Presentation by DUY NGUYEN OF P&T Consultants Pte Ltd A BRIEF STUDY OF USING MEMBRANE, SHELL OR PLATE IN ASSIGNING A SLAB/WALL
P&T Consultants Pte Ltd Civil & Structural Engineer FOREWORD: * When using Etabs we may recognize that their are 3 options for a Wall/Slab section assignment. They are Shell, Membrane and Plate. * Each type of section has different features. * Since Slab or Wall is main element so its section assignment will has significant effect into our model output. * In this brief doc, we will go through major meanings of each type, what it will affect to our model and some useful recommendations.
If a slab is assigned as Membrane section, it will has no out-of-plane stiffness then can not contribute to resist bending moment.
In other hand, if it is assigned as Plate section, it has full out-of-plan stiffness that mean can support to cater for bending moment but it will has no in-of-plan stiffness to receive horizontal load.
Lastly, if it is assigned as Shell section, it has full out and in-of-plan stiffness as what we understand for a normal slab.
Conclusion 2 : As a result of no contribution from Membrane, Beams in membrane assignment model have bigger bending moment than in others. The thicker slab, the lesser bending moment in Shell or Plate model beams.
For your reference and checking, when assigning Membrane, as it only has in-plane stiffness then also cause the Building global stiffness reduced. As a result, the displacement, story drift and period of a Membrane model are also larger than Shell model.
Conclusion 3 : The displacement, story drift and period of a Membrane model are also larger than Shell model .
3. ADVANTAGES AND DISADVANTAGES OF USING MEMBRANE, SHELL OR PLATE
When choosing membrane, in Load distribution option, tick on “Use special one-way Load distribution to assign for one way slab.
After running, go to menu Display/Show load/Frame line. Tick on “all loading that is Tributary to the Line Object (Force)’ to see the loading transferred to beams.
P&T Consultants Pte Ltd Civil & Structural Engineer “ Different companies will have different styles of using Etabs. It will be depended on the common practice of each company. However, understanding the philosophy of Etabs is the most important so that the modeler can ensure that his model is under controlled.” 1. Membrane is used to transfer the load to the beams without considering the strength contributed by the slab. It distributes the load in a yield like manner i.e. trapezoidal and triangular loads to the beams. One thing to note is that membrane will only work if there are 4 surrounding beams. If there is a missing beam, the program divides the triangular or trapezoidal loads to point loads at the supporting columns. 2. Defining slab as membrane sometime generate a lot of errors, especially when the model is not orthogonal. Membrane element may not transfer loading to the beams accurately if it has more than 4 sides.
P&T Consultants Pte Ltd Civil & Structural Engineer 3. Be carefully to check the arrangement of membrane and beams so that loading can be transferred to the correct column and correct quantity similar to load take down by loading area method. 4. For cases where we know that membrane will not work and we wish for our design philosophy to be consistent I.e. ignoring the slab strength, you could always define a shell with a reduced out of plane bending stiffness at locations where the membrane assumption cannot be met.
P&T Consultants Pte Ltd Civil & Structural Engineer 1. Etabs manual and help 2. P&T Consultant PTE Practice Note