The role of surrogate models in combined aeroelastic and … · 2016. 10. 11. · M. Cid Montoya,...

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The role of surrogate models in combined aeroelastic and structural optimization of cable-stayed bridges with single box deck M. Cid Montoya, S. Hern´ andez, I. Kusano, F. Nieto & J. ´ A. Jurado Structural Mechanics Group, School of Civil Engineering, Universidade da Coruna, Spain ˜ Abstract The aeroelastic phenomena represent a crucial issue in the design of long span bridges and their relevance grows as the length of the spans is increased, which is the current trend in these mega-structures. The search for the best performing deck cross-section, as well as the search of the most economic bridge design, are two of the most relevant goals for bridge designers. Hence, the pursuit of good aerodynamic performance of the bridge deck cross-section and the structural optimization of the bridge are destined to converge in a unified process to achieve more efficient bridge designs, given their interdependence. This paper presents an integrated methodology based on a surrogate-based optimization process to obtain an optimum bridge design considering structural and aeroelastic constraints. The deck cross-section used in this work is the well-known G1 section, in which two deck shape variables are considered. The aerodynamic behavior of the cross sections analyzed by the optimization algorithm is given by a surrogate model trained with a set of designs calculated with CFD techniques. The structural performance is analyzed by means of finite element analysis. The areas and prestressing forces of the stays and the deck shape and plate thickness are considered as design variables, and the optimization of the bridge is carried out by a gradient-based optimization algorithm. Keywords: structural optimization, bridge aerodynamics, cable stayed bridges, CFD, flutter analysis. www.witpress.com, ISSN 1743-3509 (on-line) WIT Transactions on The Built Environment, Vol 166, © 2016 WIT Press doi:10.2495/HPSM160011 This paper is part of the Proceedings of the 2 International Conference on nd High Performance and Optimum Design of Structures and Materials (HPSM 2016) www.witconferences.com

Transcript of The role of surrogate models in combined aeroelastic and … · 2016. 10. 11. · M. Cid Montoya,...

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The role of surrogate models in combinedaeroelastic and structural optimization ofcable-stayed bridges with single box deck

M. Cid Montoya, S. Hernandez, I. Kusano, F. Nieto & J. A. JuradoStructural Mechanics Group, School of Civil Engineering,Universidade da Coruna, Spain˜

Abstract

The aeroelastic phenomena represent a crucial issue in the design of long spanbridges and their relevance grows as the length of the spans is increased, whichis the current trend in these mega-structures. The search for the best performingdeck cross-section, as well as the search of the most economic bridge design,are two of the most relevant goals for bridge designers. Hence, the pursuit ofgood aerodynamic performance of the bridge deck cross-section and the structuraloptimization of the bridge are destined to converge in a unified process to achievemore efficient bridge designs, given their interdependence. This paper presentsan integrated methodology based on a surrogate-based optimization process toobtain an optimum bridge design considering structural and aeroelastic constraints.The deck cross-section used in this work is the well-known G1 section, in whichtwo deck shape variables are considered. The aerodynamic behavior of the crosssections analyzed by the optimization algorithm is given by a surrogate modeltrained with a set of designs calculated with CFD techniques. The structuralperformance is analyzed by means of finite element analysis. The areas andprestressing forces of the stays and the deck shape and plate thickness areconsidered as design variables, and the optimization of the bridge is carried outby a gradient-based optimization algorithm.Keywords: structural optimization, bridge aerodynamics, cable stayed bridges,CFD, flutter analysis.

www.witpress.com, ISSN 1743-3509 (on-line) WIT Transactions on The Built Environment, Vol 166, © 2016 WIT Press

doi:10.2495/HPSM160011

This paper is part of the Proceedings of the 2 International Conference on nd

High Performance and Optimum Design of Structures and Materials (HPSM 2016) www.witconferences.com

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1 Introduction

The main challenge in the bridge engineering field is the increasing span lengthof the new bridge projects that are currently built around the world. This trendinvolves taking into account the aeroelastic phenomena as some of the mostrelevant design constraints, keeping in mind of the structural behavior of thebridge.

