Loads and Seismic Design National Building Code Wind and Snow Importance Factors

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1 Loads and Seismic Design 2005 National Building Code Wind and Snow Importance Factors Russ Riffell, P.Eng. Chair, Standing Committee on Structural Design Part 4 of the National Building Code of Canada National Building Code of Canada Q What Is It? h Model Code that is essentially a set of provisions for the safety of the public in buildings zsafety zhealth zaccessibility z Fire and Structural Protection of Buildings h Intended for use throughout Canada zconsistent set of rules 1

2 National Building Code of Canada Q Where Does It Come From? h National Research Council publishes the NBC h All Canadians have input to it: zprovincial/territorial Policy Advisory Committee on Codes (PTPACC) zpublic Review z Membership in Committees National Building Code of Canada Q Under the Constitution Act, provincial and territorial governments regulate building Q PTPACC h Established by provincial and territorial code authorities to provide policy guidance to the NBC h Liaison with provinces/territories and the NBC h Intended to ensure relevance of the NBC Q NBC not law unless officially adopted by a territory or province or Vancouver or Montreal 2

3 National Building Code of Canada National Research Council Canadian Commission on Building and Fire Codes Provincial/Territorial Policy Advisory Committee on Codes (PTPACC) National Building Code of Canada Standing Committee on Structural Design (SCSD) Part 4 Structural Design CANCEE User s Guide Structural Commentaries NBC Q Objective-based Code h CCBFC declared a partial moratorium on code changes to allow standing committees to concentrate on conversion to objective-based format. h Part 4 not affected h Created delay in publishing of NBC Q Code objectives clearly defined Q Promote innovation Q Revisions easier to implement 3

4 2005 Code Format Q Two Divisions, A and B Q Division A h Compliance h Objectives h Functional Requirements Q Division B h Acceptable Solutions zpart 4 zother solutions with safety >= Part 4 Harmonizing Loads Q Wind (1995) h Annual probability of being exceeded z1/100 for strength of post-disaster buildings z1/30 for primary structural action z1/10 for cladding and deflection or vibration Q Earthquake (1995) h Probability of exceedance of 10% in 50 years h Annual probability of being exceeded z 1/475 4

5 Harmonizing Loads Q Earthquake (cont d) h Specified load includes a Seismic Importance Factor, I zi = 1.5 for post-disaster buildings zi = 1.3 for schools zi = 1.0 for all other buildings Q Snow (1995) 30 year return period, i.e.1/30 Harmonizing Loads Q SCSD established a task group to study this issue of different methods of accounting for loads h Earthquake includes a seismic importance factor h Wind varies the return period which is another method of implementing an importance factor h Account for buildings used for shelter in time of disaster h Review of loads, load factors and load combinations 5

6 Loads Q Dead Load, D h permanent load due to weight of building components (and vertical loads due to earth) Q Live Load, L h variable load due to intended use and occupancy, including loads due to cranes and pressure of liquids in containers Q H h permanent load due to lateral earth pressure, including groundwater Loads Q Snow Load, S h variable load due to snow + ice + included rain, or rain Q Wind Load, W h variable load Q Earthquake Load, E h rare load Q P and T h permanent load due to prestress h temperature, shrinkage, moisture, creep, settlement 6

7 Limit States Design Q Ultimate Limit States Limit state just satisfied Limit States Design Q Statistical Analysis of Safety 7

8 Limit States Design Q Statistical analysis of safety Let X = ln R/S V x = standard deviation of X Set distance from 0 to the mean value as E times V x. Thus, E is the reliability index. Reliability Index Q 1995 Code examined to calibrate new loads (return periods) and load factors. h Live load that includes both use and occupancy and snow load is conservative h Reliability indices for the combination of dead load and snow load are smaller than other load combinations. OK for concrete but not for steel or timber. h Combination of dead, wind and snow for steel: reliability index > 3, except where snow load dominates 8

