Virtual vs Real Neutral Axis 3

[hocho]

The neutral axis in the interaction diagram doesn't really correspond to the physical location of the column. Or do they?

For example. Below the balanced point. At zero moment. The neutral axis is very small (or infinitely small). At zero moment (edit: I meant at zero axial load). It acts like a beam. Yet in a beam. The real neutral axis is the middle of the beam. So the interaction diagram neutral axis doesn't correspond to the physical location of the neutral axis. What is the formula that relates the virtual and real neutral axis (what official terms distinguish these two)? Is there a software that can show or distinguish them and plot them both?

 

[BAretired]

Given a similar situation, I would want to determine the magnitude of axial load and moment at the base of the column in question and compare it to a reasonable estimate of the column capacity with epoxy in the void.

If the column appeared inadequate, I would study the effects of a redistribution of moment to other members of the frame. If that appeared reasonable, I would take no further action.

If the nine columns acting together provide insufficient lateral resistance, I would try to find a way of improving lateral resistance.

BA

 

[hocho]

Given a similar situation, I would want to determine the magnitude of axial load and moment at the base of the column in question and compare it to a reasonable estimate of the column capacity with epoxy in the void.

If the column appeared inadequate, I would study the effects of a redistribution of moment to other members of the frame. If that appeared reasonable, I would take no further action.

If the nine columns acting together provide insufficient lateral resistance, I would try to find a way of improving lateral resistance.

Ok. We'll first put the roofing before deciding whether to remove the waterproof topping (the reason for the topping was to let rain flow to drain). Btw.. you were wondering yesterday in the message

"Why does C1 have a large moment at Story 3 (-22.17 or -20.88kN-m), C2 has less than half as much (about 8) and C6 has zero moment?"

The following is the layout in the third floor for the graphics you commented the above on. I got an old file 2 years ago from the designer when the building was designed for 4 storey. I removed the 4th storey and partially removed some beams (middle longitudinally) in the third storey to test out the concept of column superposition (so the values were not meant to depict the real structure.. but the superposition curvature is still there). Right now in actual it's only columns that actually rise up above second storey. Roof will be deliver tomorrow and put on Monday.

Anyway. In your drawing of the curvature from third floor to ground. Why didn't etabs show the 2 storey combined curvature? etabs only show the column moment in each storey. Your drawing shows curvature for 2 storey. Maybe it's called second order moments? What is the exact term for it that I could find in books? I have 5 days to read the book to decide whether to remove the topping. The designer said it's up to me to decide because he believes the epoxy void functions like concrete.

 

[hocho]

Hi Ba.. i'm a bit confused by your drawing

Is the curved lines supposed to show the moments? Why are they showing it pinned? What would happen if you draw it double curvature in one beam that is originaly depicted in etabs (I'm referring to the simple beam column span sample. not the complex one.. just ignore the complex one to avoid confusion).

Again

with 5 10kn load on top of second beam:

Etabs is showing the S curvature, you are showing one curvature only. Or is your curved lines not moments but deformation? But isn't moments curvature already how they actually deformed?

 

[BAretired]

Superposition is a legitimate technique whereby moments and forces are calculated separately for each load, then combined by adding some or all of them together to find various load combinations listed in the code.

Anyway. In your drawing of the curvature from third floor to ground. Why didn't etabs show the 2 storey combined curvature? etabs only show the column moment in each storey. Your drawing shows curvature for 2 storey. Maybe it's called second order moments? What is the exact term for it that I could find in books? I have 5 days to read the book to decide whether to remove the topping. The designer said it's up to me to decide because he believes the epoxy void functions like concrete.

I am not familiar with etabs but some frame programs show deformation if you request it. The moments we have been discussing are not second order. Second order moments result from deformation of the structure which changes the value of some moments because the geometry of the structure has changed. Second order effects are usually small and are often neglected in practice.

There is no special term for moments resulting from a frame analysis. Each part of the structure simply responds in accordance with its stiffness. Any book on the analysis of statically indeterminate structures would be suitable. You can likely find a lot of information online.

The designer is wrong on two counts. First, it is not "up to you to decide" because you are not the designer, you do not have the training to make such a decision and you do not have full access to his design calculations or computer printouts. It is up to him to make a recommendation. Responsibility for design belongs to the designer and if there is a problem with the structure, he is the one who must accept liability for his design, not you.

Second, epoxy does not perform like concrete. It has adequate strength but it has a much lower modulus of elasticity; a much larger strain is required to develop that strength. The presence of the epoxy filled void changes the stiffness of the column, bringing into question the results of the frame analysis.

If the slab was intentionally roughened before placing the topping, they will be well bonded together. Removal will not be easy without damaging the slab below.