The aeroelastic behavior of a bridge is mainly driven by two capital aspects, thedynamic response of the bridge and the aeroelastic properties of the deck cross-section. The relative relevance of these two factors in the aeroelastic responsesis influenced by the length of the main span of the bridge, which is the maincause that determines the global stiffness of the structure. While in bridges withmoderate span length the aeroelastic phenomena can be controlled by the stiffnessof the bridge, in cases with longer spans, the global stiffness of the structurebecomes insufficient, and the only way to avoid undesirable wind induced effectsis to modify the deck cross-section. It must be noticed that the interdependenceof these two aspect has to be considered, given that modifying the deck cross-section affects to its structural stiffness. Besides, both aspects present conflictingtendencies, given that good aeroelastic performance of deck cross-sections impliesslender geometries, which reduces the mechanical properties of the section.

The influence of the deck cross-section shape on the aeroelastic response of acable suspension bridge is well known since many years ago. Several researchershave written reports where the influence on the aerodynamic responses due toshape modifications is studied, as is the case for instance of [1]. However, heuristicapproaches to improve bridge design considering only deck shape changes cannot reach the optimum, as they disregard other considerations that also influencethe solution such as structural responses. This is the main reason that justifies theuse of methodologies that consider both aspects simultaneously, as it is the caseof optimization process with both types of constraints. One consequence of thisapproach is the need for obtaining all the results required computationally in everystep of the bridge design.

The use of optimization algorithms in structural design is currently a widespreadtechnique, and some insights about this field can be found for instance, in[2, 3]. The application of structural optimization is also widely employed, andan interesting application to optimize the cross-section of the cables of a cable-stayed bridge can be found in [4]. In [5], optimization algorithms were used tooptimize the deck size and the main cable cross-section area of the Messina Bridgeproject in Italy. However, deck shape modifications were not considered yet. Somepreliminary results of optimization of stays and deck shape and size in a cablestayed bridge considering a set of semicircular cross-sections defined with oneshape design variable have been reported in [6]. The aerodynamic coefficients CL,CM and CD were obtained for a set of values of the design variables and thenfitted to a smooth spline, which allowed to predict the aerodynamic coefficientsof the designs proposed by the optimization algorithm. However, deck girders ofreal bridges present complex shapes and they can be modified by means of several

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G1 G1 +10H +10B G1 -10H -10B G1 +10H -10B G1 -10H +10B

0.85B

0.57B

0.9B

B

1.1B

0.1B0.1B

0.04B

0.1B0.9HH1.1H

Figure 1: Definition of the considered design domain.

shape changes, giving place to more complicated problems. This work present thedefinition of the optimization problem for obtaining a more realistic deck shape(see figure 1) including more shape design variables in the definition of the crosssection.

The section studied in this work is the well-known G1 section, as describedin [7]. The chosen domain is defined by percentage variations of the depth Hand width B of the G1 section, which is considered the base or initial design,as it can be seen in figure 1. The variations are up to a percentage of 10% forboth dimensions, which modifies the geometrical properties of the section inthe quantities indicated in table 1. The modification in the dimensions involveschanges in the depth of 0.84 m and 6 m in the width, which means a variation onthe dimensions ratio from 5.84 to 8.73. The angles reported in table 1 describe thetotal angle between the inclined sides of the deck, and the top and bottom anglesare measured with respect to the horizontal direction. The relationship betweenthe top and bottom perimeter is also reported. The aerodynamic coefficients ofthe cross-sections of the deck will be obtained for a set of values of the designvariables and afterwards a Kriging technique [8] is used to produce a surrogatemodel that allows to evaluate them for any given values of the design variables.

Step: Step−1Increment 17: Step Time = 1.000

Deformed Var: U Deformation Scale Factor: +1.000e+00

ODB: Job−1.odb Abaqus/Standard 6.13−2 Tue Jan 12 20:12:26 CET 2016

XY

Z

Figure 2: Front view of the finite element model of the cable-stayed bridge.

