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  3. Why Steel Structures ? • Reduced construction time & no seasonal effect. • Light weight and reduced foundation cost. • Durable , Long Lasting and Recyclable. • Easier to modify and reinforce if required. • Fabrication off-site possible. • On site erection is a time saving process. 3
  4. Alcoa Building San Francisco California The Bow Alberta, Canada 4
  5. Why Bracing ? • Steel structures are widespread. • They exhibit ductile behaviour when subjected to transient lateral loading, caused by wind or earthquake action. • Steel bracings are lateral load resisting system. 5
  6. Why Braced Frames ? • Braced frame is the economical method of resisting lateral load in multi storey structural frame. • Bracing creates triangular configuration in the structures. • Bracing increases structural response in steel moment resisting frames. 6
  7. Types Of Braced Frames Concentrically braced frames (CBF) Eccentrically braced frames (EBF) 7
  8. Arrangement Of Bracings Diagonal Cross / X - type 8
  9. V - type Chevron 9
  10. Methods Used For Analysis • Equivalent Static Analysis • Response Spectrum Analysis • Time History Analysis 10
  11. Equivalent Static Analysis • It is the simplest method of analysis. Here, force depend upon the fundamental period of structures defined by IS Code 1893:2002 with some changes. • First, the design base shear is computed for the whole building. • It is then distributed along the height of the building. • The lateral forces at each floor levels thus obtained are distributed to individual lateral load resisting elements. 11
  12. Response Spectrum Analysis • It is a linear dynamic analysis method. • In this approach multiple mode shapes of the building are taken into account. • For each mode, a response is read from the design spectrum, based on the modal frequency and the modal mass. • They are then combined to provide an estimate of the total response of the structure using modal combination methods. 12
  13. Time History Analysis • To perform such an analysis, a representative earthquake time history is required for a structure being evaluated. • In this method, the mathematical model of the building is subjected to accelerations from earthquake records that represent the expected earthquake at the base of the structure. • The method consists of a step- by- step direct integration over a time interval. 13
  14. AnalysisModel - 1 (Plan) 14
  15. (Front elevation for cross bracing model) (Side elevation for the model) 15
  16. (3D view of the typical steel frame with diagonal bracing) 16
  17. The three earthquake used for analysis are as follows: 17
  18. Lateral Load Profile In Equivalent Static Analysis : 18
  19.  Cross bracing has the highest lateral stiffness as compared to diagonal bracing, and obviously to frame without bracing. Graphical Observation : 19
  20. Story Drift Of The Model In Equivalent Static Analysis : 20
  21.  On an average, 87% decrement in story drift is observed by installing cross or diagonal bracing on the model as compared to that of the model without bracing. Now, cross bracing and diagonal bracing undergo almost same drift. This is because, one of the diagonal of cross bracing remains inactive during the analysis. Graphical Observation : 21
  22. Story Drift Of The Model In Response Spectrum Analysis : 22
  23.  On an average, 28% decrement is observed by installing cross bracing instead of diagonal bracing. Cross bracing is obviously more laterally stiffer than diagonal bracing, and hence the decrement is observed. Graphical Observation : 23
  24. Story Drift Of The Model In Time History Analysis : X D W 24
  25.  Time history is a linear analysis and hence the effect of decreased roof loading doesn’t affect the final drift profile. IS code ground loading has the highest peak ground acceleration as compared to the other two earthquake loadings. Therefore, highest story drift is observed in IS Code as compared to the other two earthquake loading.  Cross bracing has the most lateral stiffness and hence in both the earthquake loading it shows least story drift. Graphical Observation : 25
  26. W X Shear Force At The Corner Columns : X D W In Time History AnalysisIn Response Spectrum Analysis 26
  27.  Both the graphs, represent a lower value of shear force for cross bracing as compared to diagonal bracing and frame without bracing. For both the analyses, it can be concluded that by increasing the bracing, or by increasing the lateral stiffness shear force in columns tend to decrease. Graphical Observation : 27
  28. W Bending Moment At The Corner Columns : X In Response Spectrum Analysis In Time History Analysis 28
  29.  Both the graphs, represent a lower value of bending moment for cross bracing as compared to diagonal bracing and frame without bracing. So, by increasing the lateral stiffness of the moment resisting frame, increasing the bracing bending moment force applied at the columns tend to decrease. Graphical Observation : 29
  30. Axial Force At The Corner Columns When Subjected To IS Code Loading : 30
  31.  The graph represents a higher value for axial force at column ends for cross bracings followed by diagonal bracing and frame without bracing. So, with increase in bracing axial forces in the columns tend to increase. Graphical Observation : 31
  32. CONCLUSION : • Braced steel frames have more base shear than unbraced frames. • Cross bracings undergo more base shear than diagonal bracings. • Bracings reduce the lateral displacement of floors. 32
  33. • Cross bracings undergo lesser lateral displacement than diagonal bracings. • Axial forces in columns increases from unbraced to braced system. • Shear forces in columns decrease from unbraced to braced system. • Bending moment in column decreases from unbraced to braced system. 33
  34. REFRENCES • Tremblay, R.; et al., Performance of steel structures during the 1994 Northridge earthquake, Canadian Journal of Civil Engineering, 22, 2, Apr. 1995, pages 338-360. • Khatib, I. and Mahin, S., Dynamic inelastic behaviour of chevron braced steel frames, Fifth Canadian Conference on Earthquake Engineering, Balkema, Rotterdam, 1987, pages 211-220. • Meher Prasad: “Response Spectrum”, Department of Civil Engineering, IIT Madras. • David T. Finley, Ricky A. Cribbs: “Equivalent Static vs Response Spectrum – A comparison of two methods”. • IS 1893 (Part 1):2002, “Criteria for Earthquake Resistant Design of Structures”. • Hassan, O.F., Goel, S.C.(1991).”Modelling of bracing members and seismic behaviour of concentrically braced steel frames”. 34
  35. Thank You PREPARED BY : SAJID IQBAL Class – B.C.E-4 Roll - 001410401094 UNDER THE GUIDANCE OF : DR. KALYAN KR. MANDAL Assistant Professor Department of Civil Engineering Jadavpur University 35