values of the load effect are denoted through the subscripts "ann" and "apt." With the exception of E, the nominal loads are all defined by the values specified in the ANSI A58.1-1972 load standard. The nominal snow and wind are the 50-year mean recurrence

interval values. The nominal earthquake load

E is the value from the 1976 edition of the Uniform Building Code. Values given in

n

parentheses are characteristic extreme and shape parameters of extreme value distributions

0.73

(2.3)

rather than mean and c.o.v.

V.
X

includes uncertainties due to inherent variability,

load modeling and analysis.

n

Two values of the nominal live load L are of interest in this study. The first is the value in ANSI A58.1-1972, which was used to determine the values of B which correspond to existing accepted practice. The corresponding L is,

Ꭰ

n

= [1 – min { 0.0008A, 0.6, 0.23(1+)}] L。

L

(3.9)

n

=

in which A tributary area (see glossary, Chapter 9) and Lo basic (unreduced) live load given in Table 1 of ANSI A58.1-1972. The second nominal live load is that proposed

=

=influence area. This nominal value happens to equal the 50-year mean value, L. The live load factor in the new load criterion is derived so as to be compatible with the 1980 nominal live load. Similarly, for the arbitrary point-in-time live load,

The resistance of structural members, cross sections, cross-sectional elements and connectors is generally expressed by an analytical formula which has been derived from theory or experiment. In most cases of importance to structural design specifications, a clearly defined analytical model exists which has its origin in structural mechanics theory and which has been verified by experiment. It is possible, however, to cite cases where the basis of the model is purely theoretical or solely experimental. While it is evident that many types of analytical models exist in the design specifications of the various structural material groups, only a representative sample of them could be considered within the scope of this report. Enough models were considered, however, to arrive at representative parameters for the development of load factors. Detailed descriptions of these models are presented in the Appendices (B for reinforced and prestressed concrete, C for metals, D for masonry, and E for glulam and heavy timber), together with the collection of the available statistical information.

In most cases, the resistance was assumed to take the following product form:

in these equations is the nominal resistance based on the model used to best predict

R

n

the resistance, and on the nominal material properties and the nominal ("handbook")

geometric properties. For example, for a "compact" steel beam R = F Z, where F specified yield stress and Z is the plastic section modulus.

n

У

FyZ, Fy

is the

The factor P is the ratio of test capacities, representing actual in-situ performance, to the prediction according to the model used. The modeling of the capacity is thus defined by P (P standing for "professional"). Similarly, M and F (M defining "material" and F "fabrication") denote ratios of actual to nominal material properties and crosssectional properties.

where (Mp) test

(3.15)

y

= the mean plastic moment obtained from tests of beams, F = the mean static yield stress and Z = the mean plastic section modulus. In Appendix C it was found

The simple resistance model of Eq. 3.12 suffices for most cases which we have considered, although more complex models were used also (see especially reinforced concrete beamcolumns in Appendix B and masonry walls in Appendix D).

The rationale for selecting the material statistics for each particular structural material is discussed in detail in the Appendices, where the origin and the significance of the data is also considered. Most of this material for reinforced concrete structures

and for metal structures has been previously treated quite extensively in the literature. However, little has been previously presented for masonry and wood structures. 3.5.1 Resistance Statistics for Reinforced and Prestressed Concrete Structures

Table 3.2 presents representative statistical data (from Appendix B) for reinforced and prestressed concrete members. The probability distributions are assumed to be normal; R/R and V were obtained by fitting a normal distribution to the lower tail of the simulated

n

R

3.5.2 Resistance Statistics for Metal Structural Members

Following are some representative samples of resistance statistics for metal members and components (from Appendix C). Probability distributions were assumed to be lognormal in each case.

3.5.3 Resistance of Engineered Brick and Concrete Masonry

Statistical characteristics of unreinforced masonry walls in compression plus bending

are derived from data on full size walls tested in the laboratory, augmented by a factor to account for differences between fabrication and curing conditions in situ and in the laboratory (Appendix D).

The strength of brick and concrete masonry walls in compression plus bending appears to be modeled satisfactorily by a lognormal distribution. The mean and c.o.v. of strength,

measured in terms of vertical load, are summarized in Table 3.4 for two common wall slendernesses. The mean values depend on eccentricity ratio, e/t, and on slenderness, h/t. Variations in these estimates among individual sets of data naturally are to be expected; however, these values are representative and are suitable for the reliability analyses leading to the load criterion development.

referred

n

R

As discussed in Appendix D, there is some question as to whether R/R and V

to vertical load capacity are the most realistic statistical parameters for characterizing Calibrations were also performed for 'pure flexure, which provides an estimate of the reliability at very large eccentricities.

resistance when e/t becomes large.

In pure

The behavior of glued-laminated (glulam) structural members in bending, tension and compression has been determined from laboratory tests of large specimens, adjusted for load duration and, in the case of flexural members, for size. These data are discussed in detail in Appendix E, along with some problems in analyzing reliability of wood structures. Dimension lumber and light frame construction have not been included in this study.