Pozi Joist deflection and cantilevers

[BartW]

43 minutes ago, PeterW said:

Yes that was my point - just lighten the cantilever area and leave the rest as Durisol. 

Great, thanks!

 

[Nickfromwales]

With regards to the pozi joists, has nobody ever doubled up on the joists ( sistered together ) to a) reduced hoist depth 

b) beef up the load capacity 

c) avoid steel / intermediate beams etc

 

?

 

Seems a very simple solution which would also further reduce deflection.  

[Gone West]

4 hours ago, Nickfromwales said:

With regards to the pozi joists, has nobody ever doubled up on the joists

Yes my joist plan had sistered  joists on both sides of the stair well.

[Oz07]

@Nickfromwales @PeterStarck there's special fixing plates you get which connect them

[Gus Potter]

It may be possible to "sister" the poszi joists on each side to take the weight of the cantilever. Maybe what you would do here is to use solid timbers at a good length spanning back into the main floor zone to get the back weight. They (solid timbers) are more torsionally stiff and you can brace them more easily to partly resist the torsion (twisting) effect that is prevelant in cantilevers. You can also take the shear out more easily and practically with a solid joist at the support position, as Peter alludes to.

 

To all, pozi joists etc don't perform that well as cantilevers as they are "prone to twisting" which significantly reduces their load bearing capacity. Sometimes you can just add a few solid timbers in to skirt around the problem while keeping the ethos of an "engineered timber design"

 

It takes a bit of extra thought as the "engineered joist" suppliers don't often have to much flexibility with their software, if it's a one off job then they will charge accordingly. Most SE's will just split the problem into two to recognise this cost implication and get on with an economic design.

Edited November 17, 2020 by Gus Potter
to add a bit

[BartW]

Another interesting advice that has been provided by one of the TF suppliers was that the I-beams have less deflection than pozi over the same span.

 

What are your thoughts guys?

 

[Oz07]

I can believe that

[Thorfun]

6 hours ago, BartW said:

Another interesting advice that has been provided by one of the TF suppliers was that the I-beams have less deflection than pozi over the same span.

 

What are your thoughts guys?

 

I had a timber frame company that refused to use posi-joists as they'd had a bad experience with deflection on them. it goes without saying that I didn't use them as I don't want to be drilling holes in I-beams to run cables, pipes and MVHR ducting!

 

I'm sure that if that TF company had just designed for a less deflection on the posi-joists it would've been fine.