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

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430 Can. J. Civ. Eng. Vol. 30, 2003dards including the CEB (1976), AISC (1986), and ASCE(2000). Structural design based on this format uses loadcombinations of the form[1] α + α S + α S∑p i i ij ji≠jwhere α P is the load factor applied to the effects due to permanentload, P, such as dead load; and α i is the load factorapplied to the effects due to the principal transient load, S i ,assumed to be acting at its maximum value during the lifetimeof the structure. The load factors α ij applied to thecompanion-action transient loads S j represent thecompanion-action load effects that occur during the periodwhile the principal action is acting at its maximum value.An early objective of the calibration process was to determinewhether specified snow and wind loads should correspondto 50 year or 500 year return periods. The Task Groupon Snow and Wind Loads decided in October 2000 to recommendthat specified loads be based on 50 year returnperiods, in part because (i) these values could be readily obtainedfrom existing databases; (ii) preliminary calibrationindicated that load factors less than 1.0 were required if500 year loads were specified; and (iii) 50 year return periodsare used for the loads specified in ASCE7-98 (ASCE2000).In this paper, the bases for the statistical parameters usedto characterize dead, live, snow, and wind loads are presented.These statistical parameters differ slightly from thevalues used to verify the 1977 NBCC reported by Nowakand Lind (1979). A companion paper summarizes results ofthe calibration, describes the evolution of the recommendedload factors in response to comments from the Task Groupon Snow and Wind Loads and others, and summarizes the finalload factors and load combinations recommended for the2005 NBCC.Dead loadDead load consists of the weights of a structural memberand the structural components that it supports, partitions,and permanent equipment. Although uncertain, it is commonlyassumed to remain constant during the service life ofa structure. There is a tendency for the dead load to exceedits nominal value because the designer may overlook somethingin the dead load takeoff, which is typically estimatedto within ±10% for buildings. Dead load uncertainty is becauseof dimensional tolerances and the uncertainty of unitweights of materials. Additional uncertainty is introducedthrough the process of converting the load into a load effect,which for indeterminate structures can be done accuratelyonly if the construction sequence is known. It is commonlyassumed that dead loads are normally distributed, perhapsbecause tolerances tend to be normally distributed, althoughno actual data seem to be available to verify this assumption.The dead load statistical parameters depend to some extenton the size of structure, the construction material used,and the quality control implemented. The weight of largestructural components is relatively insensitive to absolute dimensionaltolerances, so the dead load bias coefficients andcoefficients of variation are reduced. Improved qualitycontrolprocedures have the same effect. Geometric andmaterial unit weight variations are less for steel or precastconcrete components than for cast-in-place concrete construction.In particular, formwork deflections in cast-in-placeconcrete floor construction can often cause the dead load tobe larger than the nominal value.A literature review indicated dead load itself has a bias, orratio of the mean to nominal values, from 1.00 to 1.05 and acoefficient of variation (CoV), the ratio of the standard deviationto the mean value, between 0.06 and 0.09. Modellingand analysis are typically assumed to be unbiased, withCoVs of 0.03–0.07, and increase the CoV of the dead loadeffect to between 0.05 and 0.10. Most investigators (StandardsAssociation of Australia 1985, South African Bureauof Standards 1989, and European Committee for Standardization1994, all reported by Kemp et al. 1998; Tabsh 1997;Ellingwood 1999) adopted a bias of 1.05 and a CoV of 0.10,as reported by Ellingwood et al. (1980), which were used forthe current investigation. For cases where the dead loadcounteracts the effects of other loads, the dead load wasassumed normally distributed with a bias of 1.00 and a CoVof 0.10.In response to comments from members of the TaskGroup on Snow and Wind Loads and the Canadian StandardsAssociation (CSA) Technical Committee on ReinforcedConcrete Design, the statistical basis for dead loadfactors, including the accuracy of the analysis of dead loadeffects, was investigated. Findings are presented in this paperconcerning the original dead load statistics (Ellingwoodet al. 1980) and data from a new survey of concrete floorthickness variability. The companion paper summarizes relatedproposed revisions to NBCC Commentary G that reflectthe accuracy of the analysis of dead load effects.“Original” dead load statistics and load factorsFor the past two decades, code calibrators around theworld have characterized dead load effects as having a biasof 1.05 and a CoV of 0.10. The most detailed rationale forthese numbers is presented in Appendix B of Ellingwood etal. (1980, p. 145) and is based on the statistics shown in Table1. The mean dead load is 1.0 × 4.8 + 1.1 × 1.9 =6.89 kPa, with a standard deviation of [(1.0 × 4.8 × 0.06) 2 +(1.10 × 1.9 × 0.15) 2 ] 1/2 = 0.43 kPa and CoV of 0.43/6.89 =0.062. The bias for the dead load effect, accounting for theload model and analysis parameters, is therefore 6.89/6.70 ×1.0 × 1.0 = 1.03, with a CoV of (0.062 2 + 0.05 2 + 0.05 2 ) 1/2 =0.094. In these computations, the bias and uncertainty of thesuperimposed dead load were relatively large, and the magnitudeof the superimposed dead load is also large. Theimpact of the superimposed dead load was diminished, however,because it is a small fraction of the total dead load.In the derivation of the load factors for the 1975 edition ofthe NBCC, the dead load effect was assumed to have a biasof 1.00 and a CoV of 0.10, which included a CoV of 0.07for the transformation of dead load to dead load effect(Allen 1975). This calibration suggested a dead load factorof 1.30, but this was reduced to 1.25 to maintain the sameratio of dead load factor to live load factor as was used at thetime for ultimate strength design of concrete buildings.Survey in 2000 of concrete floor thickness variabilityMembers of the CSA Technical Committee on ReinforcedConcrete Buildings were invited to estimate typical floor© 2003 NRC Canada
