Beam and load question

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Hi .. for a simple beam with uniformly distributed load and moment formula of w * ln /8 at center.. is it independent of thickness of the beam such that even if a beam is 1 meter depth by half meter width compare to beam half or even twice its size..the formula and moment is still the one for a given length and load?
 

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  • #2
haruspex
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Hi .. for a simple beam with uniformly distributed load and moment formula of w * ln /8 at center.. is it independent of thickness of the beam such that even if a beam is 1 meter depth by half meter width compare to beam half or even twice its size..the formula and moment is still the one for a given length and load?
Of course.
The characteristics of the beam only affect its ability to withstand a given load. The load is whatever it is regardless.
Does your weight depend on the details of the floor you stand on?
 
  • #3
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Of course.
The characteristics of the beam only affect its ability to withstand a given load. The load is whatever it is regardless.
Does your weight depend on the details of the floor you stand on?
But weight is weight. unless you mean it is ok to ask a person what is his/her moments on a 3 meters fixed beam?

I mean even if you are standing in a 1 mile thick iron.. the moment is the same as a small wooden beam? What principle produce the same moment/bending?
 
  • #4
haruspex
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But weight is weight. unless you mean it is ok to ask a person what is his/her moments on a 3 meters fixed beam?
Once you know where on the beam they stand, yes.
 
  • #5
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Once you know where on the beam they stand, yes.
I added this that you missed...

I mean even if you are standing in a 1 mile thick iron.. the moment is the same as a small wooden beam? What principle produce the same moment/bending?
 
  • #6
haruspex
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the same moment/bending?
Same moment, different bending.
 
  • #7
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Hi .. for a simple beam with uniformly distributed load and moment formula of w * ln /8 at center.. is it independent of thickness of the beam such that even if a beam is 1 meter depth by half meter width compare to beam half or even twice its size..the formula and moment is still the one for a given length and load?
I think, the bending moment is independent of the cross sectional dimensions of the beam.
Bending moment is just a resisting moment offered by the beam.Bending moment will be equal to the moment due to the applied loads as long as the bending stress induced is less than the yield stress(ductile) or fracture strength(brittle).Once it reaches the yield stress, the beam fails(It cant resist anymore).
Assuming same material,same length of the beams, same load applied,
for a beam with higher section modulus(roughly represents cross sectional area) lesser bending stress is induced but for a beam with lower section modulus(smaller cross section), higher bending stress is induced.
So the stress in the beam with lower section modulus can easily reach the failure stress with a lesser load.so they fail easily than the thicker beams.
 
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haruspex
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Same moment, different bending.
Whats the best definitions of the difference between moment and bending anyone has come across?
 
  • #10
haruspex
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Whats the best definitions of the difference between moment and bending anyone has come across?
It's not that subtle a difference. The moment describes the load applied and its distribution; the bending is what results, and that will depend on attributes of the beam.
Generically, these are called stress and strain. If you hang a weight from a rope, the weight is the stress and the stretch of the rope is the strain.
 
  • #11
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Offered to the beam, surely?
I think it is offered by the beam.Because bending stress is a resistance or opposition by the beam to the externally applied loads and moments at a cross section.
On one half of the cross section, the stress is tensile (surfaces are elongated) so this produces a net resisting tensile load on that half.On the other it is compressive so net compressive resisting load.
These two equal opposite parallel loads cause a resisting couple .This couple is called bending moment.(FYI:I cant understand the difference between couple and moment)
Afterall in static equilibrium condition, for every applied load or moment there should be equal an opposite reactions else the beam wont be stationary.
Initialy when you apply load on an unloaded beam, it starts bending.During bending, the internal resisting moment starts increasing and when it becomes equal to the moment due to external applied load, the bending stops and the beam is held at that position(beam comes to rest because net moment acting is zero).This is the state of static equilibrium which we usualy study about.
 
  • #12
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Whats the best definitions of the difference between moment and bending anyone has come across?
Initialy a beam will be flat(straight).When you apply transverse shear loads(loads perpendicular to the longitudinal axis of the beam) on the beam, the loads cause moments (rotating effects of the loads).These moments try to rotate the beam but since the beam resists the rotation(by applying a resisting moment opposite to this external moment), its shape will change.This process in which the shape of the beam changes is called bending and the rotating effect of the applied loads is called moment(external moments due to applied loads).The resisting moment by the beam is called bending moment.
 
  • #13
haruspex
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bending stress is a resistance or opposition by the beam to the externally applied loads
Ok. But in a static arrangement it is equal and opposite to the applied load's moment, so is dependent only on that load, not on any characteristics of the beam.
 
  • #14
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Ok. But in a static arrangement it is equal and opposite to the applied load's moment, so is dependent only on that load, not on any characteristics of the beam.
Bending stress is dependent on the cross sectional area or section modulus(higher cross sectional area, lower stress).But the net reaction load caused by the stress is independent of the cross sectional area because the ne resisting force is F=(Stress)×(Area).This means lower stress over a larger cross section and higher stress over a smaller cross section will give the same resisting force and hence the same resisting couple(bending moment) irrespective of the cross section.
The reaction load depend only on the external applied loads
(Its because of this reason,in case of axial loading we assume the magnitude of the external load as the magnitude of the resisting force and calculate the magnitude of the stress by using STRESS=EXTERNAL LOAD÷AREA OF CROSS SECTION
but the actual formula is stress=Resisting force÷area)
The rsisting moment depend only on the moments caused by the external loads.
But the stress induced will depend on the dimensions of the beam also.
 
