Ground Motions and Structures - Earthquake Engineering for Structural Design - page 336

336 Earthquake Engineering for Structural Design

8.4.2 Concrete

Concrete, as a material for seismic resistant structures, has two important deficiencies:
low strength to unit weight ratio and verylow tensile resistance. In order to improve the
concrete qualities, the research is directed to obtain the so-called high-performance
concrete with high compression and tension strengths. The performance limit of high
strength concrete changed with time: 34 MPa (1950), 52 MPa (1960) and 62 MPa
(1970). More recently, compressive strengths approaching 138 MPa have been obtained
in cast-in-place buildings. Today, high strength concrete with 250 MPA compression
strength is produced using high strength aggregates (Husem and Pul, 2007). High-
performance concrete is used mainly in the columns of high-rise buildings, obtaining
the most economical way to transfer the vertical loads to the foundations. Using
highstrength concrete, with the strength as high as 100 MPa, the dimensions of
columns are reduced, lowering, consequently, the structural weight, which is a
favourable aspect in seismic design, as also the seismic forces are reduced.
There are many advances in using high strength concrete instead of the normal one.
However, due to its brittleness, many engineers are still reluctant to use it in seismic-
resistant structures. The theoretical and experimental research works showed that well-
confined high strength columns in these zones, which are potentially susceptible to
develop large plastic deformations, can provide a satisfactory ductile behavior with
large gains both in strength and ductility, provided that adequate transverse
reinforcement is used (Cusson et al, 1996, Sharma et al, 2005, Claeson, 2008, Ngo et
al, 2008). The material models for unconfined and confined concrete are shown in
Figure 8.35. If the ascending branch is the same for both concrete, differences consist
just in strength, the descending branch for a well-confined column shows a smooth
falling.
In order to assure a good behaviour against potential plastic zones in a column, the
transverse column reinforced must vary along the column length, as illustrated in
Figure 8.36a. Good, improved and poorly confinements are presented in Figure 8.36b.
The second important advantage in using the high-performance concrete is the
increasing of the tensile strength. The high- performance fibre reinforced concrete is a
very suitable material in the regions where a large inelastic deformation capacity is
necessary to withstand the requirements induced by severe earthquakes (Canbolat et al,
2005, Parra-Montesinos, 2005). It has the important property to increase the shear
resistance, ductility and energy dissipation in members subjected to reversal cyclic
loadings.
There are two typical fiber reinforced concretes: a regular one, characterized by a
softening response after the first cracking and the high performance one, with multiple
cracking.