Theory Behind the Seismic Performance of Steel Frame Structures

Theory Behind the Seismic Performance of Steel Frame Structures

During an earthquake, a structure must be able to resist flexural, axial, and shearing actions, which come from the swaying motion of the building. This swaying causes inelastic displacement and requires specially designed framing. Such frames are referred to as Special Moment Frames. Special Moment Frames include specific proportioning and detailed requirements that allow the structures to remain safe in the event of an earthquake. Other frames such as Intermediate or Ordinary Moment Frames follow the same principle and objective, yet not quite to the standard of the Special frames.

Steel moment frames have been used for over nearly a hundred years. The first building to have the steel frame carry the vertical loads was the Home Insurance Building in Chicago. It was constructed in 1884 and stood 138-ft tall. This seemed to pioneer the way for many other buildings of its type. Tall buildings were then being built with load bearing steel frames supporting concrete floors with masonry walls on the perimeter of the structure. These early structures usually used “H” shapes built up from plates, and “L” and “Z” sections. The Manhattan Building was the first to have large stiffened triangular gusset plates that were joined to beams and columns with angles and rivets, shown in the figure below. After World War II, it was starting to cost too much money to build perimeter walls out of infill-unreinforced masonry, especially tall buildings. Glass and aluminum curtain wall systems were becoming popular amongst architects. These new, modern wall systems consisted of larger windows that made large gusseted framing connections highly undesirable. Engineers were then required to design new connections without gussets. So they began using angles or split tees in order to connect the top and bottom beam flanges to columns. However, as welding gained popularity in building construction in the 1950s, those angles and flange plates that were welded to the column flanges then replaced split tees, then riveted to the beam flanges. In the 1960s, riveting became too expensive and was then replaced by high strength bolting. And by the 1970s, the connection type known as the welded unreinforced flange bolted web was created is still used today. This new connection used joint penetration groove welds to join beam flanges to columns and field bolted shear plates joining beam webs to columns.

The Home Insurance Building Connection used in the Manhattan Building

Riveted, seat angle connection Welded flange, bolted web connection

Engineers begin to notice that steel moment frames exhibit excellent performance in earthquakes. Over 20 structures were subjected to the 1906 San Francisco earthquake and surprisingly survived the rumble and fires. Many of said buildings are still standing today. Steel frames along with the composite interaction of encased masonry and concrete, became famous for their resistive nature against earthquakes after 90 more years of abuse. Seeing the incredible performance of the structures, building codes started to be revised in the 1960s. They adopted preferential design crtieria for steel moment frames. These new codes required buildings that have complete vertical load-carrying space frames as their lateral froce-resisting systems, be designed for two-thirds of the seismic forces specified for braced frames and half the forces specified for all bearing wall structures. The codes also required moment frames in buildigns taller than 160-ft. In the 1970s, Professor Ego Popov at the University of California at Berkeley began performing cyclic laboratory testing of steel moment framing. Professor Popov and researchers discovered that control of the proportioning and detailing of the structures was crucial to obtain superior inelastic behavior during earthquakes. The buildings codes then adopted these new findings and developed recommendations that required special design, configuration, and detailing of steel moment frames used for seismic resistance in areas that are prone to earthquakes. The first frames to conform to these standards were designated as Ductile Moment Resisting Space Frames, and then in the 1988 Uniform Building Code, as Special Moment-Resisting Space Frames. These frames were assigned the highest response modification factor, Rw. They were called special, due to the special criteria necessary for earthquake resistance and their expected special performance during strong earthquakes.

Steel frame buildings that survived the 1906 earthquake in San Francisco

A large portion of the infrastructure in Canada was designed before modern seismic engineering. Design codes and building procedures have been put in place since 1970. All buildings that were built before 1970 are at risk of not meeting the current codes that have been put into place to prevent a collapse or failure caused by an earthquake. It is recommended that all older buildings that were designed and built prior to these codes, be examined and upgrades be made so that they may comply with the current safety codes and not be a danger to the general public. One-story buildings are designed so that wind and seismic loads are transferred and distributed throughout diagonal braces located in the walls and in the roof. Prior to 1960, the code stated that the braces