Slab deflection in ETABS

[Omar Khalid]

Hello,

While I was modeling a 20 story building with symmetrical concrete slab sections and loads in each story, I noticed that the deflection of the slab increases the more I go higher in elevations despite having the same sections, dimensions and loads.

I know that floors with symmetrical shapes and loads must endure the same stresses so it has the same reinforcement.

Can someone explain this behavior or explains why it occurs if I was wrong ?

Thanks in advance 

[Ayesha]

This sounds odd. What load combinations are governing slab design at different levels?

 

[Omar Khalid]

Regardless the combination I choose to look for settlements. it's always increasing the higher I go. I've seen people modeling on youtube having the same problem except they don't talk about it like it's a normal thing. does it have anything related to vertical settlements of columns and walls ?

You can try it by yourself on etabs to check I'm right or not.

[Ayesha]

You shouldn't have it. If you want to investigate it please provide what combinations are governing.

Here is what you should do:

1) Do a gravity only analysis use load factor of 1 to see what results you are getting.

2) Do a lateral only analysis to see what results you are getting without any gravity load.

3) Compare results and check model for errors.

Just because other people are getting the same problem, doesn't mean the problem is there. We need to define a basis. I don''t use ETABS so I can't do checks on my own. If you want my input, please provide the results as requested above.

[miqureshi77]

this deflection shows in Etabs is global deflection, this will increase due to axial and lateral deformation of the members globally,

if you check deflection individuallly with respect to each floor value than u will find the same deflection

[Omar Khalid]

1 hour ago, Ayesha said:

You shouldn't have it. If you want to investigate it please provide what combinations are governing.

Here is what you should do:

1) Do a gravity only analysis use load factor of 1 to see what results you are getting.

2) Do a lateral only analysis to see what results you are getting without any gravity load.

3) Compare results and check model for errors.

Just because other people are getting the same problem, doesn't mean the problem is there. We need to define a basis. I don''t use ETABS so I can't do checks on my own. If you want my input, please provide the results as requested above.

1) due to dead load only (own weight) the building tends to undergo a displacement in the x axis (horizontal displacement) the maximum displacement is 1 cm at the roof. Cause I have a stair wall with 2 elevator cores at one side of the building.

2) No earthquake or wind loads being applied yet. Only dead and live and slab deflections still increases when you go up.

3) No errors in my model.

[Omar Khalid]

35 minutes ago, miqureshi77 said:

this deflection shows in Etabs is global deflection, this will increase due to axial and lateral deformation of the members globally,

if you check deflection individuallly with respect to each floor value than u will find the same deflection

Can you tell me how to check it ?

[Ayesha]

3 hours ago, Omar Khalid said:

1) due to dead load only (own weight) the building tends to undergo a displacement in the x axis (horizontal displacement) the maximum displacement is 1 cm at the roof. Cause I have a stair wall with 2 elevator cores at one side of the building.

That is due to unsymmetrical stiffness of vertical members. This however isn't the subject of our discussion.

3 hours ago, Omar Khalid said:

2) No earthquake or wind loads being applied yet. Only dead and live and slab deflections still increases when you go up.

Are you using any live load reduction factors? What is the difference in vertical deflection of the slab in 2 consecutive floors and what is the location of that maximum vertical deflection?

 

Edited November 15, 2017 by Ayesha

[Omar Khalid]

Maximum deflection in the first floor is 8.4 mm
2^nd floor is 9.4 mm

3^rd is 10.6 mm

4^th is 11.65 mm

5^th 12.6 mm
etc the 20^th 20.5 mm

 

These are maximum deflections in each slab (same place) with dead load only (self weight only)

[Omar Khalid]

The location is in the panel with the largest span in a story which is predictable nearly in the center of the panel

[Ayesha]

1 hour ago, Omar Khalid said:

These are maximum deflections in each slab (same place) with dead load only (self weight only)

Omar, when you are reporting maximum deflections I assume that you mean maximum vertical deflections and it is relative not absolute. I still can't get my head around the fact that you have so much deflection difference even for gravity load case. 

I will look further in this. I don't have an answer on top of my head. I also request other forum members to share there thoughts about this question.

Edited November 15, 2017 by Ayesha

[W. Alkady]

If your model is correct, this increasing value of vertical displacement is due to the elastic shortening of the vertical supporting elements ( columns and cores). These elements are mainly loaded with compression stresses, which causes the reduction of its height and consequently, the increase of the slab deflection (vertical displacement).   

[Omar Khalid]

11 hours ago, Ayesha said:

Omar, when you are reporting maximum deflections I assume that you mean maximum vertical deflections and it is relative not absolute. I still can't get my head around the fact that you have so much deflection difference even for gravity load case. 

I will look further in this. I don't have an answer on top of my head. I also request other forum members to share there thoughts about this question.

No. It's maximum vertical displacement absolute not relative

[WR1]

If the building is regular as you said with same column sizes, axial shortening should be same provided loading and gemoetry all are same and symmetrical. Gravity deflection on same combination should be same at each level. And by deflection i mean z deflection not x.

[Omar Khalid]

4 hours ago, W. Alkady said:

If your model is correct, this increasing value of vertical displacement is due to the elastic shortening of the vertical supporting elements ( columns and cores). These elements are mainly loaded with compression stresses, which causes the reduction of its height and consequently, the increase of the slab deflection (vertical displacement).   

That's what I was thinking, but the maximim reinforcement of a column at the base is 1.3-2 % which indicates which indicates small loads

 

I will try to screen shot it when I get home

[Omar Khalid]

2 minutes ago, Rana said:

If the building is regular as you said with same column sizes, axial shortening should be same provided loading and gemoetry all are same and symmetrical. Gravity deflection on same combination should be same at each level. And by deflection i mean z deflection not x.

 It's not entirely symmetrical I have 4 L shaped shear walls at the corners. Same thickness and nearly close lengths

 

the columns are symmetrical right and left sides with the same dimensions but not symmetrical in the Y axis

That's why I have a displacement of 1 cm at the roof in the y axis

 

but that shouldn't cause vertical displacement

 

note that the first floor has 8.4 mm vertical deflection while when I export the story to safe it gives me 2 mm deflection

[Omar Khalid]

This is the slab it continues like this to the 20th story. 

Columns 60 X 60 cm

Walls 30 cm

Slab 20 cm

Beams 60 X 30 cm

Edited November 16, 2017 by Omar Khalid

[Omar Khalid]

Hey guys, I think I have found what's wrong but don't know how to fix it.

I drew the slab on SAFE and I have obtained the beams moments and slab deflections and they were very far away from ETABS results. However, there is an option to check it for columns and walls in SAFE it's called :

''Automatic rigid-zone area over column''  When I uncheck it, it gives nearly the exact deflection and beams moment in ETABS

This option means the following from CSI america's website :

''Rigid zones are used to model the physical overlap between columns/walls and slab, which prevents deformation of the slab at the column location. This also will have the effect of producing maximum design moments at face of columns/walls instead of center line of columns/walls.  This in turn will produce less reinforcement and hence a more economical design, and allowed by most codes(i.e ACI-318). Deformations also will be more realistic in this case.''

So I assume applying a rigid zone is more realistic but somehow it's not assigned in ETABS and I don't know how to assign it in ETABS.

What are your thoughts ?

[WR1]

Hey man, hold on. Your building isnt symmetrical. Also the theres difference between axial stiffness of cols vs walls. This will effect z direction deflections from bottom to top stories.

2nd thing..safe..in safe deflections will be different than in etabs. This has been discussed no of times here and you can search.