Load factor calibration for the proposed 2005 edition of the National ...

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436 Can. J. Civ. Eng. Vol. 30, 2003Table 4. Statistical parameters for the exposure coefficient, combined gust and pressure coefficients, and overall wind load transformationfactor reported by others.Source Bias CoV CommentExposure coefficient, C eAllen 1975 — — Bias = 0.85; CoV = 0.13 assumed for combination of C e C g andmodellingEllingwood et al. 1980 1.0 0.16 Exposure factor different from NBCCDavenport 1981 1.0 0.10 As reported by Kariyawasam 1996Davenport 1982 0.80 0.16 Low buildingsEllingwood and Tekie 1999 0.93 0.129 Low buildingDavenport 2000 0.80 0.20 For simplified method; tall buildingCurrent investigation 0.80 0.16Gust coefficient and pressure coefficient, C g C pAllen 1975 1.0 0.10 Pressure coefficient only; model error factor of 0.85 included separatelyEllingwood et al. 1980 1.0 0.11 Gust1.0 0.12 Pressure1.0 0.16 CombinedDavenport 1981 (as reported by 1.0 0.05 GustKariyawasam (1996))1.0 0.10 Pressure1.0 0.11 CombinedDavenport 1982 0.80 0.15 For low buildingsEllingwood and Tekie 1999 0.97 0.093 Gust0.88 0.067 Pressure (same nominal values as NBCC)0.85 0.115 CombinedDavenport 2000 0.80 0.21 For simplified method; tall buildingCurrent investigation 0.85 0.15Overall wind load transformation factorAllen 1975 0.72 0.174 Basis for calibration of NBCCEllingwood et al. 1980 0.85 0.239Davenport 1981 0.85 0.150 Not specifically for low buildings (as reported by Kariyawasam (1996))Davenport 1982 0.54 0.325 Low buildings; only external pressuresRosowsky and Cheng 1999 0.73 0.264 For low buildings, high-wind regionsEllingwood and Tekie 1999 0.70 0.208 Not specifically for low buildings; all factors have normal distributionsDavenport 2000 0.57 0.250 Simple method0.71 0.150 Detailed method1.00 0.087 Wind tunnel and meteorological studyCurrent investigation 0.68 0.22 Log-normalvalues in Appendix C of the 1995 NBCC. Annual maximumsnow depth data were provided for 1278 data-collection sitesby the Engineering Climatology Section of the CanadianMeteorological Centre. The CoV of the annual maximumdepth ranges from 0.08 to 1.34, with a mean of 0.491 and amedian of 0.47. Approximately 95% of the CoV values liebetween 0.22 and 0.95. Statistical parameters for maximumsnow depth in a 50 year reference period are summarized inTable 5. The ratio of the mean value of the maximum depthin a 50 year reference period to the 1 in 50 year specifiedsnow depths were computed using eq. [7]. The ranges ofbias coefficients and CoV values for the normalized maximum50 year distribution are quite small, so the valuesshown in the last row of Table 5 were adopted.Snow densities used to derive the 1 in 30 year valuesspecified in the 1995 NBCC were assigned based on thetype of forest in the region of the site. Kariyawasam (1996)has summarized statistical parameters for snow density, ρ,from Newark (1984), as reproduced in Table 6. In theTable 5. Statistical parameters for maximum snowdepth in 50 year period.CoV a Bias CoVLower 3% fractile 0.220 1.064 0.132Mean 0.491 1.099 0.197Upper 3% fractile 0.953 1.126 0.244Current investigation 1.10 0.20current investigation snow densities were assumed normallydistributed with a bias of 1.0 and a CoV of 0.17. The CoVvalue adopted is the average for all regions except tundraand taiga, which are not densely populated.Associated rain loadThe associated rain load represents the weight of rain fallingon snowpack, and values for Appendix C of the 1995NBCC have been derived based on historical data for winter© 2003 NRC Canada