BA

 

[hocho]

Superposition is a legitimate technique whereby moments and forces are calculated separately for each load, then combined by adding some or all of them together to find various load combinations listed in the code.

I am not familiar with etabs but some frame programs show deformation if you request it. The moments we have been discussing are not second order. Second order moments result from deformation of the structure which changes the value of some moments because the geometry of the structure has changed. Second order effects are usually small and are often neglected in practice.

So those moments already take into account the deformation as one of the superpositions. Makes good sense. How many structural engineers intentionally makes the second floor having more load to kinda decrease the moments in the columns in ground floor? Have you done it?

Between the topping and slab is the waterproof membrane. So they are not really connected. In our country. We mostly use Bitumen membrane. See:

http://www.ikogroup.co.uk/Products/Flat-Roofing/Bituminous-Membrane-Systems/Torch-On-Membranes/

It's described as "Torch-on membranes have an underside which has been pre-treated with a covering of thermofusible bitumen. This covering is then heated with a propane gas torch and the membrane is applied to the surface of the roof while the bitumen is still hot."

On top of the slab is the waterproof membrane then we put wire mess so concrete topping (to let rain sloped to drain) can bind together. To remove the topping. I don't want to use jackhammer but lifting the topping (again it's not directly connected to slab because of the waterproof member between them) but don't know how easy when the wire mesh bind all topping together. Any ideas or experiences?

After removing the topping. I wonder if the waterproof would still be smooth. Because the tenant want to put wooden tile and they want smooth surface.. I wonder if the membrane could still be smooth surface and if rough.. I wonder if thin concrete can bind to the membrane (have you tried it in 60 years?).

Since the total roof area is about 175. And each 2" of topping weights 23.56x0.05 = 1.178 MPA * 175 = 206 kN.

206 kN of unnecessary SD load is large, huh? (unnecessary SD load because roof will be put into the floor next week) But then.. since the building has 8 edge columns. The weight above the second floor can decrease the moments of the 8 columns in the ground floor. Again. Are their structural engineers anyway who took advantage of this moment decreasing effect of increasing loading in the second floor?

Many many thanks.

 

[BAretired]

So those moments already take into account the deformation as one of the superpositions. Makes good sense. How many structural engineers intentionally makes the second floor having more load to kinda decrease the moments in the columns in ground floor? Have you done it?

No! I don't know anyone who has.

On top of the slab is the waterproof membrane then we put wire mess so concrete topping (to let rain sloped to drain) can bind together. To remove the topping. I don't want to use jackhammer but lifting the topping (again it's not directly connected to slab because of the waterproof member between them) but don't know how easy when the wire mesh bind all topping together. Any ideas or experiences?

No experience with that.

After removing the topping. I wonder if the waterproof would still be smooth. Because the tenant want to put wooden tile and they want smooth surface.. I wonder if the membrane could still be smooth surface and if rough.. I wonder if thin concrete can bind to the membrane (have you tried it in 60 years?).

Not that I can remember.

206 kN of unnecessary SD load is large, huh? (unnecessary SD load because roof will be put into the floor next week) But then.. since the building has 8 edge columns. The weight above the second floor can decrease the moments of the 8 columns in the ground floor. Again. Are their structural engineers anyway who took advantage of this moment decreasing effect of increasing loading in the second floor?

Not so far as I am aware.



BA

 

[hocho]

No! I don't know anyone who has.

Ok. But with the simple beam and column sample hit with lateral force USB97. The one with additional 5kN uniform load in the top beam has lower ground floor column seismic moment.
See:

Without the additional 5kN uniform load in top beam, there is greater ground seismic column moment. See:

In the example. The top beam need to have twice the loading of the lower beam for there to be significant lowering seismic moments in the ground columns. So I guess this is not practical in real application. The topping may not significantly affect it. So I guess I have to remove the topping. But tenant want smooth surface for wooden tile. Anyway. In your experience. What is the minimum thickness of smooth cement polish surface for it to be stable on top of raw rough slab surface? Do you just use adhesive or wire mesh? I think half inch of topping may break easily. I don't want to end up duplicating the topping by putting a new wiremesh and putting another topping. If one inch is the minimum thickness. I may not have to remove it or just grind some part to make it even. Thanks.

 

[BAretired]

I agree that the addition of load on the top level decreases the moment at the base. It also decreases the positive moment in the lower beam. That is to be expected.

It seems foolish to remove existing topping, then add a new one. I would think it best to grind the surface flat, then apply the tile on an adhesive.

BA

 

[hocho]

BA.. thank you so much for all the help and enlightening tips on the theoretical side. I couldn't have understood some concept for many months without you. Appreciate it so much. I guess I now have to ask all local contractors about grinding and polishing.

Beers and cheers to you

 

[BAretired]

Beers and cheers!

BA