Settlement

[Engineer22]

what will happen in ettltment

 

[Engineer22]

Assume R3 is column 3. Thank you.

 

[bassplayer45]

It is my understanding, on something like this, is that when R3 settles, it typically cracks, and no longer supports as much load anymore, therefore the load gets re-distributed to the nearest support, so I think negative bending at 4 increases, but don't hold me to that, im a bridge guy

 

[tj_PE]

draw your shear and moment diagrams (or refer to the handy tables in the steel manual) - is there a moment at 4?

 

[Engineer22]

Thanks Bass! 

Julie, there's just a column at 4.

 

[tj_PE]

Thanks Bass! 

Julie, there's just a column at 4.

I can see there's no applied moment at 4. What I'm asking is if you draw your shear and moment diagram for the continuous beam over four columns, do you get a moment at 4? If they're pinned connections, you shouldn't. but you still wouldn't if you put a downward force at 3 (say if the column ends up settling enough to "hang" from the beam)or simply removed that support, so the moment at 4 would still be 0. Therefore, my thought is that there would be an increased negative moment at 3. I could very well be overthinking this, though. 

 

[DoctorWho-PE]

I agree with thejulie.  The end span has positive moment towards joint 4, and negative towards joint 3.

Maybe I can pass this thing after all.   

 

[smahurin]

The moment would lessen at joint 3, therefore the right answer is the moment would increase at joint 4.  

Initially the continuity means you have a negative bending moment at joint 3.  If theoretically, the settlement were severe enough that you lost all support at joint 3, the member would function as a single span between 2 and 4 with a large positive moment at midspan (where joint 3 used to be).  As it settles the moments will redistribute in this manner, shifting from a negative moment at 3 to a positive moment at 3 which means if joint 4 is fixed you'll end up with a larger fixed end moment there.

 

[smahurin]

Uplift at 3 would be the only thing that would cause joint 3's negative moment to increase.

 

[Engineer22]

That makes sense about creating the moment diagram and then evaluating!

What if you have the attached condition? Downward load only at 3. In this case, which has the increased negative bending moment?

1, 2, 3, or 4?

 

[tj_PE]

The moment would lessen at joint 3, therefore the right answer is the moment would increase at joint 4.  

Initially the continuity means you have a negative bending moment at joint 3.  If theoretically, the settlement were severe enough that you lost all support at joint 3, the member would function as a single span between 2 and 4 with a large positive moment at midspan (where joint 3 used to be).  As it settles the moments will redistribute in this manner, shifting from a negative moment at 3 to a positive moment at 3 which means if joint 4 is fixed you'll end up with a larger fixed end moment there.

I agree with this as well - that the negative moment wouldn't necessarily get more negative. i don't like this question. haha. but the fixity is exactly what i was trying to address. if it's a concrete beam with concrete columns then there will likely be some moment at 4. 

 

[tj_PE]

That makes sense about creating the moment diagram and then evaluating!

What if you have the attached condition? Downward load only at 3. In this case, which has the increased negative bending moment?

1, 2, 3, or 4?

View attachment 10104

if support 3 settled and had this load, i would want to say negative moment at 2 would increase assuming pinned connections

 

[Engineer22]

Uplift at 3 would be the only thing that would cause joint 3's negative moment to increase.

Settlement at 3 would lead to increased negative bending moment at 2 or 4 then? I am having a difficult time picturing this. I think it would lead to increased negative bending moment at 2 and a smaller negative bending moment at the right (3).

 

[bassplayer45]

It all depends how they are connected. If they are all fixed connections that can take moment, then the settlement will result in moment being distributed to 2 and 4 evenly (in theory). If 4 can't take moment, then the moment has to get distributed to support 2 (assuming it can support moment).

Think of it like a sticker you placed on a hard hat that got an air bubble. You push down on it to remove the air bubble (the support settles), and you usually get 2 smaller bubbles on each side of it (assuming you pushed in the middle), that is kind of like what is going on here. Putting a downward force like people have shown above is a way of simplifying what is happening from the settlement. The negative moment is the bubble, downward force pushes on it, it spreads to each side.

 

[Engineer22]

That makes alot of sense bassplayer!  Thank you for the visual analogy. Yes, all fixed.

 

[Engineer22]

Also, please ignore the uniform load, and look at my sketch above: there should only be one point load downwards at 3, with 4 fixed columns, and 3 experiences settlement.. I've received a variety of answers, so I am stumped.