Engineering Know-How: Lessons from Earthquakes and Rules for Seismic Design - Dynamic Behavior of Concrete and Seismic Engineering - page 363

develops, mainly by response of the first vertical mode. Such motion gives rise to a
dynamic normal force , and this should be taken into account in a building's design,
as it is relevant to overall strength. This dynamic normal force should not be
confused with the normal force due to an earthquake's vertical component. Its
amplitude is still not well understood, but foundation conditions are thought to
determine the first vertical mode.
As for strength verification, the bending moment derived from dynamic
calculation can be used without significant overestimation. With the horizontal
component of the earthquake, the normal force increases or decreases with the
dynamic normal force: Eurocode 8 gives this an absolute value equal to 50% of the
permanent normal force. The shear force increases with the (1+q)/2 coefficient and
ensures that a premature brittle failure due to the shear force will not take place. As a
rule, verifications with regard to the composed bending and the shear force are in
keeping with Eurocode 2 verifications. Nevertheless, as the dynamic normal force is
taken into account, the limit strain of non-confined concrete is raised to 0.5%, which
will restrict some practical applications.
With this type of wall, the density of the reinforcements is restricted: capacity
design is not suitable (except to verify the increased shear load) and it is not
generally necessary to confine the concrete, as the strain does not concentrate inside
a plastic hinge, except if the dynamic normal force brings about excessive concrete
strains. As concrete sections are usually over-abundant, shear stresses are often
weak anyway, and web reinforcements are useless if the concrete shear strength is
sufficient.
Three main failure modes can play a part, depending on the influence of the
shear load:
- failure due to a bending moment, with formation of horizontal cracks and/or
reaching the ultimate strain of the concrete or of the vertical reinforcements at the
ends: this arises when the shear stresses are weak. Verification corresponds to the
composed bending type within horizontal sections;
- diagonal cracking failure resulting from a composed-bending shear load
combination. Analysis of such a mode is carried out by a truss method, as bracing
struts are formed by the floors and several truss rods may potentially form in a level
between two floors according to the level height/length ratio;
- failure due to sliding on a horizontal plane, verification of which is carried out
according to Eurocode 2, as the main strength terms are the same as for ductile
walls.
The method developed in Eurocode 8 originated in France, and emerged from a
series of experiments on little reinforced walls: the CASSBA test, then the CAMUS