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  • #15
haruspex
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Bending stress is dependent on the cross sectional area
The original question is regarding bending moment, not stress. However the stress is distributed in the beam, it results in a resistive moment equal and opposite to the applied moment, and this is independent of the beam characteristics.
 
  • #16
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The original question is regarding bending moment, not stress. However the stress is distributed in the beam, it results in a resistive moment equal and opposite to the applied moment, and this is independent of the beam characteristics.
Yes.I think it is right.
Earlier, I said that" bending moment is the resisting moment"(i.e.it is applied by the bar).But it might be wrong.I thought of two possibilities.
I think the reason behind calling the resisting moment as the bending moment is that the magnitude of the resisting moment at any cross section will be equal to the resultant moment due to all the externally applied loads and couples about that cross section(but opposite in direction since it resists them).
i.e., Since its magnitude is equal to the net external moment that tries to bend the beam about that cross section, it is called the bendding moment.
(OR THE OTHER POSSIBILITY MIGHT BE)
Bending moment and resisting moment might be two different things.As in the case of normal tensile stress(stress=resisting force÷Area), the formula for bending stress might be "resisting moment÷section modulus" but since the magnitude of the resisting moment is equal to the net external bending moment we might be using the formula "bending moment÷section modulus"
These might even be wrong.
what do you think is right?
 
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  • #17
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RvPEQc.jpg


How is moment of P*length/8 exactly derived above? why divided by 8 and not 6 or others?

Does it depend on the gravitational constant or any Spacetime feature?

What kind of spacetime where there is no moment even if there is a loading?
 

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  • #18
haruspex
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How is moment of P*length/8 exactly derived above?
To compute the moment at some point, we only consider those loads between one end (the left, say) and the point.
Since P is uniformly spread along l, only P/2 is applied to the left of the midpoint.
We could find its moment about the midpoint by an integral, but it is easier than that. We can just look at the average point of application. This is halfway between the left end and the midpoint of the beam, so at l/4 from the midpoint.
The moment is therefore (P/2)(l/4)=Pl/8.
 
  • #19
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To compute the moment at some point, we only consider those loads between one end (the left, say) and the point.
Since P is uniformly spread along l, only P/2 is applied to the left of the midpoint.
We could find its moment about the midpoint by an integral, but it is easier than that. We can just look at the average point of application. This is halfway between the left end and the midpoint of the beam, so at l/4 from the midpoint.
The moment is therefore (P/2)(l/4)=Pl/8.
But I think it is not a distributed load, it is point load.
 
  • #20
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View attachment 228308

How is moment of P*length/8 exactly derived above? why divided by 8 and not 6 or others?

Does it depend on the gravitational constant or any Spacetime feature?

What kind of spacetime where there is no moment even if there is a loading?
Beams with fixed supporsts at both the ends are called statically indeterminate beams.Because, you cant directly find the reaction forces and reaction moments at the supports.In this case you may easily find the reaction forces(R=P/2) because the load acts exactly at the midpoint.
Iam not sure about how to find the reaction moments.Refer some books.
 
  • #21
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To compute the moment at some point, we only consider those loads between one end (the left, say) and the point.
Since P is uniformly spread along l, only P/2 is applied to the left of the midpoint.
We could find its moment about the midpoint by an integral, but it is easier than that. We can just look at the average point of application. This is halfway between the left end and the midpoint of the beam, so at l/4 from the midpoint.
The moment is therefore (P/2)(l/4)=Pl/8.
But to get moment.. why do you need to multiply the load by the length.. it's as if moment is load in circle or with circular radius?
 
  • #22
haruspex
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But I think it is not a distributed load, it is point load.
Sorry- didn't read the diagram title.

As you say, it is not, strictly speaking, determinable, so they are assuming symmetry.
If the reaction force at the left is V and the moment is M then the moment at offset x is Mx=M-Vx=-ky" for some constant k.
(I take anticlockwise as positive.)
Integrating: ky'=Mx-½Vx2+c.
By fixture this is 0 at x=0 and by symmetry 0 at x=l/2:
c=0, M=Vl/4.
So Mx=V(l/4-x)=P/8(l-4x)
Looks like the book takes clockwise as positive.
 
  • #23
haruspex
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But to get moment.. why do you need to multiply the load by the length.. it's as if moment is load in circle or with circular radius?
My initial post was wrong - please read post #22. But to answer your question above, that's what a moment is: it is the applied force multiplied by its perpendicular displacement from the axis.
 
  • #24
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I detect a troll-like inclination to involve pseudo-philosophical irrelevancies.
"Epistemological" means pertaining to a theory of knowledge. "Ontological", pertaining to the essential nature of things. Those terms could be applied to a line of reasoning or philosophical stance but not to a concept such as moment or force.

I have explained how the expression for Mx is obtained.
I will not waste my time further on this thread until you ask serious scientific questions.
I am studying structural engineering.. i'm just figuring out how certain things are derived so I can understand them at first principles. Thanks a lot for your help. I'm digesting it. So I guess moment is like acceleration due to gravity. If we are standing on ground, we are not falling although there is a tendency.. so likewise with moment.. if the beam is made of diamond.. there is tendency too but stopped from full expression.
 
  • #25
haruspex
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I guess moment is like acceleration due to gravity. If we are standing on ground, we are not falling although there is a tendency
Indeed, and if you take away whatever is preventing motion then a force, such as gravity, produces linear acceleration, whereas a moment produces angular acceleration.
 

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