Bartlett et al. 437Table 6. Statistical parameters for snow density (Kariyawasam 1996).Forest region Mean (kg/m 3 ) SD (kg/m 3 ) CoV Typical sitesAcadian 220 50 0.23 Fredericton, Halifax, St. John’s, CharlottetownAspen grove 220 40 0.18 Edmonton, Saskatoon, Winnipeg, Thunder BayBoreal 190 60 0.32 Northern Prairies, mid-northern Ontario and QuebecCoast 430 25 0.06 Coastal British ColumbiaColumbia 360 35 0.10 Southeastern British ColumbiaGreat Lakes 220 60 0.27 Southern and central Ontario, southern QuebecMontane 260 25 0.10 Interior British Columbia and YukonPrairie 210 40 0.19 Southern Prairies, ReginaSubalpine 360 30 0.08 Vancouver and Fraser ValleyTundra 300 80 0.27 ArcticTaiga 200 80 0.40 Subarctic, YellowknifeCurrent investigation 0.17Note: SD, standard deviation.rainfall (Taylor and Allen 2000). Subjectivity was requiredin the process of sketching 0.1 kPa isolines on maps(Newark et al. 1989). The difficulty of deriving statisticalparameters for this load is further exacerbated because thenecessary data are not readily available and, even if theywere, would require careful filtering to eliminate winterrainfall on nonexistent snowpacks. It was therefore assumedthat probability distributions for the roof snow load includeany further bias and uncertainty due to the associated rainload.Ground-to-roof transformation factorAlthough there is a significant body of literature concerningthe ground-to-roof snow load transformation factor, fewstudies provide results in a format that is compatible withcode calibration. Ellingwood and O’Rourke (1985) cite thefollowing relationship, from O’Rourke et al. (1982):[10] Cgr = 047 . ETεwhere E is a wind exposure factor ranging from 0.9 to 1.3; Tis a thermal characteristic factor ranging from 1.0 to 1.2; andε is the error term, which has a log-normal distribution witha mean value of 1.0 and a CoV of 0.44. The transformationfactor computed using eq. [10] ranges from 0.53 to 0.91times the basic roof snow load factor of 0.8 specified in the1995 NBCC.For the present investigation, parameters were adoptedfrom a recent study by Taylor and Allen (2000) that are consistentwith the definition of the ground-to-roof snow loadfactor as defined in eq. [8]. Various results, summarized inTable 7, consider the ratio of the maximum measured roofsnow load to the corresponding maximum ground snow loadas converted to an equivalent roof load using the NBCC criteria.The statistics are based on 13 years of data for 112roofs in four Canadian cities. The bias for data obtainedin Halifax, Chicoutimi, and Ottawa and the drift locationin Saskatoon tend to fall within the ranges suggested byEllingwood and O’Rourke (1985). The CoV values are alsosimilar.Point-in-time snow loadPoint-in-time snow load parameters were derived assumingthat the snow accumulation is a stochastic process andTable 7. Statistical parameters for ground-to-rooftransformation factor (Taylor and Allen 2000).the factors that transform the depth to a load and the groundsnow load to a roof snow load are time-independent randomvariables. It was assumed that the point-in-time pulse has amagnitude that follows the Weibull distribution and a durationof 14 days during only 3 months of the year, so thereare six events per year. The central tendency and dispersionfor the point-in-time distribution represent a bias with respectto the specified 1 in 50 year snow depth of 0.196 and aCoV of 0.882.Summaryn Bias CoVSheltered locationsHalifax 35 0.61 0.45Chicoutimi 8 0.71 0.44Saskatoon 8 0.30 0.11Recommended value 0.60 0.42Exposed locationsHalifax 11 0.47 0.46Ottawa 12 0.44 0.41Saskatoon 5 0.33 0.22Recommended value 0.50 0.42Drift locationsChicoutimi 4 0.86 0.30Ottawa 6 0.60 0.38Saskatoon 23 0.61 0.43Recommended value 0.60 0.42Current investigation 0.60 0.42Note: n, number of roofs in the sample.This paper summarizes statistical parameters for deadload, live load due to use and occupancy, snow load, andwind load that have been adopted for calibration of load andload combination criteria for the 2005 National BuildingCode of Canada (NBCC). Table 8 summarizes the bias andCoV values and distribution types adopted for the calibrationpresented in a companion paper. A new survey of typicalconstruction tolerances indicates that statistical parametersfor dead load presented by Ellingwood et al. (1980) are ade-© 2003 NRC Canada
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