9 Reliability Index & Load Factors Q 2005 h Combinations of dead and live (use & occupancy) and dead and wind loads from the 1995 NBCC have 50- year reliability indices of 3.0 or larger. h Return period of 50 years h Target Reliability Index of 3.0 Wind Load Calibration Q Canadian Meteorological Society has 223 sites with records for at least 10 years. Q Data analyzed by fitting a Gumbel distribution to wind velocities. Q 50 and 500 year return period values calculated from this distribution. Q Importance values calculated from distribution. h Mean value for 100 year return period is

10 Wind Load Calibration in-10 1-in-50 1-in in mean 90% of values in this range COV of Maximum Annual Wind Velocity Wind Loads Q Exposure factor, Ce h Two conditions: zopen Terrain - level with scattered buildings, trees or other obstructions, shoreline, open water. (Same as 1995.) zrough Terrain - suburban, urban or wooded for at least 1 km or 10 x the building height. Q Gust Effect Factor for Internal Pressures, Cgi h Was 1.0 to 2.0. Now 2.0 unless a detailed calculation yields a lower value. 10

11 Wind Reference Pressure Reference Velocity Pressure (kpa) Location 1-in-10 1-in-30 1-in-50 1-in in-500 Victoria (Gonzales) Winnipeg Mississauga Halifax Wind Reference Pressure Design Wind Velocity Pressures, q 30 and q 50 City q 30 (q 50 ), kpa Victoria 0.58(0.63) Vancouver 0.44(0.48) Chilliwack 0.63(0.72) Kamloops 0.37(0.40) Kelowna 0.43(0.47) Revelstoke 0.29(0.32) Calgary 0.46(0.50) Edmonton 0.40(0.45) 11

12 Internal Pressures Q Category 1 (small, uniformly distributed openings) h 1995 Cpi = 0.0 to -0.3; Cg = 1.0 h 2005 Cpi = 0.0 to -0.15; Cgi = 2.0 or calculated Q Category 2 (significant openings will be closed) h 1995 Cpi = 0.7 to -0.7; Cg = 1.0 h 2005 Cpi = 0.3 to -0.45; Cgi = 2.0 or calculated Q Category 3 (gusts transmitted to interior) h 1995 Cpi = 0.7 to -0.7; Cg = 2.0 h 2005 Cpi = 0.7 to -0.7; Cgi = 2.0 or calculated Wind Loads Q Cgi Detailed Calculation h Cgi = 1 + 1/(1 + τ/10)^1/2 h τ = Vo/(695 x A) x ( * As/Vo * δ) zvo = internal volume (m 3 ) za = total area of all openings in the envelope (m 2 ) zas = total exterior surface area (m 2 ) zδ = the average flexibility of walls and roof, and is equal to the interior volume change in m 3 per kpa of pressure change divided by the total surface area in m 2 of slabs and walls excluding slabs on grade 12

13 Wind Load Comparison Fig. 1: Code Loads - 4 sq.m. of Edge Wall Cladding - Open Terrain Wind Load, kpa NBC2005 qaverage NBC1995 qaverage ASCE 2002 NBC2005 qtoronto NBC1995 qtoronto Building Height, m Wind Load Comparison 4.0 Fig. 2: Code Loads - 4 sq.m of Edge Wall Cladding - Rough Terrain Wind Load, kpa NBC2005 qaverage NBC1995 qaverage ASCE 2002 NBC2005 qtoronto NBC1995 qtoronto Building Height, m 13

14 Wind Load Comparison Fig. 3: Code Loads - 4 sq.m. of Mid-Wall Cladding - Open Terrain Wind Load, kpa NBC 2005 NBC 1995 ASCE 2002 NBC2005 qtoronto 0.5 NBC1995 qtoronto Building Height, m Wind Load Comparison Fig. 4: Code Loads - 4 sq.m. of Mid-Wall Cladding - Rough Terrain Wind Load, kpa NBC 2005 NBC ASCE NBC2005 qtoronto NBC1995 qtoronto Building Height, m 14

