Departament of Quality
Management and Technology of Building Production
«Penza State University
of Architecture and Construction»,
Russia
Appearance AND DEVELOPMENT OF
CRACKS multi-row grillage FOR THE pillar AT FAILURE
in the compressed zone
On the basis of experimental studies revealed the nature of the
formation and development of cracks in the multi-row pile raft
foundation under pillar. The article contains pictures cracking grills under
pillar with the number of piles
equal to eight.
Prototypes designed in the form of models in
scale 1: 3. The destruction
by the compressed zone was achieved by increasing the amount of longitudinal tension reinforcement at the lower edge of the
grillage. Hydraulic jack transmitting
load on a grillage through the
metal plate that mimics the pillar.
Consider the research of pile foundation
grills c number
supports equal to eight. Two samples were tested: ÐÊ-1, ÐÊ-1â. They differed loading schemes. The load was transferred to the eccentricity
of 5 cm (ÐÊ-1â). Freight area was taken smaller sizes (20´20 cm) and shifted relative
to the axis of symmetry 5 cm. Characteristics of samples and their reinforcement are presented in table and in figure 1.
Table
Characteristics test
samples
|
¹ |
Brand |
Characteristics of concrete |
Class |
The geometrical dimensions |
||||||
|
Râ, MPa |
Åâ ´
10 -3, MPa |
Rât, MPa |
Â, cm |
L, cm |
h, cm |
d,
cm |
pillar b´h |
|||
|
1 |
ÐÊ–1 |
10,6 |
19,2 |
0,86 |
À–²²² |
60 |
110 |
22 |
10 |
20´30 |
|
2 |
ÐÊ–1â |
12,2 |
20,7 |
0,94 |
À–²²² |
60 |
110 |
22 |
10 |
20´20 |
Fig.1. Reinforcement scheme samples ÐÊ - 1, ÐÊ – 1â (μs = 1,2 %);
1,3,4 -reinforcement Æ10 ÀIII,
2 - Æ12
ÀIII.
Formation
and development of cracks sample ÐÊ-1 is
shown in Figure 2, 3, (photo). The first - is a crack in the central span in
the lower longitudinal edge of the grillage with a load equal to 60% of the
breaking load. The height of the crack
was 1/4 of the grillage.
With further increase in load its growth stopped. With
a load equal to 82% of the
breaking load vertical cracks have appeared at the
bottom face grillage in the
transverse direction. At the same time were formed sloping cracks
at the inner faces of the bearing surfaces oriented to column. With further increase of the load
increases the width of the cracks
in the area of the central pillars. With a load equal to 90% of the breaking load there
was a crack at the outer edge of the middle supporter. It was developed by the
height of grillage towards extreme pillar.
Fig.2. The character of crack formation and failure
mode
of the sample ÐÊ-1 (failure load of 121,6 tons)
Fig.3. The character of crack formation and failure
mode
of the sample ÐÊ-1 (top view, bottom view)
Fig.4. The character of crack formation and failure
mode
of the sample ÐÊ-1â (failure load of 104 tons)
Development of cracks in the concrete when changing loading scheme
When
loading the sample ÐÊ-1â character cracking has not changed (Fig.4.
(Photo)). The first cracks appeared earlier than in the sample ÐÊ-1. The load in this case was less 1.4 times. Their development was observed with further increase of the load. With a load
equal to 70% of the failure load have appeared sloping cracks in
the central pillars. Next, crack growth was
observed in both the direction from the upper to the lower
die, and vice versa. Transverse cracks
in the tension zone in this sample
no.
Scheme of destruction the ÐÊ-1 è ÐÊ-1â
The destruction occurred by the
compressed zone of concrete. The first cracks that were formed in the central span, further
development of the load increases have not received. The most dangerous steel sloping cracks
that were formed from central piles. Schema change of the load led to the earlier
development of cracks in the most loaded part of the grillage. No cracks at the end supports. This
testifies to the existence of small
stresses.