Friction reducing deflection of lower beam

[hoshang]

Hi all

https://www.eng-tips.com/viewthread.cfm?qid=379229#post

In this picture

https://res.cloudinary.com/engtips/...ostimg.org/z7k3miifr/Two_Beam_Shared_Load.jpg

Can friction in the place where the upper beam remain in contact with the lower beam reduce the deflection of lower beam?
I mean if we imagine a slow motion or animation of the deflection of the lower beam, gradually the lower beam will not remain in contact with the upper beam in most places. So, I think the only place where the upper beam can reduce the deflection of the lower beam by friction will be the place where the beams remain in contact at the ends of the upper beam. Isn't it?

 

[r13]

If the friction is equal to, or greater than the shear flow at the inter face. Then that's a beam with varies thickness, thus the deflection of the lower beam would be reduced to that without the upper beam (note, it may or may not, all depends on the added weight vs added section property). If composite action is not there, then no.

 

[hoshang]

Hi retired13
You know shear flow is when there's fastener or glue in contact surface between the two beams acting as composite beam. But in our case there isn't fastener or glue in contact surface between the two beams.

 

[jayrod12]

Hoshang,

retired is actually correct. Because until the shear flow stress is higher than the friction between the plies, friction is acting as the "glue".

 

[r13]

So you got my answer.

 

[hoshang]

Hi retired13 and jayrod12
Thanks.
I think I got the point.

 

[rb1957]

I wonder. the response to the OP's question was a long way of saying "in your example, yes". In the example given I see very little friction action, the "rigid" white board is distributing the load to two points on the deflecting beam. deflections of this set-up are smaller than without the white board, but not due to friction.

another day in paradise, or is paradise one day closer ?

 

[Blackstar123]

In the example given I see very little friction action, the "rigid" white board is distributing the load to two points on the deflecting beam. deflections of this set-up are smaller than without the white board, but not due to friction.

I agree. That's what I was thinking.

 

[r13]

rb and Blackstar,

My interpretation of the question was, if the weight is gradually increased, shear friction will be present as the contact areas increase, and then will the shear friction reduce the deflection. For short, the answer is "yes and no".