The proposed change revises Article 4.1.8.1. by adjusting the triggers that determine the locations where the simplified method for seismic design applies and modifying the approach for determining the design spectral acceleration.
Problem
Article 4.1.8.1. of Division B of the NBC was introduced in the 2015 edition to provide a simplified method for the seismic design of buildings in localities where the seismic hazard is low. Due to the general increase in the seismic hazard values for the NBC2020, the number of localities in the country where this method is permitted to be used for seismic design in compliance with the NBC has shrunk considerably.
In general, designers and enforcement professionals in localities where the simplified method was previously permitted are not familiar with the full (detailed) seismic design requirements. Therefore, the inability to use the simplified method presents a significant hardship for them.
Justification
The reduction in the number of localities where the simplified method can be used presents a hardship for Code users who have relied on this method since it was introduced in the NBC2015 to satisfy seismic design requirements in localities where the seismic hazard is low. The use of the simplified method by designers in such localities ensured that buildings were designed for estimated earthquake loads without designers having to navigate the complexity of the seismic provisions in the rest of the Subsection4.1.8.
However, the number of localities where this method is permitted be used has decreased significantly with the NBC2020 seismic hazard values. If nothing is done, Code users in some localities where the simplified method was previously permitted will have to apply the full seismic design requirements. For localities with relatively low seismic hazard, this is an avoidable hardship. To mitigate this hardship, the proposed change increases the values of the hazard thresholds to expand the application of the simplified design method to match, to the extent possible, its application with the NBC2015 seismic hazard values. It also harmonizes the determination of the values of the thresholds with the definitions in Article4.1.8.2. to improve consistency between the simplified and full design methods.
PROPOSED CHANGE
[4.1.8.1.] 4.1.8.1.Analysis
[1] 1)Except as permitted in Sentence (2), the deflections and specified loading due to earthquake motions shall be determined according to the requirements of Articles 4.1.8.2.to 4.1.8.23.
[2] 2)Where IEFsSa(0.2,X450S) and IESa(0.5,XS) are less than 0.2 and IEFsSa(21.0,X450S) areis less than 0.16and 0.03respectively0.1, the deflections and specified loading due to earthquake motions are permitted to be determined in accordance with Sentences (3)to (15), where
[a] a)IE is the earthquake importance factor and has a value of 0.8, 1.0, 1.3 and 1.5 for buildings in the Low, Normal, High and Post-disaster Importance Categories respectively,
[b] b)Fs is the site coefficient based on the average 60 or , as defined in Article 4.1.8.2., for the top 30 m of soil below the footings, pile caps, or mat foundations and has a value of
[i] i)1.0 for rock sites or when 60 > 50 or > 100 kPa,
[ii] ii)1.6 when 15 ≤ 60 ≤ 50 or 50 kPa ≤ ≤ 100 kPa, and
[iii] iii)2.8 for all other cases, and
[c] c)Sa(T,X450S) is the 5%-damped spectral acceleration value, expressed as a ratio to gravitational acceleration, at period T for site designation X450S, as defined in Article 4.1.8.2.Clause (d), determined in accordance with Subsection 1.1.3. and corresponding to a 2% probability of exceedance in 50 years,and
[d] --)XS is the site designation in terms of Site Class, where S is the Site Class determined using the average 60 or , as defined in Article 4.1.8.2., for the top 30 m of soil and has a value of
[i] i)XC for rock sites or where 60 > 50 or > 100 kPa,
[ii] ii)XD where 15 < 60 ≤ 50 or 50 kPa ≤ ≤ 100 kPa, and
[iii] iii)XE for all other cases.
[3] 3)The structure shall have a clearly defined
[a] a)seismic force resisting system (SFRS) to resist the earthquake loads and their effects, and
[b] b)load path (or paths) that will transfer the inertial forces generated in an earthquake to the supporting ground.
[4] 4)An unreinforced masonry SFRS shall not be permitted where
[a] a)IE is greater than 1.0, or
[b] b)the height above grade is greater than or equal to 30 m.
[5] 5)The height above grade of an SFRS designed in accordance with CSA S136, "North American Specification for the Design of Cold-Formed Steel Structural Members (using the Appendix B provisions applicable to Canada)", shall be less than 15 m.
[6] 6)Earthquake forces shall be assumed to act horizontally and independently about any two orthogonal axes.
