How does ideCAD control the diaphragm discontinuity irregularities, according to ASCE 7-16?
The diaphragm discontinuity irregularity is determined by the user.
In buildings with diaphragm discontinuity irregularities, story slabs are automatically modeled with two-dimensional plate (membrane) or shell finite elements to show that they are able to safely transfer earthquake forces between vertical bearing system elements within their own planes.
Diaphragm Discontinuity Irregularity
The cases where there is a diaphragm with an abrupt discontinuity or variation in stiffness, including those with a cut-off or open area greater than 50% of the gross closed diaphragm area, more than 50% effective diaphragm stiffness change from one story to the next.
Diaphragm discontinuity irregularity condition occurs, as shown below figure.
The ratio of the total area of all spaces (including stairs and elevators) in the story plan to the total story gross area is more than 1/2.
Local story gaps make it difficult to transfer earthquake loads to vertical bearing system elements safely.
Sudden decreases in in-plane stiffness and strength of slab.
Buildings' diaphragm discontinuity irregularities are modeled with two-dimensional finite elements. The following picture shows the analysis model of an example whose tiles are modeled with two-dimensional finite elements (shell).
As a result of a three-dimensional analysis of shell finite elements (slabs, shearwalls, and polygonal walls), they generate stresses and forces per unit length. The directions of these stresses and forces per unit length are determined according to the local axes of the shell finite element.
Shell finite element forces are obtained as a result of stresses formed by finite shell elements in three-dimensional analysis.
M11, M22: Bending moments per unit length (tfm / m or kNm / m) formed around axes 1 and 2. They are also called out-of-plane bending moments.
M12: Means unit length planar torsion moment (tfm / m or kNm / m). It is also called the in-plane torsion moment.
V13, V23: Shell is the unit-length shear force (tf / m or kN / m) on the surface of the finite element and perpendicular to the plane of the finite element. It is also called the out-of-plane shear force.
F11, F22: Tensile and compressive forces per unit length (tf / m or kN / m) parallel to the plane of the acceptance finite element in the respective direction. It is also called in-plane pressure-pull forces.
F12: Unit length shear forces (tf / m or kN / m) parallel to the shell finite element plane. It is also called in-plane shear forces.
Shell finite element results can be viewed from the "Shell Results" tab in the Analysis Model.
Deformation results from shell finite element results can also be examined in the Analysis Model.
Floor spaces cause accumulation in floor stresses caused by vertical loads and earthquake loads. In the picture below, a floor plan is shown in which the case of "the presence of local floor gaps that make it difficult to transfer earthquake loads to vertical bearing system elements" in the definition of type 2 irregularity. Since there is a gap in the floor connected to the shearwall, the in-plane shear stresses (F12) under the effect of an earthquake in the (Y) direction increase significantly at the gap edges, as seen on the right side of the picture.
Type 2 irregularity occurs due to floor gaps. For this reason, slab in-plane deformations were neglected. Rigid diaphragm solution is not applied in this type of irregularity. In buildings with type 2 irregularities, a semi-rigid diaphragm solution should be made in which the floors are modeled with finite shell elements.
According to ACI 318-11, Chapters 11 and 13, earthquake loads must be safely transferred from floors to vertical bearing elements. A floor plan in which type 2 irregularity definition consists of "the presence of local floor gaps that make it difficult to transfer earthquake loads to vertical bearing system elements safely" is shown in the above picture.