Forces acting on a horizontal slat immersed in a fluidized bed during its start-up stage were investigated systematically. Both stress and pressure signals were measured by using strain gauges and pressure transducers to describe the forces and their relationship with gas-solids hydrodynamics. The main influencing parameters are the increment of superficial gas velocity, installation height, inclination angle, static bed height, and installation method of the slat. The experimental results indicated that there is a dangerous upward impulse with a high peak and a long duration in the measured stress signal during starting up a fluidized bed. The effects of various operating and structural parameters on the forces acting on the test slat were elucidated in detail. Effective measures to alleviate the forces and critical conditions where the strength of internals needs to be re-enforced are proposed. The obtained knowledge is helpful to engineering design to ensure the safety of immersed internals in fluidized bed reactors. (C) 2017 Elsevier Ltd. All rights reserved.