load path purlin bridging and floor joist blocking

[Enhineyero]

Floor joist and purlins are installed with blocking/bridging to act as lateral restraint, where then does the force on the blocking/bridging end up?

 

[human909]

What force?

Normally all you are after is twist restraint which isn't a 'force' it is providing sufficient torsional stiffness to prevent buckling. Sure the are some minor forces but in a normal system these will cancel out and resolve themselves as a group. In the case of blocking any lateral force will end up in the flooring.

The twist restraint is from member to member turns the local buckling problem of a member into a global buckling of multiple members and twisting of the twist restraints. All of which will take more energy (force) to instigate buckling.

Unless of course it doesn't....


EDIT:
I notice you are from Australia. I deal with AS4100. It is intently focussed on lateral restraint of the compression flange and doesn't at all address twist restraint of a member. The result is an easy recipe to follow that works for most structures but by ignoring twist restaint of the section in the absence of lateral restrain it kinda ignores the 'torsion' in lateral tortional buckling.

 

[Tomfh]

Floor joist and purlins are installed with blocking/bridging to act as lateral restraint, where then does the force on the blocking/bridging end up?

Into adjacent members and into connected diaphragms e.g. roof sheeting.

 

[Tomfh]

I deal with AS4100. It is intently focussed on lateral restraint of the compression flange and doesn't at all address twist restraint of a member

What do you mean by this? It talks a lot about twist restraints, with the presence of absence of twist restraint being a key factor in determining whether you have full, partial, lateral or no restraint at a given cross section.

 

[sdz]

I deal with AS4100. It is intently focussed on lateral restraint of the compression flange

It talks about restraint of the critical flange. For a cantilever this is actually the tension flange.
e.g. 5.3.2.2 (c) the lateral restraints act at the critical flange (see Clause 5.5),

 

[human909]

What do you mean by this? It talks a lot about twist restraints, with the presence of absence of twist restraint being a key factor in determining whether you have full, partial, lateral or no restraint at a given cross section.

Sorry I could have wording my statement better. Obviously it mentions twist restraint, but it REQUIRES lateral restraint in all cases.
In my reading and in my understanding of the way it is interpreted by others, is it requires some lateral restraint, twist restraint alone isn't sufficient in AS4100 be considered a restraint.

But feel free to continue the debate, I'm happy to be shown that I am incorrect.

 

[Tomfh]

but it REQUIRES lateral restraint in all cases.

Yeah fair enough. It’s odd how the code ignores pure twist restraint. What sort of beams is AS 4100 so concerned about that can laterally-torsionally buckle without twisting? And at the same time the code treats lateral restraints as capable of inhibiting buckling, end of story, even though in some circumstances beams can buckle through L-restraints, resulting in real buckling lengths longer than what AS4100 assumes.

 

[Enhineyero]

Into adjacent members and into connected diaphragms e.g. roof sheeting.

What if all adjacent members are working (whole floor is subjected to design load) and require a restraint as well? This restraining force has to go somewhere.

Is it just assumed it goes to the Floor? Some lateral restraints are not directly fixed to floor/roof (roof bridging, diagonal timber joist bridging).

 

[Tomfh]

What if all adjacent members are working (whole floor is subjected to design load) and require a restraint as well? This restraining force has to go somewhere.

You are right that these accumulated force has to go somewhere.

Human909 photo shows what can happens when the force has nowhere to go (red circle).

Normally it is a floor or roof diaphragm, or cladding, that stops this happening, or perhaps a grid of bracing. Anything stiff and strong enough to arrest the buckle.

Full depth blocking will work too, as adjacent beams stop each other from rolling over. Some codes will prohibit this, eg human909 comments above where lateral restraint is mandatory regardless of the degree of torsional restraint.

 

[human909]

Yeah fair enough. It’s odd how the code ignores pure twist restraint. What sort of beams is AS 4100 so concerned about that can laterally-torsionally buckle without twisting?

The beams I'm talking about are ones with no lateral load path to prevent LTB but with torsional restraint. This is often found in pairs of beams where for whatever reason a lateral load path can't be readily utilised or just isn't necessary.

Im on my phone so I can draw a diagram but the type of configuration I'm meaning is similar in layout to a ladder lying horizontally.. those regularly spaced rungs provide significant and meaningful twists restrain that prevent LTB of the stiles. If spaced regularly enough then the first buckling mode will be the entire ladder undergoing LTB which is likely an order of magnitude higher.

 

[Tomfh]

The beams I'm talking about are ones with no lateral load path to prevent LTB but with torsional restraint. This is often found in pairs of beams where for whatever reason a lateral load path can't be readily utilised or just isn't necessary.

Yes, however what I mean is, what are they worried about? According to AS4100 the ladder stiles are completely unbraced when the ladder is laid as a plank. Yet we know this is nonsense. The ladder works fine as a plank as far as lateral torsional buckling of the stiles is concerned, because each one is twist restrained by the other.