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

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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

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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

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Fundamental Buckling Shape

X

Y

Z

Buckling Multiplier(FOS against buckling) = 1.32

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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…

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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

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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

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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?