Calculate the stresses in this 4-member wooden frame

[Vladimir_Kitanov]

Why force at D need to be parallel to force at E and F?

 

[DrClaude]

Why force at D need to be parallel to force at E and F?

Because otherwise there would be rotation around D.

 

[Lnewqban]

Why force at D need to be parallel to force at E and F?

Force at D does not need to be, it happens to be parallel as a consequence of the coincidential direction of the only other forces acting on that member DEF.

BCEF forms a quadrilateral of which only the opposite sides and angles are equal, making the directions of forces at D, E and F parallel to each other.

 

[Vladimir_Kitanov]

Can this be solved without assuming that forces are parallel?

 

[Lnewqban]

Can this be solved without assuming that forces are parallel?

I believe so.
Have you done any solution work that we could see?

 

[Vladimir_Kitanov]

I believe so.
Have you done any solution work that we could see?

How ever I tried I did not managed to solve it.

 

[Vladimir_Kitanov]

I believe so.
Have you done any solution work that we could see?

I brake that in pieces but always get 2 unknowns and 1 equation.
That is impossible to solve.

 

[Lnewqban]

I brake that in pieces but always get 2 unknowns and 1 equation.
That is impossible to solve.

Could you please show us what you have done so far?

 

[Vladimir_Kitanov]

Could you please show us what you have done so far?

I can't send picture now it say that I have low memory

 

[Vladimir_Kitanov]

 

[Lnewqban]

Thank you!
Hard to see, but I will try a little later.

 

[Vladimir_Kitanov]

Thank you!
Hard to see, but I will try a little later.

Okay thanks

 

[Lnewqban]

While you wait:
https://mathalino.com/reviewer/engi...members-frames-containing-three-force-members

 

[Lnewqban]

Sorry for later than promised response.
The two horizontal members, AC and DF are not working purely under compression or tension, like it would happen in a truss.
This is an armature, in which some members are supporting bending and shear loads.
That is the case for those two horizontal members.

You could try estimating those loads by calculating moments and forces also about the intermediate points B and E respectively.
The reactions at those points, aligned with the 3-4-5 triangles, would be the asked axial loads of members CF and BE (both under pure axial loads, no bending or shear).

The rest would be to estimate the stresses that those axial loads impose on the 2x4 cross sections.

 

[Vladimir_Kitanov]

Sorry for later than promised response.
The two horizontal members, AC and DF are not working purely under compression or tension, like it would happen in a truss.
This is an armature, in which some members are supporting bending and shear loads.
That is the case for those two horizontal members.

You could try estimating those loads by calculating moments and forces also about the intermediate points B and E respectively.
The reactions at those points, aligned with the 3-4-5 triangles, would be the asked axial loads of members CF and BE (both under pure axial loads, no bending or shear).

The rest would be to estimate the stresses that those axial loads impose on the 2x4 cross sections.

View attachment 317262

Still can't do that.

 

[Vladimir_Kitanov]

Did you solve it?

 

[Lnewqban]

Did you solve it?

No, sorry, I have had no time to properly do it.
Do you know the official responses?

Now you have two more moment equations to use (about B and E joints).

If you can, please, study problem 003 of link provided in post #13 above.

 

[T1m0]

It looks like your structure is not statically determinate; e.g., you could make the support at D a roller support to eliminate Dy, and the structure would still be stable. To solve the original problem, you would need to account for the deformation of each member. This would not be a fun problem to solve by hand. You could solve this pretty quickly using an FEA program if you have one available.

 

[Vladimir_Kitanov]

I think that I proved that forces must be parallel if system is in static.

 

[Lnewqban]

I think that I proved that forces must be parallel if system is in static.

Do you need to do that to solve the original question about maximum internal loads of members BE and CF?

 

[Vladimir_Kitanov]

Do you need to do that to solve the original question about maximum internal loads of members BE and CF?

No.
I just need proof

 

[Lnewqban]

No.
I just need proof

I see, but why?
Aren’t you only trying to solve the problem as described in the original post?
Sorry, I am confused.

 

[Vladimir_Kitanov]

I see, but why?
Aren’t you only trying to solve the problem as described in the original post?
Sorry, I am confused.

I tried to solve it without assuming that forces are parallel, but failed.
I beleaved that it is true but i wanted to see that proof.

 

[Lnewqban]

OK, let’s start from scratch.
In general terms, what have you learned about solving structures so far?

 

[Vladimir_Kitanov]

OK, let’s start from scratch.
In general terms, what have you learned about solving structures so far?

I already knowed how to solve those problems but that confused me.
That is first time I needed to know that forces are parallel.