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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 2
T.E.T. BUTTIGNOL |
L.C ALMEIDA
occurring significant reduction in the inferior nodal zones due
to struts compressive effect, as shown in Figure 13. The re-
duction of the supporting area of the piles caused an increase
on pile cap’s inferior surface strains due to the rotation of the
piles axis. Consequently, the ties steel bar tensile stresses
augmented.
Horizontal stirrups have absorbed part of the tensile stresses in
the struts region. In model 3 stresses in the superior longitudinal
steel bars achieved 360 MPa, as shown in Figure 13 (c). Besides,
in model 3, due to stress concentration near the piles supports, an
increase in piles stirrups stresses located near the supports was
observed, as shown in Figure 13 (c).
Splitting reinforcement of the model 4 absorbed part of pile cap’s
stresses, proving to be effective against pile caps tensile stress, as
shown in Figure 14.
In Table 10 maximum strain and stress values of both numerical
analysis and experimental models are presented. The results dem-
onstrated a good approximation between them. In Figures 15 to 19
the pile caps inferior nodal zones are shown in details, demonstrat-
ing the ties anchorage region (l
anc
) and the ties stress graphic values.
In all models (Figures 15 to 19) strains at the ties steel bar ends
were very low. Notwithstanding, as the piles supporting area was
being reduced, there was a progressive increase in the area of
the nodal zones. An increase of the stresses in the ties steel bar
at the end of the inferior nodal zone also occurred with repercus-
sion in the steel bar ends as can be seen in Figures 15, 16 and 17.
Despite that, the stress in the ends of the ties steel bar remained
low. This confirms Clarke’s [5] statement that ties anchorage is
positively influenced by struts confinement action, which excludes
the use of tie hooks.
In model 4, with splitting reinforcing bars, intensive stresses in the
ties steel bars at the beginning of the inferior nodal zone were ob-
served. However, a considerable reduction in steel bar stresses
occurred along the nodal zone. Very low stress values rose at the
steel bar ends, as shown in Figure 18.
In model 5, despite the reduction of the piles cross-section area,
zero strain value occurred at the end of the ties steel bars, as
shown in Figure 19.
4.5 Stiffness and bearing capacity
In relation to the experimental specimen, model 1 was more rigid,
presenting lower displacements. This was a result of the vertical
restraints imposed to the piles in the numerical models, which has
produced a clamping effect, limitating the rotation of the piles and
the displacements of the pile caps.
The reduction of the pile’s supporting area caused a decrease in
the pile caps structural stiffness, leading to a growing convergence
of the force versus displacement curve among the models 1, 2 and
3. In model 3, as shown in Figure 20, the force versus displace-
ment curve overlapped with the experimental model’s curve which
is already adjusted to expunge displacements due to specimen
accommodation in the first load stages.
This proves that in numerical models the supports of the piles di-
rectly affect the stiffness of structural elements. At the same time,
load bearing capacity was not significantly changed. Both numeri-
cal and experimental models achieved ultimate load capacity with
very similar load intensity and crack pattern.
In addition, as the supporting area of the piles was reduced, there
was a shift in the maximum plastic strain location. In Figure 21 it is
possible to notice plastic strain shift from the inferior nodal zones to
the superior nodal zones. If this reduction in the pile caps support-
ing area, on one side, allowed increased structural displacements,
on the other side, it generated a high compressive zone in the pile
cap’s superior nodal zone. In general, there was a reduction up
to 30% in stresses in the piles-pile cap contact surfaces with an
increase up to 28% in the stresses in the column-pile cap’s contact
surface, as shown in Table 8.
In model 4, splitting reinforcement contributed to load bearing ca-
pacity and to the increase in the pile cap resistance.
Table 8 – Maximum stress values (MPa) on ultimate load of numerical models and Delalibera [1]
experimental specimen
Pile caps specifications
Delalibera
[1]
Model
1
Model
2
Model
3
Model
4
Model
5
Maximum
compressive
stress (MPa)
Sruts
-
35
35
32
35
35
Inferior
nodal zone
-
30
27
28
35
34
Superior
nodal zone
-
36
37
34
35
34
Column-pile
cap
interface
58,3
52
50
67
54
48
Piles-pile
cap
interface
58,3
52
50
37
54
62
Ultimate tensile strength
-
2,5
2,5
2,3
2,4
2,3