truss analysis (2 Viewers)

[shell.q]

anyone able to help me with this, just basics. i need major help plz!!!!!!

 

[mitchell.j.bell]

anyone able to help me with this, just basics. i need major help plz!!!!!!

Do the method of joints. Find a part of the truss where members join with two unknowns and either graphically solve it in force diagram or analytically.

Don't start me on the method of sections. pffffttt.

 

[Kaos1]

i agree, method of joints is much simpler then method of sections... joints is all about simple trig. there's nothing to it. sure, it may be long and tedious, but at least u know what your doing, and is simple equation after equation... where as sections is... well... its fu*ked. whoever came up with it must have been smoking something funny... im suprised it evem works though... my teacher took us through how to do a few examples of it... but i couldnt for the life of me remember what to do...

the best gift question that you could be given in the HSC is a symetrically loaded equilatral-triangled bridge. they are so freaking easy

ooh, and obtain and read the TALE books. they are exelent in every way. there is a defferent TALE book for each section of Engineering (ie. bio-engineering, civil engineering, aernatuical engineering (my fautae), and even lifting devices, household appliances, etc. theres heaps of them)

got any questions?

 

[AndreRieu]

if you like taking moments, which is pretty much the whole course then method of sections
simply place line through, member which you would like to find and find a suitable place to take moments from then do so neglecting everything to one side of that line.

 

[theundefined]

Okay..
Well In summary. Truss analysis is always going to be moments.
Moments is, M=F x D, force times distance.

Most questions always ask you to find the reactions, meaning both ends of the truss. But this is usually the First step you do. I'd say its a must.

To find a reaction, you take "moments" about one end.

When you do this you have to know about "equilibrium", where a force, has a reaction force, meaning that they cancel each other out...like when you stand on the floor, the reaction force is the same the other way around to keep u standing... anways it shud be a little arrow, the "SUM of" symbol, and the reaction, (usualy Ra or Rb) , equals zero,so its the sum of the moment force blah equals zero. When you have that, you pick which way the arrow is positive, meaning which direction are you taking as the pivot point, and where it is going to spin. For forces that are pulling down, (lets say the we take the left side reaction) on the right, then they are positive, cus they are pulling the truss, in the direction of the arrow and they are positive as you assumed they were, wheras the "reaction" forces are negative, which in this case is Rb, because you took Ra as the pivot point. So basically its ...moment of ra= (f1 x d) + (f2 x d) ... (f3 x d) - ( Rb x D), and remeber distance, changes, as it is calculated as the force AWAY from the pivot point. Since rb is a unknown constant, u eventually find RB by simple equation and manipulation. THEN the ra is found, by the SUM of all forces. Remeber i said before about, equilbrium, cancelling out the forces. So the sum of ALL (vertical) forces equals zero to be in equilbrim, so itd be f1+f2-Ra-Rb=0 (depending on which way u take the arrow again...so thats just the basics. Hope u can understand all this LOL

 

[Kaos1]

thats very helpful, yes, in finding the reactions on either side of the trussed structure... but what if after that, the question asked: 'find whats happening in member d-e'

assuming its a simple truss configuration, and the members are named with a letter at each of the joints, and a member is the line connecting two joints...

and member d-e is in the middle of the structure... what do you do then?

 

[tommykins]

wow i dont know how you kids prefer joints, sections is like 10x easier

 

[arman]

and member d-e is in the middle of the structure... what do you do then?

The line is placed so that it cuts de.
When you take one side of the truss for analysis, d-e will act as a force and that can be found by taking moments to find that force. (this is kinda hard to explain!, sorry)

HINT: even if you are considering one side of truss with least forces but cant work out which point to take moments from, it may be easier to take moments about an external point - that is on the other side of the truss (good old copeland)

 

[mitchell.j.bell]

Okay..
Well In summary. Truss analysis is always going to be moments.
Moments is, M=F x D, force times distance.

Most questions always ask you to find the reactions, meaning both ends of the truss. But this is usually the First step you do. I'd say its a must.

To find a reaction, you take "moments" about one end.

When you do this you have to know about "equilibrium", where a force, has a reaction force, meaning that they cancel each other out...like when you stand on the floor, the reaction force is the same the other way around to keep u standing... anways it shud be a little arrow, the "SUM of" symbol, and the reaction, (usualy Ra or Rb) , equals zero,so its the sum of the moment force blah equals zero. When you have that, you pick which way the arrow is positive, meaning which direction are you taking as the pivot point, and where it is going to spin. For forces that are pulling down, (lets say the we take the left side reaction) on the right, then they are positive, cus they are pulling the truss, in the direction of the arrow and they are positive as you assumed they were, wheras the "reaction" forces are negative, which in this case is Rb, because you took Ra as the pivot point. So basically its ...moment of ra= (f1 x d) + (f2 x d) ... (f3 x d) - ( Rb x D), and remeber distance, changes, as it is calculated as the force AWAY from the pivot point. Since rb is a unknown constant, u eventually find RB by simple equation and manipulation. THEN the ra is found, by the SUM of all forces. Remeber i said before about, equilbrium, cancelling out the forces. So the sum of ALL (vertical) forces equals zero to be in equilbrim, so itd be f1+f2-Ra-Rb=0 (depending on which way u take the arrow again...so thats just the basics. Hope u can understand all this LOL

Thank-you for refreshing moments for all of us.

Now where we.... oh that's right! truss analysis....

 

[Kaos1]

im not sure how to upload an image onto this page... could someone please find a picture or a simple trussed structute, and give us a question? because that way we can actually practice abit.

cheers

 

[ShowStopper]

i prefer sections is heaps more convenient and faster.