Undergrad Truss analysis problem

[jojosg]

TL;DR

determine the internal force of member OA

Hi I'm struggling with this problem. I have calculated the angle to be 45 degrees with tan(theta) = a/a. A is a roller which only has Ay and O has Ox & Oy as a fixed pin. (see fbd.png) is my FBD correct?

 

[Lnewqban]

TL;DR: determine the internal force of member OA

Hi I'm struggling with this problem. I have calculated the angle to be 45 degrees with tan(theta) = a/a. A is a roller which only has Ay and O has Ox & Oy as a fixed pin. (see fbd.png) is my FBD correct?

In order not to have any movement or displacement in this system, the effects of both applied forces must be counteracted by those two supports, which are the only connection of that structure to the solid ground.

Therefore, your FBD should include all the ways in which those effects could reach those supports.

The directions of the forces potentially reaching each support coincide with the geometry of the connecting links, which are only able to transfer traction or compression forces.

Your FBD is missing the possible influence of links HO and HA.

 

[jojosg]

In order not to have any movement or displacement in this system, the effects of both applied forces must be counteracted by those two supports, which are the only connection of that structure to the solid ground.

Therefore, your FBD should include all the ways in which those effects could reach those supports.

The directions of the forces potentially reaching each support coincide with the geometry of the connecting links, which are only able to transfer traction or compression forces.

Your FBD is missing the possible influence of links HO and HA.

what if I take method of sections and use the left side?

 

[Lnewqban]

Please, see:
https://mathalino.com/reviewer/engineering-mechanics/method-sections-analysis-simple-trusses

https://mathalino.com/reviewer/engineering-mechanics/problem-435-transmission-tower-method-sections

 

[wrobel]

A reaction in the hinge O has two components. A reaction in the hinge A has only a vertical component. We have three unknowns. Find them first by considering the equations of equilibrium for the whole construction as a rigid bod

 

[jojosg]

A reaction in the hinge O has two components. A reaction in the hinge A has only a vertical component. We have three unknowns. Find them first by considering the equations of equilibrium for the whole construction as a rigid bod

ok for Fy=0; Oy + Ay = 2F, Fx=0; Ox + F = 0, Ma=0; 2F(2a) - Oy(2a) - F(2a) = 0; which gives Oy = F & Ay = F & Ox = -F.

 

[wrobel]

which gives Oy = F & Ay = F & Ox = -F.

I have got the same and my guess is that the internal force must be Ox

 

[Steve4Physics]

I have got the same and my guess is that the internal force [in member OA] must be Ox

That would only be true if the tension in member OH were zero. But in the x-direction, the force-balance at point O is:
$-F~ +~ T_{OA}~ +~ T_{OH} \cos 45^ \circ = 0$