Structural Engineering Question

I am confused about this exam question and would like some guidance if possible to how i should go around tackling it. I am usally ok with these questions but its the cantilever beam that is confusing me. thanks. :biggrin:

structural problem.png339.8KB

It's a fairly standard statics question. Firstly, I'd calculate the second moment area of the beam, because you'll need that to do some of the other questions, and - at least at my uni - it will get you are mark or two. Then just go through the sections one by one, really straight forward.

Reply 2

Dude Where's My Username

15

For the cantilevered support, wouldn't the force around support B just be a clockwise moment of (250x12=3000kN)? :unsure:

Then continue from right to left using what we call the "cut, bin, one end in" technique to work out the bending moments (in relation to support B) along all the points where a force rests. That's what I'd do but the likes of Smack, thefish_uk and Plato are more experianced here so I'd take their advice first. :o:

Btw, is it the shear force or bending moment diagrams you're having trouble with here?

This question and thread will be good revision for me too so correct me where I'm wrong.

(edited 13 years ago)

Reply 3

thefish_uk

14

Original post by xperty

As Smack says, it walks you through it in quite a straightforward way.

a.) Take moments about each support to find reactions. You then just have a beam with various forces acting up / down on it.

b.) Work along the beam from left to right. Be careful to be consistent with your signs. Remember a point load will cause a jump and the distributed load will be a slope - you should just be able to make a "cut" in a few carefully chosen places and join up the dots. As a little check, remember that when you get to the 250kN load at the far end this should be exactly enough to return the shear force to 0.

c.) Again, work along the beam from left to right. You can work out the bending moments at each point load and, for the leftmost and rightmost bits (with no distributed loads) you can join those points with straight lines. As for the distributed load, however, you're probably going to have to make a moving cut at distance x from the left hand end and find M(x), and use this to find the maximum bending moment (clue: it'll be where shear above is 0). Check the diagram to make sure it looks sensible - there should be a point somewhere along the UDL where there is zero bending moment, and left of it should be sagging (with a maximum moment somewhere along the UDL) and to the right should be hogging (with a spike above the right hand support).

You can now do a quick idiot check knowing that S=dM/dx. So positive S means positive gradient on M, increasing S means that M gets steeper, so make sure your two diagrams are consistent.

d.) and e.) should then be simple once you have read your maximum values off the graphs (for moment be aware that you have two maximum moments - so choose the largest).

Original post by Dude Where's My Username

For the cantilevered support, wouldn't the force around support B just be a clockwise moment of (250x12=3000kN)? :unsure:

Then continue from right to left using what we call the "cut, bin, one end in" technique to work out the bending moments (in relation to support B) along all the points where a force rests. That's what I'd do but the likes of Smack and Plato are more experianced here so I'd take their advice first. :o:

Btw, is it the shear force or bending moment diagrams you're having trouble with here?

This question and thread will be good revision for me too so correct me where I'm wrong.

Yup, as long as it is statically determinate you can always do this by working out the support reactions and then making cuts. (If you don't know what I mean by "statically determinate", you don't need to worry about it yet.)

(edited 13 years ago)

Reply 4

xperty

OP

Ok thanks guys you were really helpful :smile:

Reply 5

bluenoxid

13

Original post by thefish_uk

Lots of stuff that bluenoxid has forgotten

Oh my God, I cannot believe how much stuff I've forgotten.

How is life treating you thefish_uk? Got anywhere with getting work?

Reply 6

thefish_uk

14

Original post by bluenoxid

Oh my God, I cannot believe how much stuff I've forgotten.

How is life treating you thefish_uk? Got anywhere with getting work?

I still don't have a job, which is why I'm spending so much time on TSR reading the Engineering and Careers subforums...

I've had a couple of interviews so far - on the first they basically moved the goalposts after the interview; they looked at their business and decided they didn't want fresh grads after all. The second was only a few days ago and I'm waiting to hear back. Part of it was a little written test which was mainly moment and shear force diagrams which is why I'm quite fresh on them! Interview seemed to go quite well so I'm fairly hopeful, but it would require a relocation to somewhere I hadn't really considered moving to before (though on the bright side it isn't that far away from where I live currently).

I spotted on the Careers subforum that you have found a job though, which is good news!

Reply 7

bluenoxid

13

Original post by thefish_uk

I still don't have a job, which is why I'm spending so much time on TSR reading the Engineering and Careers subforums...

I've had a couple of interviews so far - on the first they basically moved the goalposts after the interview; they looked at their business and decided they didn't want fresh grads after all. The second was only a few days ago and I'm waiting to hear back. Part of it was a little written test which was mainly moment and shear force diagrams which is why I'm quite fresh on them! Interview seemed to go quite well so I'm fairly hopeful, but it would require a relocation to somewhere I hadn't really considered moving to before (though on the bright side it isn't that far away from where I live currently).

I spotted on the Careers subforum that you have found a job though, which is good news!

Cheers. I've been really lucky in the present environment although some of my mates are slowly finding jobs. Good Luck with the second job and keep holding on.