STRUCTURE PROBLEM

[isb07]

Greetings everyone
i have attached a picture of my problem...

i want to know how can i calculate bending moment for the same.????

how can i calculate moment of inertia and center of gravity for structure(made of channels and beams) shown????

as i think if i can calculate the above mentioned it will be easy to check whether my design can take that load or not

 

[rb1957]

FE is the best way to solve this highly redundant problem.

as a hand calc, i'd start with the reactions, it looks as though the load is slightly right of Centre, so the three reactions on the left side will be slightly higher than the three on the right, maybe 55:45; the middle reactions are going to be highest and the far end, away from the load, may be -ve ... that's how it'd be if the side edge beam worked as a beam, but looking as a grillage (sort of like a plate) then they'd be +ve (ie up).

then you've got what looks like two beam setions to stress.

 

[SteelPE]

Did you "moment" connect every piece together?

I don't know exactly what this frame is for, however, I have done plenty of frames that support large roof top equipment that look similar to this. Normally the connections at the end of the beams are simply supported. I apply the loads on the beams and then calculate moments and deflections. Then I would make a model in the computer and check to make sure my calculations were correct.

 

[Brad805]

I assume you have some load distribution for the large mass. After that, it is a simple grill problem. The stiffness of the various members will affect how the load travels to the support points. Given the scope of the project you need a structural guy to help you out.

Brad

 

[delagina]

It's not just getting the moment, but also putting the right connection releases(shear or moment), the correct unbraced length, etc..

 

[TXStructural]

I think it is simpler than that. The load will follow the stiffer elements back to their supports. Your design moves the load to the channels at the ends though the longer channels, then to the corner supports across the short ends. None of the main load would go to the center supports. This is particularly true since the central cross pieces/channels are discontinuous and not aligned across the center section. The smaller channels connected to the web will rotate significantly as the web bends (possibly promoting buckling of the webs.)

This should probably be framed with the deeper, continuous members running the short direction to girders across the long sides. This would get the load to all of the supports according to the location of the load, and would reduce deflection significantly, member sizes being equal.

To reach a more accurate answer using software, make the ends of smaller channels pinned. This will place the main load on only the four long channels under it.

 

[trainguy]

Looks like 4 different beams to design.

This could easily be done by hand calcs, if the 16 ton load is evenly distributed.

If the short beams are pinned, it's a piece of cake.

tg

 

[isb07]

thanx everyone for ur support

its actually a kinda chassis that carry load from one place from other

what i got in mind to calculate Moment of inertia and center of gravity of whole structure and hence i can get section modulus of whole structure

and by calculating maximum bending moment i can equate both to get maximum bending stress

tell me if m wrong???

but i dont know how to calculate Moment of inertia, COG and bending moment...

if there s better way please share??

i will be highly indebted if anyone can do rough calculation for same

or can anyone suggest a better software for analysis??

i use catia... is catia a better software to do analysis??

thanx

 

[rb1957]

"calculate Moment of inertia and center of gravity of whole structure and hence i can get section modulus of whole structure"
that does IMHO make any sense. if this is your thinking, then i suggest you need to review mechanics of solids texts.

you need to analyze the loadpaths, the individual beams. you have lots of asusmptions to make if you're going to hand calc this ...
1) you could assume that the short beams distribute the load the the long edge beams, and the long beams take these loads to the corners.
2) you could assume the corner reactions (from solving the structure as a whole) and then devise internal loads that make each member balance.
3) then you'll have the maximum moment in each member and you can analyze each loadpath.
4) you can simplify by assuming each joint is a pinned connection.
5) you can use the minimum number of beams to be conservative.

...

 

[delagina]

lol i have a feeling this thread will be close or deleted anytime soon.

 

[RUS777]

To figure out whether this structure is adequate or not I would suggest assuming that the 16t load will be carries byt longer channel beams. Distrubute the load to beams using their tributary areas, then you will get a simply supported beam with a uniform distrubuted load on it. Check bending moment and shear. Smaller cross members will act as lateral braces to held brace the beams so your Lb will be whatver the spacing of the infill beams is. Exterior beams will only see point loads from the load carrying beams (shear reactions) therefore you need to analyse them that way.

Make sure though that you restrain the moment at every connection. Company where I work everything would be wellded all arround fillet therefore resisting bending.

 

[BAretired]

what i got in mind to calculate Moment of inertia and center of gravity of whole structure and hence i can get section modulus of whole structure

and by calculating maximum bending moment i can equate both to get maximum bending stress

tell me if m wrong???

You're wrong!

Beams in the vicinity of the load will carry more than those further away. Properties of the whole structure would give incorrect values of bending stress.

BA

 

[trainguy]

Actually,

This could even be reduced to 3 separate beams to design, because the shorter perimeter beam that is far from the load could be made the same section as the closer one for simplicity.

tg

 

[rb1957]

three cross beams (along the short side) joining the supports (assuming that they are aligned, and the pic is slightly 'rong); and two long beams, distributing load onto the supported beams.