Bartlett et al. 431Table 1. Dead load statistics for cast-in-place concrete (Ellingwood et al. 1980).Variable Bias CoVConcrete density 1.00 0.03Dimensions and density combined150 mm slab 1.00 0.08Slab and beam floor 1.00 0.07Column 1.04 0.04Total (100 pound-force per square foot = 4.8 kPa) 1.00 0.06Superimposed dead load: (40 pound-force per square foot = 1.9 kPa) 1.10 0.15Load model 1.00 0.05Dead load analysis 1.00 0.05thickness deviations for cast-in-place concrete construction.Three responses were received, two with quite detailed (andconsistent) information as summarized in Table 2. The valuesfor one- and two-way slabs are consistent with thicknesstolerances specified in standard CSA-A23.1-00 (CSA 2000).Bias values shown are based on the average of the reporteddeviations. Coefficients of variation are determined assumingthe range represented by the typical difference representstwo standard deviations, and the range represented by theworst difference represents four standard deviations.The tolerances shown in Table 2 indicate that the thicknessof cast-in-place toppings on precast construction andcast-in-place cover slabs on unshored composite constructioncan be extremely variable. It is assumed that the excessthickness is not uniform, but varies parabolically from amaximum, ∆, at midspan to zero at the ends. The extra load(or shear) in this case is equivalent to a uniform excessthickness of 2∆/3 along the full length of the member. Theextra midspan moment for a simply supported member isequivalent to a uniform excess thickness of 5∆/6 along thefull length of the member. If a 50 mm topping is placed on a200 mm precast hollow core slab, then the overall dead loadof topping and precast has a bias of 1.044 and a CoV rangingfrom 0.053 to 0.132. If a 50 mm topping is placed on a400 mm precast double tee beam, then the overall dead loadof topping and precast has a bias of 1.068 and a CoV rangingfrom 0.068 to 0.154. The bias and CoV for these casesare slightly greater than those shown in Table 1.Unshored composite construction was further identified asa potential problem, even though the definition of dead loadin clause 7.1.1 of CSA-S16.1-95 (CSA 1995) was amendedto include “the additional weight of concrete and finishesresulting from deflections of supporting members”. No guidanceabout acceptable tolerances for the cover slab thicknessis given in the CSA-A23.1-00 (CSA 2000) or CSA-S16.1-95(CSA 1995) standards. As noted previously, the impact ofgreater-than-specified slab thickness is more significant forbending moment than for shear force or reactions. Combiningthe worst values shown for cover slabs in Table 2with data for typical steel beams and deck, the overall deadload (or shear) of the concrete slab, deck, and steel beam hasa bias of 1.22 and a CoV of 0.25. The moment of the slab,deck, and beam has a bias of 1.33 and a CoV of 0.25. Theseparameters would require dead load factors in the order of1.8–2.0 to achieve acceptable levels of reliability.The survey responses also indicated that the weight ofconnections in steel roofs is known only after the roof is detailedby the fabricator and can exceed the 10% allowancethat some (but probably not all) designers use. For example,the allowance necessary for connection weight of the roof ofthe Skydome in Toronto was 30% (C.M. Allen, personalcommunication, 2000).The present study assumed a CoV for dead load analysisof roughly 0.050–0.075, which implies that the actual deadload force effects should generally be within 10–15% ofcalculated values. Although this margin may seem small,current codes and standards generally require the user to designductile members and structures that have the capabilityto redistribute the load if the actual distribution of force effectsis not exactly as predicted by an elastic analysis.Live load due to use and occupancyIn the present study, live load refers to that associatedwith the use and occupancy of a building. It includes theweight of people, equipment, and furnishings and materialsin storage. The total live load may be represented by a sustainedlive load component, which remains essentially constantfor a period of time that is often associated with theduration of a tenancy, and a transient (or extraordinary) liveload component that has a short duration or is instantaneous.The magnitude of each component is conventionally modelledby a Gamma distribution (Peir and Cornell 1973;Chalk and Corotis 1980; Ellingwood et al. 1980), and the arrivalof each is modelled as a Poisson process.Maximum lifetime live loadFor most reliability analyses or code calibrations, themaximum live load during the life of the structure has beenrepresented by a time-independent random variable with aGumbel probability distribution (Allen 1975; MacGregor1976; Ellingwood et al. 1980; European Committee forStandardization 1994; Kariyawasam 1996; Tabsh 1997;Ellingwood 1999). The reliabilities obtained by consideringthe live load as a Poisson process may in some cases differfrom those obtained by modelling the maximum live loadduring the life of the structure using an “equivalent” Gumbeldistribution. In practice, however, it is acceptable to representthe maximum live load as a Gumbel variate, and thisrepresentation was adopted.The mean value of the maximum equivalent uniformlydistributed live load for office floors during a 50 year referenceperiod was derived following Kariyawasam (1996).From simulation results (Ellingwood and Culver 1977), the© 2003 NRC Canada
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