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-1
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-2
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
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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-9
GTSTRUDL Users Group Annual Meeting, June 2010, page-10
FEA Model
FIXED JOINT
RESTRAIN FX FZ MX
Y
SPRING 0.5000 0.5000
Z
X
0.5000
GTSTRUDL Users Group Annual Meeting, June 2010, page-11
Test Lifting Load (792kips)
o
o
o
o
o
o
o
o
o
-88.00
-88.00
-88.00
-88.00
-88.00
-88.00
-88.00
-88.00
-88.00
Y
Z
X
GTSTRUDL Users Group Annual Meeting, June 2010, page-12
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-13
Fundamental Buckling Shape
Buckling Multiplier
(FOS against buckling) = 1.32
GTSTRUDL Users Group Annual Meeting, June 2010, page-14
Fundamental Buckling Shape
Buckling Multiplier
(FOS against buckling) = 1.32
Y
Z
X
GTSTRUDL Users Group Annual Meeting, June 2010, page-15
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-16
Pushover Analysis
o
0.1000
-0.1000
o
X
Joint 12
Y
Z
X
Small out-of-plane loads
applied to initiate the
desired buckling shape
o
0.1000
GTSTRUDL Users Group Annual Meeting, June 2010, page-17
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
…
GTSTRUDL Users Group Annual Meeting, June 2010, page-18
Pushover Analysis – Original Lifting Truss
Out-of-Plane Displ v.s. Applied Load at Joint 12
120
Applied Load (kips)
100
80
60
Buckling Load at JNT_12 = 88kips
Total Buckling Load = 88 x 9 = 792kips
FOS against Buckling = 792/792 = 1.0
40
20
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Out-of-Plane displ. (in)
GTSTRUDL Users Group Annual Meeting, June 2010, page-19
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-20
Reinforced Top & Bottom Chords
HSS18x6/5/8
Top & Bottom chords
KL/R = 419 for bottom chord
HSS12x12x1/2 welded
on both sides
KL/R = 127 for bottom chord
GTSTRUDL Users Group Annual Meeting, June 2010, page-21
Reinforced Top & Bottom Chords
Reinforced Cross Section
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Pushover Analysis - Retrofitted Lifting Truss
Applied Load (kips)
Out-of-Plane Displ. v.s. Applied Load at Joint 12
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
-0.05
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
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Out-of-Plane Dipsl. (in)
GTSTRUDL Users Group Annual Meeting, June 2010, page-23
Comparison: FOS Against Buckling
Linear Buckling
Analysis
Pushover 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-24
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-25
Questions or Comments?
GTSTRUDL Users Group Annual Meeting, June 2010, page-26