A very large floating structure has hydroelastic responses that are an elastic deformation and a slowly varying drift motion. Usually, wave drifting forces increase when an elastic deformation is reduced by a motion reduction system. In this study, characteristics of the hydroelastic responses of the backbone type elastic floating models with air chambers are investigated experimentally. The model tests were carried out in irregular waves using the five models. The responses include the elastic deformation and wave drifting forces. The air chambers can absorb the wave energy. It is confirmed that the reduction system is effective for the motion reduction from the experimental results of the vertical displacements and the bending moments. Then, we know that there is no direct causal relation of the motion reduction and the wave energy absorption. And more, it is difficult to estimate the reduction effect of the slowly varying wave drifting force from the reduction characteristic of the steady wave drifting force directly.