785 Eighth Avenue is a 43-story concrete residential building, 484 feet high at the top of the bulkhead roof, and 516 1/2 feet above ground level to the top of the triangular screen wall. The residential floor plate is 85 feet in the east-west direction and varies from 23 1/2 feet to 44 feet in the north-south direction. The gravity load-bearing system is composed of reinforced concrete columns and shearwalls. Floor slabs are typically 9" thick made of flat plate construction. The typical floor to floor heights are 10'-4". The floor height is 11'-7 1/2" below the 8th floor. Because of its 15:1 slenderness ratio (max ratio 18:1), making this building one of the most slender structures in the world, the building was designed with due consideration given to its dynamic behavior. Designing an effective lateral system was a challenge, achieved by judicious placement of shear walls and the maximum possible utilization of slab-frame action. High strength concrete was used to increase stiffness, if not fully utilized for strength. Concrete strengths in the columns and shearwalls were 12,000 psi from foundation to the 21st floor, 10,000 psi from the 22nd to the 35th floor, and 8000 psi from the 36th floor to the roof. With the shearwall as the main lateral resisting system, coordination with the architect was particularly essential in configuring the wall system, allowing the walls to traverse the building from north to south. The walls were allowed to meander through the apartment partitions. Coupling beams were used to connect many parts of the wall system being interrupted by doors and other openings through it. To achieve our design objective of stiffening the building, walls running along the north and south property lines of the building were connected by three north-south walls from the foundation to the 9th floor. The north and south walls became the flanges of the "box system". Above the 9th floor, and up to the 30th floor, the shear wall system still traverses the entire short width of the building, but the south flange was opened up for the enclosed 270 sq.ft. of area using air rights from the adjacent property. This area cantilevers approximately 7 feet over the wall below. From the 31st floor up to the roof, one of the three north-south walls remained, projecting south of the elevator core which still connects to the north flange of the shearwall. Reinforcement was typical 60 ksi grade steel. Designing such a tall and thin building in a challenging site and being able to maximize the use of space was the biggest feat achieved by the "785 Eighth Avenue" team. In this building, concrete is utilized to its full advantage, providing stiffness, weight and damping that are essential in designing not only for strength and stability, but also for acceptable perception of motion.