Calculation example on the analysis of box beam

[wadeshade]

Hi all, i am a junior engineer who just started my career in a material handling company as a design engineer. Now i am facing a difficult question, my boss had given me a task on designing the main girder for a double girder overhead crane. However, as i am newly fresh graduated, i have no knowledge in this field at all. Furthermore, the on-site engineer of my company trying to hide the information on how to carry out the design and calculation for these main girder. Can anyone help me on this? Pretty much thanks.

The crane data:
-25 ton safety work load
-Span length 18.7m
-Hoist weight 2 ton
-Distance between wheels 1.5m

I am required to find out the dimension for the girder needed. It should be in a welded box shape as illustrated as the photo attached.

https://www.researchgate.net/profil...on-of-the-main-girder-of-the-bridge-crane.png

I am required to carry out:
-stress analysis
-buckling check
-deflection check

Please help me guys. Thanks !

 

[kingnero]

I believe you have bigger problems in your company than designing a crane...

Seeing you're using metric units, you might want to take a look at the eurocodes:
EN 1993-6 and EN 1991-3 I believe.

 

[wadeshade]

Yes i am really having huge problems in my company.

Yes. I am refering to eurocodes en 1993-6 and en 1991-3. However, the eurocodes doesnt mention how to analysis the buckling of a box beam..... Or maybe eurocode has classified box beam to another terms which i do not know....... It would be helpful if you can teach me or guide me or carry out sample calculation on these. Anyway, thanks for the reply and information! You are more helpful than those engineers in my company.

 

[wadeshade]

Now my biggest problem is by refering to eurocdes manual, i know how to design for a universal beam. But when the universal beam had been replaced by a box beam. i do not know where to start. I can carry out the moment of inertia things, but i do not know how to analyze the local stress and etc.

 

[kingnero]

Local stresses = due to wheel forces? That's cookbook formula in EC3-6.
You have the diemnsions and forces? Follow the guidelines from EC3-6.
Girders have a max. vert. deflection of L/600.
I suggest not to take into account the advantage of the rail itself, as there are wear limits you need to impose, and the final result will be marginal.
Buckling of the plate you'll find somewhere else in EC3, you'll need to red through EC3-1-5 and EC3-1-8 for starters.
As for the eccentricity, I'll leave that to someone else, I'm not overly familiar with this.

 

[FLCraneBuilder]

I know these quote well
but Not willing to go into this in this forum

suggestion
1 - refer to previous projects the firm has done that use box girders. extrapolate from them
2 - if they do not exist, refer to CMAA for designed criteria
3 - if your firm has not done these before, I would seek work elsewhere... this is a dangerous situation

 

[wadeshade]

@flcranebuilder

Sir, my firm has done these before but the engineer doesnt want to share the information to me because of office politics...... I hope that you can understand my situation and help me in a way. Thanks a lot sir and wish you have a nice day.

@kingnero

Thanks for the information. I would download those codes and grind through it!

 

[Agent666]

Talk to your boss immediately about the other engineers attitude to helping you, time is money and you'll waste a lot of it if no one is showing you the ropes.

 

[ThomasH]

wadeshade
Are the Eurocodes applicable for your particular project? Are you familiar with the Eurocode and the methods thet apply?
The reason I ask is because I find some of your questions a bit odd.

There might be some typos in your posts but the starting point is usually EN 1993-1-1. You may also need EN 1993-1-5, 1993-1-8, 1993-1-9, 1993-6.

The questions you mentioned are covered by the Eurocode, and also some questions that you didn't mention.

I agree with others, you need to talk to your boss. Call it office politics or whatever, as a junior engineer you need proper guidance and a better "discussion" partner than a Internet forum can provide.

Thomas

 

[wadeshade]

@Agent666

I have already told my boss. My boss asked me try to learn it myself , and have some patience......

@ThomasH

Yes. I study eurocode during university, however we only study how to design universal column or universal beam but my company they using box girder as main girder for crane....

And they using FEM standard and British standard. They do the calculation of design girder in terms of stress analysis for every point of the girder (Top flange, bottom flange, left web and right web) and the buckling check by using Euler stress.... They also carry out the deflection check.

Actually for my task, I am required to provide the dimension required for the box girder which can resist buckling and deflection.. I am also required to provide stiffeners spacing for longitudinal and transverse direction.

 

[dhengr]

Wadeshade:
It seems that you are kinda being thrown to the wolves by your boss and senior engineers, when you are given no help and guidance. That’s a pretty poor way for them to help a young, aspiring engineer to learn the ropes, or to make them a valuable asset to the company. At the same time, you learn and retain much more when you put in some real effort to learn it, spend some of your own free time in study, to improve yourself. While your design problem is a significant real world problem, your questions are pretty basic, so you might also ask your Uni. or your Prof. why they didn’t teach you anything given what they cost you. Finally, nobody here on E-Tips cares to help you if you way oversold yourself, and now are being asked to prove your worth. You dug that hole, you dig yourself out of it.

