Engineering Know-How: Lessons from Earthquakes and Rules for Seismic Design - Dynamic Behavior of Concrete and Seismic Engineering - page 365

- mere chemical “sticking”, which only gives low strength and does not play any
effective part in energy transfer;
- conventional friction, which can make important contribution if compression
stress is applied perpendicularly to the reinforcement axis. Such compression stress
can be either active (i.e. due to pressure applied to the anchoring area) or passive
(linked to the confinement of the anchoring area), because its shear strains (caused
by Poisson's effect) are prevented by transverse reinforcements called confining
reinforcements . Friction strength is linked to the reinforcement surface condition. It
determines the pull out strength of plain reinforcements , though cannot account for
the pull out strength of high adherence reinforcements by itself. Conversely, a
normal traction condition on the reinforcement axis (generated by Poisson's effect
when the bar elongates, for example) decreases the pull out strength;
- by concrete buttressing on the reinforcements acting as high adherence
reinforcements . This phenomenon involves formation of conic cracks initiating in
the excrescences. The transverse reinforcements sew the cracks. Once the maximum
strength has been reached, two failure mechanisms can develop: either crack
propagation through the coating that ends up in its separation, or cylindrical crack
formation around the bar. In both cases, strength decreases very rapidly.
The phenomena described above are influenced by different parameters,
including the geometry of the bar, concrete strength, confinement, the design of the
transverse reinforcements, coating and bar spacing. The Eurocode 2 formulae that
allow determination of the anchoring length take these different influences into
account. In a seismic situation, the strong extension or shrinking cycles of the bar
that is to be anchored can cause variations in both the confining condition and the
behavior of concrete. Sliding strength is modified in the same way as the
compression strength or the ultimate tensile strength of concrete by the application
of high amplitude cycles. The deterioration of that strength becomes increasingly
significant, by increasing either the amplitude or the number of cycles. With cycle
amplitudes set at about 80% of the static sliding strength, the deterioration is
important and must be taken into account. Consequently, as anchoring in the critical
areas is stressed according to the real capacity of the reinforcement because of the
potential rotation of the plastic hinge, it is better to design in such a way as to avoid
it. It will be the same for overlapping lengths as it is for anchoring lengths. Eurocode
8 takes overcapacity of reinforcements in critical areas into account and introduces
specific regulations for bar anchoring into the nodes.
9.6.5.2. Compressed reinforcement buckling
As was discussed in section 9.2.3 (see also Figure 9.12), the reinforcements may
buckle when they are strongly compressed, especially in plastic hinge regions. It
also arises in conventional column layouts where concrete shrinking is higher than