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tooma SEFI Member
Joined: 27 Aug 2017 Posts: 6

Posted: Sun Aug 27, 2017 3:09 pm Post subject: nails and glue in pure bending 


If a composite beam consists of layers stacked on each but not connected to transfer shear between the layers was subjected to pure bending, will there be sliding between layers?
We know that when there is only pure bending, no shear exists, does that eliminate the need to glue or nail the layers together? Please explain with details. 

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Dr. N. Subramanian General Sponsor
Joined: 21 Feb 2008 Posts: 5228 Location: Gaithersburg, MD, U.S.A.

Posted: Sun Aug 27, 2017 6:35 pm Post subject: Re: nails and glue in pure bending 


Assuming only udl and s.s. beam, the beam will not have SF only at the center. Near the supports you will have. Max SF. Hence there will definitely be sliding between the layers. To prevent it there should be some connection between the layers.
Best wishes
NS
tooma wrote:  If a composite beam consists of layers stacked on each but not connected to transfer shear between the layers was subjected to pure bending, will there be sliding between layers?
We know that when there is only pure bending, no shear exists, does that eliminate the need to glue or nail the layers together? Please explain with details. 


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tooma SEFI Member
Joined: 27 Aug 2017 Posts: 6

Posted: Sun Aug 27, 2017 8:05 pm Post subject: Re: nails and glue in pure bending 


Dr. N. Subramanian wrote:  Assuming only udl and s.s. beam, the beam will not have SF only at the center. Near the supports you will have. Max SF. Hence there will definitely be sliding between the layers. To prevent it there should be some connection between the layers.
Best wishes
NS
tooma wrote:  If a composite beam consists of layers stacked on each but not connected to transfer shear between the layers was subjected to pure bending, will there be sliding between layers?
We know that when there is only pure bending, no shear exists, does that eliminate the need to glue or nail the layers together? Please explain with details. 

Dr. N. Subramanian,
Thank you for the answer. The beam is not subjected Uniformly Distributed Load. It is subjected to Pure Bending. 

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Dr. N. Subramanian General Sponsor
Joined: 21 Feb 2008 Posts: 5228 Location: Gaithersburg, MD, U.S.A.

Posted: Mon Aug 28, 2017 1:29 am Post subject: Re: nails and glue in pure bending 


Even if it is subjected to pure bending, it has to deflect and this deflection will make the sliding possible!
Best wishes
NS
tooma wrote:  Dr. N. Subramanian wrote:  Assuming only udl and s.s. beam, the beam will not have SF only at the center. Near the supports you will have. Max SF. Hence there will definitely be sliding between the layers. To prevent it there should be some connection between the layers.
Best wishes
NS
tooma wrote:  If a composite beam consists of layers stacked on each but not connected to transfer shear between the layers was subjected to pure bending, will there be sliding between layers?
We know that when there is only pure bending, no shear exists, does that eliminate the need to glue or nail the layers together? Please explain with details. 

Dr. N. Subramanian,
Thank you for the answer. The beam is not subjected Uniformly Distributed Load. It is subjected to Pure Bending. 


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tooma SEFI Member
Joined: 27 Aug 2017 Posts: 6

Posted: Mon Aug 28, 2017 10:38 am Post subject: Re: nails and glue in pure bending 


If sliding is possible then shear forces exist! For me this is not comply with the fact that shear forces are zero for pure bending loading. It is confusing to me.
Dr. N. Subramanian wrote:  Even if it is subjected to pure bending, it has to deflect and this deflection will make the sliding possible!
Best wishes
NS
tooma wrote:  Dr. N. Subramanian wrote:  Assuming only udl and s.s. beam, the beam will not have SF only at the center. Near the supports you will have. Max SF. Hence there will definitely be sliding between the layers. To prevent it there should be some connection between the layers.
Best wishes
NS
tooma wrote:  If a composite beam consists of layers stacked on each but not connected to transfer shear between the layers was subjected to pure bending, will there be sliding between layers?
We know that when there is only pure bending, no shear exists, does that eliminate the need to glue or nail the layers together? Please explain with details. 

Dr. N. Subramanian,
Thank you for the answer. The beam is not subjected Uniformly Distributed Load. It is subjected to Pure Bending. 



