Shear flow connector spacing

[WARose]

I am working on connecting a channel to some existing boards with bolts. I am trying to minimize the spacing of the bolts because there are a lot of things to miss. This means using the shear at various points along the composite beam rather than the max. (at the supports).

Is there any downside to this? I was brainstorming earlier and couldn't think of any. Was wondering if there were any serviceability concerns (or anything else).

 

[Struct123ure]

Depends on the size of the wood member, I had a similar design to do with an existing old church having to be reinforced by 2 sandwiched channels.
The Modulus of elasticity of steel is 200GPa compared to wood which is 7GPa and I found that relying on old wood (which I don't know the properties of, or how much rot, mold damage it has sustained) was not really worth the mental gymnastics. The steel is so much stiffer that it will attract all the load, so I designed the channels for all the load. 2 channels back-to-back sandwiching a wood member with through bolts.
I had to rework the supports so that they can also support the channels.

As for using shear at various points, I don't see an issue. Be careful if its a continuous beam in which case you should look at all the pattern loading to get worst shear at the various points.

 

[canwesteng]

That's often how I design composite beams, although with welds not bolts. If you want fewer bolts, I would terminate the shear connectors at the point you no longer need moment reinforcement, and then make sure you provide anchorage for MQ/I with the bolts past this point.

 

[KootK]

First off, I'm generally not a fan of trying to make wood and steel behave compositely with bolt-like fasteners alone. End annoying disclaimer.

The only risk that I see in dialing in the fastener spacing to match the shear demand is that there may be cases where partial loading situations generate a shear demand locally that exceeds this value. I'd be tempted to do as the steel joist folks do and design for 0.25 Vmax everywhere as the minimum. No doubt you already plan to do more than this anyhow.

 

[WARose]

The only risk that I see in dialing in the fastener spacing to match the shear demand is that there may be cases where partial loading situations generate a shear demand locally that exceeds this value. I'd be tempted to do as the steel joist folks do and design for 0.25 Vmax everywhere as the minimum. No doubt you already plan to do more than this anyhow.

I do. It's just that: once you get close to the support....the connector spacing starts getting pretty tight.....and I just didn't want to do that everywhere.

 

[Agent666]

From the description, I'm assuming you're bolting timber to the back of the web (or bolting a channel to some existing timber in an effort to strengthen something) and trying to attain some composite longitudinal force transfer to create the composite action. If not a picture would help.

Remember to account for the bolt deformation in the timber under loading and the likelihood of slack due to oversized holes in the timber and steel, also allow for longer-term creep in the timber members (as this will shed load from the timber to the steel). In summary best to discount any composite action due to shear flow as all these things are working against you and the mechanics of it mean it doesn't really work as you're perhaps assuming in practice. i.e. refer KootK's annoying disclaimer about trying to make wood and steel behave compositely by utilising longitudinal shear mechanisms.

Just rely on transformed stiffness and share loading accordingly, make sure you check the timber doesn't fail in flexure or shear before the steel gets anywhere near yield, if it does determine at that point where the timber fails and work out the load in the steel. Also consider in sizing the fixings the transfer of load between materials, like is the load introduced to the steel only, both timber and steel in different proportions for example. Due to the issues with fixings noted above the fixings only really distribute the external loading between plies in my opinion in the absence of any meaningful longitudinal shear being able to be developed.

Also if this is a strengthening scenario, you need to also account for the forces locked into the timber before attaching the steel. Most flitch beams are designed this way (this has been discussed in quite a few other threads, so a search may prove helpful). Alternatively, designing the steel to carry all the load may be a scenario to look at.

https://engineervsheep.com

 

[dik]

If running two or more 'rows' of fasteners, be careful about lumber dimensional issues. Restraining the edges may 'force' a longitudinal crack.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik