Compression/Tension in Steel I Beams

[Alena Selone]

Homework Statement

Why is a steel beam shaped in the form of the letter "I" still strong enough to bear a load?
1) Its compression area is located on the top of the I beam.
2) Its tension area is located on the top of the I beam.
3) Its neutral area is located on the top of the I beam.
4) Its compression area is in the middle of the I beam.
5) Its neutral area is located in the middle of the I beam.

Most of the material in these I-beams is concentrated in the top and bottoms parts, called the flanges. The piece joining the bars, called the web, is thinner. Stress is predominantly int he top and bottom flanges when the beam is used horizontally in construction. One flange tends to be stretched while the other tends to be compressed. The web between the top and bottom flanges is a region of low stress that acts principally to hold the top and bottom flanges apart.

Homework Equations

n/a

The Attempt at a Solution

I think its #5, only because the neutral layer is in the middle of the block and that's where neither compression nor tension occurs. I figured that's why it can support a load. I'm unsure though and would like a second opinion.

 

[berkeman]

Homework Statement

Why is a steel beam shaped in the form of the letter "I" still strong enough to bear a load?
1) Its compression area is located on the top of the I beam.
2) Its tension area is located on the top of the I beam.
3) Its neutral area is located on the top of the I beam.
4) Its compression area is in the middle of the I beam.
5) Its neutral area is located in the middle of the I beam.

Most of the material in these I-beams is concentrated in the top and bottoms parts, called the flanges. The piece joining the bars, called the web, is thinner. Stress is predominantly int he top and bottom flanges when the beam is used horizontally in construction. One flange tends to be stretched while the other tends to be compressed. The web between the top and bottom flanges is a region of low stress that acts principally to hold the top and bottom flanges apart.

Homework Equations

n/a

The Attempt at a Solution

I think its #5, only because the neutral layer is in the middle of the block and that's where neither compression nor tension occurs. I figured that's why it can support a load. I'm unsure though and would like a second opinion.

But the question is asking about why I-Beams are strong. #5 is a true statement, but doesn't really answer the problem's question, IMO. Try again?

 

[Alena Selone]

But the question is asking about why I-Beams are strong. #5 is a true statement, but doesn't really answer the problem's question, IMO. Try again?

Then what's the answer? There are only 5 choices

 

[berkeman]

Then what's the answer? There are only 5 choices

You tell us!

Are any of the other statements true, and dealing with the "strength" issue?

 

[Alena Selone]

You tell us!

Are any of the other statements true, and dealing with the "strength" issue?

I wouldn't be asking if I knew, haha

 

[Alena Selone]

You tell us!

Are any of the other statements true, and dealing with the "strength" issue?

I really don't know and that's why I posted the question.

 

[berkeman]

I really don't know and that's why I posted the question.

Well, it's against the PF rules for us to give answers on homework / schoolwork questions. We can give hints, ask questions, etc.

So, say in words what you think about each of the choices #1-#5. Which are true and which are false?

 

[Alena Selone]

Well, it's against the PF rules for us to give answers on homework / schoolwork questions. We can give hints, ask questions, etc.

So, say in words what you think about each of the choices #1-#5. Which are true and which are false?

oh my godddddddddddddddddd I'm just trying to finish this assignment and y'all tripping
1) Its compression area is located on the top of the I beam.
this one is true because that's where the weight is

2) Its tension area is located on the top of the I beam.
no, the bottom is where the tension area is

3) Its neutral area is located on the top of the I beam.
no

4) Its compression area is in the middle of the I beam.
that doesn't even make sense, there is little to no stress on the web of an i beam

5) Its neutral area is located in the middle of the I beam.
yeah but you said this wasn't right

there are my reasonings

 

[berkeman]

oh my godddddddddddddddddd I'm just trying to finish this assignment and y'all tripping
1) Its compression area is located on the top of the I beam.
this one is true because that's where the weight is

2) Its tension area is located on the top of the I beam.
no, the bottom is where the tension area is

3) Its neutral area is located on the top of the I beam.
no

4) Its compression area is in the middle of the I beam.
that doesn't even make sense, there is little to no stress on the web of an i beam

5) Its neutral area is located in the middle of the I beam.
yeah but you said this wasn't right

there are my reasonings

Is there a figure that goes along with this question? How is the beam loaded? I was assuming it was being used in a cantilever configuration, but it sounds from your answers like it is center-loaded with a weight down...