The bridge proposed to test the methodologies presented in this work is a projectof a cable-stayed bridge designed in La Coruna, Spain, with a main span length of658m and two lateral spa s of 270 . The finite element model is shown in figure 2n m

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Tabl

e1:

Sum

mar

yof

the

geom

etri

cal p

rope

rtie

sof

som

ede

sign

sin

the

limits

ofth

ede

sign

dom

ain.

Sect

ion

Dim

ensi

ons

Are

a[m

2]

Ang

les

[◦]

Peri

met

er[m

]

∆B

∆H

B[m

]H

[m]

B/H

ratio

Top

Bot

tom

Tota

lTo

pB

otto

mTo

tal

Rel

atio

n[%

]

00

30.0

04.2

07.1

4103.9

528.0

724.9

453.0

130.6

031.3

361.9

32.3

8

+10

+10

33.0

04.6

27.1

4120.7

717.7

423.2

841.0

233.3

734.4

167.7

83.1

0

-10

+10

27.0

04.6

25.8

4106.9

157.9

934.6

492.6

328.3

329.1

357.4

62.8

3

+10

-10

33.0

03.7

88.7

399.7

317.7

417.9

835.7

233.3

733.8

267.1

91.3

2

-10

-10

27.0

03.7

87.1

488.3

957.9

927.5

385.5

228.3

328.2

656.5

9−

0.2

3

+10

033.0

04.2

07.8

6110.2

517.7

420.6

738.4

133.3

734.0

967.4

72.1

6

-10

027.0

04.2

06.4

397.6

557.9

931.2

289.2

128.3

328.6

857.0

11.2

2

0+1

030.0

04.6

26.4

9113.8

428.0

727.9

356.0

030.6

031.7

062.3

03.6

0

0-1

030.0

03.7

87.9

494.0

628.0

721.8

049.8

730.6

030.9

961.5

91.2

9

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and consists of bar elements for the deck and towers and truss elements to simulate the stays. The simulation code used to analyze the model was Abaqus/Standard 6.13 [9] software.

This work presents some improvements in the methodology developed in [6] tomake it more general, particularly in the aeroelastic characterization process. Inthe following sections, the process for obtaining the aeroelastic properties of eachdesign proposed by the optimization algorithm is described, which is based on theuse of surrogate models, and some results are presented. Later, the optimizationapproach is also described and commented.

2 Aeroelastic characterization of a single box deck

Given that in the optimization process described before some of the designvariables are shape parameters of the deck cross-section, the aeroelastic behaviorof each section proposed by the algorithm has to be characterized. As the cross-sections considered in the design domain can be accepted as streamlined shapes,the quasi-steady theory formulation [10] can be applied to obtain the flutterderivatives from the aerodynamic coefficients of each shape. Therefore, under thisassumption, the only required task is to obtain these aerodynamic coefficients.This can be done computationally by means of Computational Fluid Dynamics(CFD) techniques (see for instance [11, 12]) for a set of designs and afterwardsapplying surrogate models [8] to generate these coefficients for every sectionrequired during the optimization process as described below.

2.1 Computational evaluation of aerodynamic coefficients

In order to build the surrogate model, some designs for selected values of B andH of the design domain have to be evaluated at two angles of attack α = [0◦, 2◦]to obtain the values and slopes of the aerodynamic coefficients, as required by thequasi-steady theory. These coefficients are given by

CL =L

12ρU

2B, CD =

D12ρU

2Band CM =

M12ρU

2B2, (1)

where L, D and M are the lift and drag forces and moment, respectively,considering the sign convention indicated in figure 3, ρ is the air density, U isthe wind velocity and B is the width of the deck cross-section.

The CFD analyses were conducted in the open source code OpenFOAM [13]using a 2D unsteady Reynolds-averaged Navier–Stokes (URANS) approach. Theturbulence model considered was Menter’s SST k − ω model, first developedin [14] and improved in [15].

