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GTSTRUDL Users Group Annual Meeting, June 2010, page-1
Stability Analysis of 400 Ton Lifting Truss
Luis M. Moreschi
Hongchun Liu
Peter J. Carrato
Bechtel Power Corp.
GTSTRUDL Users Group Annual Meeting, June 2010, page-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
GTSTRUDL Users Group Annual Meeting, June 2010, page-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
GTSTRUDL Users Group Annual Meeting, June 2010, page-4
GTSTRUDL Users Group Annual Meeting, June 2010, page-5
GTSTRUDL Users Group Annual Meeting, June 2010, page-6
GTSTRUDL Users Group Annual Meeting, June 2010, page-7
GTSTRUDL Users Group Annual Meeting, June 2010, page-8
GTSTRUDL Users Group Annual Meeting, June 2010, page-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
GTSTRUDL Users Group Annual Meeting, June 2010, page-10
GTSTRUDL Users Group Annual Meeting, June 2010, page-11
FEA Model
X
Y
Z 0.5000
SPRING 0.5000 0.5000
RESTRAIN FX FZ MX
FIXED JOINT
GTSTRUDL Users Group Annual Meeting, June 2010, page-12
Test Lifting Load (792kips)
X
Y
Z -88.00
o
-88.00
o
-88.00
o
-88.00
o
-88.00
o
-88.00
o
-88.00
o
-88.00
o
-88.00
o
GTSTRUDL Users Group Annual Meeting, June 2010, page-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
…
GTSTRUDL Users Group Annual Meeting, June 2010, page-14
Fundamental Buckling Shape
Buckling Multiplier(FOS against buckling) = 1.32
GTSTRUDL Users Group Annual Meeting, June 2010, page-15
Fundamental Buckling Shape
X
Y
Z
Buckling Multiplier(FOS against buckling) = 1.32
GTSTRUDL Users Group Annual Meeting, June 2010, page-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
GTSTRUDL Users Group Annual Meeting, June 2010, page-17
X
Y
Z
0.1000
o
-0.1000
o
0.1000
o
XX
Joint 12
Small out-of-plane loads applied to initiate the desired buckling shape
Pushover Analysis
GTSTRUDL Users Group Annual Meeting, June 2010, page-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 0.000100MAXIMUM NUMBER OF CYCLES 50>END
>PERFORM PUSHOVER ANALYSIS…
GTSTRUDL Users Group Annual Meeting, June 2010, page-19
Pushover Analysis – Original Lifting Truss
Out-of-Plane Displ v.s. Applied Load at Joint 12
0
20
40
60
80
100
120
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Out-of-Plane displ. (in)
Ap
plie
d L
oad
(ki
ps)
Buckling Load at JNT_12 = 88kips Total Buckling Load = 88 x 9 = 792kips792kips FOS against Buckling = 792/792 = 1.0
GTSTRUDL Users Group Annual Meeting, June 2010, page-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.
GTSTRUDL Users Group Annual Meeting, June 2010, page-21
Reinforced Top & Bottom Chords
HSS18x6/5/8Top & Bottom chords
KL/R = 419 for bottom chord
HSS12x12x1/2 weldedon both sides
KL/R = 127 for bottom chord
GTSTRUDL Users Group Annual Meeting, June 2010, page-22
Reinforced Top & Bottom Chords
Reinforced Cross Section
GTSTRUDL Users Group Annual Meeting, June 2010, page-23
Pushover Analysis - Retrofitted Lifting Truss
Out-of-Plane Displ. v.s. Applied Load at Joint 12
0200400600800
100012001400160018002000220024002600
-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Out-of-Plane Dipsl. (in)
Ap
plie
d L
oad
(ki
ps)
Selected Buckling Load at JNT_12 = 2,024 kips Total Buckling Load = 2,024 x 9 = 18,216kipsFOS Against Buckling = 18,216 / 792 = 23
GTSTRUDL Users Group Annual Meeting, June 2010, page-24
Comparison: FOS Against Buckling
Linear Buckling
AnalysisPushover Analysis
(Geometric nonlinearity)
Original Lifting Truss 1.32 1.00
Retrofitted Lifting Truss 27.02 23.00
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!
GTSTRUDL Users Group Annual Meeting, June 2010, page-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
GTSTRUDL Users Group Annual Meeting, June 2010, page-26
Questions or Comments?