Jonathan Ochshorn's Structural Elements for Architects and Builders, Third Edition
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Chapter 5 — Reinforced concrete: Design approaches

The so-called strength design method (originally called "ultimate strength design") was pioneered by the American Concrete Institute (ACI), which adopted a methodology in 1956 that both incorporated load factors and considered the ultimate (failure) stress, rather than an allowable stress, as an alternate and more rational strategy for design. A working (allowable) stress design method remained the dominant methodology, however, for many years. This latter method did not distinguish between uncertainties inherent in various load types (e.g., dead vs. live loads), and did not consider the actual strength of a structural element subjected to these loads. Instead, it accounted for risk using a single factor of safety based on an assumed elastic limit state. This is problematic for concrete on two counts: first, because the material itself does not exhibit clearly defined elastic behavior (see Figure 5.3) and second, because — in general and not just for concrete — the simple addition of load values without consideration of the probabilistic nature of their distribution within a structure is not entirely consistent with a risk-based approach to structural design.

By the early 1960s, strength design for reinforced concrete structures had matured to the point where both loads and resistances were given their own, independent sets of safety factors that were equivalent, at least in theory, to current versions of "Load & Resistance Factor Design" (LRFD) used for wood and steel. While the traditional working stress design method was, at that time, still the featured methodology for the design of reinforced concrete elements, strength design gradually began to displace the older method. The first incarnation of strength design did not yet have explicit strength reduction (resistance) factors and was presented somewhat tentatively in the 1956 edition of ACI 318, the "Building Code Requirements for Reinforced Concrete" that is updated by ACI every few years. A short note referred those willing to try this new method to the appendix, which contained a concise description of the requirements for "ultimate strength design." In 1963, working stress and strength methods achieved separate but equal status within the body of ACI 318. By 1971, strength design had become the featured method, with working stress design still included, but only as an "alternate design method." In 1989, working stress design no longer appeared in the main text of ACI 318 at all, but was moved to the appendix, where it remained as an alternate method for another decade: by the time ACI 318 was updated in 2002, working stress design had been consigned to a small note in the manual's Commentary stating that anyone still interested in it would need to consult the appendix of the 1999 edition, where it had last appeared.

Remarkably, it took 30 years after strength design was first presented in the ACI Code before the steel industry adopted its own version, called load and resistance factor design (LRFD), in 1986. Up until quite recently, however, load factors differed between steel and reinforced concrete. Those adopted by ACI had been calculated on the basis of "engineering judgment" rather than on more solid empirical studies and probabilistic research. Initial values from 1963, for example, included load factors of 1.5 and 1.8 for dead and live loads respectively; these were "adjusted" to 1.4 and 1.7 in 1971, where they remained for more than 30 years. Meanwhile, dead and live load factors for steel structures were set at 1.2 and 1.6 respectively, values that appeared in the very first LRFD edition of the American Institute of Steel Construction's (AISC) Manual of Steel Construction in 1986, and that have been sanctioned by the American Society of Civil Engineers (ASCE) in their Minimum Design Loads for Buildings and Other Structures since 1988 and by the American National Standards Institute (ANSI) in the precursor to this standard dating from 1982. Since safety factors for loads ought to be completely independent of particular material properties, it was something of an embarrassment for the concrete and steel institutes to be seen arguing in this way, and something of a relief when the ACI finally reconciled their strength design load factors with those of the AISC and ASCE in the 2002 edition of ACI 318. In order to maintain a comparable level of safety with these newly reduced, and therefore less conservative, load factors, ACI 318-02 also adjusted its strength reduction factors — i.e., made them more conservative.

While LRFD, even today, is not universally used for steel design — and certainly not for wood design — strength design has almost completely superseded older working stress methods used to design reinforced concrete elements and is therefore the design method that will be considered exclusively in this chapter.