Calculate web bending from eccentric load

[howdoyoudoit]

Please see attached scenario, sorry about the rubbish sketch!

Assume the UC in the section is 4m long and fixed at both ends. The cantilever arm is bolted to the bottom flange in the centre of the beam. Ignoring all other elements, how does the moment created by the cantilever effect the web of the supporting beam? Is torision being applied to the beam and does this bend the web? Fogive me if the question doesn't make much sense but I am just a student :-s

 

[BAretired]

Forces are being applied to the flange which result in bending of the web and torsion in the beam. It is incorrect to say that torsion bends the web.

BA

 

[howdoyoudoit]

So the flange bends the web, but how do we calculate if the web is thick enough to resist the bending. Where does the bending act, is it on the centre of the web. So the centre of the web is fixed and the bottom bends. Or does the bending in this case start at the top of the web where it meets the top flange?

 

[csd72]

the torsion becomes a horizontal push and pull in the flanges, the web acts as a cantilever to transfer the force to the top flange.

The hard bit is judging what length of the web to use.

Also, this is a very flexible arrangement and would not be suitable for most applications where deflection is critical.

 

[BAretired]

Use a stiffener each side of the web of the carrying beam and forget about calculating the capacity of the web in bending. This will get the torsion into the entire beam section without bending the web. Bending in the stiffeners would be maximum at the bottom and zero at the top varying linearly in between.

A UC us not a very good member to select for torsional load. Maybe a square hollow section would be a better choice.

BA

 

[CorBlimeyLimey]

howdoyoudoit,

From this site's policies; (

http://eng-tips.com/market.cfm)

Students are not allowed to post homework problems in the Eng-Tips forums for the purpose of getting answers to their homework. This is considered cheating. Offending posts will be removed from the site and offending members will lose their membership privileges.

Currently, your questions infer that you are just wanting someone to do your homework for you, in which case this thread will be removed very soon.

However, if you post what you have done so far, someone here may be able to point out where you may be going wrong, or confirm that you have it correct.

 

[msquared48]

I agree with BA and CBL here on both issues.

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

 

[BAretired]

GBL and Mike,

The OP quite candidly admitted that he is a student. I did not get the impression that he was trying to get someone to do his homework. I got the impression that he wanted to understand the behaviour of a beam in a particular situation. Accordingly I tried to assist in a general way.

BA

 

[CorBlimeyLimey]

BA,

That's why I made the post. Consider it a pre-emptive strike (or a chance for proof of intent) before some trigger happy soul Red Flags the thread for deletion. As you know, for some here, just the mention of being a student is sufficient cause for a Red Flag.

BTW, the "but how do we calculate if the web is thick enough" question makes it a borderline "do-my-homework-for-me" request.

 

[BAretired]

CBL,

I appreciate the point you are making.

howdoyoudoit,

Are you asking us to solve a homework problem? Please clarify.

BA

 

[dhengr]

Howdoyoudoit??

There isn’t anything wrong (rubbish) with your first sketch, it shows the general problem, along with your verbiage pretty well. You just stopped sketching way too soon, and deprived yourself of the real learning process. And, after all, this is your education, not ours. How do the short canti. and the bolts support the 5kN load, and what are the forces in the bolts, moments and forces on the canti.? Draw us a free body diagram (FBD) of this canti., with dimensions and forces, etc., and show it to us. Next, how are the bolt forces imparted to the flg. of the beam above, can these flgs. take these loads, where do they go? Draw us a FBD of this flg. and the web immediately above the radius btwn. the flg. and the web. What are the forces and moments on the web to support the 5kN load? Why don’t you cut the web down in the radius, right at the top of the flg.? Next, extend this second FBD and cut the web just below the radius at the top flg., again show dimensions, forces and moments on the web. Show us this much and lets talk again. This is a test, show all your work steps.

You need to do this a few times, at every step along the load path, to start to get a feel for how this works. BA and Cds have done this problem so often that they can skip these intermediate steps. You can’t if you want to really understand. We don’t give answers here, to students, we give more questions, to stir the gray matter btwn. your ears.

 

[rb1957]

i'd second that ... draw individual FBDs of each piece .. the short cantilever supported by the two bolts, the next chain in the loadpath is the flange, loaded by the two bolts.

as drawn there is no stiffener bracket to help distribute the load from teh flange into the overall section, so the flange will be doing a lot of bending.

I'd look at a piece of the UC beam, loaded by the two bolts, and look at a section where the I-beam is working as a section ... there'll be vertical shear carried by the web, bending (tension/compression in the flanges) and torque on top; the shear flows due to shear are straight forward, the shear flows due to troque (on an open section) not so obvious (i think they resolve into a couple between the I-beam flanges.

 

[howdoyoudoit]

schools out for summer... and I am not this far on in my studies, I was looking at some calculations and trying to understand what was happening. I have a calculation showing how the flange is checked for bending and wondered how the forces then effect the web.

 

[howdoyoudoit]

I will work it through and let you know how i get on. thanks!