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

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434 Can. J. Civ. Eng. Vol. 30, 2003listed in Appendix C, which represent the centres of urbanareas and so generally differ from the sites where data werecollected, 1 in 30 year wind velocities and dispersion parameterswere obtained from the maps by interpolation. Then 1in 10 year and 1 in 100 year velocities were determined foreach site, from which 1 in 10, 1 in 30, and 1 in 100 year referencepressures were computed using eq. [6]. Thus the referencepressures listed in Appendix C of the NBCCgenerally do not exactly fit a Gumbel distribution, though itis possible to back-calculate wind velocities from the publishedreference pressures that do. There is no documentationavailable concerning the factors used to transform thevelocities and dispersion parameters from the data collectionsites to the locations listed in Appendix C. Generally theCoVs of maximum annual wind velocities back-calculatedfrom the wind pressures in Appendix C seem greater thanthe corresponding CoVs for nearby sites where data werecollected.The present investigation considered the wind velocity,not the wind pressure, as a random variable with a Gumbeldistribution to be consistent with the methodology and assumptionsadopted to compute the NBCC reference pressures.The ratio of the mean maximum velocity expected ina 50 year design life, V 50, to the specified 1 in 50 yearvelocity, V 50 , depends on only the CoV of the maximumannual wind velocity, CoV a , and isV 50 1[7] = + 3.050 CoVaV 1 + 2.592 CoV50aAnnual maximum wind velocity data were provided for311 sites by the Engineering Climatology Section of theCanadian Meteorological Centre in Downsview, Ontario, ofwhich 223 had at least 10 years of record. The CoV of themaximum annual wind velocity varies as shown in Fig. 3.The horizontal axis in Fig. 3 is the difference in degrees betweenthe longitude of the site and the Yukon–Alaska border,so the points on the left-hand side of the figurecorrespond to sites in Yukon and British Columbia and thepoints on the right-hand side correspond to sites in the Atlanticprovinces. The CoV a values range from 0.028 (OldGlory Mountain, British Columbia) to a maximum of 0.394(Squamish Airport, British Columbia), with an overall meanvalue of 0.134. For the 223 sites with at least 10 years of record,CoV a ranges from 0.055 (Vancouver Harbour, BritishColumbia) to 0.230 (Iqaluit, Nunavut), and the overall meanis 0.135. For these 223 sites, 90% of the CoV a values arewithin the range 0.087–0.183. There is a marked tendencyfor the standard deviation of the maximum annual wind velocityto increase as the mean value of the maximum annualwind velocity increases, as shown in Fig. 4. A linear regressionline forced through the origin has a slope, which correspondsto the CoV a , of 0.135 as shown.Figure 5 shows the relationship between the referencewind pressures for various return periods and the CoV of themaximum annual velocity distribution. The reference windpressures have been normalized using the 1 in 30 year windpressure. Figure 5 can be used to estimate the wind load factorsfor specified 1 in 50 year reference wind pressures thatgive factored wind loads identical to those in the 1995NBCC, where the load factor of 1.5 is applied to specified 1in 30 year reference wind pressures. For the average CoV aFig. 3. Coefficient of variation (CoV) of maximum annual windvelocity.Fig. 4. Standard deviation of maximum annual wind velocity.Fig. 5. Normalized specified wind pressures.of 0.135, the load factor applied to the 1 in 50 year referencewind pressure is 1.38, and for the CoV a limits of 0.087 to0.183 it corresponds to 1.35 and 1.42, respectively. If theload factor selected for the 1 in 50 year specified wind pres-© 2003 NRC Canada
Bartlett et al. 435sure exceeds these values, and the resistance factors areunchanged, designs to the 2005 NBCC will require a greaternominal resistance than designs to the 1995 NBCC.Figure 5 can also be used to estimate the importance factorfor wind load for post-disaster buildings, which in the1995 NBCC are designed for the 1 in 100 year wind load.These provisions are equivalent to an importance factor thatranges from 1.14 to 1.26, with a mean value of 1.21.Calibration for wind load was carried out for Regina,Rivière du Loup, and Halifax. The annual maximum windvelocity was treated as a Gumbel variate with bias computedusing eq. [7] and CoV a values derived from the CanadianMeteorological Centre data. These values are summarized inTable 3.Transformation factorsAs implied by eq. [5], the reference velocity pressuremust be transformed using gust, exposure, and pressure coefficientsto obtain the pressure applied to the surface of abuilding. Given that the transformation is represented as aproduct of independent random variables, the Central LimitTheorem implies that the overall transformation factor hasa log-normal distribution. It was further assumed that