welded flush type girts end support type?

[hamy08]

What are the end support types of flush type girts or purlins if they are welded to the columns.

In my view it should be fixed type. Moment also act at the end in all three direction.

Correct me if i am wrong?

 

[hokie66]

I don't think cold formed girts should be welded, to columns or beams, so won't comment further on your question.

 

[hamy08]

hokie: yes they can be welded if hot rolled channels are used as purlins

 

[hokie66]

Yes, they can, advisable or not. I understood you to be talking about cold formed girt and purlin sections, but apparently I misunderstood. But as to your end restraints, that is up to you to determine your design assumptions.

 

[hamy08]

The end supports are not design assumptions. They are taken according to the connection type. Connection type determines whether it should be pinned or fixed.

 

[hokie66]

No. You are trying to be like a computer programmer, not like an engineer.

 

[CANPRO]

In my experience, girt connections are considered pinned 100% of the time. Maybe a sketch showing what a "welded flush type girts" looks like would be helpful.

The end supports are design assumptions - and then you either need to design the connection to match your assumption, or communicate the connection requirements to the fabricator (if connection design is delegated). I'm not really understanding your reasoning in your last post - are you evaluating an existing connection?

 

[KootK]

Consider that, if you weld the connection every which way, it's still not fixed since you've welded to what is likely an I-profile section with very little torsional stiffness. The advice given to you above is sound. Moreover, I struggle to envision a situation where welded connections make sense for this, even if it is a hot rolled section. Those are field welds made at height and the rotation restraint unintentionally generated at them may cause stress concentrations in the welds.

 

[Ron247]

They can be welded and not be considered fixed. Even cold-formed can be welded, they do it all the time. As far as cold-formed girts or purlins, I have never seen any that were fixed-ended and I have been around metal buildings a really long time.

You had several posts about a shed-type building you are designing and most of them relate back to a computer program. Do you have formal training in designing structures or are you relying on the computer program? I am not intending to sound insulting but due to what appears to be a reliance on a computer program, you need to proceed with caution.

 

[hamy08]

hookie:
Engineers do assumptions but where facts are present should utilize them.

 

[Ron247]

Hamy, If your last post had to do with CANPRO's comment about "end supports are design assumptions", you appear to not understand the statement. In design, we establish some structural system that is composed of members and their connections. Once we do that, we apply the design loads and perform an analysis. If we analyze the purlins as pinned (an assumption), then we must design a pinned connection in the final product. If we analyze as pinned and provide a fixed connection, our analysis is incorrect when compared to the final product.

Also, designing all these assumed connections may be harder than designing the structural components in your computer model. You will find that making a truly pinned or truly fixed connection is almost impossible. You can get close, but not exact.

 

[hamy08]

CANPRO: I am evaluation an existing structure.

I have attached a simple sketch just to show how purlins are attached to the side columns. The purlins are welded to the side columns.

SO in this case what should be the end conditions of purlins fixed-fixed or pinned-pinned.

Note: Please ignore the bad sketching i made it quickly.

 

[hamy08]

Ron:

The designing of purlins is also important in terms of end connections because it has an effect on effective length. Which is required for the calculation of slenderness ratio. If purlins are acting like struts than we have to consider slenderness ratio. The value of Lambda is based on end connections. Which is used in formula of effective length ( L X Lambda)

 

[jayrod12]

Are the flanges of the girt welded? or just the web?

 

[hamy08]

jayrod : also the web with all around weld

 

[BridgeSmith]

New design or existing structure, no connection is ever fully fixed, and rarely can a welded connection be considered as pinned. They are always somewhere in between. A detailed finite element model of the connection configuration can get fairly close to the actual amount of rotational restraint. Outside of that, however, I find it prudent to analyze the area of the connections using the assumption that they are fixed, and analyze the rest of the structure using a pinned-end assumption. It's commonly referred to as enveloping the analysis - getting the worst-case stresses from the two extremes to be sure that the actual behavior in between is captured by the analysis, and the structure is safe for the actual loading.

 

[KootK]

Your terminology is likely causing some confusion here. What you're calling "purlins", many engineers would call "girts". Or perhaps moment frame beams. If it's the latter, then it makes sense that you're having slenderness issues. Such beams draw=meaningful axial loads. And channels aren't a great section choice without additional bracing along the span.

 

[Ron247]

If the flanges and web are welded all around, you are closer to a fixed connection than a pinned, provided the member (or element of a member) it is attached to it capable of reasonably resisting the end-moments. For example, if you weld the devil out of it to a "very thin web", it may still not be fixed. In essence, it can be like bolting a steel beam to a marshmallow.

Your recent sketch appears to show the Z-shape turned so it looks like an upright "Z" if you cut a section through it rather than a Z rotated 90 degrees. Which is the correct orientation? If it is upright, then I am wondering who designed this existing structure you are analyzing. You have so many things in this structure that are unconventional it is hard to believe the original designer did much thought. The columns appear to be rotated 90 degrees from the more common orientation. The baseplate of the columns appear closer to fixed than pinned due to the anchor bolt locations in your sketch.

Take some pictures of this existing structure, they would be more helpful than sketches.

 

[hamy08]

Ron: its just a sketch showing the connection of purlins, how they are connected to columns.

Yes you are right columns are rotated to 90 from conventional situation.

The columns are fixed with base plate to the anchors.