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

 

[skeletron]

Fixed vs. Pinned is all about the rotation of the joint. So while a connection with every surface connected is intuitively "more fixed" than just a simple connection that allows rotation, the classification is unknown unless you look at the rotation-moment curve. The designer should also consider the stiffness of the supporting member...

All that being said, my intuition suggests that you can never "fix" a channel and it's nearly always a "pin" connection. This stems from channels have very little flange material, always being Class 3 (Sx), and more or less being used as lightly-loaded members. Without running any numbers to back my opinion up, I would classify this as a "pin".

 

[rowingengineer]

I would have to idea statica that connection, you can get a 2 week trial. A bean in strong axis connected to a column in weak isn't something that is really covered by standard details.

I also hope you are doing an analysis that takes into account stiffness of the weak column for you moment transfer and not just assuming fixed end moment transfer.

 

[phamENG]

Yeah...partially restrained at best. If you make all of those bolts slip critical, you can work out a "moment resistance" from the couple. But good luck with that - channels have sloped flanges and you'd have to investigate those inclined slip surfaces pretty closely. Then there's all sorts of places for displacement within the connection that would reduce the stiffness.

How critical is the application? You may need to hire a connection engineer with access to a FEM or CBFEM software package to investigate the actual joint stiffness. Or you could approximate it by hand, but that'll probably take a while.

 

[BAretired]

The hangers appear to be channels of approximately the same size as the double channel beams, so the limiting negative bending moment for the double channels is the minor axis bending capacity of one channel. The connection is, to all intents and purposes, a pin.

BA

 

[Tomfh]

It’s a flexible connection to a flexible member - PINNED

 

[kaffy]

Thank you very much guys. Any idea about how can I make it a fixed connection? Vertical member will be designed accordingly but at this point, I am interested in beam only..

 

[phamENG]

What are you using it for? What are the loads?

If this thing is 3 feet tall and being used as a rack to store long wooden dunnage or something similar, what you have drawn would probably work. If it's a moment frame to support a 3 story building, no chance.

 

[kaffy]

I am using it for a frame in elevator sling. There will be two flat plates welded on the bottom flange of C channels (Symmetrically placed at L/4 from the end) and Both plates will experience two reaction loads loads (6000lb)

Something similar to this (C channels are under the plywood in attached link) :

https://www.elevatorequipment.com/sling-and-platform

 

[human909]

Thank you very much guys. Any idea about how can I make it a fixed connection? Vertical member will be designed accordingly but at this point, I am interested in beam only..

A thick endplate with a suitable number and tensioned bolt setup could approach a fixed connection.

But really why are you bothering? The connection is more expensive and you still need an extremely rigid beam to hang from otherwise you aren't getting much benefit.

The easier solution is a different beam. Channels can be made with fixed connections but it rarely makes sense to do so.

 

[canwesteng]

I'm with BA - that connection is fine to count as fixed for lateral stability purposes, but no connection will allow you to design this beam as if it has fixed ends. The weak axis of the channel hangers are neither stiff enough nor strong enough for that.

 

[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.

 

[kaffy]

Thank you very much guys....

 

[rb1957]

but how does the sling attach to the endplates? but a single pin ?? then the sling is pinned, and puts a shear load onto the endplates.

if the line of action of this sling load is through the center of the weld group, then no moment.
if signifacntly offset then you need to include the offset moment in the weld loads.

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

 

[BAretired]

rb1957

BA

 

[kaffy]

@skeletron Do u have any reference/reading material about moment rotation curves? Also where can I find the rotation moment curve for different joint such as in this case?

 

[kaffy]

@Rowingengineer Thank you very much. I will look into the trial of Idea statica. Do they good tutorial series for beginners?

 

[kaffy]

@BAretired The hangers appear to be channels of approximately the same size as the double channel beams, so the limiting negative bending moment for the double channels is the minor axis bending capacity of one channel. The connection is, to all intents and purposes, a pin.

Yes the hanger is same size as the double channel beams. Under the effect of load, the hanger will have a negative bending moment and it will be on minor axis. But how do u figure it is pin based on that? Or u would prefer it to be pin?

 

[kaffy]

@Tomfh Sorry I didn't understand. I attach both the web and flanges of C channel. So why is it flexible ?

 

[kaffy]

@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.