answers varied widely, with designers using in-house standards, looking at past performance of foundations, obtaining values from published literature or obtaining recommendations from local geotechnical consultants. Kalaga and Yenumula (2016) point out the importance of understanding the effects of angular rotation at ground level on t r ansmi s s i on l ine s t ruc tur e performance. Even small angular rotations can induce large displacement at the top of these tall transmission structures, leading to possible reductions in horizontal or vertical conductor clearances, poor aesthetic appearance, and additional stresses on the pole structure. The authors note there are no universal standards for de f l e c t ion or rot a t ion l imi t s on transmission structure foundations, leaving designers to use their best judgment. Complicating the analysis further for the average practitioner, recent parametric studies on the effect of combined pole-foundation deflection found this type of analysis to be difficult, requiring the use of multiple software models (Bowland et al., 2015). The U.S. Department of Agriculture’s Rural Development department provides guidance for the design and construction of rural utility services. Their specifications allow designers the option to specify foundation rotation either as a maximum for all load cases or as a certain value for each load case. Alternatively, the engineer can opt to simply specify a fixed base with no allowable movement at any load value. The standard does recognize the need for varying performance parameters, matching types and probability of loads with foundation response, and also requires the designer to include such effects in calculations as the final deflected pole stresses but gives no guidance on how to approach the problem (USDA RDUP, 2016). With all the confusion, it is not surprising that many utilities and their consultants develop internal design manuals. The DFI survey indicated that nearly 60% do this in response to the lack of uniform guidance or to consolidate learned knowledge. Both the DFI and EPRI survey results showed current design practices vary widely, and that these variations apply to all aspects of design, including subsurface investigations, foundation design process and design of foundations for all types of transmission line structures. In response, EPRI produced a transmission structure foundation design guide in 2012 with suggested standard design processes for monopoles, H-Frame and lattice tower foundations within the framework of a Reliability-Based Design (RBD) format (EPRI, 2012). Improving Design Methodology It is clear that there is little consistency in the approach to transmission foundation design among designers, consultants and utilities. Load factors (primarily safety factors) and performance factors used by designers vary greatly. Foundation design methodologies are not consistent. Methods to integrate structure load factors used for foundation design with reinforced concrete code factors differ or are not used at all (Kanda r i s and Da v idow, 2015) . Foundation design loads are developed by enveloping worst case loading conditions that do not occur simultaneously, and strength and serviceability design checks are not consistently evaluated at separate loading levels. The consensus among transmission line design professionals is that there is a need to develop a guideline design document with recommendations for a uniform analysis and design approach for deep foundations. Load-Deformation Response The goal of the engineer is to design a foundation that performs as expected under the anticipated range of applied loads. At a conceptual level, the mechanics of materials dictates a relationship between stress and strain as a function of soil-structure interaction. The nonlinear nature of the load-deformation response for typical transmission line foundation types is well documented from full scale testing, laboratory scale tests, and most foundation design models in both axial and lateral load modes (Davidson, 1982; Kulhawy et al., 1983; DiGioia and Rojas-Gonzalez, 1994). The relationship offers an opportunity to examine var ious load level s and performance at those levels in terms of settlement, deflection or rotation. Using either Allowable Stress Design (ASD) or RBD approaches, foundation dimensions are typically sized to reach some ultimate limit of applied resistance capacity. This value is well above the factored applied load, accounts for variability in soil properties, and results in large foundation movements as plastic motion has likely been achieved and foundation failure is imminent. Alternately, foundation movement at factored maximum applied loads tends to be in the elastic-plastic transition range, while service (unfactored) applied loads are expected to be within the elastic range of foundation movement, where normal deformation is mostly recoverable. Nonlinear load (resistance) to performance relationship DEEP FOUNDATIONS • JULY/AUG 2018 • 75
