Basic Statics Problem Help 7

[Techne]

I am charged with designing a catwalk that will support not only the weights of the beams and grating, the people, but also a 5800 lb filter press.

The press rests on two beams as shown in the picture below.

Does all 5800 lbs rest on each beam? I can't seem to remember how to determine where the weight is in order to size the beam. I've been out of college 2 years now and the cobwebs are pretty thick. Thanks for any help in the right direction.

http://img151.imageshack.us/img151/2061/filterpress.jpg

 

[jhardy1]

Ummm ...

If you can't work out where the weight goes for a simple structure like this, you need to think about engaging the services of a qualified engineer, rather than attempting to tackle this problem yourself.

Otherwise, you will possibly end up getting sued, or worse still, killing someone ...

(Seriously - if you are having trouble with the basic statics of a problem like this, seeking advice on a forum is NOT the way to go!)

 

[Techne]

I have a facilities manager above me who will ultimately check all designs.

I am familiar with statics and strengths of materials, it's just that I'm used to doing only 2D free body diagrams from school. This is my first real world problem and I'm having trouble remembering how to draw a load that spans two beams (3D) in 2D.

I know that it's a distributed load across the end but I didn't think that it was the full load. Just looking for a refresher is all.

 

[steellion]

The point is, if you can't figure out how the load gets in to the beam, I doubt that you will properly size and detail the beams for moment, shear, deflection, lateral torsional buckling, connection design, fatigue, etc etc.

It is unethical to practice engineering outside of your field of expertise. I blame the company because they shouldn't have asked you to do it in the first place, and I know you're just trying to be a good employee. This is not a homework problem though, peoples' lives are on the line.

 

[rb1957]

2D free bodies are ok ...
start with the load, it'll be reacted at the front and the back (a 2D FBD looking onto the side of the load).

then at the front and back there look as though there are two legs, so divide the load between them (2D FBD looking onto the front/rear faces, with the appropriate load reaction).

then another 2D FBD looking onto the front of the structure, showing the two leg loads calc'd above being reacted to the ground ...

then check your ground reactions, they Should balance your load

 

[paddingtongreen]

The problem that rb1957 hasn't tackled is how the load gets to the columns. Which pair of beams are you talking about?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Techne]

Woah. Lots of consultants defending their jobs or do you guys think I'm really incompetent?

I have just been out of practice for 2-3 years since college. I'm a young engineer. If you don't use it, you lose it, I suppose.

What I have been doing so far is creating a 2D diagram of the each side of the structure. The problem I'm having is whether it really IS all 5800 lbs on one beam in the end view.

I'll hit the books this weekend. I apologize for thinking I could ask for some direction here. Thanks anyway.

 

[paddingtongreen]

For equilibrium, the following must be true: ?F=0, ?M=0

Does the center of load fall on the one beam? No. Where does it fall?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Techne]

Ah right, all forces need to be in equilibrium. The center of gravity of the unit falls between the 2 beams. But I can imagine the load shifts with the sliding plates.

I also have the 2 beams connecting both ends beams which would transmit the load between the two.

Is what I'm looking for Moments about both beams originating from the center?

Would the method of Sections help here?

 

[cap4000]

You first need to indicate the center of gravity for the 2 sections with all the dimensions. Post this diagram and I can help you.

 

[rb1957]

the load is going to be reacted at 6 points, right?

start with the load, take a section, looking side on. your 2D FBD will show you how the load is reacted between the forward and rear ends of the load ... so you've replaced the original load with two loads.

now you can do another 2D FBD to see how the load is reacted between the two legs (at the front and rear) ... probably 50:50.

and you can keep chasing the load down to the final reaction points.

not so sure the books will help so much, i'd see this as a more practical problem (rather than a really simple book problem).

but a 3D FBD isn't that difficult to manage ... 6 equations of equilibrium (3 forces and 3 moments) ... if you can do 2D you can do 3 !

bear in mind massive simplifications ... the biggest redundancy looks to be how the load will be reacted at the rear, between the middle frame and the rear frame. there are quite complicated ways of dealing with this redundancy (matching the deflection of the side-to-side frame and the fore-aft beam) but i'm sure there are reasonable conservative assumptions that can simplify the problem.

you're dealing with static loads ... are there any dynamic loads ? torque from start-up ??

 

[MiketheEngineer]

Sorry if people have been beating you up a bit - but this is basic statics. If you need to rely on your boss to correct - what is he going to think?

Suggest you privately implore the help a friendly engineer who can guide you and recommend some remedial action to strengthen your competency in this area.