thetransformation factor is time independent. The current literaturepresents a considerable range of statistical parametersfor these coefficients; rationale for the particular values selectedis summarized briefly in the following paragraphs.The literature pertaining to the wind load approach inASCE7-98 (ASCE 2000) has limited relevance becausesome factors differ markedly from those in the NBCC.Statistical parameters for the exposure coefficient, C e , andthe combined gust and pressure coefficients, C g C p , reportedby others are presented in Table 4. The C e parametersadopted for the current investigation are consistent withthose proposed by Davenport (1982, 2000). The C g C p biasadopted is conservative with respect to values proposed forlow (Davenport 1982) or tall buildings (Davenport 2000),and the CoV adopted is at the higher end of the range of valuesreported by others.ASCE7-98 (ASCE 2000) specifies a factor of 0.85 fordirectionality that was neglected in the present investigationbecause the gust and pressure coefficients used for lowbuilding design in the NBCC have been reduced to accountfor directionality and other factors. Also, the pressure coefficientsin the NBCC for taller buildings are less severe thanthose in ASCE7-98 because they permit the designer tocompute the leeward suction at the mid-height of the building,not at the top.Thus the overall bias coefficient accounting for exposure,gust, and pressure coefficients, ignoring wind directionality,is 0.80 × 0.85 = 0.68, with a CoV of 0.219, say 0.22. Asshown in Table 4, these values are similar to those proposedby Ellingwood and Tekie (1999) and are comparable to valuesproposed by Davenport (1981, 1982).Point-in-time wind velocitiesPoint-in-time wind loads were derived assuming that thewind velocity (and therefore pressure) is a stochastic processand the factor that transforms the reference pressure to aload against the building surface is a time-independent randomvariable. The duration of each wind pulse was assumedTable 3. Statistical parameters for maximum wind velocity in50 year and 3 h periods.to be 3 h, so the annual number of pulses is 365.25 × 24/3 =2922. The central tendency and dispersion parameters forthe point-in-time distributions are summarized in Table 3.Snow loadMaximum 50 yearMaximum 3 hSite CoV a Bias CoV Bias CoVRegina 0.108 1.039 0.081 0.130 0.785Rivière du Loup 0.170 1.054 0.112 0.066 1.095Halifax 0.150 1.049 0.103 0.080 1.001The actual snow load on a roof is the difference betweenthe quantity of snow or rain that has accumulated and thequantity that has been removed by wind, melting, or evaporation(Ellingwood and O’Rourke 1985), but data are notreadily available to quantify snow load in this manner. Instead,the model used for design, which expresses the snowload on the roof of a structure, S, as a snow component anda rain component, was adopted:[8] S =(C b C w C s C a )S s + S rwhere S s is the ground snow load, S r is the associated rainload, C b is the basic roof snow load factor and equals 0.8,C w is the wind exposure factor, C s is the slope factor, and C ais the accumulation factor. The transformation factor, C gr ,that converts the ground snow load at a given site to an appropriateroof snow load is[9] C gr = C b C w C s C aAccording to the 1995 NBCC, S r need not be takengreater than C gr S s .Ground snow loadThe ground snow load, S s , is determined in Appendix C ofthe 1995 NBCC as the product of the depth of snow, d, andthe unit weight of snow, γ. These values were computed(NBCC 1995; Newark et al. 1989) by first fitting Gumbeldistributions to maximum annual accumulated snow depthdata for 1618 stations with 7–38 years of record and calculating1 in 30 year values. Then snow densities were determined,for various geographical regions with commonclimatic features as manifested by forest type, that averaged2.01 kN/m 3 east of the continental divide, 2.55–4.21 kN/m 3west of the continental divide, and 2.94 kN/m 3 above thetreeline in the Northwest Territories and Nunavut. The loadswere normalized to account for the site elevation, assuminga linear variation of load above sea level, and smoothed contourmaps were prepared. The final loads corresponding tothe various geographical locations in Appendix C were derivedby interpolation and rectified to account for elevation.There is no documentation available concerning the interpolationweighting factors used to transform the ground snowload values from the data collection sites to the locationslisted in Appendix C.The present investigation considered the maximum annualsnow accumulation depth to have a Gumbel distribution, forconsistency with the basis for computation of the specified© 2003 NRC Canada
Page 1 and 2: 429Load factor calibration for the
Page 3 and 4: Bartlett et al. 431Table 1. Dead lo
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Page 9 and 10: Bartlett et al. 437Table 6. Statist
Page 11: Bartlett et al. 439Ellingwood, B.R.

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