1
$\begingroup$

My instructor said that if a joint has reaction support attached to it, then it is not a zero force member. However I saw in this figure that joints E and A are attached to a pin and a roller, respectively but it is indicated in the answer key that members DE and AE are zero force members. What is your insight about this?

enter image description here

source: Engineering Mechanics - Statics (R. C. Hibbeler)

$\endgroup$
1
3
$\begingroup$

Use graphics to visualize the force flow and determine zero-force members. (Draw member forces from joint D)

enter image description here

$\endgroup$
3
$\begingroup$

DE and AE are zero are indeed zero.

However you need to start from point C to reason about that.

If you look at this question, when you have a node without external forces with only two members which are not colinear, the forces on the nodes should be zero. So from this $DC$ and $CB$ are equal to zero.

Node D:

Now if you take point D and write the equations of equilibrium you obtain:

  • for x $$ ED - DC =0 \rightarrow $$ $$ ED = DC =0 $$

  • for y: (so all the force is carried from ED $$ -DB - 3kN =0 \rightarrow $$ $$ DB = - 3kN $$

Node A:

Again for Node A if you take the equilibrium in the y-axis you get:

$$ +AE =0 $$

because A has rolling support and it produces only a horizontal reaction.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.