The Ontario Building Code | Structural Configuration Structural Configuration

(1) Structures having any of the features listed in Table shall be designated irregular.

(2) Structures not classified as irregular according to Sentence may be considered regular.

(3) Except as required by Article, in cases where IEFaSa(0.2) is equal to or greater than 0.35, structures designated as irregular must satisfy the provisions referenced in Table

Structural Irregularities(1)

Forming Part of Sentence


Column 1


Column 2

Irregularity Type and Definition

Column 3




Vertical Stiffness Irregularity

Vertical stiffness irregularity shall be considered to exist when the lateral stiffness of the SFRS in a storey is less than 70% of the stiffness of any adjacent storey, or less than 80% of the average stiffness of the three storeys above or below.




Weight (mass) Irregularity


Weight irregularity shall be considered to exist where the weight, Wi, of any storey is more than 150% of the weight of an adjacent storey. A roof that is lighter than the floor below need not be considered.



Vertical Geometric Irregularity


Vertical geometric irregularity shall be considered to exist where the horizontal dimension of the SFRS in any storey is more than 130% of that in an adjacent storey.



In-Plane Discontinuity in Vertical Lateral-Force-Resisting Element

Except for braced frames and moment-resisting frames, an in-plane discontinuity shall be considered to exist where there is an offset of a lateral-force-resisting element of the SFRS or a reduction in lateral stiffness of the resisting element in the storey below.




Out-of-Plane Offsets

Discontinuities in a lateral force path, such as out-of-plane offsets of the vertical elements of the SFRS.




Discontinuity in Capacity – Weak Storey

A weak storey is one in which the storey shear strength is less than that in the storey above. The storey shear strength is the total strength of all seismic-resisting elements of the SFRS sharing the storey shear for the direction under consideration.




Torsional Sensitivity (to be considered when diaphragms are not flexible)

Torsional sensitivity shall be considered to exist when the ratio B calculated according to Sentence exceeds 1.7.




Non-orthogonal Systems

A non-orthogonal system irregularity shall be considered to exist when the SFRS is not oriented along a set of orthogonal axes.


Notes to Table

(1) One-storeypenthouses with a weight of less than 10% of the level below need not be considered in the application of this Table.

(2) See Article

(3) See Article

(4) See Article

(5) See Sentences and (10) and

(6) See Article