Fracture and Fatigue Design of the Wilshire Grand Tower

Abstract

The 1,100-foot Wilshire Grand Tower in Los Angeles owned by the Hanjin Group is the tallest building in the United States west of Chicago. The building, whose architect is AC Martin Inc., has a slender elevation in one direction, necessitating the use of heavy steel box columns filled with concrete on the perimeter, connected to the concrete core with outrigger trusses. The building has a slender, 272-ft-tall spire, also constructed from steel plates. The height and slenderness of the tower, as well as the spire, and its location in a seismically active zone motivated comprehensive analysis to mitigate the risk of steel fracture within a performance-based design framework. This analysis includes the following components: (1) development of acceptance criteria for earthquake-induced fracture, especially when it may follow years of wind-induced fatigue crack growth; (2) wind tunnel testing; (3) linear and nonlinear time-history simulations to determine stress demands in critical components under appropriate wind and seismic hazards; and (4) fracture mechanics simulations to characterize trade-offs among various design variables to meet the acceptance criteria. The analysis indicates that wind-induced oscillations have the potential to grow fatigue cracks in some components, affecting their performance during a subsequent maximum considered earthquake. This situation is unusual for steel buildings, being the result of the extraordinary height, geometry, heavy steel members, and location of this particular building. The analysis also indicates that fracture risk may be successfully mitigated through existing design and detailing approaches and acceptance criteria, along with use of steel material exhibiting high, but commercially available and affordable, specified Charpy V-notch toughness values.