The meshes created have a number of elements of about 2.8 · 105, and details oftwo of them can be seen in figures 5 and 6 corresponding to the designs +10%H+10%B and +10%H -10%B, respectively. They are structured meshes and weregenerated by the in-house software FLUSINI [16]. The analyses were carried out

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αB

O

M

DLU

Figure 3: Sign convention for theaerodynamic forces.

Inlet Outlet

Slip wall

Slip wall

U

15B 25B

15B

15B

B

Figure 4: Size of the domain andboundary conditionsconsidered in the CFD

Figure 5: Detail of the mesh of thesection +10%H +10%B.

Figure 6: Detail of the mesh of thesection +10%H -10%B.

considering a Reynolds number of 105, and the walls are modeled using a lowReynolds approach. The boundary conditions considered in the flow domain andits size are shown in figure 4.

The CFD results obtained for three of the samples considered in the surrogatemodel were validated by means of wind tunnel test conducted at the aerodynamicwind tunnel of the University of La Coruna.

2.2 Aerodynamic response by means of surrogate modeling

Although computational power of digital computers and capabilities in the field ofCFD are continuously increasing [11], the idea of iterative processes which includeCFD evaluations is far for being a suitable alternative in terms of computationaltime. However, approximations using surrogate models to avoid recursive CFDcalculations represents a good alternative to obtain the aerodynamic coefficients ofeach design requested by the optimization algorithm. The application of surrogatemodels in the field of multi-objective optimization is tackled for instance, in [17],

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analyses .

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and in the field of optimization in aerodynamics, its application has already startedin the area of tall buildings. An interesting application case can be found in [18],where the effects of wind on a building subject to shape modifications by cross-section twisting are studied. More information about surrogate modelling and allthe tasks involved in this process are widely described for instance, in [8].

The surrogate model chosen for this work is the Kriging surrogate model, whichwas first developed in [19], and later on, it was applied to the engineering designin [20]. This technique is nowadays one of the most widespread methods and itsperformance is widely demonstrated in the literature, for instance, in [21]. In thefield of aerodynamics, some applications can be found, as for instance, the workof [22], which employs Kriging surrogate models for optimizing the shape of abuilding by means of changing the corners of the shape.

The formulation of the Kriging surrogate model can be found for instance, in [8],and can be written as

fK (x) = κ (x)Tρ+ ε (x) , (2)

where κ (x)Tρ is a trend function which is combined with a stationary gaussian

process error model given by ε (x) that is used to correct the trend function.This stationary gaussian process present a zero mean, constant variance, andthe stationary autocorrelation function r (x,x′), in which the most used is theanisotropic generalized exponential model, given by

r (x,x′) = exp

(−

D∑k=1

θk|xk − xk′|γ

). (3)

The most important task of this process is defining the sampling plan to generatethe realization cases for the surrogate model training. The accuracy of the surrogatemodel is highly conditioned by how these samples represent the full domain.Several techniques to establish a sample plan have been created over the years,mainly based on random generations. Further information about this issue canbe found in [8]. In this work, a non-random sample plan based on the authorsexperience has been carried out, which is shown in figure 7. Vertical and horizontalaxes contain the range of variation of B and H with regards to the baseline valuesof G1 cross-section. The initial design corresponds to the black square and theremaining sample designs are represented by blue circles. It can be observed thatthe point locations are adequately distributed in the domain considered.

With the values obtained for these designs, a Kriging surrogate model withreduced quadratic trend was built using the Dakota framework [23]. The surrogatemodel was designed to work with two inputs (the shape parameters B and H) andsix outputs (CL, CD, CM , C ′L, C ′D and C ′M ), and a representation of the outputswith respect to the 2 design variables are shown in figure 9.

2.3 Aeroelastic response by means of the quasi-steady theory

Hence, from the aerodynamic coefficients given by the surrogate model for eachpair of shape design variables (H,B), the flutter derivatives of each design can

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Figure 7: Design of experiments for thesurrogate model construction.

