MEMBRANE in ETABS
Load Distribution
When we go with the modeling type of the slab in ETABS as MEMBRANE then the load distribution from the slab to the beam will be based on YIELD LINE THEORY
When a Load is applied to MEMBRANE objects in ETABS, the load (100% of the applied load) is transferred directly to supporting structural objects.
Stiffness
In-plane stiffness of the slab will be accounted in the analysis when an engineer goes with the slab’s modeling type as MEMBRANE.
Out of plane stiffness of the slab will not be accounted in the analysis, wherein reality the structural slab also exists (Conventional RC Beam-Slab Structures) in a building.
Excluding the out of plane stiffness of the slab in the analysis, the results such as Bending Moments, Shear Forces &, etc. will be much higher when we go with the slab’s modeling type as MEMBRANE instead of SHELL ELEMENT.
Applications of Membrane in the Design Industry
An engineer generally goes with the slab’s modeling type as MEMBRANE when analyzing a structure which consists of following slabs
Conventional RC Beam-Slab Structures
Precast Slabs
Ramps
Deck Slab
Profile Slab & etc.
Dos & Don’ts
Do not mesh the slab using the finite element meshing option in ETABS while opting for the slab’s modeling type as MEMBRANE in any case.
SHELL THIN/THICK ELEMENT IN ETABS
When it comes to the modeling type of the slab as SHELL ELEMENT there is generally two types of SHELL OBJECTS available to be used in ETABS which are of SHELL THIN & SHELL THICK.
Both SHELL THIN and SHELL THICK ELEMENTS have almost the same properties with only one difference which will be mentioned at the end of this article.
Load Distribution
When we go with the modeling type of the slab in ETABS as SHELL THIN/THICK then the load distribution from the slab to the beam/column will be based on FINITE ELEMENT ANALYSIS THEORY
When a Load is applied to SHELL objects in ETABS, very less load is transferred to supporting structural objects due to the bending stiffness present in the slab & due to the bending stiffness, some part of the load which is applied on the slab will be resisted through flexural deformation.
Stiffness
In-plane stiffness of the slab will be accounted in the analysis when an engineer goes with the slab’s modeling type as SHELL THIN/THICK.
Out of plane stiffness of the slab will also be accounted in the analysis, which brings in the realistic behavior as the structural slab also exists (Conventional RC Beam-Slab Structures) in a building.
Including the out of plane stiffness of the slab in the analysis, the results such as Bending Moments, Shear Forces & etc. will be much less when we go with the slab’s modeling type as SHELL THIN/THICK instead of MEMBRANE as the flexural deformation of the slab will also come into the picture.
Applications of Shell Thin in the Design Industry
An engineer goes with the slab’s modeling type as SHELL THIN when analyzing a structure which consists of following slabs
Conventional RC Beam-Slab Structures
Slabs which are Irregular in Shape
Shear Walls
Structures with flat slabs, Flat Plates &, etc.
Post Tension Slabs
Applications of Shell Thick in the Design Industry
An engineer goes with the slab’s modeling type as SHELL THICK when analyzing a structure which consists of following slabs
Raft Foundations
Combined Foundations
Isolated Foundations
Structures with Transfer Slabs
Column Head in case of Flat slabs
Dos & Don’ts
Always do mesh the slab using finite element meshing option in ETABS while opting for the slab’s modeling type as SHELL THIN/THICK in any case.
DIFFERENCE BETWEEN SHELL THIN ELEMENT AND SHELL THICK ELEMENT
The SHELL THIN analysis is based only on bending deformations and neglects shear deformations.
Whereas,
The SHELL THICK analysis is recommended to use when the shear deformations become significant. The SHELL THICK analysis captures both shear and bending deformations.
