Stability Analysis of 400 Ton Lifting Truss

1. Stability Analysis of 400 Ton Lifting Truss Luis M. Moreschi Hongchun Liu Peter J. Carrato Bechtel Power Corp.

2. Outline • Background: Heavy Load Lifting in Power Plant Construction • Lifting Truss Buckling Problem • Stability Evaluation of the Original Lifting Truss • Root Cause and Proposed Solution • Stability Evaluation of the Retrofitted Lifting Truss • Conclusion

3. Heavy Lifting in Power Plant Construction • Wall panels are assembled at ground to reduce # of components being lifted • Heavy wall panel assembly weights up to 400 Tons • Strand jacks are provided on the structure • Lifting beam / truss is used to spread the load evenly to multiple lifting points • An approved Rigging Plan is required for critical heavy lifts

9. Lifting Truss Buckling Problem • Lifting truss was tested to 125% of the design capacity • The truss exhibited out-of-plane deflections during testing • Home office engineering was consulted for retrofit solution

11. FEA Model

12. Test Lifting Load (792kips)

13. Linear Buckling Analysis (Engenvalue) • … • >Stiffness analysis • >Perform buckling analysis • >Eigen parameters • >Shift 1.0 $ Required to find the realistic buckling mode shape for $ this application. Try-and-error needed to find the $ lowest positive buckling multiplier • > End parameters • >Perform buckling analysis • …

14. Fundamental Buckling Shape Buckling Multiplier (FOS against buckling) = 1.32

15. Fundamental Buckling Shape Buckling Multiplier (FOS against buckling) = 1.32

16. Further Analysis – Pushover Analysis • Linear Buckling Analysis predicts the classical ‘Euler’ load, which cannot be directly used in real-life. • Pushover Analysis is an automated nonlinear incremental load analysis that searches for structural instability or collapse load. • Geometric nonlinearity was considered for the truss • Small out-of-plane loads were applied at selected joints to Initiate the desired buckling shape

17. Pushover Analysis Small out-of-plane loads applied to initiate the desired buckling shape

18. Pushover Analysis Commands • … • >NONLINEAR EFFECT • >GEOMETRY MEMBER EXISTING • >PUSHOVER ANALYSIS DATA • CONSTANT LOAD ‘2’ $ 0.1kips load at selected lifting point • INCREMENTAL LOAD ‘1’ $ 88kips lifting load at each lifting point • MAXIMUM NUMBER OF LOAD INCREMENTS 200 • MAXIMUM NUMBER OF TRIALS 20 • LOADING RATE 0.2 • CONVERGENCE RATE 0.200000 • CONVERGENCE TOLERANCE COLLAPSE 0.000100 • MAXIMUM NUMBER OF CYCLES 50 • >END • >PERFORM PUSHOVER ANALYSIS • …

19. Pushover Analysis – Original Lifting Truss Buckling Load at JNT_12 = 88kips Total Buckling Load = 88 x 9 = 792kips FOSagainst Buckling = 792/792 =1.0

20. Root Cause and Proposed Solution • Both linear and nonlinear analyses indicated that the buckling load is very close to the actual test load (792kips). • Original design did not consider the unbraced length correctly for top and bottom chords. KL/r = 419 for bottom chord! • Out-of-plane stiffness shall be significantly increased to eliminate out-of-plane buckling.

21. Reinforced Top & Bottom Chords

22. Reinforced Cross Section Reinforced Top & Bottom Chords

23. Pushover Analysis - Retrofitted Lifting Truss Selected Buckling Load at JNT_12 = 2,024 kips Total Buckling Load = 2,024 x 9 = 18,216kips FOS Against Buckling = 18,216 / 792 = 23

24. Comparison: FOS Against Buckling Practically, a FOS in the magnitude of 10~20 should be used to account for imperfections, out-of-plane loads, residual stresses, and etc. Remember: Buckling is a wild beast to deal with. Be cautious!

25. Conclusions • Buckling could occur if lifting devices are not properly designed for heavy lifts • Linear Elastic Buckling Analysis is just a starting point for buckling evaluation • Pushover Analysis should be performed to further investigate the situation • FOS against buckling should be set high (10~20) for real-life applications

26. Questions or Comments?