Seismic Design of an Irregular Wharf Utilizing ASCE 61

ASCE/COPRI 61-14 Seismic Design of Piers and Wharves (ASCE 61), published in 2014, offers a uniform methodology for multi-level performance-based seismic design that relies heavily upon the substitute structure methodology. Prior to this document, marine structural engineers have drawn from a collection of reference documents when performing seismic design. The publication of this document coincided with the design of the Pier 4 Reconfiguration Project at the Port of Tacoma and was adopted as the criteria document for seismic design. The project is a pilot project for the Port of Tacoma for use of this standard. A specific challenge that was present in the design was the extreme stiffness irregularity along the length of the wharf. The southern portion of the wharf is designed as a typical wharf structure, with shorter exposed pile lengths inboard, and longer pile lengths outboard. Due to the existing slope configuration, the northern portion of the wharf structure is located in an area where all of the piles have an exposed length above the mudline of approximately 68 feet. This configuration creates slender piles at the northern end of the wharf. To control deflections during vessel mooring/ berthing, the northern portion of wharf utilizes batter piles in the transverse direction. To provide better seismic response, lead rubber bearings (LRBs) have been incorporated into the design of the portion of the structure utilizing batter piles to reduce response differentials along the length of the wharf. A comparative nonlinear, direct integration, time-history analysis was completed in select transverse locations to evaluate crane-wharf interaction and to develop displacement amplification factors for use in a nonlinear, direct integration, time-history analysis of a simplified spine model for the entire pier. This paper discusses how ASCE 61-14 was used in the design and how the methodology was adapted for use in this complex structure.