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Design of a four-bar truss

Stadler, W., Dauer, J., Multicriteria optimization in engineering: A tutorial
and survey. Structural optimization: status and future. AIAA Journal, 1992,
pp. 209-249.

Costa, M.F.P., Fernandes, E.M.G.P., Practical evaluation of an interior
point three-D filter line search method using engineering design problems.
8th World Congress on Structural and Multidisciplinary Optimization.
June 1-5, 2009, Lisbon, Portugal.
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Scalars L     /200/
        F     /10/
        E     /200000/
        sigma /10/;

Variables x1,x2,x3,x4, obj;

Equations e1, eobj;

* Objective functions:
eobj.. obj =e= (2*x1+sqrt(2)*x2+sqrt(2)*x3+x4)*L;

* Constraints:
e1.. (F*L/E)*(2/x1+2*sqrt(2)/x2-2*sqrt(2)/x3+2/x4) =l= 0.04;

* Bound on variables:
x1.lo = F/sigma;           x1.up = 3*F/sigma;
x2.lo = sqrt(2)*F/sigma;   x2.up = 3*F/sigma;
x3.lo = sqrt(2)*F/sigma;   x3.up = 3*F/sigma;
x4.lo = F/sigma;           x4.up = 3*F/sigma;

Model truss2 /all/;

Solve truss2 minimizing obj using nlp;

$ifThenI x%mode%==xbook
file out /tr2.dat/
put out;
put x1.l:15:7, x2.l:15:7, x3.l:15:7, x4.l:15:7 /;
put obj.l:15:7 /;
$endIf
* End truss2