Keywords: shear strength, beam-column joints, cyclic load, earthquake resistant structures, reinforced concrete, structural design.

It is now generally believed that beam-column joints can be critical regions in reinforced concrete frames under severe seismic effects. Beam-column joint failures were the main cause of many structural failures in the 1999 Kocaeli earthquake in Turkey [1]. A heavy damage can not be tolerated in beam-column joints because the gravity loads are carried by the joints and the joints are difficult to repair after damage.

Over the last 30 years significant amount of research has been carried out on cyclically loaded exterior beam-column joints. In spite of this cumulated data, there is still no consensus between the research communities regarding the factors affecting the joint shear strength. The New Zealand Code [2] is based on the assumption that joints resist shear by two mechanisms. The first is the strut mechanism, which accounts for the concrete contribution to the joint shear and the second is the truss mechanism, which accounts for the contribution of the transverse reinforcement. The truss mechanism can only exist when there is good bond in the beam longitudinal reinforcement. Thus, the contribution of stirrups to the joint shear strength is dependent on the good bond conditions in the beam bars in the New Zealand Code Design Philosophy. To achieve this purpose, the bar sizes are strictly limited relative to the joint dimensions. The New Zealand Code also requires that intermediate column bars are necessary to equilibrate the vertical component of the force in the inclined struts of the truss mechanism. However, recent Japanese research by Fuji and Morita [3] has shown that intermediate column bars are ineffective in resisting the joint shear because there is a considerable amount of tensile shift in the column bars from Bernoulli's assumption of plane sections remain plane. These recent findings invalidate the existence of the truss mechanism in cyclically loaded beam-column joints. The ACI Code [4] is based on the strut mechanism and allows some bond deterioration in the joint zone. Thus, the Code assumes that the joint shear strength is not influenced by the stirrup ratio and stirrups are only necessary in the joint zone to provide confinement under reversed cyclic loading.

In this paper, the factors influencing the shear strength of cyclically loaded exterior beam-column joints are investigated using an experimental database consisting of cyclically loaded exterior beam-column joints without transverse beams and slabs. The parametric studies show that the joint shear strength is related to the square root of the concrete cylinder strength and as the concrete cylinder strength increases, the joint shear strength increases proportional to the square root of the concrete cylinder strength. The parametric studies also show that as the beam longitudinal reinforcement ratio increases, the joint shear strength increases. As the joint aspect ratio increases, the joint shear strength decreases. The joint shear strength of cyclically loaded exterior beam-column joints is independent of the column axial stress and the column longitudinal reinforcement ratio. The authors have made a similar study [5] for monotonically loaded exterior beam-column joints. It is apparent that the same factors are influential on the joint shear strength of monotonically loaded joints. However, the stirrups also contribute to the joint shear strength in monotonically loaded beam-column joints. In cyclically loaded beam- column joints on the other hand, the strain in the stirrups is dependent on the loading history and as the cycle number increases, the strain in the stirrups increases.

1

Bakir P.G. and Boduroglu M.H., "Earthquake risk and hazard mitigation in Turkey", Journal of Earthquake Spectra, August, 2002. doi:10.1193/1.1503341

2

NZS3101: 1995, Concrete Structures Standard, Standard Association of New Zealand, Wellington, 1995.

3

Fuji S., and Morita S., "Comparison between interior and exterior reinforced concrete beam-column joint behaviour", Design of beam-column joints for seismic resistance, SP123, J.O.Jirsa ed., American Concrete Institute, Farmington Hills, Michigan, 167-185, 1991.

4

ACI Committee 318, "Building Code Requirements for Structural Concrete (ACI 318-95)", American Concrete Institute, Farmington Hills, Michigan, 369pp, 1995.

5

Bakir P.G., and Boduroglu M.H., "A new design equation for predicting the joint shear strength of monotonically loaded exterior beam-column joints", Engineering Structures Journal, 24, 1105-1117, 2002. doi:10.1016/S0141-0296(02)00038-X

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