Concrete Slender Wall Design – Back to the Future

Slender concrete walls incorporated into tilt-up construction over the past 60 years have performed remarkably well under out-of-plane wind and seismic loads. While issues associated with seismic wall anchorage gave this form of construction a black-eye in the early days, the concrete walls themselves have always performed very well even as far back as their first use in the early 1900s. Yet the height-to-thickness limitations in 1985 and earlier model building codes were proved irrational and were removed. We as engineers expect building codes to evolve and advance the state of the art with each successive edition. The latest edition of ACI 318-08 has significantly revised the slender walls design procedures, yet not necessarily advancing the state of the art. The latest ACI 318 edition is largely revising slender wall design back to match equations found in the old 1997 UBC. This paper revisits the historical effort SEAOSC played in the development of the original slender wall provisions in the late 1970s, and why ACI is now revising their slender wall design provisions to agree with concepts developed over 30 years ago by SEAOSC. While these original concepts were based on empirical data from full-scale tests conducted in the early 1980s, only within the last five years have we really begun to fully understand the behavior of these thin concrete members when subjected to combined axial load and large horizontal forces. Historical Background Tilt-up panels were developed in the pre-WWI era as an erection technique to facilitate construction of large concrete wall panels without forming both sides in-place. In 1909, Col. Robert Aiken, described an innovative method of casting panels on tilting tables and then lifting them into place by means of specially designed mechanical jacks [Spears, 1980]. During the ensuing years, this technique was used for constructing target abutments, barracks, ammunition and gun houses, a mess hall, low cost housing, factory buildings and churches [Leabu, 1980.] After WWII, from the late 1940s to the 1960s, reinforced concrete tilt-up walls became a great innovation and construction advancement. Walls could be built, not with expensive wall-forms but cast on the concrete slabs on the ground and lifted into final position serving as architectural enclosures or elements. Developer clients on industrial and commercial buildings, particularly supermarkets and large warehouses, demanded taller walls than the maximum 16 feet 8 inch allowed for an 8-inch thick concrete wall, based on a height-to-thickness limit of 25. The first report “Technical Bulletin Number 2,” on tilt-up wall construction, covering the design and construction practice of tilt-up at that time, was published in 1949 by the Structural Engineers Association of Southern California. Height-to-Thickness Ratio Prior to 1985, building codes requirement for the design of concrete wall panels were based on arbitrary height-tothickness limitations. A minimum wall thickness of six inches was required. A height-to-thickness ratio limitation of 25 was imposed on bearing walls, and 30 for non-bearing walls. Such height-to-thickness ratios increased to 36 when second-order effects were accounted for in the wall panel design in accordance with ACI 318 Chapter 10’s requirements for slender compression members. The increased use of tilt-up concrete walls for commercial and industrial buildings led to trends toward designing slender walls using pseudo second-order analyses. At that time, a second order analysis was permitted by the building code, and that led to a trend towards designing slender walls with little or no concern for stiffness, thus possibly affecting long-term serviceability. In 1974, Portland Cement Association developed design aids for load bearing walls [Kripanarayanan, 1980]. The design aids were compatible