best purlin connection: over trusses or in the field?

[lolobau]

Hi

why do people always want to connect / extend roof purlins over trusses?
Is it not more efficient to connect them rather in the middle of the span?
Your deflection would be less and your cut offs would be reduced

see attached sketch

Lolobau

 

[dcarr82775]

It is cheaper to splice them over the frames. I do not know the exact reasons, but Metal Building guys do things as cheap as they can. It is one of the few sure things in life such as death and taxes.

 

[jayrod12]

It's also much easier. you don't have as many conflicts because the purlin can extend as far beyond the truss as required without requiring cutting.

If you save them trimming each purlin they're happy.

 

[lolobau]

I somehow don't agree that this is cheaper because you will have so many cut off from purlins which are actually passing the frame and then you cut them back to the frame instead of just connecting it there where they end

 

[jayrod12]

Why cut the ends off if they aren't long enough to do anything? don't they just let them extend past? saves a bunch of cuts.

 

[KootK]

Perhaps it's unpleasant for installers to position themselves out at midspan to make the connection.

Are you proposing a moment connection at midspan?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

I actually think KootK nailed that one on the head. No one is going to suspend themselves to make that connection. And using a lift for each connection is also too costly.

 

[BAretired]

People do not always do it that way. Sometimes, they lap the purlins at the supports and develop a moment connection by bolting.

BA

 

[Ingenuity]

What BAretired said. Use Z purlins and over-lap at support (trusses or portal beams) with bolted splices.

 

[lolobau]

@KootK: you might have a point, the fixing procedure would be more tricky

by long buildings the standard length of the purlins hardly lines up with the trusses so they always need to be trimmed back

why moment connection in the midspan? what about the theory GERBER BEAM ?

 

[jgailla]

I did steelworking for about a year. Think about how it's being put together.
You have a guy sitting on a beam bolting the purlins together. Where are you going to sit to bolt them together in between the beams?

 

[KootK]

why moment connection in the midspan? what about the theory GERBER BEAM ?

I was just curious because the detail that you provided looked as though it could be a moment connection if desired and the deflected shape that you drew in your sketch suggested continuity across the mid-span splice. I like the Gerber system as much as the next engineer but that's not what you drew in your original sketch. Is Gerber something that you're actually proposing here? I can't imagine why one would want two mid-span splices in a purlin unless these purlins are a lot longer than I'm envisioning.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

If the purlins were ever getting to that length I would expect them to be better suited as Steel joists.

 

[lolobau]

@jgailla: You have a point. Thisis what KootK also said. i think it is just not practical in terms of mounting, unless you have a scaffolding under the roof structure and easily accessable everywhere

Deflection and force wise I still belive that the "Gerber" solution for purlins also efficent would be

@KootK: It is such an mix from Gerber system and present purlin length on site. Tomorrow i will upload some photos from wronk from the real structure. Then you guys will see what I am refering to. I hope there are some good photos from the site

Thnaks so far for the valuable tipps ;-)

Lolobau

 

[ajh1]

Z-purlins lapped over the supports create a continuous beam. Any joint in the purlins that is not lapped in some fashion is going to create a pinned condition, regardless of whether it is at a support or somewhere out in the span. A continuous beam obviously reduces the maximum moments present by shifting to a positive and negative moment situation rather than all positive or all negative depending on load direction, thus reducing the size requirements. I'm not sure what type of purlins were envisioned at the start of this thread, but a Z-purlin is typically rolled to exactly the length that is necessary to make it fit the structure and design. There is no waste, scrap, or field cutting typically. Lap lengths are generally set as no less than 1 1/2 times the depth of the purlin on either side of the support up to potentially 4 or 5 feet beyond the support. A general rule of thumb would be somewhere in the range of 10% of the span between any given set of trusses. The negative moment region at the support will generally have an extremely sharp spike in the moment as compared to the relatively flat curve of the moment diagram near the center of the span. The lapped section is set to a length to balance the strength of a lapped section with the peak moment versus the midspan moments with a single section. Z-purlins are generally lapped at every support, since an unlapped support creates sizing problems with the peak negative moment region at the support.