Experimental Study on Seismic Behavior of High-Performance Fiber-Reinforced Cement Composite Coupling Beams

Experimental Study on Seismic Behavior of High-Performance Fiber-Reinforced Cement Composite Coupling Beams. Paper by B. Afsin Canbolat, Gustavo J. Parra-Montesinos, and James K. Wight

The motivation for this discussion stems from some 40 years of interest in the seismic design of reinforced concrete coupled walls. It was interesting to learn about beneficial effects of fiber reinforcement. However, it was disappointing that issues of behavior, necessary for engineering understanding, and determination of nominal strengths, necessary for the applications in design of experimental findings, were not addressed in the paper. It is hoped that the comments offered may encourage the authors to revise some of their conclusions and remind designers of important behavioral features applicable to coupling beams.

1. The identification in the mid-60s of the inappropriateness of established procedures, relevant to the prediction of the nominal strength of conventionally reinforced squat coupling beams (Fig. 1(a)), then initiated some research. Among specimens tested in New Zealand, four were geometrically similar to those shown in Fig. 1(a) and 2(b). In 1969, that study concluded that such elements possess limited seismic displacement and energy dissipation capacity.

Sources of ductility in such beams were traced by extensive strain and displacement measurements (Park and Paulay 1975). Features, partly illustrated in Fig. A, demonstrated that, contrary to the conformity with the illusory deformation configuration perceived by the authors in Fig. 2, all horizontal bars over the clear span s of squat beams were subjected only to tension.

The consequently greatly reduced bond transfer along horizontal bars implies that traditionally defined shear stresses are insignificant. Diagonal compression stresses along trajectories (sketched in Fig. A) rather than shear stresses, initiated a potential diagonal failure crack. When vertical stirrup reinforcement is provided to transfer without yielding the maximum shear force V between the triangular bodies A to B, no failure along any diagonal crack occurs. In the absence of steel strain reversals, significant strength can be extracted from such beams only when an imposed seismic displacement is larger than the previously accumulated plastic deformations. The results are a dramatic reduction of stiffness and inferior energy dissipation.

Identified internal forces, associated with the nominal strength of the anti-symmetric panel, are recorded on the left-hand side of Fig. A. In spite of the total horizontal internal forces, C^sub c^ =T^sub s^ + T'^sub s^, being larger than those derived for ordinary beam sections, because of the reduced internal lever arm z, nominal strength is reduced. Relative displacements of the right-hand boundary of the beam, based on Villiot's technique, and their components, Δ^sub c^ and Δ^sub t^, respectively, are also shown. After a few inelastic displacement reversals, the entire shear force V needs to be transferred by friction across previously extensively cracked compression zones. All specimen in which diagonal tension failure was prevented, failed by sliding shear (Park and Paulay 1975) with subsequently mobilized dowel mechanism having inadequate shear strength.

2. The negative outcome of the studies summarized previously prompted a search for radical changes in the detailing of coupling beams. This led to the experimental work of Binney reported in 1974, and its further exploitation by Santhakumar, using tests of coupled walls. The vastly superior performance of such coupling beams resulted in the abandonment in New Zealand (1973) of conventionally reinforced beams. The relevant local code in 1982 formally restricted their use. Much earlier than 1999, several astute American structural consultants adapted diagonally reinforced coupling beams in their buildings. As Fig. B shows, the behavioral model used in New Zealand was that of symmetrical diagonal steel bracing, capable of sustaining large inelastic strains both in tension and compression. During the first tensile strain excursion beyond yield, the concrete surrounding the diagonal compression bars (shown by the shaded area in Fig. B) contributes to force transfer. At this stage, diagonal compression bars perform in the elastic domain. The associated components and beam displacements, at the onset of yielding along the tension diagonal, Δ^sub vy^ and Δ^sub hy^, are identified in Fig. B. This enables the nominal yield chord rotation, θ^sub by^ = Δ^sub vy^/s, and shear stiffness, k^sub b^ = V^sub y^/Δ^sub vy^ = (2T^sub s^sinα)/Δ^sub vy^, of such beams to be realistically estimated. Bar elongation due to uniform yield strains along a diagonal, with some allowance of strain penetration into the anchorage regions in the walls (Paulay 2002a), is denoted as Δ^sub t^. After the first reversal of inelastic seismic displacements, the diagonal bars in compression replace the contribution to strength of the surrounding concrete. Hence the shortening of the compression diagonal is Δ^sub c^ [asymptotically =] 0. Energy dissipation is furnished only by the diagonal bars, leading to a Ramberg-Osgood type of hysteretic response. As expected, inelastic deformations will be proportional to post-yield tensile strains imposed. A displacement ductility, μ^sub Δb^ = Δ^sub vu^/Δ^sub vy^, in the order of 15 can be sustained without approaching an acceptable limit on steel tensile strains, for example, 5 to 6%.