR demands this type of connection you must oblige.

You must not oblige! It is absolutely wrong for a fabricator's engineer to oblige an EOR who doesn't know what he or she is doing.

Why don't you advise the EOR what you believe should be done in order to resolve the situation. If she is not prepared to accept your solution, then you must demand an instruction in writing as to what detail is to be used. Needless to say, she must accept full responsibility for that instruction.

BA

 

[hokie66]

If the EOR does not understand connection design for torsion, he/she probably does not understand torsion design for the beam either. If there is significant torsion, an open shape like a W8 is not much good in resisting torsion. Good luck!

 

[Valstone]

I totally agree!! These issues I will be taking up with the EOR this Monday.

 

[csd72]

I agree with Hokies comments regarding the EOR and connection design. I have worked in both systems and personally believe that when the EOR designs the connections it is better for all involved.

The problem arises when the fabricator has their own special connections that they insist on doing and then the EOR has to redesign to suit.

Sizing beams is usually monkeys work, making connections work is often the hardest part.

That debate aside, I believe that you will find in one of the best practice guides that the EOR should provide a generic detail of any unusual connections to show their intent.

I had a similar situation and as an EOR I agreed to do the connection design for some of the more complex connections as that was easier than conveying all the requirements.

 

[DaveAtkins]

No one has answered Valstone's question!

Yes, I generally check the outstanding angle legs as two rectangles each taking 1/2 of the torsion. Keep in mind you can't make the angle legs thicker than 5/8", per AISC, to allow the connection to act as a pinned support.

You will find that you can't get much torsional resistance out of two rectangles.

DaveAtkins

 

[hokie66]

Dave,

I thought we did a fairly good job of discussing Valstone's problem, and he seems happy. Why would you be concerned with a connection acting as pinned when it has to resist torsion?

 

[BAretired]

Perhaps the entire question is academic since the beam web is likely unable to carry much torsion, but it seems to me that the parallel legs of the two angles would provide substantially more torsional resistance than the sum of two slender rectangles in torsion.

The legs are separated by a distance of t(web) + t(leg), so major axis bending comes into play, one leg bending up, the other down.

If the beam web is adequate to resist the applied torsion, minor axis bending of the angle legs comes into play as well.

But the weak part of the whole connection is going to be the beam web, so the question is definitely academic.

BA

 

[DaveAtkins]

The double angle connection needs to act as a pinned support, even as it resists torsion.

But I agree there are other issues which may render the point moot.

DaveAtkins