Maintain joint positions when simulation is started

I just finished importing a robotic from a URDF file using the URDF importer: I have a fixed base followed followed by a series of revolute and prismatic joints. After adding collisions to each mesh, a ground plane and physics scene, I proceeded to add an Articulation Root to the fixed joint. When I press ‘Play’ all the joints seem to be affected by gravity and fall towards the ground instead of staying fixed like any usual robotic arm. How would I go about maintaining initial position when pressing play? I have damping/stiffness set for all the joints