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dimensioning deck chairs

  • Thread starter Thread starter Mito125
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Mito125

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Good morning, I'm preparing for the examination of machine constructions... in one of the past exams the prof asked to design a deck chair for the sea, having only the schematization of the structure and the length of the arms to which is connected the canvas that supports the person... Of course I know the maximum weight that must support the structure... But I don't know how to schematize and solve the structure internally... I tried to divide, by chance, the weight in two equal parts on the two arms... I found external binding reactions, but then I can't "disassemble" the structure to calculate internal reactions to make the relative diagrams and find the most stressed point... I tried to look at whether on the violet or on the belluzzi there was a similar exercise, but nothing... Do you have any advice? Thank you:
 
I don't understand the problem.. .
Don't you know what loads to apply? We can help you.
Don't you know what shape to give to the structure? we can suggest a way
but if you know the two previous points, you can't calculate the internal tensions to the structure, you're put wrong, but really bad.. .

maybe put a single and very precise point of the structure that we can illustrate a generic method.
 
Wow. This is an examination theme that leaves freedom to the student.
once baptised in the direction and form the forces due to the weight (even here there is wide freedom, I would say) to obtain the other forces should suffice some triangle of the forces applied to the two main pieces to solve the structure
 
we always start by assuming that the weight affects little on the structure, so as not to consider it in the first approximation. . I completely miss the double zipper that is there... i can't schematize it with a set of forces. . .
 
Mito125 said:
we always start by assuming that the weight affects little on the structure, so as not to consider it in the first approximation. . I completely miss the double zipper that is there... i can't schematize it with a set of forces. . .
Click to expand...
I guess you're talking about the weight of the structure. The approximation you say is plausible.

If a dot is between two hinges, you know that it can generate forces exclusively aligned with itself. and therefore you have already identified at least direction and direction.
then make the knot balance.
 
the compressed rod between the two hinges I managed to set it, is a typical example sopratt in the flag cranes. . what makes me trouble is the "hole" of the structure, there creates all the problem.. .
 
However, I did not understand whether the general procedure for the solution of hyperstatic structures is clear or not...

I tried to make an example, without any claim to correctness. try to see the annex:

1. do the free body diagram of the three pieces
2. replace binding reactions with forces. you find nine unknowns.
3. Write the balance equations of the three pieces. you find six equations.

there are three more unknowns; in fact the structure was three times hyperstatic.

4. Write a bit of congruence equations, for example impose zero rotations and shifts to the inks, until you get the three equations you miss.

5. Now you have nine equations in nine unknowns. or you put a half day trying to solve it, or you make it symbolically solved by a calculator, or by matlab, or, much better, you write it in the form of a matrix, you reverse the matrix from the calculator and transcribe the result.

p.s.
If you have matrix rank degradation, try replacing a couple of equations with balance equations to the whole structure. Maybe simplify the accounts.
 

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Before I put myself to solve it, I would like to propose a further simplification... and if instead of 2 frames I put a zipper and a cart down??? I tried to fix it so...
 
Fulvio Romano said:
However, I did not understand whether the general procedure for the solution of hyperstatic structures is clear or not...
Click to expand...
Mito125 said:
Before I put myself to solve it, I would like to propose a further simplification... and if instead of 2 frames I put a zipper and a cart down??? I tried to fix it so...
Click to expand...
I mean?
Do you want to remove the incastries and replace the binding reactions removed with incognite active forces, or do you want to remove the incastris and just make the isostatic structure?

In the second case... you answered the question above.
 
Maybe I explained badly... I would not have considered the two supports as two frames, a sunbed leaning on the ground to the max I can consider it as supported-supported, rather than set-up... then I simplify saying to consider one of the two supports as a cart, so it becomes isostatic... is it??? ?
 
Mito125 said:
Maybe I explained badly... I would not have considered the two supports as two frames, a sunbed leaning on the ground to the max I can consider it as supported-supported, rather than set-up... then I simplify saying to consider one of the two supports as a cart, so it becomes isostatic... is it??? ?
Click to expand...
ah ok. as you say it would have been more correct, in the sense that schematization would have been more adherent to reality.

However you are not solving the sizing of a deck chair, but an exercise with data imposed...right?
 
the problem I would consider it in two dimensions.... in pdf of fulvio where it put the forces p1 and p2 also acts a flente moment due to the rod to which the roof of the deck is hung
 
Perhaps I have misunderstood myself.... I did not mean size but according to two separate floors.... a plan that includes all the rods as in the schematization porposta while another normal plan to this and that includes the cross rod that supports the roof of the deck
 
I am so free to do what I want... That's what the prof says when the exam starts... do what you want but do not call us (it means him and the assistants). .

thanks hairitano della dritta... important note... But until I can solve in a plan, I can't try to add a torque that obviously there is...

I still haven't solved the problem cmq...
 
peloritano said:
Perhaps I have misunderstood myself.... I did not mean size but according to two separate floors.... a plan that includes all the rods as in the schematization porposta while another normal plan to this and that includes the cross rod that supports the roof of the deck
Click to expand...
I really considered it a slow problem.
actually there is the tart. we can imagine that it transfer to the design structure is a cut (which turns into the force that I put) and a couple, which would turn into a twisting moment.

However be it, given the principle of overlapping effects, solve your structure, then add the torque moment you always do in time.
 

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