Design of Joist Extension(s)

[dwf123]

Hi All
I am trying to design a mechanical connector arrangement to extend a series of roofing joist in length by about 500mm. A structural wall is being moved (by the same 500mm) and rather than replacing all the joist I want to extend the existing by this 500mm.

The plan is to use toothed plate connectors with bolts with a identical size timber as the original joists used to form the extension pieces.

I can easily spec the TPCs to deal with shear, but struggling a bit to understand how to deal with the BM in the beam. One TPC alone can do the work that I need to it to, but surely this is also placing a rotational force onto the connector which it is not designed for?

Any help would be greatly appreciated.
D

 

[dik]

In addition to the shear at the non extended end, the flexural moment may increase enough for failure. It's not just a matter of the attachment of the extension.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[kissymoose]

500mm isn't too bad, but providing bending moment connections in sawn lumber is not a good idea. Any sort of dowel connection will compress the wood slightly at the edges under load, making the joint loose. Definitely not a good idea for members under constant loading and unloading conditions. And any glue that you'll use will creep over time. If you absolutely have to (as a last resort), go with lots of smaller fasteners (and AFG-01 glue, which still helps) instead of a few bolts. In your case, the toothed plate is not going to cut it. Sister the rafters with new members that lap the existing by a certain amount. And get a structural engineer to design it for you.

 

[kipfoot]

A sketch would be helpful because, if I'm interpreting what you're describing correctly, the extension alone won't work.

I think you'll need a 'sister' piece that I expect will need to be nearly full length of the span, with close attention paid to the fasteners between the new and original.