One would need either some idea of the prevalent L/D for this situation or the relative

The value of corresponding to the desired ẞ could then be

When the c.o.v.

through 5.10e linear interpolation is perfectly acceptable.

or values are between those which are presented in Figs. 5.10a Further resolution in (e.g.

5.7

A comprehensive example of the selection of is presented in the following section. Resistance Factors Compatible with Selected Load Factors

The following discussion will focus on the methods by which material specification writing bodies can arrive at resistance factors compatible with the load factors presented in this report. The factors discussed below are presented for purposes of illustrating concepts and should not be considered as being recommendations by American National Standard Committee A58. The final choice of reliability indices and resistance factors rests with the specification writing groups.

5.7.1 Metal Structures

The following data illustrate two kinds of information that may be developed by a specification writing committee. The case considered is a steel beam: R/R = 1.07, V

=

R

0.13.

n

R

can be obtained by interpolation from the

Resistance factors for a given R/R and V charts relating 8, , R/R, VR and L/D (Figs. 5.10a - e). For illustration, values of B

for a given, L/D, R/R V. are:

n

R

From such a tabulation, considering a desired level of B, the committee might choose

0.80 or, perhaps &

=

0.85, as the basis for designing steel beams.

=

Similar data are given

in Table 5.6 for other types of structural elements.

The committee might next want to consider typical designs to compare current design practice with the future design practice based on the new load factors.

Parametric studies of the type discussed below might be performed, where the ratio R/R (subscripts f and nf' nc

c refer to "future" and "current," respectively) is determined from the relationships

Table 5.7 and Fig. 5.11 give the results for steel beams. If, for example, = 0.85

is selected, the required section modulus for the new design will be 1.04 times the value for the current design for S/D

n n

=

2 (typical roof beam); it will be 0.96 times the

current value for L/D = 1.5 and AT

O n

= 1000 ft2 (93 m2) (typical floor beam). For D L +

2

W the ratio will be somewhat larger than unity if = 1000 ft' and the live load reduction

is permitted; in other instances, it may be less.

Should the committee decide that =

0.9, with a corresponding 8 of approximately 2.5, is desirable for beams, then the ratios

of R/R would reduce, as shown in Figure 5.11a. nf nc

5.7.2 Reinforced and Prestressed Concrete Structures

The first step in selecting factors for concrete members is to select a target B.

In the calibrations presented in Appendix B, current reliability levels calculated for D +

L were

Reinforced concrete beams in flexure, current 8 = 2.6 to 3.2.

Plant Produced Pretensioned Beams in Flexure, Current B = 3.2 to 4.0.

Tied Columns, compression failures, current 8 = 3.0 to 3.5.

Spiral Columns, compression failures, current B = 2.6 to 3.3.

Shear, beams with stirrups, current B 1.9 to 2.4.