18-03-2014, 10:57 AM
Seismic design of beam-column joints in RC moment resisting frames – Review of codes
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Introduction
Beam column joints in a reinforced concrete moment resisting frame are crucial zones for transfer
of loads effectively between the connecting elements (i.e., beams and columns) in the structure. In
normal design practice for gravity loads, the design check for joints is not critical and hence not
warranted. But, the failure of reinforced concrete frames during many earthquakes has demonstrated
heavy distress due to shear in the joints that culminated in the collapse of the structure. Detailed
studies of joints for buildings in seismic regions have been undertaken only in the past three to four
decades.
Joints in reinforced concrete moment resisting frames
Beam column joints are generally classified with respect to geometrical configuration and
identified as interior, exterior and corner joints as shown in Fig. 1. Theoretical background on
design of beam column joints has been reviewed in a number of publications (e.g., Uma and Meher
Prasad 2005). There are basic differences in the mechanisms of beam longitudinal bar anchorages
and the shear requirements in two types of joints such as interior joint and exterior joints in
relevance to code recommendations. With respect to the plane of loading, an interior beam-column
joint consists of two beams on either side of the column and an exterior beam-column joint has a
beam terminating on one face of the column.
Depth of member for interior joint
In an interior joint, the force in a bar passing continuously through the joint changes from
compression to tension causing push-pull effect with distribution of bond stress as shown in Fig. 2.
The severe demand on bond strength necessitates that adequate development length for the bar be
made available within the depth of the member. In recognition of this, the codes limit the ratio
between the largest bar diameter and the member depth expressed as db / h c ratio. This limit is to
provide reasonable control on the amount of potential slip of the longitudinal bars through the joint
that can eventually reduce the stiffness and energy dissipation capacity of the connection region.
Longer development lengths are desirable, particularly when associated with high shear stresses and
low values of ratio of column flexural strength to beam flexural strength (Leon 1990). The axial
compression load on column improves the confinement of joint core to some extent which improves
the bond condition within joint core (Paulay and Priestley 1992). The codes NZS and EN recognize
contributions from various factors such as effect of axial load, material strength and ratio of
compression to tension reinforcement whereas ACI gives the ratio as a constant (Table 1).
Flexural strength of columns
The codes give a design check expressions to preclude formation of plastic hinges in columns
which essentially aim at ensuring the design values of the moments of resistance of columns more
than that of beams including overstrength factors. The NZS performs the check with respect to the
centre of the joint including the moments due to the shears at the joint faces apart from the
moments acting on the faces of the joint. However, ACI and EN codes accept the check considering
only the latter as the loss in accuracy is minor and the simplification achieved is considerable if the
shear allowance is neglected. With regard to the flexural strength of column being influenced by the
axial load acting on it, all three codes consider axial load that resulted in the minimum flexural
strength. Table 1 gives the checks suggested by each code where ACI and NZS provisions compute
the nominal flexural strengths of the members and EN code checks with the design values of
minimum moment of resistance.
Shear strength of joint
The shear forces in vertical and horizontal directions develop diagonal compressive and tensile
forces within the joint core. Conflicting views on shear transfer mechanisms and design parameters
of the joint exist between researchers because of the interplay of shear, bond and confinement
within the joint. The model proposed by Paulay et al. (1978) considers that the total shear within
the joint core being partly carried by a diagonal concrete strut (Fig. 6(a)) and partly by an idealized
truss consisting of horizontal hoops, intermediate column bars and inclined concrete bars between
diagonal cracks (Figs. 6(b),©). The strut mechanism is associated with a diagonal force, Dc within
the concrete strut developed by major diagonal concrete compression forces formed at the corners
of the joint and is contributing to the substantial portion of total shear. However, the strength of the
strut is reduced by tensile strains developed in perpendicular direction of the strut wherein the
confinement of the joint core helps in improving the strength of the strut.