Investigating Performance of Plastic Hinge in Steel Frames by Knee Bracing

Author Name(s): *Behzad Dezhkam
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Constructing plastic hinges and the way of their distribution and failure mechanism can play an important role on seismic structures design. Mechanism type affect frame sensitivity toward secondary effects, total and local ductility, energy absorption and structure resistance before damage, general instability and destruction. Failure mechanism of moment frames under earthquake effect could be mentioned as three general types (first, second and third). The first type resulted from constructing hinges in beams and columns in the first few -story up, the second one resulted from constructing hinges in beams and columns of few upper -story and the third type including mechanism of one middle floor. Failure mechanism of general type is a special mode of the second type mechanism in which, plastic hinges locating at the two ends of beams and the first floor columns near the connection to the foundation. Based on researches, this type of mechanism shows the most amount of energy absorption against earth quack. Knee bracing frame is appropriate as an energy dissipation system composed of ductility and lateral stiffness which has good performance against lateral loads specially earthquake. In this paper, forming plastic hinges of components and the base shear of different steel systems and comparing them with knee bracing lateral load system in three, five and seven -story frames using regulations of FEMA356 and ATC-40.


For better understanding structures of knee bracing, steel frames including moment frames, knee bracing, coaxial bracing and Chevron bracing with 3, 5 and 7 -story were evaluated. In this research, members performance and the base shear of steel structures was evaluated. Results show that structures of knee bracing have more power of energy absorption and ductility than the other bracing systems; as comparison done between frames and knee brace and coaxial brace, Chevron brace and moment frame, knee members causes plastic hinges being created in knee members before formation in brace member in moment form. In most of the cases, plastic hinges being formed in knee member and decrease braces destruction. Structures with knee brace under uniform load pattern experience more base shear than structures with knee brace under load pattern of vibration modulus. So, equally values of the base shear for designing knee brace is less than X bracing and reduce steel consumption.

  1. In five -bay structures because of having more knee members, the base shear is less than three -bay which is show that the more number of knee member have more energy absorption that leads to the base shear reduction.
  2. Knee member in KBF frames has drastic drop after yielding because of ductile fuse formation on them. Accordingly, energy depreciation is done for forming moment plastic hinges and damages small portion of the building which can be repaired or replaced easily. So, in this sense, KBF frames are better over the similar CBF frames.
  3. Structures with knee brace under uniform load pattern experience more base shear over the structures with knee brace under modulus load pattern of vibration.
  4. The base shear of three -story has 8% decrease over the three -story with moment frames.
  5. The base shear of three -story has 10% increase over the three -story with Chevron frame and 13% increase over the three -story with coaxial brace.
  6. The base shear of five -story with knee brace has 11% decrease over the five -story with moment frame.
  7. The base shear of five -story with knee brace has 11% decrease over five -story with Chevron frame and 5% increase over the five -story with coaxial brace.
  8. The base shear with seven -story with knee brace has 26% decrease over the seven -story with Chevron frame and has 7% increase over then coaxial brace. Generally, the knee bracing system is a bracing system which can supply appropriate hardness and high ductility for structure if effective parameters are chosen and after earthquake, it can be reused with replacing knee member.

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