Diaphragm in Structural Engineering and Its application in ETABS
By Er. Saunak Sharma
B.E. (Civil), M.E. (Earthquake Engineering)
Diaphragm is a structural element that transmits lateral loads to the vertical-resisting elements of a structure such as Shear wall, columns, braces. These are Roof, floor, or other membrane or bracing system acting to transfer the lateral forces to the vertical resisting elements.
Diaphragms are typically horizontal but can be sloped in a gable roof on a wood structure or concrete ramp in a parking garage.
Diaphragms play an important role in the distribution of seismic and wind forces across the structure, ensuring that these loads are transferred efficiently to the building’s lateral force-resisting system.
Let us understand more detailed concept about diaphragm
The concept of diaphragm come from type of joint
- Restraint joint: Restraint joints restrict movement in one or more degrees of freedom (DOF). For example, a fixed support can restrain all six DOFs—three translations (along the x, y, z axes) and three rotations (about the x, y, z axes). These joints are designed to control the behavior of a specific joint under loads by limiting its movement. These are like applying boundary condition in a single point.
- Constraint joints: Constraint joints involve relationships between two or more joints, ensuring that the connected joints move together in a specified manner. Constraints are used to model the interaction between different parts of a structure, such as ensuring that certain structural elements move in a coordinated way. These are like applying boundary condition between group of points.
In general, a node has 6 degrees of freedom (3 translational and 3 rotational) before applying any constraint. So, if a structure, let’s say slab, has 10 nodes, that means total degrees of freedom in the structure is 60. But when a constraint is assigned, the degrees of freedom reduce
The constraint is basically a relation among certain degrees of freedom. Let’s say, translation along x axis in above structure is same for all the 10 nodes. So, the degrees of freedom (independent) now will be 60-10+1 = 51 only. If similar constraint is applied along y direction for the same slab, now the degrees of freedom reduce to 51-10+1 = 42.
Here no constraint has been applied in z direction (vertical direction). So, the slab can still have 10 different values of displacement along z axis, here we have achieved rigid diaphragm.
Type of Diaphragm.
Diaphragms: | Behavior | Materials | Applicable in |
Rigid Diaphragms: | Assume that the diaphragm is rigid compared to the vertical elements, and the lateral forces are distributed to these elements in proportion to their stiffness. | Steel decks with concrete topping, or heavily reinforced concrete slabs. | Concrete buildings and steel structures with concrete decks, |
Semi-Rigid Diaphragms: | Semi rigid diaphragm is something new that CSI have implemented in ETABS. But it has not been well documented. Semi-rigid diaphragms simulate actual in-plane stiffness properties and behavior this is what happens also in case of ‘no diaphragm’. But there is a major difference that when you assign semi rigid diaphragm, you can also assign eccentricity while defining the seismic load. And eccentricity is required to be defined as per most of the building codes. So assigning semi-rigid diaphragm is always safer than not assigning diaphragm at all. | Could be composite materials or partially reinforced concrete slabs | Often used in structures where precise load distribution is necessary, and neither flexible nor rigid assumptions are entirely appropriate |
Flexible Diaphragms | These diaphragm does not provide significant resistance to lateral deformation and that it deforms easily compared to the vertical elements. In this case, the lateral forces are assumed to be distributed to the vertical resisting elements based on their tributary areas. | Wood or thin metal decks without concrete topping. | Suitable for buildings where the diaphragm is relatively flexible compared to the vertical elements, such as wood-framed structures. |
Code Provision
IS 1893:2016 (Part 1) – Criteria for Earthquake Resistant Design of Structures:
In ETABS
Seismic loads
For rigid diaphragms, the accidental eccentricity associated with auto seismic loading is concentrated and applied at the center of mass.
For semi-rigid diaphragms, accidental eccentricity is applied to every node.
If no diaphragm is assigned eccentricity will not be applied to any node.
Wind loads
For rigid diaphragm, loading is applied at geometric centroid,
For semi-rigid diaphragm auto wind loads are distributed in 10 nodes, so that the summation of these forces with respect to centroid will be equivalent to lateral and torsional wind cases.
How to Define and Assign in ETABS
Result in ETABS
Rigid Diaphragm Semi Rigid Diaphragm
Main Points to Remember
The base shear and story shears due to earthquake (EQ) loads remain consistent across cases, regardless of whether a rigid, semi-rigid, or no diaphragm is defined.
EQ Load Application Based on Diaphragm Type:
Rigid Diaphragm: EQ loads are applied at the center of mass (CM) of the diaphragm.
No Diaphragm and Semi-Rigid Diaphragm: EQ loads are applied at each internally and externally meshed joint.
Distinction Between No Diaphragm and Semi-Rigid Diaphragm:
Semi-Rigid Diaphragm: Allows for the application of eccentricity, which generates moments at each joint.
No Diaphragm: Eccentricity cannot be applied, so no moments are generated at the joints.
It is recommend assigning a semi-rigid diaphragm unless you are certain that the floor possesses very high in-plane stiffness, in which case a rigid diaphragm can be assigned to reduce analysis time.