Figure 8: Sign convention for theaeroelastic forces.

be obtained using the quasi-steady theory [10, 24], following the sign conventionshown in figure 8 and according to the following expressions,

P ∗1 = −2CD,0K

P ∗2 =C ′D,0 − CL,0

KµP P ∗3 =

C ′D,0K2

P ∗5 =C ′D,0 − CL,0

K

H∗1 = −C ′L,0 + CD,0

KH∗2 =

C ′L,0 + CD,0

KµH H∗3 = −

C ′L,0K2

H∗5 = −2CL,0K

A∗1 =C ′M,0

KA∗2 =

C ′M,0

KµA A∗3 =

C ′M,0

K2A∗5 = −2CM,0

K(4)

where A∗i , H∗i and P ∗i (i = 1, ..., 6) are the flutter derivatives, CL,0, CM,0, CD,0,C ′L,0,C ′M,0 andC ′D,0 are the aerodynamic coefficients and derivatives at zero windincident angle α = 0 and µP , µH and µA are the distances between the elasticcenter of the cross section and the point of application of the aerodynamic forcesexpressed as a fraction of the width of the section B. Once the flutter derivativesare obtained, the flutter velocity of a bridge can be assessed using the Scanlantheory [25] solving the eigenvalue problem considering the aeroelastic forces. Thedynamic equilibrium of the bridge is given by

Mu+Cu+Ku = fa =Kau+Cau (5)

where M is the matrix of masses, C is the damping matrix, K is thestiffness matrix, u, u and u represent the vector of displacements, velocitiesand accelerations, respectively, the subindex a indicates aeroelastic, and fa is the

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B [%]

105

0

5

10

H [%]

10

5

0

5

10

Cd (α

=0)

0.05

0.06

0.07

0.08

0.09

B [%]

105

0

5

10

H [%]

10

5

0

5

10

Cd'

0.05

0.00

0.05

0.10

0.15

0.20

B [%]

105

0

5

10

H [%]

10

5

0

5

10

Cl (α

=0)

0.06

0.04

0.02

0.00

0.02

0.04

0.06

B [%]

105

0

5

10

H [%]

10

5

0

5

10

Cl'

5

6

7

8

9

B [%]

105

0

5

10

H [%]

10

5

0

5

10

Cm

=0)

0.025

0.030

0.035

0.040

0.045

0.050

0.055

0.060

B [%]

105

0

5

10

H [%]

10

5

0

5

10

Cm'

0.8

1.0

1.2

1.4

1.6

Figure 9: Kriging surrogate model for the aerodynamic coefficients and slopes.

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aeroelastic forces vector, which can be obtained from the flutter derivatives as

fa =1

2ρU2K2

P ∗4 −P ∗6 −BP ∗3−H∗6 H∗4 BH∗3

−BA∗6 BA∗4 B2A∗3

v

w

φx

+1

2ρUKB

P ∗1 −P ∗5 −BP ∗2−H∗5 H∗1 BH∗2

−BA∗5 BA∗1 B2A∗2

v

w

φx

,(6)

where v and w are the displacements in the y and z directions, respectively, φx arethe rotations in the x direction and K represents the reduced frequency, which isgiven by K = Bω/U where ω is the vibration frequency. Further informationabout this process, and some insights about its implementation can be foundin [26].

3 Approach for the optimum design of cable supported bridges

The design process of the bridge by means of the surrogate-based optimizationapproach described in this work consists of two main stages. The first stagerequires CFD evaluations of some possible deck cross-sections designs andthe construction of a surrogate model that will be recursively evaluated duringthe optimization process. The second stage is the surrogate-based optimizationprocess, which in each iteration carries out static structural analyses and eigenvalueanalyses of the bridge, evaluates the surrogate model, applies the quasi-steadytheory and finally solves the eigenvalue problem considering the aeroelastic forcesto obtain the flutter velocity. This process can be summarized in the steps listedbelow.

1. Definition of the lower and upper bounds of the shape design variables.2. Definition of the sampling plan, establishing the points required for the

construction of the surrogate models of the aerodynamic coefficients andslopes for angle of attack α = 0◦.

3. Obtaining the aerodynamic coefficients for α = [0◦, 2◦] by means of CFDof the cross-sections proposed in the previous step.

