FEA beginner, fundamental question

[rikkyc]

Hi, Ive just started reading a book on FE and it has a description of how FE theoretically works. It says that it creates truss members and approximates them as being springs and works out deflection according to Hookes law. My question is that it when the spring constant is represented as K=EA/L what values are used for the Area and the Length? Is it the length of the elements or the length of the whole object being analysed, and if it is the elements then what is the area of an element? I was always taught that a truss member had no cross sectional area. Any help you could offer is much appreciated, Thanks.

 

[GBor]

Not sure what book you are reading and would question your "teacher", but truss elements do have cross-sectional area and the length is the length of elements descretized within your model. If truss elements do not have physical properties, then bridges are falling...particularly in the NorthEast US and under trains that are crossing rivers all over the world.

 

[rb1957]

agree with the above ... truss elements have area (if they didn't how could they react endload?) and their length (L in the stiffness equation) is from node to node (joint to joint if you'd rather).

i think this question could have been clarified by reading a FE text book

 

[rikkyc]

Ok so how do you know what the cross sectional area of the elements are? Or is it a case of they are always identical, say 0.1mm^2 just specified by the program being used?

 

[rb1957]

the cross section area is the area of a section normal to the axis of the truss element. not all truss elements will be the same size (that would be very inefficient design). each element has it's own stiffness based on its E, A, and L.

this has got to be spelt out in an text book

 

[jen4950]

The simple answer to your question is that the entire truss is split into each individual element between nodes. So the A, E, and L components are for each element.

The fundamental definition of a truss is that the loads can only be applied at the nodes- therefore the resulting internal forces can be idealized as pure axial loads. And Finite Element Analysis is the analysis of a defined structure- so you need to specify these parameters for each element for the analysis to be effective (and meaningful).

 

[GBor]

Area is generally an input into an FEA for a truss element type. Truss elements are usually 3 degree of freedom elements meaning that they don't transfer rotational motion. To transfer rotations, you change to beam elements, which are another form of line element.

Have to agree with rb1957, this should be spelled out in virtually any textbook. I don't think any of us are prepared to teach FE101 over a web forum.

 

[Denial]

Perhaps Rikkyc was envisaging a statically determinate truss, where the force distribution (but not the deflected shape) is not dependent upon the cross-sectional area of any of the individual members.