Find the Reactions

Structural Layout Vignette and Multiple Choice

Find the Reactions

Postby Lindalou84 » Mon May 01, 2017 3:58 pm

Hi - please see the attached image. It asks to find the reactions. Is this correct (I circled the three reactions)? If so, why doesn't the diagonal take on any of the 7k load? The directions mention that the diagonal bracing is tension only (it is a cable) - is that why? If it were a strut would the answer be different?
Thank you.
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Re: Find the Reactions

Postby corbismyhomeboy » Wed May 03, 2017 9:17 am

Yes your reactions are correct. The diagonal doesn't take any of the 7k load because the diagonal is only there to handle the lateral load (aka: the 5k). Think about if you didn't have the 6k or 5k applied (aka: your structure is in equilibrium) and you put a load only where the 7k is. Only the vertical member below the 7k would deform.

The cable or strut thing is kind of a tricky / important thing to consider. The cable is only able to handle tension. In the example you provided, if the 5k load was applied to the right side (near the 7k load), the structure would not be able to handle it because the cable would be put into compression. (Think of two people holding a jump rope. Both can pull and it will stretch vs. both walking near. It would just become floppy.) If the same diagonal was representing a strut, it could handle both tension and compression, therefore it wouldn't matter if the 5k load was applied on the left or right side of the structure; the strut would be able to transfer the load. All that to say, if the problem at the beginning stated the diagonal was a cable and the 5k load was applied to the right side of the structure, you'd know that it was being put into compression and therefore was doing nothing, and to factor your reactions as if there was no diagonal.

Make sense?
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Re: Find the Reactions

Postby Lindalou84 » Wed May 03, 2017 12:52 pm

@corbismyhomeboy, Thanks that does clarify it a bit.
So with this image the truss (attached), the diagonals do have a force within them even though there is no lateral force - such as member E-K.
Why does E-K take on some of the load from the forces but in my original post that diagonal does not? Is it because the adjacent forces are creating a moment? If this truss only had the 10k load over joint E as shown, and none of the other adjacent forces, would that diagonal (E-K) be 0?

I really appreciate you taking the time to explain this. Thank you.
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Re: Find the Reactions

Postby Coach » Wed May 03, 2017 1:26 pm

Lindalou84 wrote:Why does E-K take on some of the load from the forces but in my original post that diagonal does not?

Among other things, the frame on the ground and the truss is in space.
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Re: Find the Reactions

Postby corbismyhomeboy » Wed May 03, 2017 2:39 pm

Remember that the in your first example, the cable is only taking load when it's engaged, and in this case, that would be when it's in tension.

In a truss, the diagonals are designed so they are always engaged in either tension (if they are downward pointing "V" shape) or compression (if they are upward pointing "V"s). Therefore they are always transferring the applied load. Trusses are really good at handling axial loading, but don't do well in lateral loading so, if the loads are applied correctly (aka: at a panel point) then all of the parts of the truss will only either be in compression or tension, and not in bending. (There are zero members that actually don't really do anything so keep an eye out for those as well.)

If this truss only had the 10k load over joint E as shown, and none of the other adjacent forces, would that diagonal (E-K) be 0?

No, because if diagonals of the truss are always engaged, then the diagonal will always be taking some of the 10k.
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Re: Find the Reactions

Postby Lindalou84 » Wed May 03, 2017 5:27 pm

Oh I see - I guess I didn't consider that members in a truss are designed to constantly be engaged (other than 0 members).
Thank you for the explanation - this helps a lot.
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