4. Construction of the surrogate model.5. Definition of the initial values of the design variables. These are the shape

and size dimensions of the deck (widthB, depthH and thickness t), and thecables cross-section areas and prestressing forces.

6. Optimization process. The stages carried out in each iteration are:(a) Evaluation of the objective function.(b) Structural analyses of the static load cases that are included in the

design. In this case, five load cases are analyzed, including fourload cases representing traffic loads and the self-weight load case ofthe bridge. These analyses provide the responses for the structuralconstraints.

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(c) Dynamic analyses of the bridge by means of solving the eigenvalueproblem.

(d) Evaluations of the aerodynamic coefficients and slopes, provided bythe surrogate model previously built in step 4.

(e) Evaluations of the flutter derivatives using the quasi-steady theory andthe coefficients and slopes obtained in the previous step.

(f) Evaluations of the flutter speed of the bridge by solving the eigenvalueproblem having into account the aeroelastic forces, as described insection 2.3. These analyses provide the response of the aeroelasticconstraint, which is the flutter velocity of the bridge.

(g) Modification of the design variables.It must be noticed that the steps (a), (b), (c) and (d) can be run in parallel, while

the step (e) has to be carried out from the results of (d), and finally, the step (f) isbased on the results of (c) and (e). Besides, the load cases considered in (b) canalso be parallelized. This enables a considerable reduction of computing time oneach iteration and consequently in the whole optimization process.

Initial designH0, B0, t0, A0, N0

Step: Step−1Increment 17: Step Time = 1.000

Deformed Var: U Deformation Scale Factor: +1.000e+00

ODB: Job−1.odb Abaqus/Standard 6.13−2 Tue Jan 12 20:12:26 CET 2016

XY

Z

G1

Analysis Optimumdesign

Optimum designH∗, B∗, t∗, A∗, N∗

Optimization algorithm

No

Yes

Modified designH, B, t, A, N

DE

SIG

NVA

RIA

BL

ES:

H,

B,

t,A

,N

RE

SPO

NSE

S:F

,ut l,

wd l,σ

d l,σ

c l,U

f

Eigenvalueanalysis

Step: Step−1Mode 1: Value = 3.5102 Freq = 0.29818 (cycles/time)

Deformed Var: U Deformation Scale Factor: +7.469e+03

ODB: Job−1.odb Abaqus/Standard 6.13−2 Mon Jun 15 19:27:22 CEST 2015

XY

Z

Step: Step−1Mode 7: Value = 33.068 Freq = 0.91522 (cycles/time)

Deformed Var: U Deformation Scale Factor: +9.124e+03

ODB: Job−1.odb Abaqus/Standard 6.13−2 Mon Jun 15 19:27:22 CEST 2015

XY

Z

Step: Step−1Mode 9: Value = 89.830 Freq = 1.5084 (cycles/time)

Deformed Var: U Deformation Scale Factor: +9.507e+03

ODB: Job−1.odb Abaqus/Standard 6.13−2 Mon Jun 15 19:27:22 CEST 2015

XY

Z

Aerodynamic

coefficientsB [%]

105

05

10

H [%]

10

5

0

5

10

Cd (α

=0)

0.05

0.06

0.07

0.08

0.09 Flutterderivatives

QS

theory0 2 4 6 8 10 12 14

0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0A1*

ExperimentalQuasi-steadyFlat plate

0 1 2 3 4 5 6 7 8 90.8

0.6

0.4

0.2

0.0

0.2A2*

0 1 2 3 4 5 6 7 8 9

0.0

0.5

1.0

1.5

2.0A3*

0 2 4 6 8 10 12 141.0

0.5

0.0

0.5

1.0

1.5A4*

0 5 10 15 20

U*

1.0

0.5

0.0

0.5

1.0A5*

0 2 4 6 8 10 12 14 16

U*

1.0

0.5

0.0

0.5

1.0A6*

Flutter analysisMu + Cu + Ku = fa

fa = Kau + Cau

0 20 40 60 80 100 120Wind velocity [m/s]

