of duration of load on strength is in a state of flux (see Appendix E). Regardless of how sophisticated theoretical models become, however, the results will have to be reduced to the LRFD format for design office use, since structural designers in the United States appear to be unwilling to work with anything more complicated than this. Third, the data presented in Appendix E is insufficient to determine whether any statistically significant differences in R/R and VR (upon which n depends) exist among species. If possible, it would appear desirable to allow any differences to be ironed out n Douglas Fir and Southern Pine beams in flexure. Fourth, it should be decided whether should depend on whether the timber members or laminating stock is visually or machine graded. 5.7.4 Masonry Structures Current design of engineered brick and concrete masonry structures uses working stress principles. Masonry specification writing groups moving toward limit states design n First, the specification of the factor and nominal resistance R for different members and limit states are interrelated, as discussed in connection with wood structures in Section 5.7.3. Second, the substantial reduction in 8 which occurs in unreinforced masonry walls as the load eccentricity increases, discussed in Chapter 4 and Appendix D, is of concern. Such a large variation does not appear to be desirable. If the mode (ductile or brittle) and the consequences of failure of such a wall are relatively uniform for all eccentricities, then ẞ should also be relatively uniform and some relative adjustments should be made in methods of computing R It seems that some reduction in conservatism would be possible n at small eccentricities, and that perhaps an increase in conservatism could be desirable n If the failure mode and consequences are relatively uniform, the adjustments should probably be Third, the standard governing engineered brick masonry distinguishes between inspected and uninspected workmanship. When the workmanship is inspected, wall alignment, thickness Figure 5.12 - Comparison of Designs Using Existing and Proposed 2 of joints, effects of partially filled joints and other factors which would reduce the probable strength and increase its variability are more carefully controlled. It appears desirable that this distinction be made in a limit states criterion. Data on the effect of inspection on R and V, and on the variability in construction practice across the n R United States would be useful. The upsurge in the use of engineered masonry and in masonry research may well provide additional data on this aspect. The specification writing group has a choice as to whether workmanship should be reflected in or in R. n Structures. This report has described the development of a set of recommended load factors and load combinations for use with loads in the proposed 1980 version of American National Standard A58, Building Code Requirements for Minimum Design Loads in Buildings and Other The scope of the resulting recommended load criterion is the same as that of the A58 Standard, which covers dead, live, wind, snow and earthquake loads. The criterion does not apply to vehicle loads on bridges, transients in reactor containments, and other loads which are considered outside the scope of the A58 Standard. A series of aids to material specification writing groups to assist them in their selection of resistance factors is also presented. The method of arriving at the resulting load factors is an advanced reliability analysis procedure. Earlier versions of this method have been used in the development of the Canadian Limit States Design specifications for steel structures for buildings, the Ontario Bridge Code, and the proposed Load and Resistance Factor Design criteria for structural steel in the United States. The method used in this work employs information on the probability distributions of the random variables, while the earlier methods only considered mean values and standard deviations. It was reassuring to find that the less sophisticated process gave results which are similar to those from the more advanced method. The procedure by which the load factors were developed consisted of: 1) Collecting and evaluating statistical and probabilistic information on various types of structural loads (dead, live, snow, wind, earthquake) and structural capacities (resistances). Much of this material was already available in the literature, but additional data evaluation and probabilistic analysis was necessary for the environmental loads (wind, snow, earthquake), for glulam members, and for masonry walls. The input from the load subcommittees of American National Standard Committee A58 was especially helpful, as was the previous research of the authors. The details of the data evaluation are presented in the Appendices. 2) Evaluating the relative reliability implied in current design. The measure of reliability was the reliability index B. This is consistent with previous work in this field. Values of ẞ were determined using a computer program. The basis of the method is described in Chapter 2 and the description of the program is presented in Appendix F. 3) Selecting target reliabilities and developing load factors consistent with these target reliabilities. It was not surprising that values of the reliability index 8 varied a great deal, depending on the type of structural load (e.g., gravity versus wind), the type of structural material, the limit state and the kind of element within a structure. In selecting the target reliability it was decided, after carefully examining the resulting reliability 3 is a representative average value for indices for the many design situations, that B = many frequently used structural elements when they are subjected to gravity loading, while B。 = 2.5 and 8 1.75 are representative values for loads which include wind and earthquake, respectively. = The recommended load combinations and load factors are as follows: The load combinations assume that the simultaneous occurrence of maximum values of snow, wind, earthquake and live loads is not likely. The smaller load factors in these combinations are a reflection of the fact that the factored arbitrary-point-in-time load is less than the nominal load. It was felt that while the determination of the resistance factor in the design was not within the purview of the A58 Standard, it would be helpful to specification writing groups if a method was given that they would find relatively easy to apply. Accordingly, charts are presented which permit the determination of values of ◊, given a desired B-level and material statistics, which are consistent with the load factors recommended in this report. Material specification writing groups can thus select their own target B values reflecting the particular situation of interest to them, and can determine a consistent with the selected B; conversely, they can choose and determine the resulting B. The computer program given in Appendix F may, of course, also be used for this operation. |