Varying passive crowds can cause considerable changes in the dynamic properties of civil structures, such as sport venues. The effectiveness of passive control systems for the reduction of vibrations in these structures is limited as the dynamic properties of the structure change depending on the occupancy level and postures. Hence, a semi-active system is proposed for reducing structural vibrations regardless of the off-tuning due to passive crowd effects. This paper describes the experimental implementation of a semi-active control strategy for a pressurized tuned liquid column damper (PTLCD) on a grandstand-type steel structure. Additionally, an analytical model is proposed for the human–structure–damper system, and numerical results are compared with those obtained experimentally. Only the effects of passive anthropic behavior were considered, and results show that individuals, when sitting, produce more damping in the main structure than when they are standing. Overall, the semi-active PTLCD had a satisfactory performance for the different configurations considered, achieving reductions in peak and root mean square acceleration of up to 74% and 57% with respect to the grandstand without the control system.