2 Irregularities based on NSCP 2015 Section 208

A soft story irregularity occurs when a story's stiffness is less than 70% of the story above it, or less than 80% of the average stiffness of the three stories above. To prevent this, lower floors should be stiffer than upper floors. An example of failure due to lack of walls on the ground floor making it less stiff is shown.

Weight/mass irregularity occurs when the effective mass of any story is more than 150% of the effective mass of an adjacent story. This is often visually observed similarly to a soft story, as greater mass usually implies greater stiffness. Designers must ensure the mass of a story does not exceed 150% of the adjacent story's mass.

This irregularity exists if the dimension of the lateral force resisting system at any story is more than 130% of that for any adjacent story. The dimensions between stories should not exceed this 130% threshold.

The video introduces configuration irregularities, which are significant physical discontinuities in a structure's configuration or lateral force resisting system, as described in tables 2-8-9 and 2-8-10 of the NSCP 2015. Structures in occupancy categories four and five in seismic Zone 2 only need evaluation for vertical irregularities of type 5 and horizontal irregularities of type 1.

This section details several types of vertical structural irregularities: stiffness (soft story), weight/mass, vertical geometric, in-plane discontinuity in vertical lateral force resisting element, and discontinuity in capacity (weak story) irregularity.

In-plane discontinuity irregularity occurs if an offset is greater than the width D, or if there's a reduction in stiffness of the story below. Weak story irregularity exists if the lateral strength of any story is less than 80% of the strength of the story above, emphasizing that lower floors should be stronger.

The video then moves to horizontal structural irregularities, which include torsional, re-entrant corner, diaphragm discontinuity, out-of-plane offset, and non-parallel system irregularity. Torsional irregularity is considered when diaphragms are not flexible.

Horizontal torsional irregularity is present if the maximum displacement exceeds 1.2 times the average displacement. This irregularity is undesirable as it necessitates additional computations to increase lateral force values and can lead to catastrophic damage.

Re-entrant corner irregularity exists if the certain opening (Py) is greater than 15% of Ly, and Px is greater than 15% of Lx.

Diaphragm discontinuity irregularity occurs if the open area is greater than 50% of the floor area, or if the effective diaphragm stiffness varies by more than 50% from one story to the next. Openings should not exceed 50% to prevent this.

Out-of-plane offset irregularity involves a discontinuity in the structural resisting member. Non-parallel system irregularity exists when vertical lateral force resisting elements are not parallel to or symmetric about the major orthogonal axis of the seismic force resisting system.

The video concludes by explaining how to determine diaphragm flexibility. Diaphragms are semi-rigid unless classified as flexible or rigid. Examples include steel decking and wood structural panels as flexible if vertical seismic force resisting systems are steel/composite frames or shear walls, and concrete slabs as rigid if the width-to-pressure ratio is less than 3 and no horizontal irregularity exists.