As for sample calcs., drawings, and a general plan of attack for approaching your problem, there should be no better place to look than your company’s own files. Find a crane of similar span and cap’y., and review, understand and generally follow those plans and calcs. Then, there is some amount of iteration to arrive at a final solution and sizing, etc. This is also the best way to learn how to do it, try and try again till you reach a good solution. You should also dig out your Engineering Mechanics, Strength of Materials and Structural Analysis and Design textbooks and for the first time, really study, learn and understand them. Your boss’ saying…, “asked me try to learn it myself , and have some patience,” would seem to indicate that he expects you to dig a little, not just be spoon fed, take some self-initiative in getting old designs out of the files, review your textbooks, then ask some meaningful questions. And, finally have some patience with yourself in this learning endeavor, and he will have some patience in the time it takes, if you show the initiative. Try that approach on, for size.

 

[ThomasH]

Wadeshade:
I agree with a lot of what dhengr wrote.

I am unfamiliar with FEM standards, (FEM is a method for me), but I do know that British Standard and Eurocode should be the same except for a National Annex. I have also seen your other posts and some of the things you ask is basic beam theory. I hope that you don't use Euler stresses as design stresses (at least not the definition of Euler stress that I am familiar with).

I can see two explanations for this situation. Either the company is testing you to see how you handle a completely new assignment. I would call this bad mentoring but I don't know how long you have been struggling. It is also the possibility that you haven't yet properly asked for help. Perhaps you have oversold yourself and somebody is now checking your hand.

Not knowing everything and learning new things, thart is part of being an engineer. And asking or discussing on forums like this one can be very valuable. But I would not expect anyone to share their calculations, one reason is because I can't do the same. I don't own my calculations, they belong to my employer.

I think you have some learning to do. And perhaps not only in technical matters.

Good Luck

Thomas

 

[rb1957]

I can't believe this !

You go to your boss, saying "I need help".
He says "go learn it".

You say your lead engineer is "hiding the information" … not a very pleasant thing to do.
I guess he's thinking "I'm not paid to mentor you" or "no-one helped me".

I hope the only saving grace in this situation is that they won't build what you're designing;
well not until they've looked it over and approved it.

I understand how hard it is to move from the theoretical world (school) to the practical world.

You say you've got the code but don't understand how to apply it ?

You say you've learnt how to design a "universal" beam. Ok, well a "box beam" is only a specific "universal beam", a beam that looks like a box (ie 4 sides).

Can you find another project your company has done where they've done something differently.

I've heard of "sink or swim", but your guys have thrown you into a piranha pond and aren't listening to your cries for help.

another day in paradise, or is paradise one day closer ?

 

[r13]

Link below will take you to an old AISC paper that may help you to some extent. Also, you can search literatures on AISC website for the design of "box girders", "plate girders", and "crane girders", the results may assist you as well.

https://www.aisc.org/globalassets/a...-girder-bridges-design-guides-and-methods.pdf

 

[wadeshade]

Thanks for all the valuable information guys.

 

[12345abc6ttyui67]

I'm not sure if you have figured out to get this far or not yet.

Apply the loads to the girder and work out the usual critical things, e.g. max. moment and max. shear.
Determine what the allowable stress (or allowable force) is based on the applicable design code.
Use both of these to determine the box girder sizes you will need (i.e. pick some trial sizes for box girder depth, width, flange/web thickness and work out "I" and "Z" required to meet the allowable stress).
Adjust sizes as necessary.
Check deflection for your chosen shape. Increase properties if required to meet deflection limits. Note this is probably the critical sizing requirement for your box girder.
Check for local buckling of the flanges and/or webs vs. the applicable code requirements. I'm not familiar with Eurocode so can't say much more about that.

As you can tell this may take a bit of trial and error, so it would be efficient to set up a spreadsheet where you can tweak things and have everything automatically recalculate.

There are other issues with cranes to be wary off, e.g. localised wheels loads, lateral loads, load factors on lifted weights, and many others, but maybe the above will get you started.

Note for others: "universal beam" or "UB" is the name for the standard "I" shaped profile in the UK.

 

[edison123]

Very interesting discussion, thanks guys.

Is there any reputed software that does these pesky crane design calculations?

Muthu

http://www.edison.co.in

 

[Taras.D]

Wadeshade, as to my experience, problems like that are not in the area of construction specialists. And the drawings are usually made using some SolidWorks, Inventor, etc.
As soon as you perform this task from your chief, you'll probably get some more concerning other parts of that overhead crane
Of course, you can perform some necessary checks (strength, deflection, local stress, local stability of beam components, by the way, LT-buckling isn't applicable for hollow sections) by using some FE-analysis software or manually, but usually mechanical engineers should do a design of this kind. And there's a list of some checks they have to follow, they account for dynamic type of load and some other features, I'm not proficient in the area. This is the first thing you should tell your boss. That you can check this crane beam as a construction element only, if it is to be certified as "load-bearing equipment"(or something like that), make sure all of the requirements are met.

I'd recommend you to look for some specific textbooks that mechanical engineers use, there should be a list of those specific checks, they usually don't use "limit state design" as construction engineers do, but rather "limit stress design" method.

Maybe, you should start a thread here

https://www.eng-tips.com/threadminder.cfm?pid=404&page=1

Good luck.