Gelöste Aufgaben/StaF

Aufgabenstellung

Wir untersuchen die Belastung eines ebenen Stabwerks. Die Stäbe haben wie skizziert die Länge ℓ bzw. ℓ/2. Die Struktur wird mit der Kraft F belastet.

Caption

Gesucht ist ein Vergleich zwischen der klassischen Stabwerkstheorie und einer Herangehensweise, bei der wir eine feste Verbindung der Stäbe in den Knoten ansetzten. Grundlage des Modells ist die FEM-Lösung der Felddifferentialgleichung im Vergleich zur Lösung in Problemstellung „Stab“.

Wir stellen das Modell des Stabwerks mit dem Prinzip der virtuellen Verrückungen auf und vergleichen, wie sich diese von der Herangehensweise aus „Stab“ mit der analytischen Lösung unterscheidet.

Lösung mit Maxima

Wir nutzen das Computer-Algebra-System Maxima zur Lösung. Das macht hier Sinn, weil wir die Herangehensweise mit der aus Stab vergleichen wollen – für die wir ebenfalls Maxima eingesetzt haben.

Declarations

Wir übernehmen alle Vereinbarungen und Parameter aus der Problemformulierung „Stab“.

Gleichgewichtsbedingungen

Für die Gleichgewichtsbedingung nach dem Delta W = 0

       = delta \Pi - \delta Wa

benötigen wir die virtuelle Formänderungsenergie \delta \Pi und die virtuelle Arbeit der äußeren Kraft F. Mit allen Konventionen für die Knoten-Verschiebungen ist Delta Wa = -delta W_{4,0} *F Für \delta \Pi gilt

${\begin{pmatrix}{\frac {2{{A}^{2}}E\eta }{3{\ell _{0}}}}&0&-\left({\frac {2{{A}^{2}}E\eta }{{\ell }_{0}^{2}}}\right)&0&{\frac {{{A}^{2}}E\eta }{3{\ell _{0}}}}&0&0&0\\0&{\frac {2{{A}^{2}}E\eta }{3{\ell _{0}}}}&-\left({\frac {{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}\right)&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}\right)&{\frac {{{A}^{2}}E\eta }{6{\ell _{0}}}}&-\left({\frac {{{A}^{2}}E\eta }{2{{\ell }_{0}^{2}}}}\right)&0&{\frac {{{A}^{2}}E\eta }{6{\ell _{0}}}}\\-\left({\frac {2{{A}^{2}}E\eta }{{\ell }_{0}^{2}}}\right)&-\left({\frac {{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}\right)&{\frac {{{A}^{2}}E\eta +3{{\ell }_{0}^{2}}AE}{2{{\ell }_{0}^{3}}}}+{\frac {8{{A}^{2}}E\eta }{{\ell }_{0}^{3}}}&0&-\left({\frac {2{{A}^{2}}E\eta }{{\ell }_{0}^{2}}}\right)&-\left({\frac {{{A}^{2}}E\eta +3{{\ell }_{0}^{2}}AE}{4{{\ell }_{0}^{3}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{{\ell }_{0}^{2}}AE}{4{{\ell }_{0}^{3}}}}&{\frac {{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}\\0&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}\right)&0&{\frac {3{{A}^{2}}E\eta +{{\ell }_{0}^{2}}AE}{2{{\ell }_{0}^{3}}}}+{\frac {2AE}{\ell _{0}}}&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{2{{\ell }_{0}^{2}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{{\ell }_{0}^{2}}AE}{4{{\ell }_{0}^{3}}}}&-\left({\frac {3{{A}^{2}}E\eta +{{\ell }_{0}^{2}}AE}{4{{\ell }_{0}^{3}}}}\right)&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}\right)\\{\frac {{{A}^{2}}E\eta }{3{\ell _{0}}}}&{\frac {{{A}^{2}}E\eta }{6{\ell _{0}}}}&-\left({\frac {2{{A}^{2}}E\eta }{{\ell }_{0}^{2}}}\right)&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{2{{\ell }_{0}^{2}}}}\right)&{\frac {4{{A}^{2}}E\eta }{3{\ell _{0}}}}&-\left({\frac {{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}&{\frac {{{A}^{2}}E\eta }{6{\ell _{0}}}}\\0&-\left({\frac {{{A}^{2}}E\eta }{2{{\ell }_{0}^{2}}}}\right)&-\left({\frac {{{A}^{2}}E\eta +3{{\ell }_{0}^{2}}AE}{4{{\ell }_{0}^{3}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{{\ell }_{0}^{2}}AE}{4{{\ell }_{0}^{3}}}}&-\left({\frac {{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}\right)&{\frac {{{A}^{2}}E\eta +3{{\ell }_{0}^{2}}AE}{4{{\ell }_{0}^{3}}}}+{\frac {{{A}^{2}}E\eta }{{\ell }_{0}^{3}}}&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{{\ell }_{0}^{2}}AE}{4{{\ell }_{0}^{3}}}}\right)&-\left({\frac {3{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}\right)\\0&0&{\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{{\ell }_{0}^{2}}AE}{4{{\ell }_{0}^{3}}}}&-\left({\frac {3{{A}^{2}}E\eta +{{\ell }_{0}^{2}}AE}{4{{\ell }_{0}^{3}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{{\ell }_{0}^{2}}AE}{4{{\ell }_{0}^{3}}}}\right)&{\frac {3{{A}^{2}}E\eta +{{\ell }_{0}^{2}}AE}{4{{\ell }_{0}^{3}}}}+{\frac {AE}{\ell _{0}}}&{\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}\\0&{\frac {{{A}^{2}}E\eta }{6{\ell _{0}}}}&{\frac {{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}\right)&{\frac {{{A}^{2}}E\eta }{6{\ell _{0}}}}&-\left({\frac {3{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{{\ell }_{0}^{2}}}}&{\frac {2{{A}^{2}}E\eta }{3{\ell _{0}}}}\end{pmatrix}}\cdot {\begin{pmatrix}{W_{1,0}}\\{U_{1,0}}\\{{\Phi }_{1,0}}\\{W_{2,0}}\\{U_{2,0}}\\{{\Phi }_{2,0}}\\{W_{3,0}}\\{U_{3,0}}\\{{\Phi }_{3,0}}\\{W_{4,0}}\\{U_{4,0}}\\{{\Phi }_{4,0}}\end{pmatrix}}={\begin{pmatrix}0\\0\\0\\0\\0\\F\\0\\0\end{pmatrix}}$

Hier kommt jetzt irgendein Text.

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Element-Steigigkeitsmatrizen mit globalen Koordinaten
Element #1
${k_{1}}={\begin{pmatrix}{\frac {8{{A}^{2}}E\eta }{\ell _{0}^{3}}}&0&{\frac {2{{A}^{2}}E\eta }{\ell _{0}^{2}}}&-\left({\frac {8{{A}^{2}}E\eta }{\ell _{0}^{3}}}\right)&0&{\frac {2{{A}^{2}}E\eta }{\ell _{0}^{2}}}\\0&{\frac {2AE}{\ell _{0}}}&0&0&-\left({\frac {2AE}{\ell _{0}}}\right)&0\\{\frac {2{{A}^{2}}E\eta }{\ell _{0}^{2}}}&0&{\frac {2{{A}^{2}}E\eta }{3{\ell _{0}}}}&-\left({\frac {2{{A}^{2}}E\eta }{\ell _{0}^{2}}}\right)&0&{\frac {{{A}^{2}}E\eta }{3{\ell _{0}}}}\\-\left({\frac {8{{A}^{2}}E\eta }{\ell _{0}^{3}}}\right)&0&-\left({\frac {2{{A}^{2}}E\eta }{\ell _{0}^{2}}}\right)&{\frac {8{{A}^{2}}E\eta }{\ell _{0}^{3}}}&0&-\left({\frac {2{{A}^{2}}E\eta }{\ell _{0}^{2}}}\right)\\0&-\left({\frac {2AE}{\ell _{0}}}\right)&0&0&{\frac {2AE}{\ell _{0}}}&0\\{\frac {2{{A}^{2}}E\eta }{\ell _{0}^{2}}}&0&{\frac {{{A}^{2}}E\eta }{3{\ell _{0}}}}&-\left({\frac {2{{A}^{2}}E\eta }{\ell _{0}^{2}}}\right)&0&{\frac {2{{A}^{2}}E\eta }{3{\ell _{0}}}}\end{pmatrix}}$
Element #2
${k_{2}}={\begin{pmatrix}{\frac {{{A}^{2}}E\eta +3{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&{\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&{\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}&-\left({\frac {{{A}^{2}}E\eta +3{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&{\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\\{\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&{\frac {3{{A}^{2}}E\eta +{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&{\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&-\left({\frac {3{{A}^{2}}E\eta +{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\\{\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}&{\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}&{\frac {{{A}^{2}}E\eta }{3{\ell _{0}}}}&-\left({\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)&{\frac {{{A}^{2}}E\eta }{6{\ell _{0}}}}\\-\left({\frac {{{A}^{2}}E\eta +3{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&-\left({\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)&{\frac {{{A}^{2}}E\eta +3{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&{\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&-\left({\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)\\-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&-\left({\frac {3{{A}^{2}}E\eta +{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&{\frac {3{{A}^{2}}E\eta +{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)\\{\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}&{\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}&{\frac {{{A}^{2}}E\eta }{6{\ell _{0}}}}&-\left({\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)&{\frac {{{A}^{2}}E\eta }{3{\ell _{0}}}}\end{pmatrix}}$
Element #3
${k_{3}}={\begin{pmatrix}{\frac {{{A}^{2}}E\eta +3{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&{\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}&-\left({\frac {{{A}^{2}}E\eta +3{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&{\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\\-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&{\frac {3{{A}^{2}}E\eta +{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&-\left({\frac {3{{A}^{2}}E\eta +{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)\\{\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)&{\frac {{{A}^{2}}E\eta }{3{\ell _{0}}}}&-\left({\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}&{\frac {{{A}^{2}}E\eta }{6{\ell _{0}}}}\\-\left({\frac {{{A}^{2}}E\eta +3{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&-\left({\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)&{\frac {{{A}^{2}}E\eta +3{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&-\left({\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)\\{\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&-\left({\frac {3{{A}^{2}}E\eta +{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta -{\sqrt {3}}{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}\right)&{\frac {3{{A}^{2}}E\eta +{\ell _{0}^{2}}AE}{4{\ell _{0}^{3}}}}&{\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\\{\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}&-\left({\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)&{\frac {{{A}^{2}}E\eta }{6{\ell _{0}}}}&-\left({\frac {{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}\right)&{\frac {{\sqrt {3}}{{A}^{2}}E\eta }{4{\ell _{0}^{2}}}}&{\frac {{{A}^{2}}E\eta }{3{\ell _{0}}}}\end{pmatrix}}$
Element #4
${k_{4}}={\begin{pmatrix}{\frac {{{A}^{2}}E\eta }{\ell _{0}^{3}}}&0&{\frac {{{A}^{2}}E\eta }{2{\ell _{0}^{2}}}}&-\left({\frac {{{A}^{2}}E\eta }{\ell _{0}^{3}}}\right)&0&{\frac {{{A}^{2}}E\eta }{2{\ell _{0}^{2}}}}\\0&{\frac {AE}{\ell _{0}}}&0&0&-\left({\frac {AE}{\ell _{0}}}\right)&0\\{\frac {{{A}^{2}}E\eta }{2{\ell _{0}^{2}}}}&0&{\frac {{{A}^{2}}E\eta }{3{\ell _{0}}}}&-\left({\frac {{{A}^{2}}E\eta }{2{\ell _{0}^{2}}}}\right)&0&{\frac {{{A}^{2}}E\eta }{6{\ell _{0}}}}\\-\left({\frac {{{A}^{2}}E\eta }{\ell _{0}^{3}}}\right)&0&-\left({\frac {{{A}^{2}}E\eta }{2{\ell _{0}^{2}}}}\right)&{\frac {{{A}^{2}}E\eta }{\ell _{0}^{3}}}&0&-\left({\frac {{{A}^{2}}E\eta }{2{\ell _{0}^{2}}}}\right)\\0&-\left({\frac {AE}{\ell _{0}}}\right)&0&0&{\frac {AE}{\ell _{0}}}&0\\{\frac {{{A}^{2}}E\eta }{2{\ell _{0}^{2}}}}&0&{\frac {{{A}^{2}}E\eta }{6{\ell _{0}}}}&-\left({\frac {{{A}^{2}}E\eta }{2{\ell _{0}^{2}}}}\right)&0&{\frac {{{A}^{2}}E\eta }{3{\ell _{0}}}}\end{pmatrix}}$

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