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Alp

ha v

alu

es

U=128.97 m/s, U ∗ =9.01, K ∗ =0.697 Mode6

Mode2

Mode6

Mode9

Mode7

Mode14

Mode5

Mode8

Mode10

Mode11

Mode12

Mode13

Mode15

Mode16

Mode17

Mode18

Mode19

Mode20

Mode21

Mode22

Mode23

Mode24

Mode25

Mode26

Mode27

Staticanalyses

Definition of designvariables and ranges Sampling plan CFD analyses

B [%]

105

05

10

H [%]

10

5

0

5

10

Cd (α

=0)

0.05

0.06

0.07

0.08

0.09

Surrogate modelconstruction

Figure 10: Flowchart of the surrogate-based optimization approach.

This process is also illustrated in the flowchart presented in figure 10, where itcan be seen how the surrogate model construction process is carried out beforethe optimization algorithm starts. Besides, the parallelization capabilities of the

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process are also illustrated in the analysis box, where it can be seen that the staticanalyses, the eigenvalues analysis and the aerodynamic coefficients evaluation bymeans of the surrogate model are independent processes and consequently can becarried out simultaneously, allowing the parallel computation of the process. Itmust be noticed that all the structural static analyses are also independent amongthem.

The design variables considered in the problem are the deck shape variables Band H , the deck plate thickness t, 40 cable areasA and 40 prestressing forcesN .The target of the problem is to minimize the total weight of the bridge. Hence, theobjective function is the sum of the volume of deck and cables, and is given by

minF (H,B, t,A,N) = AD (H,B, t)LD + 2n∑i=1

AiLC,i, (7)

whereAD is the area of the cross-section of the deck, LD is the length of the deck,n is the number of cables of the bridge, which is 40 in this case, Ai is the cross-section area of the i-esime cable and LC,i its length. The prestressing forces arecalculated by being considered as design variables, as explained in [6] or in [4].

Five static load cases are considered: the self-weight of the bridge to assessthe prestressing forces of each cable, and four uniform overloads, as shown infigure 10. The structural constraints considered are kinematic and stress constraintsfor each load case l. These are the horizontal displacements of the top of the towersutl , the vertical displacements of the deck in the locations where the stays contactwith the deck wt

l , the stress of each stay of the bridge σcl and the stress in thetop and bottom side of the deck in some control points σdl , which are 88 responsepoints distributed along the deck. Besides, in order to assess the flutter velocityof each design, a eigenvalues analysis is carried out and the flutter derivativesare obtained employing the quasi-steady theory and the aerodynamic coefficientsgiven by the surrogate model.

Furthermore, all the evaluations carried out in each iteration of the optimizationprocess are also parallelizable, leading to very assumable computational time,which can be about 3 hours for the whole process.

4 Conclusions

This paper present some improvements in a methodology previously developedby the authors [6] for the full bridge optimization considering simultaneouslystructural and aerelastic constraints. The enhancements are related to theaeroelastic characterization of the deck cross-section, and consist of usingsurrogate models to obtain a more accurate response when the optimizationalgorithm requires the aerodynamic coefficients of a proposed deck cross-section.After the description of the methodology, some conclusions can be extracted:

• The methodology proposed in [6] can be applied to box type deck, and morecomplex geometries, increasing the number of deck shape parameters.

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• Application of surrogate models for the aerodynamic characterization of theof the deck cross-section is an accurate and efficient solution, which alsoallows to predict the errors expected for the responses provided.

• The independence between the processes carried out in the presentedmethodology allows a high level of parallel computing, leading to reducedcomputational times which makes the methodology very efficient andcompetitive.

• This research is currently going on to obtain optimum designs that reducesthe total amount of material of the bridge considering structural andaeroelastic constraints simultaneously.

Acknowledgements

The research leading to these results has received funding from the SpanishMinister of Economy and Competitiveness (MINECO) with reference BIA2013-41965-P and the Fundacion Pedro Barrie de la Maza and the Universidade daCoruna. The authors fully acknowledge the support received.

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