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2D frame analysis using Cross method

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SnapSpace

SnapSpace

Structural
Oct 11, 2007
53
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!
 
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mmmm.... I'm so familiar with that kind of approach when frame is subject to lateral load and gravity load... the common practice is that, first, you analyze the frame separately with lateral load alone using portal method, and secondly, analyze the the continuous beam using moment distribution method with the assumption of continuous beam in the right sketch you posted... then in designing for example concrete or steel... you will combine the loads or stresses you get from separate analysis and that will be your basis in the design....

I hope I answered your question...

That's the common practice here when analyzing frame, If some one know more precise analysis can you give us some tips???
 
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Lateral load analysis with moment distribution is quite tedious- first, the frame has to be analysed as if it is fixed from lateral movement, second, the frame has to be re-analysed allowing lateral displacement and the moments from the first step has to be corrected.



 
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Can someone recommend me a good book on this subject, please.
 
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SnapSpace,
1.If you are located in a city that has a university with a college of engineering I suggest that you call the civil engineering department, ask to talk to a professor that teaches structural engineering and ask him for his recommendations for books about indeterminate structures.
2.If that resource (#1, above) is not available then go to Google, type in "books about indeterminate structures", read the reviews and order one that appears to meet your needs.
3. The book that I used as an undergraduate is probably out of print now.
4. If you have not taken an undergraduate course in strength of materials then you will probably have a very difficult time teaching yourself how to do an analysis of indeterminate frames that are subject to sidesway.
 
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This isn't really the place for hw help. However, if you are really stuck and need help, then you need to tell us exactly what you've done so we can find your error. Those last two posts have been especially lazy. You've obviously neglected the carry over moments in symmrr9. forcewb7 can be done by dividing into two members for the top - the force is conveniently given perpendicular to the members, so it should be straightforward.
 
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I didn't look too closely, but you managed to solve "crosshq7", yes? So, "forcewb7" really isn't much different.

Your mistake in "fixedbeamsz1" is that you are neglecting the columns. When you include the whole picture, you will see that the "ratio" for 1-2 and for 3-2 is not zero. Thus, at nodes 1 and 3 there will be distributed moments. This will also result in carry-over moments from 1-2 to 2-1 and from 3-2 to 2-3. You have to continue this cycle until the carry-over-moments are negligible. Didn't you do this, at least for a few cycles, for "crosshq7"? Again, I didn't look too closely at that one...

Also, I don't know what is the significance of your tabulating 4/EI*L...


It's going to get even more cumbersome when you have to consider joint translations, as others in this thread have mentioned.

I recommend the book by Hsieh and Mau; it is cheap on Amazon.

Hope that helps

 
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Forget the frame.
Im talking about only this problem:
where you have a beam thats fixed at both sides and its like an roof structure (angled).
You can see my calculation there, and also see that the final result are moments of 10,32kNm wich are far from the real solution.
By looking at the picture can you tell me where I did wrong.
 
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there is a joint translation/deflection at node 2.
you can convince yourself that a unit translation at the end of a beam creates moments analagous to the case of "antisymmetry" (if you've covered modified stiffness in your studies).

So you have to consider those moments, in terms of the node 2 deflection. You can use moment distribution as always, but considering fixed-end-moments as usual PLUS those from the deflection. In the end, you can solve for the deflection by simply including equilibrium (sum forces in y dir = 0), then you have all of your moment values.

I recommend using modified stiffness "ratios".
Also, that book I mentioned covers this.
 
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Please, can you write this down, like I did.
I kinnda need this in a hurry, and if I order the book it would take about 3 weeks to arrive.
Please
 
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forget the comment about modified stiffness for your prob.

-6*E*I*D/L^2 is the moment on both sides of a beam subjected to end displacement D.

g'luck, sir
 
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