Solving Simple Truss Problems: Identifying Compression & Tension

[Dave1357]

I wonder how to determine a member is in tension or compression without calculation.
For the above video starting at 3:03, how to idenfify the last 2 members are in compression and tension respectively.
Hope some more examples and diagrams to illustrate.
Thanks a lot.

 

[PhanthomJay]

You can't always tell by inspection, but in this simple case, does the leg on the right get pushed down or pulled up when the load is applied as shown? How about what happens to the other leg?

 

[Dave1357]

You can't always tell by inspection, but in this simple case, does the leg on the right get pushed down or pulled up when the load is applied as shown? How about what happens to the other leg?

I'm sorry I can't get what you mean. How does the compressive member come?

 

[PhanthomJay]

I'm sorry I can't get what you mean. How does the compressive member come?

Compressive forces are 'pushing' forces and tensile forces are 'pulling' forces. Take 2 popsicle sticks and fasten them at one end in the shape of the upside down V as in your problem ( ^ ). Set the legs on a table and apply the rightward horizontal force at the top. One of the legs will pull off the table, while the other leg will push on the table. Which is which? The one that pulls would be in tension if anchored to the table, and the one that pushes will be in compression.

 

[Dave1357]

Compressive forces are 'pushing' forces and tensile forces are 'pulling' forces. Take 2 popsicle sticks and fasten them at one end in the shape of the upside down V as in your problem ( ^ ). Set the legs on a table and apply the rightward horizontal force at the top. One of the legs will pull off the table, while the other leg will push on the table. Which is which? The one that pulls would be in tension if anchored to the table, and the one that pushes will be in compression.

Oh, I understand. Thanks a lot !
So, this kind of checking can only be applied on simple case like the above, right?
If there are complicated trusses, we must use method of joint/section to calculate.

 

[PhanthomJay]

Oh, I understand. Thanks a lot !
So, this kind of checking can only be applied on simple case like the above, right?
If there are complicated trusses, we must use method of joint/section to calculate.

They can be a bit more complex, but, yes, that is best. Try to visualize when you can, then run the numbers.