Clarification of the Performance Requirements for Post-disaster Buildings, High Importance Category Buildings, and a Subset of Normal Importance Category Buildings
Description:
This proposed change clarifies the additional performance requirements in Article 4.1.8.23. for post-disaster buildings, High Importance Category buildings, and a subset of Normal Importance Category buildings.
Problem
A lack of clarity in some of the existing requirements of Article 4.1.8.23. of Division B of the NBC could result in difficulties for designers complying with the requirements and for enforcement staff checking compliance with the requirements. The lack of clarity may also lead to post-disaster buildings, High Importance Category buildings, and a subset of Normal Importance Category buildings that do not meet their performance objective (no structural damage) under lower-intensity, more frequently occurring earthquakes. In other cases, the lack of clarity may result in overconservative designs having a higher than necessary cost of construction.
Justification
Article 4.1.8.23. was introduced in the NBC 2020 to specify additional performance requirements for post-disaster buildings, High Importance Category buildings, and a subset of Normal Importance Category buildings. The performance requirements in this Article represent a novel approach for Code users. For the purpose of compliance with Article 4.1.8.23., the buildings must behave elastically and meet reduced drift limits when subjected to lower-intensity ground motions that occur more frequently than the design ground motions (DGMs).
The novelty of the approach in Article 4.1.8.23. requires clarification and additional information to support better understanding by Code users. Guidance was added to the "Structural Commentaries (User's Guide – NBC 2020: Part 4 of Division B)," but more importantly, revisions must be made to clarify the Code for regulatory purposes. The proposed change makes the necessary modifications to improve understanding of the requirements and to ensure accurate compliance with the requirements. It clarifies how the requirements in Subsection 4.1.8. for earthquakes with a 2%-in-50-year probability of exceedance are to be applied for the purposes of compliance with Article 4.1.8.23. This clarification would reduce the difficulties encountered in the application of this Article and prevent unsafe or overconservative designs resulting from the lack of clarity in the Code.
PROPOSED CHANGE
[4.1.8.23.] 4.1.8.23.Additional Performance Requirements for Post-disaster Buildings, High Importance Category Buildings, and a Subset of Normal Importance Category Buildings
[1] 1)Except for buildings designed in accordance with Clause 4.1.8.22.(6)(b), this Article does not apply toBbuildings designed in accordance with Articles 4.1.8.19.to 4.1.8.22.need not comply with this Article.
[2] 2)The design of post-disaster buildings in Seismic Category SC2, SC3 or SC4 shall be verified using 5%-damped spectral acceleration values based on a 5% probability of exceedance in 50 years and shall satisfy the following requirements:
[a] a)the building shall be shown to behave elastically for a specified lateral earthquake force, V, determined in accordance with Sentence 4.1.8.11.(2)Article 4.1.8.11. or 4.1.8.12., as appropriate, using IE = 1.0 and RdRo = 1.3,
[b] b)the largest interstorey deflection at any level of the building, as determined in accordance with Sentence 4.1.8.13.(2) using IE = 1.0 and RdRo = 1.01.3, shall not exceed 0.005hs, and
[c] c)the connections of elements and components of the building described in Table 4.1.8.18. with Rp > 1.51.3 shall be shown to behave elastically for a specified lateral earthquake force, Vp, determined in accordance with Sentence 4.1.8.18.(1) using IE = 1.0and Rp = 1.51.3.
[3] 3)The design of High Importance Category buildings in Seismic Category SC3 or SC4 shall be verified using 5%-damped spectral acceleration values based on a 10% probability of exceedance in 50 years and shall satisfy the following requirements:
[a] a)the building shall be shown to behave elastically for a specified lateral earthquake force, V, determined in accordance withSentence 4.1.8.11.(2)Article 4.1.8.11. or 4.1.8.12., as appropriate, using IE = 1.0 and RdRo = 1.3,
[b] b)the largest interstorey deflection at any level of the building, as determined in accordance with Sentence 4.1.8.13.(2) using IE = 1.0 and RdRo = 1.01.3, shall not exceed 0.005hs, and
[c] c)the connections of elements and components of the building described in Table 4.1.8.18. with Rp > 1.3 shall be shown to behave elastically for a specified lateral earthquake force, Vp, determined in accordance with Sentence 4.1.8.18.(1) using IE = 1.0 andRp = 1.3.