[7] 7)The specified lateral earthquake force, Vs, at the base of the structure in the direction under consideration shall be calculated as follows:
where
Sa(Ts,X450)
= value of Sa(Ts,X450)design spectral acceleration at period Ts, determined byasfollows, using linear interpolation for intermediate values of Tsbetween the values ofSa(0.2,X450 ), Sa(0.5,X450) and Sa(1.0,X450),
= Sa(0.2,X450S) or Sa(0.5,XS), whichever is greater, for Ts ≤ 0.2 s, and
= Sa(0.5,XS) for Ts = 0.5 s, and
= Sa(1.0,X450S) for Ts ≥ 1.0 s,
W
= sum of Wi over the height of the building, where Wi is defined in Article 4.1.8.2., and
Rs
= 1.5, except Rs = 1.0 for structures where the storey strength is less than that in the storey above and for an unreinforced masonry SFRS,
where
Sa(0.2,XS), Sa(0.5,XS), Sa(1.0,XS)
= 5%-damped spectral acceleration values at periods of 0.2 s, 0.5 s and 1.0 s, respectively, for site designation XS, determined in accordance with Subsection 1.1.3. and corresponding to a 2% probability of exceedance in 50 years, and
Ts
= fundamental lateral period of vibration of the building, as defined in Article 4.1.8.2.,
= 0.085(hn)¾ for steel moment frames,
= 0.075(hn)¾ for concrete moment frames,
= 0.1N for other moment frames,
= 0.025hn for braced frames, and
= 0.05(hn)¾ for shear walls and other structures,
where
hn
= height, in m, above the base to level n, as defined in Article 4.1.8.2., and
N
= total number of storeys above exterior grade to level n, as defined in Article 4.1.8.2.,
except that, in cases where Rs = 1.5, Vs need not be greater than FsSa(0.5,X450S)IEW/Rs.
[8] 8)The specified lateral earthquake force, Vs, shall be distributed over the height of the building in accordance with the following formula:
where
Fx
= force applied through the centre of mass at level x,
Wx, Wi
= portion of W that is located at or is assigned to level x or i respectively, and
hx, hi
= height, in m, above the base to level x or i respectively, as defined in Article 4.1.8.2.
[9] 9)Accidental torsional effects applied concurrently with Fx shall be considered by applying torsional moments about the vertical axis at each level for each of the following cases considered separately:
[a] a)+0.1DnxFx, and
[b] b)−0.1DnxFx.
[10] 10)Deflections obtained from a linear analysis shall include the effects of torsion and be multiplied by Rs/IE to get realistic values of expected deflections.
[11] 11)The deflections referred to in Sentence (10) shall be used to calculate the largest interstorey deflection, which shall not exceed
[a] a)0.01hs for post-disaster buildings,
[b] b)0.02hs for High Importance Category buildings, and
[c] c)0.025hs for all other buildings,
where hs is the interstorey height as defined in Article 4.1.8.2.
[12] 12)When earthquake forces are calculated using Rs = 1.5, the following elements in the SFRS shall have their design forces due to earthquake effects increased by 33%:
[a] a)diaphragms and their chords, connections, struts and collectors,
[b] b)tie downs in wood or drywall shear walls,
[c] c)connections and anchor bolts in steel- and wood-braced frames,
[d] d)connections in precast concrete, and
[e] e)connections in steel moment frames.
[13] 13)Except as provided in Sentence (14), where cantilever parapet walls, other cantilever walls, exterior ornamentation and appendages, towers, chimneys or penthouses are connected to or form part of a building, they shall be designed, along with their connections, for a lateral force, Vsp, distributed according to the distribution of mass of the element and acting in the lateral direction that results in the most critical loading for design using the following equation:
where
Wp
= weight of a portion of a structure as defined in Article 4.1.8.2.
[14] 14)The value of Vsp shall be doubled for unreinforced masonry elements.
[15] 15)Structures designed in accordance with this Article need not comply with the seismic requirements stated in the applicable design standard referenced in Section 4.3.
Impact analysis
The proposed change is expected to have a positive impact as it would alleviate a hardship for Code users in localities with relatively low seismic hazard where the simplified method is no longer permitted under the NBC2020 due to an increase in the estimated values of seismic hazard.
The proposed change would provide relief for designers and enforcement professionals in such localities, as it reinstates the simplified method as a means of compliance with NBC in these localities.
Enforcement implications
The proposed change would have positive implications for enforcement, as it addresses the hardship resulting from the change of practice in the NBC2020 for localities with relatively low seismic hazard.
The continued ability to use the simplified method for seismic design in localities with relatively low seismic hazard, where compliance with the full seismic design requirements would otherwise be required, would provide relief for enforcement professionals, as it would allow authorities having jurisdiction to use existing methods and resources without the need for additional training.
Who is affected
Owners, designers, contractors and enforcement professionals dealing with the seismic design of buildings in low-seismic-hazard localities covered under Article4.1.8.1.
OBJECTIVE-BASED ANALYSIS OF NEW OR CHANGED PROVISIONS
