Changing structures over time 3

[Danielsp]

Hello folks

I am a civil Engineer, M.SC. in structures Engineering and I model and design structures, mostly using FEM.

Right now I am interested in static modeling structures that change over time. Some cases are really simple, for instance:

1) If you load a structure then remove some part of it while keeping the same load, then the new moments, shear forces and strains can be easily obtained by simply ignoring the original structure. You load the new structure and that's it.

2) If you apply Load 1 on Structure 1, then add a structural element to it and finally apply a new Load 2, then the final state of forces and displacements is obtained by adding L1 on Structure 1 to L2 on structure 2.

So far, no big deal.
But what if after the first load you add a new part to the structure while loaded and then remove an old one?

 

[Danielsp]

rb, the slabs are not in different spans. On a vertical projection, they occupy the same area, a full circle. The final shape is partly above and partly below the existing one.

 

[Danielsp]

Drift, at least for me, solving the physical problem was not easy. So much so that I came here looking for answers, and even after some discussion, someone else solved it. Apparently, it wasn't easy for most people.

To prevent the rotation of a structure node in real life and then build something around that node does not seem easy for me. Especially when compared to letting the node free to rotate.

 

[rb1957]

It sounds to me like you're reinforcing an existing slab.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Danielsp]

Rb yes, that's a reasonable description.
In that spirit, parts of the reinforcement would be on top of the existing slab and parts of the reinforcement will be under it.
So, this is how it goes:

1) Existing slab is already loaded
2) Upper reinforcement is built
3) Part of the existing slab is demolished in order to build the lower reinforcement
4) Lower reinforcement is built

That is my present version of the "load, remove and then add" problem which motivated my query.

 

[Tomfh]

What is the structure you are concerned with regarding the load sequence? The old slab? The new slab? If it’s reinforced concrete it should be ductile and this level of fuss over exactly which member is doing exactly what may not be warranted…

You’re chopping out a third of the circle at a time and replacing with a thicker slab? Is that what you mean by a hole?

How does the 400mm slab work? How is it supported? Where are these walls that it is bracing?

 

[Danielsp]

Tomfh, I'm worried about both. I'm chopping a third of the hole at a time. The hole is a rectangle of some 5,5x25,5m that starts at the perimeter and runs through the center.
The existing slab is fully supported by the soil below. The walls are at the perimeter of the circle.

 

[Tomfh]

If the slab is fully supported on soil then what load is it seeing?

Presumably the new slab will also be supported on the same soil?

And the hole is only a small portion of the original slab? How thick is the new slab outside of the hole area?

Maybe draw a sketch of what going on. It’s a bit hard to understand.

 

[Danielsp]

The slab is actually a strut for the walls. The upper level of the new slab will be on top of the existing one. The lower, under it.
The hole has around 240m² in a total of some 1000m². Outside the whole, the new slab is 2m thick.

 

[rb1957]

please stop and send a drwg. we were talking about slabs which I think we thought of as round, ok, it's rectangular ... oh and now has a hole in it.

for the life of me I don't understand why you can't put all the reinforcement on top, or why you can't disassemble the existing slab piecemeal and replace it (piecemeal) with the new one ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Tomfh]

Yes I’m really struggling to understand it myself.

I feel we’re missing some important information. A slab increasing in thickness from 400mm to 2000mm? What is really going on here?

 

[Danielsp]

Folks, unfortunartely I'm not at liberty to release more detailed data. I was really just trying to discuss general principles, not one specific structure.

 

[rb1957]

then I guess we've been as helpful as we can be.

We don't need the project name (nor particularly location, unless that is relevant).

If you can't show us the specific design, we can't talk (sensibily) in terms of generalisations. How relevant was all that discussion on beams (to reinforcing a slab) ?
Sure we can think it quite the change (from 400mm thick to 2000mm) but you can say, "yeah, that's what we want to do". Of course this can raise questions in our minds and we could say "there are things here we don't understand and don't want to mislead people (into doing dangerous things), so pass."

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Danielsp]

Rb, the problem had already been fully understood and solved by steveh49 a couple of days ago. I didn't quite understand what was everybody else's goal on the topic after this. He answered my question, but I'd be glad to continue discussing the issue if that's what you'd like to do.

As I stated on my messages, I was just addressing the absolutely general situation of how to solve any structure undergoing the situation "load, add part and then remove part". Yes, it was originally motivated by the slab situation but at no point I asked about the slab itself. Steveh49 showed how to approach this using a simple beam structure, but the principles he stated are universal and therefore can be applied to slabs, shells or whatever else. I just made a quick passing mention about the slabs to Brad because he insisted on some sketches and I pointed out that they were irrelevant since I was after general solutions, not solving my specific structure.

At the time, I said, "Brad, I didnt use a drawing because I was afraid it would tie the discussion to the specific case." And apparently I was just exactly right. After that comment, everybody started to talk just about slabs while my query was about structures in general.

 

[rb1957]

Really ? The last thing SteveH contributed was "Yeah, maybe not a good example either. Too trivial."

Since then we discovered you were talking about a slab, which I thought completely changed the subject.

But if you're happy, I'm not dissatisfied.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Danielsp]

Rb, yes... that's his last message. After he laid out the general concept and created a simple example.
As I said, I was never talking specifically about slabs or a slab. The whole time I was asking about a general problem, but suddenly everybody else changed the subject. I guess that's because most people dont want to deal with really universal principles since they are evidently harder.

 

[rb1957]

you said it was a slab ... "rb, It's a 35m diameter 2m thick slab at the bottom of a subway access well 33m deep." (17:29 on the 4th Aug)

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Danielsp]

Yes, rb... I did say it was a slab. In passing. But I wasn't discussing the slabs, even in that message.
I mentioned it for the sole reason that... you asked me to! ("what are you building that you need to jump through these hoops ?" 4 Aug 22 16:58)