Engineering Know-How: Lessons from Earthquakes and Rules for Seismic Design - Dynamic Behavior of Concrete and Seismic Engineering - page 357

Construction layouts are required to cover the uncertainties that remain at each
stage of seismic action design, especially those concerning ductility. Layouts deal
with beam and column design, minimum reinforcements on beam upper sides, the
minimum percentage of traction reinforcements inside beams, the minimum
reinforcement percentage and the limitation of the normal compression stress inside
columns.
Finally, as a general rule, because frames can be very flexible, if no other
additional layout is taken, the relative floor-to-floor displacements should be limited
to minimize P-'effects.
9.6.3.2. Verification of composed bending sections and rotation ductility
Verification of sections subjected to composed bending is carried out according
to methods laid out in Eurocode 2. The reliability of a section thus designed is quite
well controlled, as they are based on many empirical measurements. In addition to
partial safety coefficients being applied to materials, part of the safety requirements
are based on strain tolerated limits, others on the fact that the stress tolerated by
reinforcements is limited to their steel elastic limit rather than their resistant
capacity.
In the ULS calculation conditions for a simple bending section, the concrete and
steel strains are respectively limited to 3.5% and 10%. This corresponds to a
maximum 0.0135/ d bend (where d refers to the usable height of the beam). With a
length equal to the usable height, the rotation of the beam is equal to 0.0135 rads,
which is generally too small to allow justification as a plastic hinge.
Actually, and subject to proper reservations, section rotation may be far more
important, which means that the usual bending model is insufficient to represent the
behavior of a concrete plastic hinge. Plastic rotation depends on both the integration
length of the bend in the critical area and the strains reached.
As far as tensile reinforcements are concerned, the 1% conventional limit strain
is far lower than the failure strain, which is generally closer to 10%. However, the
strain limit of reinforcements is given by the bound limit, beyond which the
assumptions that make it possible to calculate bends are no longer valid.
For compressed concrete, the composed bending calculation method takes the
uni-axial behavior of the material into account. In a multi-axial stress condition, this
behavior is highly variable. When helically reinforced, concrete strength and
ductility are both improved.
Transverse reinforcements ensure confinement, and their effectiveness is
therefore directly related to the ability of critical areas to undergo plastic rotations.