15 Wind Load Comparison Fig. 5: Code Loads - Structure (Across Building) - Open Terrain Wind Load, kpa NBC2005 qaverage NBC1995 qaverage ASCE 2002 NBC2005 qtoronto NBC1995 qtoronto Building Height, m Wind Load Comparison 2.5 Fig. 6: Code Loads - Structure (Across Building) - Rough Terrain 2.0 Wind Load, kpa NBC2005 qaverage NBC1995 qaverage ASCE 2002 NBC2005 qtorontor NBC1995 qtoronto Building Height, m 15

16 Wind Load Comparison 2.5 Fig. 7: Code Loads - Structure (Along Building) - Open Terrain 2.0 Wind Load, kpa NBC2005 qaverage NBC1995 qaverage ASCE 2002 NBC2005 qtoronto NBC1995 qtoronto Building Height, m Wind Load Comparison Fig. 8: Code Loads - Structure (Along Building) - Rough Terrain Wind Load, kpa NBC2005 qaverage NBC1995 qaverage ASCE 2002 NBC2005 qtoronto NBC1995 qtoronto Building Height, m 16

17 Wind Design Flow Chart q: NBC Appendix C Structural components and Cladding Building structure (1,2) H>120 m Yes No (1,2) Yes H/w>4 No (2) Susceptible to vibrations Yes Static Procedure No Static Procedure Dynamic Procedure (3) Ce: NBC (5) Par. 7-8, Ce: NBC (5) Par. 7, Ce: Par. 7, High-Rise Cg: 2.5 Par No (1) H 20 m (1) High-Rise H 20 m and No Cg: 2.0 H/D < 1 Par Cg: Par Yes Yes Cp*: Par , 29, Fig. I-10 (4) CgCp: Par , 25-28, Fig. I-4 to I-9 (4) CgCp: Par , 25-28, Fig. I-3 (4) Cp: Par , 29, Fig. I-10 (4) Cp: Par , Fig. I-10 (4) Internal pressure Cgi: 2.0 or Par. 22 Cpi: Par pe (5) = IwqCeCgCp pe (5) = IwqCeCgCp pe (5) = IwqCeCgCp Notes: pe = IwqCeCgCp and pi = IwqCeCgiCpi Partial Loading: Par , Fig. I-11 Partial Loading: Par , Fig. I-11 Reliability Index & Load Factors Q Snow h Large variability in load h New calibration gave load factor = 1.7 zfactored snow load would increase by 25% h Decided to remain with LF = 1.5 zfactored snow load now ~ 10% larger than

18 Reliability Index Q 1995 Code h Reliability indices for the combination of dead load and snow load are smaller than other load combinations. OK for concrete but not for steel or timber. h Combination of dead, wind and snow for steel: reliability index > 3, except where snow load dominates Snow Q Reduction of wind exposure factor, Cw, tightened. Cannot reduce from 1.0 for Importance categories High and Post-Disaster. Q Drift criteria for curved roofs 18

19 Importance Factors Q Apply an importance factor to all variable loads, based on use and occupancy Q Use and Occupancy h Low Hazard h All Others h Emergency Shelter h Post-Disaster Q Importance Category h Low h Normal h High h Post-Disaster Importance Factor - Wind Importance Category I W ULS SLS Low Normal High Post Disaster

20 Importance Factor - Snow Importance Category I s ULS SLS Low Normal High Post Disaster Major Changes to Part 4 Q Harmonization of Return Periods and Importance Factors h Review of reliability index for factored loads and resistances Q Companion Action Load Combinations h Re-definition of loads, factors & combinations Q 50 year return periods for Wind and Snow Q Revised Seismic Design Requirements 20

21 Other Changes Q WSD gone h Masonry standard now only LSD, no WSD h Factored resistance of soil for design of foundations - no more allowable bearing pressures Q Steel building systems added to 4.3 Factored Loads 2005 vs Q Dead + Live combination is unchanged. Q Dead + Wind combination 1% less than Q Dead + Snow combination 10% greater than Q Dead + Live + Snow combination less than Q Dead+ Live + Wind greater than Q Dead + Wind + Snow greater than

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