Dear John, In nonlinear analysis you need to prescribe a minimum constraints to the model to remove rigid body motions that will cause singular matrix in your model, this is difficult, but you need to define some "isostatismic" that do not affect to the results, or the minimum possible. The best way is to exploit symmetry conditions, if exist, in order to reduce the model size but also to stabilize the model. If not any symmetry boundary condition exist, then you will have to design a support method by prescribing constraints in your model in order to be as much as "soft" as possible, ie, that constraints do not modify results too much (do not over-constrain!) In nonlinear contact analysis is typical the use of "soft springs" to stabilize the model in the directions of rigid body motion, using numerical penalty methods, of resources by the solver that allows to include very small values in the diagonal terms of the stiffness matrix (using MSTAB in NXSTRAT, default MSFAC=1e-12), or physically including in the model CROD elements with small stiffness to avoid singular matrix in the initial solution step when contact iteration is performed. Alternative resources could be to use "Low Speed Dynamic" in the Time Incrementation Scheme. Or fully solve your problem as advanced nonlinear dynamic (SOL601,129), defining contact over a rigid floor, the object resting in the floor, but be prepare for the cost of the solution!!. Best regards, Blas. PD Please send my best regards to Svein Gjølmesli -- we meet in Bilbao years ago!!