Wood Floor/Roof Joist cross-bridging

[StrEng007]

What is the mechanism or load path by which floor/roof cross bridging for dimensional lumber joists/rafters helps share the load? I have never been able to figure this out.

I understand the code requires some form of bridging in order to hold the members in a vertical position and prevent rotation. But how does the load sharing work?

Also, for uplift conditions, is cross-bridging sufficient to reduce the unsupported lengths (bending; beam stability factor)... and if so, what is the force requirement in the connection to ensure it works?
Does solid bridging work the same way for gravity conditions (i.e. if hypothetically you had solid bridging at 12" o.c., would this be the same as wood structural panel sheathing with nails at 12" o.c.)?

 

[phamENG]

If you have 3/4" floor sheathing, most of your load sharing will probably happen there. But the bridging shown there acts like little struts. Load one joist, and it deflects. As it deflects, half of the X goes into tension and the other in compression. They react against the adjacent joist, loading it.

Yes. That's good for beam stability. Doesn't take much. I don't know of any resource that specifically calls out a required force for wood design like we have in the steel manual. Maybe do 0.01*(M/0.67d)?

I prefer solid blocking. Less likely to cause floor squeaks, faster and easier to put in.

 

[lexpatrie]

To me the bridging and sheathing are more to allow the repetitive member stress increase than to provide any reliable load transfer. If you really want to provide load transfer, say for point loads, that would need a strongback style connection and analysis that won't work with normal joists (trusses only).

It also affects effective length so it prevents C[sub]L[/sub] from meaningfully influencing the design.

 

[BR0]

To me the bridging and sheathing are more to allow the repetitive member stress increase than to provide any reliable load transfer.

I thought the repetitive member increase was due to load sharing.

 

[Greenalleycat]

I'm not sure exactly how your code works, but a number of factors come into play

1/ We typically design with 5% values (95% of timber is stiffer) so when you have multiple members being engaged a higher value is taken to reflect the very low probability of all of them being 5% timber

2/ The floor itself will have significant load distribution capabilities to share footfall loads between adjacent joists

3/ Here the convention is for solid full depth blocking (not the herringbone look you've got there) so the blocking is a large wedge that connects adjacent members - practically, you can't deflect the middle joist without trying to engage the joists either side too. Hard to quantify, but easy enough to conceptualise.

4/ Blocking helps with twist restraint which I think provides hidden bang-for-bang at prevent additional movements that would be perceptible to occupants, even if we can't quantify that benefit

I expect herringbone blocking would be less effective at both 3/ and 4/ than full depth solid blocking

I spoke to one architectural builder who said he likes to put blocking in at tighter crs than Code as he firmly believes it gives better floors (= less chance of call back)

 

[RontheRedneck]

I never thought it for load sharing - More to prevent vibration.

 

[Greenalleycat]

I assume this is one of those cases of reverse engineering something that has been found to cause better results
So each engineer may have their own interpretation of WHY it works - but at the end of the day, the most important thing is that we follow the rules of thumb that have been provide to provide better floors

 

[XR250]

Old school framers would nail the top of the struts during framing and then nail the bottoms after the house was finished. Supposedly prevents squeaking. I have been in countless older houses where they never nailed the bottoms and they are just flapping in the breeze.