Stress flow (vertical connection) between beam and a plate

[n3jc]

Hi, Im a new member. I have been lurking around here for a while so I decided to join. I hope you can help me.
Id like to calculate shear stress flow between a wood beam and a steel plate (strenghtened wood beam). In order to get one element from two, there must be sufficient connection between this two elements. Steel plate is connected from the side (shear stress flow in vertical plane).I have to calculate stress flow first. How can I do that?

 

[Archie264]

Do a quick search on Flitch plate beams and you'll get a lot of information. Here's one article on the topic: Link

Speaking generally, I wouldn't normally use an asymmetric one like you've shown, though; I'd want the steel sandwiched between two layers of wood. If access or conditions don't allow such then perhaps you could get away with unbalanced situation but you'd have to look at it more carefully, considering torsion, shear center, etc.

 

[JAE]

There's really no stress flow between them. If they are connected together they will each deflect the same amount and the load into each will be in direct proportion to their relative stiffnesses.

The only caveat to that is that over time, the wood will creep a bit under sustained loading and as a result the steel will pick up a bit more load. So to account for that you can use the long term deflection of the wood portion to design the steel and use the immediate deflection of the wood to design the wood...sort of a bracketed design.



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[racookpe1978]

Well, the fasteners on the wood side will immediately begin creeping into the wood, then continue over time so the link between and steel and wood will tend to disappear entirely over time - unless regularly and accurately re-torqued. And that is not going to happen in the real world.

Glue (epoxy at a minimum) the two surfaces together to try to minimize the wood-steel slip. Better is to clamp with steel on both sides as mentioned above.

 

[n3jc]

So if I understand correctly, there is no stress flow between them because there is a VERTICAL plane between them. If there was HORIZONTAL plane (steel plate on wood beam) there would be shear stress flow when elements connected. But because we have vertical connection there is no such thing?

I still need to connect them properly... but how to choose bolts - diameter, spacing between them...? im little confused.

 

[KootK]

there is no stress flow between them because there is a VERTICAL plane between them.

I would say that there is no VQ/It stress between them because their centroids align. Vertical/horizontal isn't pertinent.

Based on the sketch that you provided, I'm guessing that the load is delivered to the wood member and not the steel plate. As such, you would need to transfer a portion of the load from the wood to the steel using the vertical shear transfer capacity of your bolts. That portion would be based on the relative stiffness of the two members. As Archie intimated, with an asymmetrical design, you may have torsional issued to ponder. Particularly if your steel plate doesn't make it to a vertical load bearing support element.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jec67]

One approach is to calculate the maximum stress in the plate (assuming the assembly is uniformly loaded for this discussion). You can then determine the moment in the steel at this point (Moment x section modulus). Next you can calculate the uniform load transferred to the steel beam (Moment x 8/(length^2). This load is what needs to be transferred through the bolts.

Assumptions

1. uniformly distributed load
2. Total load is applied to the wood member that then has to transfer part of the load to the steel member

For other loading situations, such as a point load, for example), the approach is the same. The correct moment equation needs to be used in calculating the load to be transferred to the beam. It is worth noting for the concentrated load case, the majority of the load will be transferred only by the bolts in the vicinity of the load.