[4] 4)For Normal Importance Category buildings in Seismic Category SC4 with a height above grade of more than 30 m, the structural framing elements not considered to be part of the SFRS shall be designed to behave elastically for a specified lateral earthquake force, V, determined in accordance withSentence 4.1.8.11.(2)Article 4.1.8.11. or 4.1.8.12., as appropriate, using 5%-damped spectral acceleration values based on a 10% probability of exceedance in 50 years and RdRo = 1.3.
[5] 5)For the purposes of applying Sentences (2)to (4), the following criteria shall apply:torsional moments due to accidental eccentricities need not be considered if B, as determined in accordance with Sentence 4.1.8.11.(10), does not exceed 1.7.
[a] --)the SFRS and structural elements not considered to be part of the SFRS are permitted to be used to resist earthquake loads and deflections,
[b] --)the structural modeling shall be in accordance with Sentence 4.1.8.3.(8) using appropriate stiffness,
[c] --)except as provided in Clause (d), the design spectral acceleration, S(T), shall be determined in accordance with Article 4.1.8.4. but using the 5%-damped spectral acceleration values specified in Sentences (2) to (4),
[d] --)the Seismic Category shall be unchanged from that determined in accordance with Article 4.1.8.5. using
[i] --)the 5%-damped spectral acceleration values having a 2% probability of exceedance in 50 years, and
[ii] --)the value of IE given in Table 4.1.8.5.-A,
[e] --)the designation of the structure as regular or irregular determined in accordance with Article 4.1.8.6. shall be unchanged from that determined for the 5%-damped spectral acceleration values having a 2% probability of exceedance in 50 years,
[f] --)the method of analysis shall be in accordance with Article 4.1.8.7.,
[g] --)for buildings with a Type 9 or 10 irregularity as described in Table 4.1.8.6., the analysis shall account for
[i] --)the effects of the vertical response of the building mass, and
[ii] --)the vertical ground motions,
[h] --)except as provided in Clause (i), torsional effects shall be accounted for using the procedures given in Article 4.1.8.11. or 4.1.8.12., as appropriate,
[i] --)the torsional moments due to accidental eccentricities need not be considered if B, as determined in accordance with Sentence 4.1.8.11.(10) using a structural model satisfying Clause (b), does not exceed 1.7,
[j] --)where the Dynamic Analysis Procedure described in Article 4.1.8.12. is used to determine the specified lateral earthquake force, Vd, the requirements of Sentences 4.1.8.12.(8), (9) and (12) need not apply,
[k] --)the increased displacements of the structure resulting from foundation movement, as described in Sentence 4.1.8.16.(1), need not be considered, and
[l] --)except as provided in Clauses (a) to (k), the requirements of Articles 4.1.8.9., 4.1.8.10. and 4.1.8.14. to 4.1.8.17. need not apply.
(See Note A-4.1.8.23.(5).)
[6] 6)For the purposes of applying Sentences (2)to (4), elements of the SFRS and structural framing elements not considered to be part of the SFRS, when included in the analysis, shall be modeled in accordance with Sentence 4.1.8.3.(8) using elastic properties.
[7] 7)All other requirements of Articles 4.1.8.2.to 4.1.8.18. shall be satisfied in meeting the additional requirements of this Article.
Note A-4.1.8.23.(5)Elastic Analysis Required by Article 4.1.8.23.
When Article 4.1.8.11. is used to determine the specified lateral earthquake force, V, for the smaller design spectral acceleration values, S(Ta), corresponding to the higher probabilities of exceedance specified in Sentences 4.1.8.23.(2) to (4), the fundamental lateral period, Ta, may need to be recalculated to account for the higher stiffness of a building that behaves elastically. In addition, the higher mode factor, Mv, and the base overturning moment reduction factor, J, may need to be recalculated to account for the different spectral ratios, S(0.2)/S(0.5), as well as the different fundamental lateral period, Ta.
Impact analysis
The proposed changes clarifies existing requirements. As a result, no cost impact is expected. The proposed clarifications would support a better understanding of the intent of Article 4.1.8.23. by Code users. Therefore, the impact on industry and enforcement is expected to be positive.
Enforcement implications
The proposed change supports a better understanding of the requirements of Article 4.1.8.23. This better understanding is expected to lead to improved compliance and smoother enforcement.
Who is affected
Owners, designers, contractors and enforcement professionals dealing with the construction of post-disaster buildings and High Importance Category buildings on sites with moderate or high seismic hazard and the construction of Normal Importance Category buildings more than 30 m tall on sites with high seismic hazard.
OBJECTIVE-BASED ANALYSIS OF NEW OR CHANGED PROVISIONS