 

[Ron]

Agree with Mike...from your questions, you are not "getting it". Have someone sit down with you before you make a poor impression on your boss (that could be hard to overcome).

 

[Techne]

Yeah it is pretty basic and that's why I'm ashamed to even be asking where to start. I got A's in both Statics and Machine Design and was even a Statics tutor. but that was 5 years ago and the last job I had I might as well have been asleep for 2 years!

Now that I'm thrown back into it, I am having trouble picking up where I left off and forget even where to start.

The comments so far related to the question have been helpful though in kick starting my brain. I'll try to take it from here and post up that diagram this weekend.

rb1957, what did you mean that books would not help much here? I've heard this before that this job even is just practical applications, but I intend to go back through my books and review the process. I don't just have textbooks at my disposal, I'm pretty dedicated to get in the library and figure out what I've forgotten.

Thanks.

 

[chrislaope]

Techne:

Mike and Ron have given you a very good advice.

Now I would like to add one more advice: don't take it for granted that your past knowledge in statics and strength of materials in school can cover all structural engineering problems. Tell you a true story, a couple of years ago, I have a mechanical engineer colledgue, he used a stress analysis software to build a 15 ton crane rail track and from stress analysis he find a W18x65 pure I-beam (without channel cap) with 30' span would be strong enough (software shows the highest stress is below 24 ksi). What's happening is that software only gives stress under limit states of yielding condition, while in reality, the failure mode is lateral torsional buckling which is beyond his previous knowledge.

I hope you will avoid such mistake.

Good luck.

 

[rb1957]

books tend to deal with ideal situations. start wth wiki (geez i shiver at recommending that) it's available. the books will help with some of the things you've dredged up (method of sections, bending stress i a beam, ...).

first you need the beam reactions. you can work with 2D views, but you should be able to get some mileage out of your iso drawing ... draw in the load, draw the reactions with a different colour pen ... keeping sense of loads and reactions is Key.

are there other loads than just weight ??

you mention "sliding plates" ??

 

[LSPSCAT]

This just is not right. Sketch looks completely out of proportion.

Most important, attitude goes a long way. Insulting those who are offering advice, even though critical shows lack of maturity and a further lack of the true responsibilities that you have as a design engineer.

The other important thing I gather from the sketch is that you have limited experience with structural design. Go look at plans, pictures, and past structual drawings of similiar designs and start working from that. Structual design is more than just statics.

You need a really good understanding of theory but also real world application before you can even design simple structures which can effectively be constructed in an economical manner.

 

[Techne]

The sliding plates are part of the filter on top of the structure.

I am aware that I am not fully capable of designing the next 15 ton crane, but I think I should be able to get by designing a simple catwalk that is 4' off the ground at this point.

I am not solely relying on my previous knowledge which is why I came here for insight. I have already been cruising books on all aspects of structural engineering and statics and plan to hit the books harder this weekend at the library. I don't plan on just finding an equation or a calculator and "plugging and chugging". If I wanted to go simple with blatant disregard for the safety and stability of this structure it would've been built last week

Google books is a great resource for me as I am able to search all texts (that have been scanned) for relevant topics. I can read material that covers the basics as well as advanced topics and really get a feel for what is going on. But as it's been said, you can't learn everything from a book and it's even harder if you don't know what you're looking for which is why I am here looking for the expertise of the community.

I know that more than one set of eyes is better and I am hoping you can all respectfully guide me down that path. I am a new generation of engineer, I would hope that the more senior members of the engineering community (which I hope frequent this board) would be willing to pass on the knowledge to the next generation and prevent the next Hyatt-Regency Collapse or Tacoma Narrows bridge.

 

[Den32]

You say you came here for insight, and that's what you have gotten. Practicing Structural Engineers have offered their honest opinion - you can choose to listen or get mad.

It doesn't matter if the catwalk is 4' or 40' off the ground, if it fails, there is an effect (your company is sued, loss of client, etc.). This is serious business here, that is what members have been trying to relay to you.

In my opinion, it will be difficult to assist you through a 'forum'. Like others have said above, get someone else at your company to sit down and 'refresh' you if you don't wish to approach your boss.

 

[Techne]

I'm not mad, upset, offended or anything of the sort.

Anyway, the picture is just a simple assembly, the size of the beams is not relative to the load, just simple beams to represent the initial design.

Also, the only problem with forums is the fact you can't see poster's credentials. I've offered up mine, how about yours?

Thanks to the on topic contributors, ill have more information up later with a more detailed sketch. Your insight is appreciated!