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sakumar79 ...
Joined: 18 Apr 2008 Posts: 649

Posted: Mon Aug 28, 2017 11:45 am Post subject: 


Dear Engineer,
Please explain how you can have pure bending without shear in real life when at least selfweight has to be taken into account? I am assuming you are talking about real life problems and not theoretical problems since you are looking at nailing and glues.
Yours sincerely
Arunkumar 

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tooma SEFI Member
Joined: 27 Aug 2017 Posts: 6

Posted: Mon Aug 28, 2017 2:13 pm Post subject: 


I want to discuss the issue theoretically.
But for real world problems, I can give an example is when a beam subjected to four point loads (The sample is placed on two supporting pins a set distance apart and two loading pins placed at an equal distance around the center), neglecting the selfweight of the beam, the middle portion of the beam between the two loading pins will be subjected to pure bending.
sakumar79 wrote:  Dear Engineer,
Please explain how you can have pure bending without shear in real life when at least selfweight has to be taken into account? I am assuming you are talking about real life problems and not theoretical problems since you are looking at nailing and glues.
Yours sincerely
Arunkumar 


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abhio ...
Joined: 08 Mar 2010 Posts: 457

Posted: Mon Sep 11, 2017 7:01 am Post subject: 


Dear Er Tooma,
This present discussion reminds me of the famous quote by Yogi Berra "In theory, there is no difference between practice and theory. In practice, there is.”
Of course, theoretically there is no need for a shear connection between layers if bending is pure and (importantly) plane sections remain plane, deformation of the layer in compression is equal (and opposite) to that of the layer in tension, and the interface between layers is exactly at the neutral axis.
However, in practice, there will always be some departure from these ideal conditions, which is why experienced teachers like N S Sir and Arun Kumar Sir hesitate to say "no need of shear connection". 

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tooma SEFI Member
Joined: 27 Aug 2017 Posts: 6

Posted: Tue Sep 12, 2017 9:15 pm Post subject: 


Dear Abhio, thank you for your answer. So, if we suppose that the neutral axis is exactly at the interface between the two layers, and the height of each layer is h and the width is b, which one of the following is the moment of inertia of the section:
(1) b(2h)^3/12
or
(2) 2(bh^3)/12
I am asking this question because your assumption for the one neutral axis leads to option (1) and for me this counteract with what I was learned in the first classes of engineering college. I was learned that option (2) is what to use when the layers are not glued or connected together.
abhio wrote:  Dear Er Tooma,
This present discussion reminds me of the famous quote by Yogi Berra "In theory, there is no difference between practice and theory. In practice, there is.”
Of course, theoretically there is no need for a shear connection between layers if bending is pure and (importantly) plane sections remain plane, deformation of the layer in compression is equal (and opposite) to that of the layer in tension, and the interface between layers is exactly at the neutral axis.
However, in practice, there will always be some departure from these ideal conditions, which is why experienced teachers like N S Sir and Arun Kumar Sir hesitate to say "no need of shear connection". 


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kunalkansara General Sponsor
Joined: 26 Jan 2003 Posts: 185

Posted: Tue Oct 17, 2017 5:40 pm Post subject: 


Dear Er Tooma
If I understood your question correctly, I suppose the confusion is due to mixing up the directions of shear that are possible. By composite beam if you meant it comprising of two or more layers staked oneovertheother and glued or nailed at the interfaces then clearly the shear that one should worry from glue point of view is the ‘longitudinal slip shear’ that occurs between the layers at the interface when the beam tends to bend.
The four point bending loading that you are talking about does create pure bending condition in between the pointloads, and most labscale testing involving beams requiring to be under purebending condition use this type of loading setup. But the shear that is absent in such cases is the ‘transverse vertical shear’ not the ‘longitudinal slip shear’.
If the beam is glued effectively, the layers comprising the beam section produces composite action. If there were two layers, each with width b and height h, then yes you are right in recalling your college teaching where you learnt the composite MI is option 1 if they were glued effectively. Otherwise the two sections will act together but not as a single composite section, and therefore the MI in this case will be that suggested by your option 2.
I hope this will be of use to you.
Thanks
Kunal 

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