This study presents inelastic nonlinear behavior of under-tensioned beam systems. Typical large span structures consisting of beam members result in increased moment and bigger member sizes. In order to reduce these excessive moment and deflection, under-tension system composed of beam, strut, and cable elements is frequently employed. In under-tension system, prestress is introduced in cables to improve force distribution with some weight reduction. Two different under-tensioned beam systems made up of structural steels and cables with $10m{ imes}2.4m$ are tested. One is made up of typical wide flange section, the other uses customized cold-form hollow section called PF500. The various magnitude of the pretension is applied at under-tension cables. The nonlinear behaviour of the under-tension systems is investigated by using finite element model, and compared to experimental results. It shows that there are very good correlation between finite element model and experimental results. It also indicates that the system with PF500 members exhibits much better structural performances than that of wide flange members.