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Double Angle Connection with Torsion

Double Angle Connection with Torsion

Double Angle Connection with Torsion

(OP)
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  

RE: Double Angle Connection with Torsion

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

BA

RE: Double Angle Connection with Torsion

(OP)
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?

RE: Double Angle Connection with Torsion

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

RE: Double Angle Connection with Torsion

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.

RE: Double Angle Connection with Torsion

I like sketches

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

RE: Double Angle Connection with Torsion

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

RE: Double Angle Connection with Torsion

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.

RE: Double Angle Connection with Torsion

(OP)
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

RE: Double Angle Connection with Torsion

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

RE: Double Angle Connection with Torsion

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

RE: Double Angle Connection with Torsion

(OP)
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

RE: Double Angle Connection with Torsion

Quote:

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

RE: Double Angle Connection with Torsion

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!

RE: Double Angle Connection with Torsion

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

RE: Double Angle Connection with Torsion

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.

RE: Double Angle Connection with Torsion

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

RE: Double Angle Connection with Torsion

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?

RE: Double Angle Connection with Torsion

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

RE: Double Angle Connection with Torsion

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

RE: Double Angle Connection with Torsion

Dave,

I'm still curious as to why you say that.  Why not a rigid support?  I don't think we know enough about the rest of the structure to define how a given connection should work.

RE: Double Angle Connection with Torsion

I will admit that if you connected the beam flanges to the girder flanges, by welding or bolted splice plates, you could take out more torsion.  But then you would be introducing some partial end moment from the beam to the girder, which I suspect is not the intent of the EOR.

So if you wanted to keep the support pinned, and not connect flanges to flanges, you could still take some torsion while maintaining the flexibility in the double angles which is required by AISC.

DaveAtkins

RE: Double Angle Connection with Torsion

I understand now.  Your reluctance to make the connection rigid is because of a code provision, not practicalities.  Anyway, if the connection is stiff, that end moment is taken by the beam in bending, not the girder in torsion.

RE: Double Angle Connection with Torsion

I'd try like hell to torsionally brace the beam at the point of application of the load causing torsion.
I guess I should assume this was attempted, but not possible.  

RE: Double Angle Connection with Torsion

I agree with a few of you on this; the detailer should post an RFI to the EOR about the desired intent of the connection. Possibly the EOR forgot to detail it specifically, thinks they can get torsion out of this connection, or doesn't know better.

RE: Double Angle Connection with Torsion

(OP)
This connection occurs at a pipeway framing plan in an industrial plant and due to the lateral forces induced by the pipes a torsion is induced at the ends of the beams. There is grating that bears on the top of steel so a flange plate would not work. I would be concerned about connecting the flanges of the beam directly to the girder flanges because it would definitely change the behavior of the beam. I know for a fact the beam web will need to be reinforced, but my main reason for asking was to see if anyone else has run into this type of problem. This is not a typical connection let alone a normal special connection. I have spoken with AISC and they were not much help other than saying that the coped out beam web acts like a rectangular section in torsion and that there is no documentation for this type of connection. I personally think the EOR hasn't fully thought this through. I think sometimes the EOR has so much pressure to get member sizes and drawings complete that they don't think about all forces involved at the connection. If I were designing the framing myself I would definitely resolve the torsion outside of the connection. I will definitely be talking to the EOR about this connection because this affects a good amount of the connections, which will be costly to the fabricator and owner.

RE: Double Angle Connection with Torsion

I don't agree with AISC that there is no documentation on this type of connection.  The torsional shear stress in a rectangle is (3*M)/(b*t^2), where M is the torsional moment, b is the long dimension of the rectangle, and t is the short dimension of the rectangle.

DaveAtkins

RE: Double Angle Connection with Torsion

You could put an end plate on the beam outside the girder flange, then weld a 5x5 HSS stub with a 5 x 10 end plate with four bolts through the girder web.

BA

RE: Double Angle Connection with Torsion

Just to be clear about the comment about the AISC requirement that a clip angle has to be 5/8" to ensure ductility. That is a comment in the manual and not the spec. The same section in the manual says that ductility checks in Part 9 of the 13th Ed can be used to justify other angle thickness.  

RE: Double Angle Connection with Torsion

On second thought, in this particular case, the angle flexibility doesn't matter very much.  Even if you were to rigidly connect flanges to flanges, what would happen is the girder would twist slightly, thus creating the pinned end condition for the beam.  The slight twist of the girder would not result in any appreciable torsion in the girder.

DaveAtkins

RE: Double Angle Connection with Torsion

Now that BA has designed the connection, all that is left is to implement it.

RE: Double Angle Connection with Torsion

Thanks, hokie.

BA

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