Flitched Beams 1

[SharpMan]

I've always wondered on how actually do Flitched Beams work.

Now there it is considered that the strain in steel and wood are equal ( it is very much visible) and the design continues with this condition of design, that, e(s) = e(w).

So, ?(s)/?(w) = E(s)/E(w)is the design criteria.

I could not clearly understand a few points:

1. How does the steel actually take up a greater proportion of the Moment acting?

Though the strength of steel is greater than wood, HOW ACTUALLY DOES THE LARGER MOMENT GET TRANSFERRED TO STEEL?

The steel and wood are not animate so that they can literally decide that steel being stronger should take up more load.

So can anyone explain this theory in detail OR give me some WEB LINKS to study a detailed theory of this (I've searched a lot inline but nothing is promising)

OR suggest a good Book or ebook for this ( just dont say try a S.O.M. book, they all str simply with the equal strain condtion but dont expain the concept or how this happens)

HElp expexcted from all the seniors and ( juniors as well )

 

[desertfox]

Hi Sharpman

Have a look at this site:-

http://books.google.co.uk/books?id=7eKu5Kh0dHcC&pg=PA70&lpg=PA70&dq=theory+of

+flitched+beams&source=bl&ots=I_A6KgOdcK&sig=
xEHdjScCA23y8rbN1QF
JrMh61Q8&hl=en&ei=OUbcS_nmIoX60wSYn8XLBw&sa=
X&oi=book_result&ct=result&resnum=1&ved=0CAsQ
6AEwAA#v=onepage&q=theory%20of%20flitched%20beams&f=false

Basically you convert the beam to the equivalent section of one or other material, which then allows you to compute stresses.


desertfox

desertfox

 

[Tmoose]

Its the difference in Es.
I'll tape each end of a piece of cut rubber band near the ends of a 1 foot length of steel pipe. Then I Pull hard on the each end of the pipe as if to lengthen it 1/16 inch. the low modulus rubber band does not feel a thing, or make it appreciably more difficult for me to (futilely) stretch the pipe.

 

[SharpMan]

Thanks Desertfox and tmoose for the esponse.

But I was not lookinfg for the solution ( iI know these methods already) BUT infact asking or wanted to know that HOW EXACTLY does steel take up more load.....WHAT HAPPENS THERE THAT THE GEATER LOAD IS TAKEN BY STEEL(I mean steel or wood cant decide by themselves that steel being stronger let it take more load ....I hope you get what I wanna say....difficult putting in words)

How is the load distribution taking place.

 

[desertfox]

Hi Sharpman

Okay I see what your saying,I'll try and explain it like this:-

Imagine the amount of force required to stretch a steel bar of a given cross section and length, by say 0.5mm, then given the same cross section in another material for instance wood would you require the same amount of force to stretch it by 0.5mm or less?
Now connect the steel and wood together such that there lengths are identical and the total cross section would be double.
Now apply a given load to the composite and bear in mind that the wood and steel are joined such that they move together and not independently and therefore the strain in each material is identical.
Now the steel is a much harder material to stretch than wood, so therefore if we apply a load to the composite to stretch it 0.5mm we need more of the applied force to act on the steel to achieve the given stretch of 0.5mm than we do on the wood.
If this was not the case then the stretch of the wood and steel would not be the same and there strains would be different.

desertfox

 

[Ron]

In the flitch process you equilibrate the stiffness of the two materials. Further, there is a shear connection between the plates and the wood (thru-bolts, etc). If the strains are truly compatible, the shear connection carries no load. When strains are slightly incompatible, the shear connection makes the transfer.

 

[Compositepro]

Well I never heard the term filtched beam before. The concept is same as sandwich construction. A large part of the flexural and buckling strength of a beam has to do with its dimensions. A steel strip can carry more tensile load than a wood beam but it can't carry much compressive load without buckling. Wood can be used to keep the steel from buckling while a little bit of steel can add significantly to the strength and stiffness of the wood alone.

 

[SharpMan]

Hi Desertfox

Thanks again for replying.

But I do understand the fact that steel being stronger than wood requires a greater force to bring about a certain strain ( say 0.5mm as you mentioned) than the force required by wood for the same strain.

But the question is actually HOW DOES IT APPEN AUTOMATICALL IN A COMPPOSITE SECTION

Lets take up the case explained by you:

Now the steel is a much harder material to stretch than wood, so therefore if we apply a load to the composite to stretch it 0.5mm we need more of the applied force to act on the steel to achieve the given stretch of 0.5mm than we do on the wood.

Here if we apply the load tot he composite section in a uniform distributed manner ( say compressive load on the total composite cross section ie two steel plates at the outer sides each and wood section in the middle with area of each steel plate being half that of wooden section)
such that the load must get automatically distributed over the cross section in the typical fashion of p/a(total)

This [ p/a(total)] should have been the load distribution ( load per sq.mm of area should be equal every where as in case of a uniform material section) but in case of a composite beam the greaer part of the load automatically gets transfered to the steel - HOW DOES THIS HAPPEN AUTOMATICALLY OR WHAT IS THE MECHANISM HAPPENING IN THE CROSS SECTION THAT BRINGS ABOUT THIS CHANGE OF TRANSFERRING GREATER PROPORTION OF THE LOAD ON TO THE STEEL.

This is actually what I want to know.

Hope now you got the actual doubt I was referring to.

 

[desertfox]

Hi Sharpman

The composite theory only works if the steel and wood interfaces are joined such that there is no relative movement between the interfaces under load.
So if I compress the composite with an applied load then the cross section of the steel and wood have no choice but to move together as one, therefore the steel by virtue of its stiffness as to carry a greater proportion of the applied load and the wood a smaller proportion so that both the materials end up the same length after compression.
If the steel and wood are not rigidly connected then there would be no control of the deflections of the wood or steel and they would end up with different lengths after compression.
Have a look at the files I have uploaded they might help.
Please don't reply in capital lettered sentances its deemed to be shouting and I do understand what your struggling with but its not easy to explain.

desertfox

 

[SharpMan]

Hi desertfox

Read the attatchment and thanks for it.

Yes its a bit difficult to explain and thanks for understanding what I am struggling with.

As for the capital letter are concerned, it was not shouting in any sense, but just a way of emphasising - highlighting the important part, just like writing in bold, so shouldnt be a problem.

 

[desertfox]

Hi Sharpman

Its okay not a problem, anyway do you understand whats happening with the composite beam now?

desertfox

 

[dvd]

The steel and wood act as springs in parallel: they are both constrained to the same amount of deflection.

Only works if the steel is properly constrained to the wood.

 

[msquared48]

...as in using periodic thru-bolts thru the wood and steel members to get them to work as one member.

The idea is to get them to deflect uniformly so they share the load proportionate to their relative EI values. Thru bolts will accomplish this.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MiketheEngineer]

Quite a few very good explanations --- But here is how I usually handle them. Since the wood doesn't help much (at least proportionately) - just design the steel plate to carry the load and deflections. Use the well bolted wood to prevent any local steel buckling and for shear/bearing requirements. You might even have to add lateral buckling bracing to the beam as a whole.

But this method is quick and virtually foolproof.

 

[boo1]

see the NAHB Flitched Beam guide

http://www.toolbase.org/PDF/DesignGuides/flitchplate.pdf

also see:

http://www.structuremag.org/article.aspx?articleID=399