page_title: Seismic retrofit
section_title: Lattice girders, beams, and ties
hierarchical_section_title: Seismic retrofit / Bridge retrofit / Lattice girders, beams, and ties
caption_attribution_description: English: Cropped version of en:Image:LatticeBeamRetrofit.jpg Bolted plate replaces hot riveted lattice in box beams as part of a seismic retrofit of this portion of the San Francisco-Oakland Bay Bridge. The four vertical cylindrical objects at the top of each vertical beam are the terminations of a pair of suspender cables of this double deck suspension bridge. Image Sept 8, 2004 by User:Leonard G..
context_page_description: Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. Prior to the introduction of modern seismic codes in the late 1960s for developed countries and late 1970s for many other parts of the world, many structures were designed without adequate detailing and reinforcement for seismic protection. In view of the imminent problem, various research work has been carried out. State-of-the-art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world – such as the ASCE-SEI 41 and the New Zealand Society for Earthquake Engineering's guidelines. These codes must be regularly updated; the 1994 Northridge earthquake brought to light the brittleness of welded steel frames, for example.
  The retrofit techniques outlined here are also applicable for other natural hazards such as tropical cyclones, tornadoes, and severe winds from thunderstorms.
context_section_description: Lattice girders consist of two "I"-beams connected with a criss-cross lattice of flat strap or angle stock. These can be greatly strengthened by replacing the open lattice with plate members. This is usually done in concert with the replacement of hot rivets with bolts.
caption: Bolted plate lattice replacement, forming box members
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