moment / fixed connection

[kaffy]

Good Morning fellow engineers,

I am working on a steel structure and trying to make a fixed connection on beam in order to reduce the required beam size as compared to pinned connection. Structural member acting as beam is a C channel. I was thinking to use two clip angles as shown in attached file. Idea is to attach the web of c channels with bolts and then flanges with clip angles. What do u guys think? Would u consider it as fixed connection? Any related reference will be appreciated.

Thank You
Kaffy

 

[kaffy]

@Canwesteng please correct me if I am wrong:
So I wont be able to get a fixed connection for beam design but while doing the design of hanger and checking it for lateral stability, I can assume hanger is fixed (Not pinned)

 

[kaffy]

@phamENg Got it. Design the beam as if the ends are free to rotate (because they are), but design the connection with enough moment to maintain lateral stability of the sling frame. It's been a few years since I designed one of these, but I think ASME 17.1 has guidance on how to do that.

I understand. yeah I think I will have to do it. Actually we don't have any wind loads inside the building and frame is supported on both top and bottom on all 4 corners. So do I actually need to worry about lateral stability?

working of frame is shown at 0:26 second in attached video (Car frame is similar to cwt frame(as in video) from the way it is attached and moves)

https://www.youtube.com/watch?v=q-gIFp9EECU

 

[phamENG]

Of course you have to worry about lateral stability. You're designing a structure. Structures have to be stable. Just because you don't have a clearly defined lateral load doesn't mean the structure can just be pinned together and free to rotate everywhere.

 

[kaffy]

@rb currently I do not have any end plates. All I have is two clip angles on top and flange of hanger connected to web of beam. If I make my beam channels shorter, weld plate on end of c channels(all around weld on both c channels) and then this plate is bolted to hanger (on web) with bolts. Would u consider this as fixed connection)

 

[kaffy]

@BAretired, the last picture u attached is for a fixed connection?

 

[kaffy]

@phamENG I will keep that in mind. Thank you very much

 

[BAretired]

@BAretired, the last picture u attached is for a fixed connection?

My last sketch is my understanding of the description by human909. It is not fixed, but as human909 says, it approaches a fixed connection. It was in response to the comment by rb1957 which mentioned an end plate. I am aware than your proposal did not include an end plate. Hopefully, you realize that a fixed connection does not mean a fixed support.

A simpler fixed connection would be to simply weld each of the double channels to the channel hanger using a [-shaped weld. Again, not a fixed support; the most you can hope to gain is the minor axis bending strength of the hanger, which in turn is shared by two channels in major axis bending.

It might be helpful if you provided a line drawing showing the configuration of the structure, along with the loads.

BA

 

[BAretired]

A fillet weld as shown below should be sufficient to fully engage the hanger in bending.

BA

 

[human909]

My last sketch is my understanding of the description by human909.

For the record that wasn't what I was suggesting. But I can understand any confusion as pictures are worth 1000 words and I had none. Personally I have no idea why somebody is wanting to use channels in this way let alone provide a connection stiff enough to be considered a moment connection when you have such a weak bending behaviour of the hanging channels.

 

[IFRs]

The connection can be designed to transmit 100% of the moments and forces without rotation but if BOTH members are not stiff enough to appear as fixed to the other member then the members should not be modelled with fixed ends. Think of fixed as embedded in concrete - can not rotate. From the horizontal beam's perspective, is the vertical member stiff enough to resist all rotation in the direction of the moment? Not from what I see...

 

[human909]

Like I said in the parallel thread. Much of the confusion here is regarding the workflow and analysis means. I generally model all my steel structures using full frame analysis. If I want a connection to be fixed I model it as fixed and design the connection to suit. However unless the members it is attached to are extremely rigid the induced moment in the beam with fixed ends won't be the same as the induced moment in a beam with fixed ends connecting to a fully restrained point.

@Human Can u please create a rough sketch of the setup u mentioned. The reason, I want to make a fix connection is to reduce the weight. I understand that I will need a stiffer column (either bigger hanger or some other way to stiffen it), As we have a welding shop so if I can achieve it with just welding a plate or something and bolting it is not really increasing the cost.

No. Because I believe you are on a fools errand. You won't achieve weight reduction required. Right now your hanger seem grossly over designed for axial load. I your goal is to reduce weight the lighten you hangers and increase the depth of the beam. Unless there is alot you haven't told us you approaching this completely the wrong way.

If I make my beam channels shorter, weld plate on end of c channels(all around weld on both c channels) and then this plate is bolted to hanger (on web) with bolts. Would u consider this as fixed connection)

No because the web of the hanger is not stiff. You will get significant flexure in the thin web. And that is before we even look at the broader flexure of the hanger in the minor axis.

 

[BAretired]

@kaffy,

You can make a rigid connection by welding beam to hanger. Then use frame analysis to determine beam moments.

BA

 

[Tomfh]

I attach both the web and flanges of C channel. So why is it flexible ?

Because the connection is only partially rigid. The connection will slip. It is hard to make rigid connections that do not slip/rotate.

Also, the vertical members are much less stiff, so even with a rigid connection you won’t have a fully continuous beam.

Why the need to make it work? Have you committed to this size already?

 

[Settingsun]

I attach both the web and flanges of C channel. So why is it flexible ?

Those flange connections are pretty nominal. With one bolt per leg as drawn, the angle clips are notionally free to rotate so provide only whatever restraint is achieved through friction. Even if that were fixed, you have the flange of the beam and hanger plus the angle legs being pried in their weak directions. These are the sort of movements usually relied on to allow more-typical bolted connections to be considered pinned for design purposes.

 

[kaffy]

Thank you very much guys. I have not committed to anything on this project. It was just an existing design and I was looking for ways to reduce the weight but as per your valuable suggestions, this idea will not work.

Thank you very much
Newbie

 

[rb1957]

the little clips with only one fastener in them are good for just about nothing.

without them them the horizontal is reasonably a simply supported beam.

i've heard about some sling, how does that fit with your original sketch ?

another day in paradise, or is paradise one day closer ?