As previously noted, the industry surveys indicate inconsistency in how transmission l ine designers select performance parameters or the load level associated with those parameters. These parameters should not be arbitrary but should relate to performance at a specified load or resistance condition. It is easy to understand how improperly applied performance parameters could result in highly-conservative designs if everyday service limits are assigned at maximum factored applied loads, or worse, at ultimate resistance capacity. Most engineers have well-developed concepts of deformation for service and design conditions, as these are in ranges of movement that would be expected. However, when asked to provided performance parameters for models at ultimate capacity, most still respond with limits they would desire at those lower load values. It is not intuitive to define deformations at failure. Studies of laterally loaded, short, rigid drilled pier transmission line foundations tested by EPRI for development of its lateral load model explored theses load-deflection and load-rotation relationships (Davidson, 1982; Kandaris et al., 2012). The earlier study determined that ultimate shaft resistance (capacity) is reasonably defined at the point where a pier rotates to an angle of about 2 degrees. The latter study found that for every 1 degree of pier rotation, the top of the pier laterally deflects from 3.5 to 4% of the pier diameter. In other civil design sectors, service limit states are well defined and the associated working loads are based on ei ther codes or experience. Wi th transmission line foundation design, there is no controlling code to follow. Few design models present service load evaluations. Unfactored transmission line structure service loads are rarely, if ever, calculated by structure designers or vendors. Compatible Load Factors Although the utility industry has not directly addressed the issues of using a probabilistic-based method to assess combined structure and environmental loads on foundations, guide documents developed by other agencies and pro- fessional organizations can be used to better understand how these can be incorporated with transmission line foundation design. These documents include load factors that vary depending on specified strength, extreme event and service load cases. • ASCE/SEI Standard 7-10 (ASCE, 2010) – Probabilistic method of assessing variable load factors for combined dead, live, roof, wind and earthquake loads on structures. • NCHRP 489 (Ghosn et al., 2003) – Probabilistic method of assessing vari- able load factors for combined dead, traffic, wind, collision and earthquake loads on bridge structures. Probabilistic foundation scour depth factors are also given for various extreme event load cases. • AASHTO LRFD Highway Bridge Specifications (AASHTO, 2012) – Prob- abilistic method of assessing variable load factors for combined loads from components/attachments, traffic and other live loads, wind on bridges, water on piers, ice on piers, collisions and earthquakes. The NCHRP report presents analysis of target reliabilities for pier scour, combined maximum wind and scour, and combined maximum ground motion and scour. Moving Forward There is a demonstrated need to provide a unified approach or comprehensive set of state-of-the-practice guidelines that assess risk and account for variability in loads and soil resistances for the design of electric transmission line foundations. The industry lacks consistent standards for design approach (i.e., allowable strength and reliability), compatible load and resistance factors, reliability level, and performance parameters. DFI ’s El e c t r i c Powe r Sys t ems Foundations Committee intends to lay the groundwork for these future guidelines and is presently preparing a state-of-the-practice white paper that summarizes current industry practices, and evaluates existing foundation types, electric system codes and civil engineering standards. Future foundation design guidelines resulting from this effort will be prepared by this group to: • Establish a consistent approach to ASD and RBD methods for all transmission line foundation types. • Determine consistent reliability levels for load and resistance probability distributions. Relationship of load and resistance probability to performance criteria 76 • DEEP FOUNDATIONS • JULY/AUG 2018 • Incorporate resistance factors consis- tent with RBD methods using a target reliability index consistent with the risk associated with this type of facility.
