Beam stuff

[Don P]

I was waiting on the missus at the docs today and brought a book to read. These were a couple of interesting pages... and yup, you know how you wake up with a book scattered on your lap and look around trying to figure out where you are, am I drooling, and who is laughing  :D. Anyway pay attention to fig 6-19 and the description of its' design.



 

[Jim_Rogers]

Thanks for sharing.

Jim Rogers

[Tom King]

I will try to stay away from statically indeterminate beams.

[kevin5055]

If you're interested in a real world application: steel bridges used to sometimes have "hung spans" using pin and hangers or just putting bearings on the cantilevered portion. These aren't great details for steel bridges and they aren't used anymore (corrosion concerns due to the joints in the deck above these details as well as fatigue concerns). You'll still see them every once in a while driving around; they just need extra care/inspection to remain safe.

https://en.wikipedia.org/wiki/Pin_and_hanger_assembly

https://en.wikipedia.org/wiki/Mianus_River_Bridge

[mike_belben]

i read a lot of technical stuff at night to fall asleep.  3rd time the book or device slaps me in the face is just right. 

[Don P]

It was 2 hours each way to and from Roanoke yesterday, I took lighter reading  :D. 

I've seen 6-19 done in glulam beam roof systems as well. The connector for that is usually a Simpson (strongtie.com), search their for their "HC" line of hinge connectors. 

Here's what I was seeing as I read that. A beam scarfed in span is something between 6-18 and 6-19. It is certainly not continuous (6-18) but does have some moment capacity as compared to being hinged (6-19) It doesn't design the joint but if the more conservative answer from both methods is used I believe it is a fair way to size the beams themselves. I have not run any comparisons to see how much difference it makes, that will be interesting.

From the steel manual I scanned a  "cheat sheet" of coefficients for continuous beams, there is another page of design properties for cantilever multi span beams... but lets see if this is legible, I have to put on my specs to read the book.



 

Undeterred, I apparently consider it my job to hand an engineer the most indeterminable problem possible :D.

[kevin5055]

From the steel manual I scanned a  "cheat sheet" of coefficients for continuous beams, there is another page of design properties for cantilever multi span beams... but lets see if this is legible, I have to put on my specs to read the book.

I was going to point this out so I'm glad you did.  You may want to check out the method of superposition since you're going down the rabbit hole.  It basically allows you to add diagrams together to get the combined response.

This article below may also be worth a read and appears to provide some insight on the moment capacity of a scarf joint.

Historical scarf and splice carpentry joints: state of the art | Heritage Science | Full Text

[Don P]

Thanks! That link sent me down a rabbit hole of its own  :D.
I haven't digested it yet. I saw 50% strength in a modern lapped scarf and ~25% bending strength in carpentry joints? I don't think i saw shear numbers. I'd rather not chase complexity, what would be great is some safe derate co-efficient to apply to the moment and shear in that scarf area or to the beam... carpenter simple, if that's possible.

[Don P]

I'll bring this back up this morning for more background. Click on that link Kevin posted just above.

[jpassardi]

If you're interested in a real world application: steel bridges used to sometimes have "hung spans" using pin and hangers or just putting bearings on the cantilevered portion. These aren't great details for steel bridges and they aren't used anymore (corrosion concerns due to the joints in the deck above these details as well as fatigue concerns). You'll still see them every once in a while driving around; they just need extra care/inspection to remain safe.

https://en.wikipedia.org/wiki/Pin_and_hanger_assembly

https://en.wikipedia.org/wiki/Mianus_River_Bridge

I remember my transportation professor (former CT DOT engineer) explaining that the pin and hanger was the culprit for the Mianus bridge collapse.
Apparently the hanger slid off the pin and let a portion of the deck fall.
They retrofitted "catching stops" all pin and hangers on existing bridges after that.

[Don P]

We were leaving a job one evening and DOT was at the old steel truss bridge barricading it. We got out and were asking what was going on. One of the pins at the other end had failed. The bridge had been twisting and hanging by a thread unbeknownst to us, our concrete trucks and pumpers, and the dairy up the road that was expanding and doing at least the same on the bridge...
Between probable overload and time we don't know who did the deed but we had sure worn out some karma. From standing at the broken bridge and looking towards home we turned around, down into Carolina and took the long way home.

[Don P]

This is the next paper the author mentioned in the paper Kevin linked above;
Materials | Free Full-Text | Experimental Investigations of Timber Beams with Stop-Splayed Scarf Carpentry Joints (mdpi.com)

Its the same author but the second time I've seen this possible rule of thumb.

"In the case of vertical loading, an element with a splice joint formed on a horizontal surface can transfer the bending moment up to a quarter of the moment transferred in a continuous beam, whereas the same joint formed on a vertical surface, with the same load, can transfer a moment of half the value of the moment transferred in the continuous beam."

I'll add, moisture content lab controlled at 12%, machined in shop conditions and tested shortly after assembly, be conservative.

[DangleSnipe267]

Interesting read but it is a little disappointing that no tests were performed without the addition of supplemental steel clamps, bolts, or screws. I was hoping to find more information from the source material of each article but I found that most were locked behind paywall.