Advanced Composite Materials in Flexural Members for Auto-adaptive Structural Response Modifcation

The response of moment resisting frame structures to seismic excitation is strongly dependent on the ability of particular structural members to sustain relatively large inelastic deformations without significant degradation of lateral and axial load-carrying capacity. Conventional reinforced concrete frame structures are typically designed according to the strong column/weak beam concept, which prescribes inelastic deformations to occur exclusively in the beam members to dissipate energy while the columns remain elastic in order to maintain stability and prevent possible collapse (Fig.1a). This ideal frame deformation mechanism, enforced by a strength differential between beams and columns intersecting at joint locations, however, usually requires the formation of plastic hinges at the base of the first story columns in order to initiate frame sway and utilize the energy dissipation capacity of the beam members. The formation of plastic hinges at the column base is anticipated and not necessarily critical for the stability of the moment resisting frame, assuming that further inelastic deformations occur exclusively in the beam members. Due to axial and shear forces at the column base, the plastic hinge regions of these members must be provided with relatively large amounts of transverse reinforcement to ensure ductility under reversed cyclic loading conditions by proper confinement of the concrete core, resistance to shear and buckling of longitudinal reinforcement. Furthermore, residual deformations in structural members and in the frame system may require extensive rehabilitation efforts. Most importantly, however, the possibility of formation of additional plastic hinges in the columns above or within the first story in conjunction with plastic hinges at the column base may lead to a kinematic mechanism and collapse of the structure (Fig.1b). The frame configuration investigated in this paper does not require the formation of plastic hinges at the column base in order to initiate frame sway and subsequent utilization of inelastic rotations in the beam plastic hinges (Fig.1c). In the suggested configuration, the formation of plastic hinges at the column base is prevented by employing advanced composite materials, in particular Fiber Reinforced Polymer (FRP) reinforcement combined with a ductile engineered cementitious composite (ECC) to substitute brittle concrete. These FRP reinforced ECC column elements have a relatively large elastic deformation capacity and sufficient flexural strength to enforce inelastic deformations in the beam members in accordance to the strong column/ weak beam concept. Engineered cementitious composites (ECC) are a fiber-reinforced cement-based composite material micromechanically designed to achieve a tensile stress-strain behavior analogous to that of metals. Unlike the dislocation micromechanics in the plastic deformation regime of metals, the inelastic deformation behavior of ECC is based on the formation of multiple cracking while undergoing pseudo-strain hardening. This composite material utilizes randomly oriented fiber reinforcement at a moderate volume fraction (Vf<2%), which are added to the cementitious matrix during the mixing process. Utilizing the particular load-deformation characteristics of steel and FRP reinforced structural members in the suggested moment resisting frame system, a bi-linear load-deformation behavior can be obtained with considerable energy dissipation capacity and reduced residual displacements at unloading. The auto-adaptive stiffness modification is expected to reduce base shear forces during a seismic event by increasing the period of the structural system at exceeding a particular horizontal displacement.