Analysis of continuous slabs separately from building model

[user277418]

Hi

It is common practice to analyse/design continuous slabs separately from building model in FEM. It is clear and easy when gravity load-bearing system (columns and walls) is the same on all stories or changes relatively uniform along height of building. But what if location of bearing elements differ each couple of stories. For example 3 stories the same pattern of columns and walls. Next 3 stories some columns from above become supported by walls/deep beams and vice versa. Then the pattern changes again after 2 stories etc. Have met such buildings quite often. For correct analysis of continuous slabs stiffness of supports matters. Therefore, sometimes, it leads to a necessity to design/analysis almost every slab in the building even they look the same.

How do you deal with that? Are there any simplification methods/rules of thumb for such cases?
How such software like SAFE deals with it?

Best regards

 

[KootK]

How do you deal with that?

I typically deal with it just as you described: model the whole building in ETABS and then pull out individual floors for detailed design in SAFE. In doing this, one needs to give some thought to how the sequence of construction would affect the distribution of load between levels. If you're not careful, you can wind up with weird stuff like columns in tension etc.

Are there any simplification methods/rules of thumb for such cases?

The advanced versions of whole building FEM software often have tools for studying construction sequence. I don't normally use them however. Rather, I tweak the model to make transfer slabs infinitely stiff such that, when I export those slabs for design, they are loaded as they would have been loaded by hand back in the pre-FEM days.

But what if location of bearing elements differ each couple of stories.

If bearing element positions are changing appreciably up the building, then I would expect the slab designs to also be appreciably different. That sounds reasonable to me. Or, at the least, I know of no generally applicable method for somehow "batch processing" such slabs.

For correct analysis of continuous slabs stiffness of supports matters.

Using the method that I described above, all of my slab supports wind up being effectively rigid axially. The penalty for this is probably some excess conservatism in the design of transfer elements. I deem that a reasonable price to pay for improved simplicity and tractability of results.

 

[user277418]

thank you KootK for the quick reply

About the pulling out the individual floors... Does SAFE actually account for the changes in support stiffness or it accounts only stiffness of supports per single storey from above and below the slab under question?

 

[KootK]

Any accounting for support stiffness occurs within the whole building FEM model (ETABS for example). In the slab only model (SAFE), this information simply becomes the design loads that were pulled from the whole building FEM model.

 

[user277418]

for whole model of a building it is clear

this information simply becomes the design loads that were pulled from the whole building FEM model.

do you mean like take-down loads? like a load from a column that is supported by a transfer structure?

 

[KootK]

Yes. Flexibility in the whole building model will alter the loads that wind out being transferred to the slab only model.

 

[user277418]

I am back

Flexibility in the whole building model will alter the loads that wind out being transferred to the slab only model.

Have read SAFE help. Basically it exports imposed loads to the floor slab under consideration and optionally can export internal forces in vertical structures at a level of the same slab. Ok. What you obtain are correct point/line loads from the vertical structures for transfer structure design.

But I am talking about how deflection of the transfer structure will affect internal forces in all those slabs above it. How correct internal forces of the slab (let's say on 20th storey) may be obtained in SAFE without consideration of effect of the transfer structure deflection (let's say on 10th storey) on a column that supports the 20th storey floor slab? By other words how SAFE may know that the column support modeled will settle down unevenly relatively to other columns due to less stiff support under it?

Or am I missing something in you answers?

PS: sorry for my English. Tried my best

 

[KootK]

But I am talking about how deflection of the transfer structure will affect internal forces in all those slabs above it.

Me too. That should all be captured in the loads exported to whatever level you choose to look at in more detail. The only question is whether or not the full building model behaves as you want it to with respect the load transfer and things like construction sequence.

 

[deemtocomply]

Kootk, my concern about this approach is that we will actually underestimate the transfer loads sometimes when there are more than one transfer slabs in the 3D model.

Say we have a three-storey building and level 1 and level 2 are both transfer slabs. If there is a transfer column C1 above level 1 that is quite close to support under (say a column above ground floor), using this method might underestimate the load that goes to this column C1 and thus causing an unsafe design for transfer slab at level 1 at that location since this column is ought to attract more loads than other transfer columns supported by slab at near mid span.

Also, overestimate the load on one span might reduce the deflection and moment at its adjacent spans. For example, if our transfer slab has a cantilever, then overestimating the transfer loads at its back span will reduce the deflection at cantilever end.