Analysis and modelling of the seismic behaviour of high ... - Ingegneria

4. SEISMIC RESPONSE OF PARTIAL-STRENGTH COMPOSITE JOINTS
Under sagging bending moments, composite T sections can be ductile if the
crushing of the concrete is avoided. The buckling failure of the steel walls normally
does not control the ultimate limit state of composite beams under sagging bending
moments, because of the connections between the upper flange and the concrete.
To avoid concrete failure under sagging bending moment, the proportions of a
composite steel-concrete section must be such that the steel on the bottom side
yields before the concrete strain εc at the top of the section are too high; for
reinforced concrete elements submitted to cycling loading, this is deemed to be
satisfied when εc < 2·10 -3 , when the strains εs in steel are high enough to obtain the
required local ductility µ. Under earthquake loading at the connection of beam to
column, a sagging bending moment on one side, and a hogging bending moment
on the other side, are transmitted to the column. This implies one transfer of forces
from the steel part of the composite section, which takes place through steel
connecting elements and for which a design practice does exist, and another
transfer of two forces Fsc and Fst from the slab. Fsc is the resulting compression
force in the slab, on the sagging moment side, while Fst is the resulting tension
force of the re-bars, on the hogging moment side (Plumier et al, 1998).
The transfer of the compression force Fsc from the slab to the column can be
realised through two mechanisms, as illustrated in Figure 4.9.
b c
112
F Rd1
h c
F Rd1
(1−β)/2 F Rd2
(1−β)/2 F Rd2
A T
A /2
S
θ = 45°
A /2
S
F Rd2
Figure 4.9. Two basic mechanisms of force transfer from the slab to the column
β/2 F Rd2
β/2 F Rd2
Mechanism 1 is the direct compression of concrete on the flange of the column.
Mechanism 2 is a truss with two compressed struts and one steel tie in tension.
This mechanism can be developed only when the column cross-section has some
concave zones or special connecting devices on the sides. The design resistance
of these two mechanisms can be estimated in a way similar to the one used in the
design of reinforced concrete structure.
The design resistance FRd1 of Mechanism 1 is simply: