Nonlinear　dynamics　of　flexible　multibeam　structures　modeled　as　an　L-shaped　beam　are　investigated　systematically　considering　the　modal　interactions.　Taking　into　account　nonlinear　coupling　and　nonlinear　inertia,　Hamilton＇s　principle　is　employed　to　derive　the　partial　differential　governing　equations　of　the　structure.　Exact　mode　functions　are　obtained　by　the　coupled　linear　equations　governing　the　horizontal　and　vertical　beams　and　the　results　are　verified　by　the　finite　element　method.　Then　the　exact　modes　are　adopted　to　truncate　the　partial　differential　governing　equations　into　two　coupled　nonlinear　ordinary　differential　equations　by　using　Galerkin　method.　The　undamped　free　oscillations　are　studied　in　terms　of　Jacobi　elliptic　functions　and　results　indicate　that　the　energy　exchanges　are　continual　between　the　two　modes.　The　saturation　and　jumping　phenomena　are　then　observed　for　the　forced　damped　multibeam　structure.　Further,　a　higher-dimensional,　Melnikov-type　perturbation　method　is　used　to　explore　the　physical　mechanism　leading　to　chaotic　behaviors　for　such　an　autoparametric　system.　Numerical　simulations　are　performed　to　validate　the　theoretical　predictions.