Eccentrically Knee Bracing: Improvement in Seismic Design and Behavior of Steel Frames

Document Type : Structural Earthquake Engineering




The use of passive control systems is widely considered as a reliable approach for controlling earthquake vibrations in steel structures. First, under frequently occurring low to moderate earthquakes, the structure should have sufficient strength and stiffness to control deflection and prevent any structural damage. Second, under rare and severe earthquakes, the structure must have sufficient ductility to prevent collapse. For this case, significant damage of the structure and non-structural elements is acceptable. In this paper, the performance and the lateral stiffness of a new eccentric and knee bracing system named Eccentrically Knee Brace (EKB) is investigated. The stiffness of eccentrically knee braced frames (EKBs) is difficult to calculate by hand because they are indeterminate and have significant shear, flexural and axial deformations in different members. EKB stiffness is important for design, because it is used to compute story drifts and the knee and link rotations, which have prescribed limits. This note presents an equation for the stiffness of an EKB story in terms of the design story shear, frame geometry, and beam depth. The equation is independent of specific member sizes, making it useful for determining appropriate geometry in design. The equation is developed numerically and verified with experimental data from code compliant. One application of the equations is the estimate of the beam depth required to ensure a link or knee will satisfy inelastic rotation limits.


  1. Popov, E.P. (1983) Recent research braced frames on eccentrically braced frames. Journal of
  2. Engineering Structures, 5(1), 3-9.
  3. Englekirk, R. (1994) Steel Structures: Controlling Behavior through Design. Wiley, New York,
  4. -484.
  5. Roeder, C.W., and Popov, E.P. (1978) Eccentrically braced steel frames for earthquakes. Journal of the Structural Division, 104(3), 391-412.
  6. Hjelmstad, K.D., and Popov, E.P. (1983) Cyclic behavior and design of link beams. Journal of Structural Engineering, 109(10), 2387-2403.
  7. Kasai, K., and Popov, E.P. (1986) General behavior of WF steel shear link beams. Journal of Structural Engineering, 112(2), 362-382.
  8. Hjelmstad, K.D., and Popov, E.P. (1983) Seismic behavior of active beam links in eccentrically braced frames. NASA STI/Recon Technical Report, 84, 18480.
  9. Aristizabal-Ochoa, J.D. (1986) Disposable knee bracing: improvement in seismic design of steel frames. Journal of Structural Engineering, 112(7), 1544-1552.
  10. Zahrai, S.M., and Jalali, M. (2014) Experimental and analytical investigations on seismic behavior of ductile steel knee braced frames. Steel and Composite Structures, 16(1), 1-21.
  11. Hosseini Hashemi, B. and Alirezaei, M. (2015) Evaluation of the use of EKBs to improve seismic performance of steel frames. Submitted to International Journal of Steel Structures.
  12. Commentary for the seismic rehabilitation of buildings (FEMA356) Washington, DC: Federal
  13. Emergency Management Agency, 7.
  14. Lee, K., and Bruneau, M. (2005) Energy dissipation demand of compression members in concentrically braced frames. Steel and Composite Structures, 5(5), 345.
  15. Hosseini Hashemi, B., Alirezaei. M. (2015) Experimental investigation of a combined system for maximum energy dissipation in braced frames. Journal of Seismology and Earthquake Engineering, 17(3), 181-191.
  16. Nateghi-Alahi, F., and Khazaei-Poul, M. (2012) Experimental study of steel plate shear walls with infill plates strengthened by GFRP laminates. Journal of Constructional Steel Research, 78, 159-172.