2D frame analysis using Cross method

[SnapSpace]

Helo all!
Im new here so be gentle
Please look at the picture


For calculating bending moments diagrams of beams in an 2d frame simplification like this one is used.
This way you get higher moments over the support and in the mid spam since your sistem is not framed.
My question is how would you get right diagrams for columns?
Can Cross method be used for the entire 2D frame (like in the picture) when calculating by hand, or you have to use another simplification for geting column diagrams?
Thanx in advance!

 

[shin25]

I am unable to open your sketch. Try to post the sketch in some other way.

 

[SnapSpace]

Try this

http://img262.imageshack.us/img262/3895/2dframely4.jpg

 

[SnapSpace]

Anyone? :-(

 

[hokie66]

Yes, moment distribution can be used to analyse the whole frame.

 

[SnapSpace]

I got different result on this problem.

http://www.freeimagehost.eu/image/3d34201277662

If you can notice on the right the results where obtained using cross program.
When I calculated by hand, I got results same as the cross program has, but they quite differ from the ones got from our standard 3d analyze&design program we use here.
Can some do the same egsampe in their application and just verify wich results are correct and wich are not.
Thanx!

 

[SnapSpace]

...but they are two differnt system...so ofcoures the results would be differnet...stupid of me...
But how else can I calculate this using Cross method?
Or Cross method cant be used when structures is subjected to lateral loads?

 

[OldPaperMaker]

SnapSpace,
I'm not real clear about what your question actually is.
If you are trying to use Hardy Cross's Moment Distribution to comparing a rigid frame to a continuous beam that is "flattened out" with a ridid frame that is subjected to sidesway you are comparing very different structural systems. I suggest that you get out your indeterminate structures text and review the section with a title something like "frames subject to sidesway". With Moment Distribution it is basically a two step process with the first step done with all joints fixed against rotation and with translation prevented. Then the moments & shear from the artificial force at the top of the frame that prevented translation is distributed to obtain the actual moments & shears.
I'm sure that your text book will do a better job of explaining this than my post has done.

 

[csd72]

Do you need a frame in this situation or can it all be pinned?

I usually dont use fixed frames unless they form part of the lateral system.

csd

 

[SnapSpace]

I tried solving this frame using cross method but my results seem to be wrong.
Heres how I started and divided the system.
Did I do someting wrong?

 

[jike]

The difference between a frame and a continuous beam is the frame is subject to sideway and additional moments are required for stability. You are comparing apples and oranges. They are not the same.

Try comparing a frame, not subject to sideway, with a continuous beam.

 

[shin25]

I finally managed to open the sketch.

I am assuming that all the connections are rigid. Then for approximate analysis- for lateral-

It is common to assume pin at the mid height of columns and mid length of beams. Find out the axial forces in the columns depending on their distances from the cg of the structure. Find out the moments.

For gravity loads, if the joints are rigid-

It's common to assume pins at both ends of a beam about .1L to .145L from the ends. 'L' being the span length. Slove for the moments. Remember any unbalanced moment will go directly into the column.

 

[msquared48]

oldPaperMaker is correct. The differece here is sidesway.

Mike McCann
McCann Engineering

 

[SnapSpace]

Shin25 can you post a sketch please

 

[shin25]

I am unable to post any sketch. But, you probably can check in any good structural analysis book and look under approximate analysis methods.

For most of the structures, exact analysis is can be supplemented with appropriate approximate methods. Remembers that for an approx. method the number of assumptions needs to be equal to DOI (degree of indeterm.) of a structure.

For an example, if DOI = 4, the assumptions need to be = 4.

 

[DaveAtkins]

You CAN use moment distribution to solve for the moments in the frame--but don't use those approximate continuous beams; use the actual frame.

shin25 describes the best approximate solution for something like this--the portal method. Once you define where moments are equal to zero (the inflection points), the frame becomes determinate.

But why not use a computer analysis program? It will give you joint deflections as well.

DaveAtkins

 

[SnapSpace]

Can someone confirm if I got the proceedure right, please.
I know that you people have mastered this, but Im a student and unfortunalty, still trying to learn something that our staff hasnt learned us:-(

http://img86.imageshack.us/img86/9410/crosshq7.th.jpg

 

[SnapSpace]

Heres the valid link

http://img86.imageshack.us/img86/9410/crosshq7.jpg

 

[SnapSpace]

Bay lenght is 6,00m and frames height is 5,00m

 

[SnapSpace]

O.k. this was wrong.
I found the right way
Will get back to you guys after Im done with practicing