Ductility of wide-beam RC frames as lateral resisting system

Abstract

Some Mediterranean seismic codes consider wide-beam reinforced concrete moment resisting frames (WBF) as horizontal load carrying systems that cannot guarantee high ductility performances.Conversely, Eurocode 8 allows High Ductility Class (DCH) design for such structural systems. Code prescriptions related to WBF are systematically investigated. In particular, lesson learnt for previous earthquakes, historical reasons, and experimental and numerical studies underpinning specific prescriptions on wide beams in worldwide seismic codes are discussed. Local and global ductility of WBF are then analytically investigated through (i) a parametric study on chord rotations of wide beams with respect to that of deep beams, and (ii) a spectral-based comparison with conventional reinforced concrete moment resisting frames (i.e., with deep beams). Results show that the set of prescriptions given by each modern seismic code provides sufficient ductility to WBF designed in DCH. In fact, global capacity of WBF relies more on the lateral stiffness of the frames and on the overstrength of columns rather than on the local ductility of wide beams, which is systematically lower with respect to that of deep beams.

Seismic performances and behaviour factor of wide-beam and deep-beam RC frames

“…Lower μ θ for WB are obtained with respect to DB: 13% and 28% lower for EC8 and NCSE-02, respectively, consistent with the average 27% obtained in the parametric study in [12]. …”

“…(6,7) DCH (q=3.5) ≈30dbi ≈48 (6,7) -0.25hc (9,11) 0.25hc 0.25hc (16) min{L/8;8hf;hb; hc·{0.5;0.75} (18) }; min{L/8;8hf;3hb} (19) 0.25hc (22) 90% (24) Spain: NCSE-02 (2002) [14] -33%) ≈36dbi ≈55 (6,7) min{0.5hb; 0.5bc (8) } (10) -0.5bc (14) 0.25hc (8) hf·{0;2;0;2} (17) - (21 Current version of EC8 does not cover flat slab, 1.5 is the basic assumption for elastic design; new version in progress (1) For DLS but obtained from ULS displacements (2) Specific for ULS (3) Obtained from specific DLS demand spectrum (4) Depending on ag and number of storeys (5) Sufficient stiffness to ensure frame -not cantilever-behaviour in all columns (6) Formulation depending in most of the cases on ductility class, material strengths, axial load, reinforcement ratios and location of the joint (7) Considering ϕw=16mm (8) Edge beams not explicitly considered (9) Not for low-ductility design (10) Referred to gross section, not to web (11) Referred to the 90% of the required flexural reinforcement; remaining 10% within (19) (12) Required transverse beam for external connections or internal connections with moment inversion (13) Not mandatory, only for taking advantage of the column compression on the bond behaviour (14) Higher values only if proper perpendicular reinforcement is placed (15) Further research is needed (16) Also reciprocal requirement for columns in the case of wide column -narrow beam connection (17) Exterior connection with and without transverse beam, and anal...…”

“…All the issues, with exception of those related to local ductility, are overcome by specific design provisions in modern seismic codes [12] [35]. As long as design to DLS -controlling IDR-and second order (P-∆) corrections are implemented, the stiffness of WBF must be rather similar to that of DBF by means of the use of larger column sections.…”

“…Even if the design hypothesis EC8 100-50 can seem not realistic, it well represents an upper bound design version aimed at providing more robust conclusions with regard to personal choices of design. EC8 does not regulate some relevant assumptions regarding stiffness of the elements: contribution of member ends within joint panels, contribution of upper slab or joists [12], contribution of outstanding (b b -b w ) in wide beams [12], and does not specify quantitative procedure for the evaluation of regularity of stiffness in elevation. Thus, EC8 100-50…”

“…Aimed at an agreement between accuracy and computational demand, the assessment of relative performances between WBF and DBF is carried out by means of the simplified approach proposed in [12], based on similar approaches already used in other studies unfavourable for WBF with respect to DBF, because higher H mec for WBF rather than for DBF are not taken into account. In all the following, subscript 1 and n refer to the storey of interest (first or last, respectively); θ u,min is the minimum θ u between members involved in the collapse mechanism;…”