overstressed walls under and above a thick transfer slab

[chekre]

Hi guys,

I am working on a structure where i have a thick transfer slab supporting walls from above and resting on walls below. I have conducted the analysis and found that a big part of the walls at the floor directly above and directly below are overstressed in shear. When u go down (or up), all the walls become ok versus shear. As a first trial, i have selected all the walls overstressed and reduced their stiffness (the one related to the shear) so they can throw some of their shear to the adjacent elements (the ones that can carry an additional shear force).

I am aware that a construction sequence should be conducted to evaluate a more correct shear. The problem is that a non linear version isnt available right now (it will take some time to arrive). so i will do it manually by deleting floors through several runs and adding shear forces (i am talking of adding of shear force of the own weight only).


Any comments or suggestions ?

thank you

 

[johnbridge231]

Are you using ETABS for the analysis. I have had similar issues in the past, when analyzing transfer slabs with walls on the basement. I managed to overcome that issue, by obtaining the wall forces and checking the critical cross sections (every 5 m) using this software.
I hope this helps,
Rgs

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[chekre]

Hi Johnbridge231,

Yes, I am using Etabs. i have verified the results given by Etabs and matched the hand calculations. I will be interested to know how did u overcome this issue specially that the more i thicken the walls, the more shear i will get.

 

[hokie66]

Is the shear you are talking about from lateral loading or gravity loading? The type shear is not clear from your description.

 

[chekre]

both gravity and lateral shears are coming very high. But, if i manage to reduce the shear coming from own weight of the structure, total shear will decrease and may become acceptable.

 

[hokie66]

Can you post an elevation of the shear walls? That may throw some light on your problem, for the rest of us who don't fully understand. Etabs is a fine tool, but understanding how structures actually work is important as well.

 

[chekre]

hI,

i have attached for u 2 screenshots of the walls below the transfer and the walls above. as u can notice, walls below and above layout is totally different. the high shear values are coming from the defections (minor) that the slab is experiencing. I tried to multiply the slab inertia by 10 and the shear values decreased 20%.

 

[chekre]

2nd attachement

 

[hokie66]

How thick is the transfer slab? Are you saying that the ends of the walls are taking all the load as the slab bends and deflects, so the corners are shearing off? Does this computational problem exist if you make the slab infinitely stiff? Maybe someone who uses ETABS all the time will comment...

 

[chekre]

the slab is very thick (around 3 meters).

 

[hokie66]

In that case, I would consider it infinitely stiff, and the loading to the walls can be considered uniformly applied. No shear from the vertical loading, which leaves only shear from the lateral loading, including any slab shortening issues.

These shear walls would likely be non-flexural elements, so the most appropriate approach in design would be strut and tie, not beam theory.

 

[slickdeals]

Post a screenshot of your transfer plate meshing. Agree with Hokie that this slab is very stiff and it should load the walls below rather uniformly.

How tall is this building? It is a shame that you have a 3m thick transfer slab.

 

[chekre]

there are 30 floors above the transfer. Which shame are u talking about ? 3m is just enough for such structure.