Beam Column Joints
» Importance of joint behavior
o Weak link theory
o Deterioration mechanisms
» Monolithic beam-column joints o In the design with the philosophy of limit states it is seen that joints are often weakest links in a structural system.
o The knowledge of joint behavior and of existing detailing practice is in need of much improvement.
o Joint behavior is especially critical for structures subject to earthquake effects.
o The shear forces developed as a result of such an excitation should be safely transferred through joints. The R/C system should be designed as a “ductile system”.
» Design of joints
o Joint types
Type I – Static loading
→ strength important
→ ductility secondary
Type II – Earthquake and blast loading
→ ductility + strength
→ inelastic range of deformation
→ stress reversal
o Joints should exhibit a service load performance equal to that of the members it joins.
o Joints should possess strength at least equal to that of the members it joins (sometimes several times more).
o Philosophy: Members fail first, then joints.
→ The joint strength and behavior should not govern the strength of the structure.
o Detailing and constructability.
» Behavior of joints
o Knee joint
• Typical example of a portal frame. The internal forces generated at such a knee joint may cause failure with the joint before the strength of the beam or column.
• Even if the members meet at an angle, continuity in behavior is necessary.
o Corner joints under closing loads
• Biaxial compression: εu > 0.003
• Full strength of the bars can be developed if there is no bond failure.
• Joint core
o Factors influencing joint strength
1. Tension steel is continuous around the corner (i.e., not lapped within the joint).
2. The tension bars are bent to a sufficient radius to prevent bearing or splitting failure under the bars.
3. The amount of reinforcement is limited to
Relative size will affect strength and detailing for practical reasons.
5. Bond force
6. Full bond strength needs to be developed to transfer shear forces into the concrete core.
Corner joints under opening loads
→ When subjected to opening moments the joint effects are more severe.
• Behavior under seismic loading
→ Concrete with joint cracks due to cycling.
→ Degradation of bond strength.
→ Flexural bars should be anchored carefully.
→ No benefit should be expected from axial loads.
→ Rely on ties within the joint.
→ Effects from both opening and closing should be considered.
→ An orthogonal mesh of reinforcing bars would be efficient.
o Corner joints under cyclic loads
→ When subjected to cyclic loading (opening moment), one should consider the interaction between tension and compression zones.
o Exterior joints
• Exterior joints of multistory plane frames
a. Bond performance as affected by the state of the concrete around anchorage.
b. Transmission of compression and shearing forces though the joint when the joint core cracks
• Also consider load reversals. This is critical for seismic effects.
• Top beam bars
• Subject to transverse tension
• The anchorage condition of the reinforcement steel
• Bottom beam bars
• Subject to transverse compression
• Outer column bars are subjected to severe stress conditions.
• Transmission of shearing and compression forces by diagonal strut across the joint
o Interior joints
→ Vc = Cc - V' = shear force transferred through concrete
→ Vs = Cs + T = shear force transferred through steel
• Combined behavior:
Shear transfer by bond
Vj = Vc + VS
• Reduction in compressive strength due to biaxiality in concrete and deterioration of bond due to load cycling are of importance in joint integrity.
• Effect of axial force
• Effect of confinement