 

[Alena Selone]

Is there a figure that goes along with this question? How is the beam loaded? I was assuming it was being used in a cantilever configuration, but it sounds from your answers like it is center-loaded with a weight down...

if the beam is standing like an I, the weight is on top of it with shear (vertically downward force) force in the middle of the top flange

 

[berkeman]

BTW, if it is center loaded with a weight down and supported on its ends, your reasonings are correct in your post #8.

 

[berkeman]

if the beam is standing like an I, the weight is on top of it with shear (vertically downward force) force in the middle of the top flange

Okay, then I agree with your answer of #1 being true and describing the strength of the I-Beam. Good job!

 

[Chestermiller]

What does the top of the beam being in compression have to do with the strength of the beam, especially since the bottom of the beam is in an equal amount of tension? I don't think any of the answers have anything to do with strength. Maybe they meant stiffness? This is a very puzzling question to me. The web part of the beam has lower tensile and compressive stresses, but the entire cross section has shear stress. Still, the highest stresses (combined shear and tensile) have to be near the top and bottom.

 

[Alena Selone]

What does the top of the beam being in compression have to do with the strength of the beam, especially since the bottom of the beam is in an equal amount of tension? I don't think any of the answers have anything to do with strength. Maybe they meant stiffness? This is a very puzzling question to me. The web part of the beam has lower tensile and compressive stresses, but the entire cross section has shear stress. Still, the highest stresses (combined shear and tensile) have to be near the top and bottom.

That's why I'm so confused, all parts of the beam contribute to it's strength

 

[Chestermiller]

That's why I'm so confused, all parts of the beam contribute to it's strength

Well, if you are ever able to get them to make sense out of this question they have given you, I would be interested in hearing about it.

 

[Alena Selone]

Okay, then I agree with your answer of #1 being true and describing the strength of the I-Beam. Good job!

My first answer was correct and as a result I got the question wrong since I put #1. Thanks, MENTOR.

 

[Alena Selone]

Okay, then I agree with your answer of #1 being true and describing the strength of the I-Beam. Good job!

For someone a part of the staff, you don't know your physics very well. I will not be returning to THIS site.

 

[berkeman]

My first answer was correct and as a result I got the question wrong since I put #1. Thanks, MENTOR.

Huh, strange. Sorry that it didn't work out this time. Were you able to ask the instructor why answer #5 was better than #1?

 

[apadonien]

The compression is in the middle of the I beam (the vertical part, between the two flanges). The top is under an equally distributed load which gets transferred to the middle and redistributed to the bottom flange. The tension would occur on the object that is providing the weight transferred to the I beam. Am I correct or no?

 

[haruspex]

The compression is in the middle of the I beam (the vertical part, between the two flanges). The top is under an equally distributed load which gets transferred to the middle and redistributed to the bottom flange. The tension would occur on the object that is providing the weight transferred to the I beam. Am I correct or no?

No.

The question is poor because it doesn’t give an alternative for comparison, but let's say the alternative is a square section beam of the same length and volume.
None of the offered answers is correct. Although 1 and 5 are true statements, they are also true of the alternative. A better answer would be that the vertical span puts the top and bottom flanges further apart, giving them more torque.

 

[256bits]

giving them more torque.

??

 

[haruspex]

??

The load creates a bending moment. The vertical span increases the distance between the tension in the lower plate and the compression in the upper plate, giving each a greater torque about the centre of the plate. This enables the beam to withstand a greater bending moment.

 

[256bits]

The load creates a bending moment. The vertical span increases the distance between the tension in the lower plate and the compression in the upper plate, giving each a greater torque about the centre of the plate. This enables the beam to withstand a greater bending moment.

I am familiar with it being discussed as a bending moment induced onto/within the beam due to the applied load, rather than as a torque. A torque I would have to assume as being an external load(s) applied to the beam. At least from my training.
What's in a name I suppose when it all comes down to it, and historical naming conventions.

 

[haruspex]

I am familiar with it being discussed as a bending moment induced onto/within the beam due to the applied load, rather than as a torque. A torque I would have to assume as being an external load(s) applied to the beam. At least from my training.
What's in a name I suppose when it all comes down to it, and historical naming conventions.

The difference between bending moment and torque is like that between tension (or compression) and force. Just as tension is an extensive property that can be thought of as pairs of equal and opposite forces at all points along the body, bending moment is like equal and opposite pairs of torques.
Considering one half of the beam, the combination of load and end support exert a torque. The combination of tension and compression above and below exerts a countering torque.

I understand that in engineering parlance "torque" is sometimes reserved to mean a twist about the central axis instead.