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 on the tension 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
on the tension zone was achieved by reducing the amount of longitudinal 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. First - ÐÊ-2, reinforced mesh. Second
- ÐÊ-5 with concentrated reinforcement. The number of rods equal in both samples. 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–2 |
12,2 |
20,7 |
0,94 |
Âð–1 |
60 |
110 |
22 |
10 |
20´30 |
|
2 |
ÐÊ–5 |
12,2 |
20,7 |
0,94 |
Âð–1 |
60 |
110 |
22 |
10 |
20´30 |
à)
á)
Fig.1. Reinforcement scheme samples:
à – ÐÊ 2 (μs = 0,18 %); , á – ÐÊ 5(μs =0,18 %);
In a sample of ÐÊ-2 first vertical
cracks formed in the span between the central piles with a load of 42% of the breaking load (Fig. 2
(photo)). With further increase of
the load, these cracks developed poorly. Simultaneously with them were sloping crack
emanating from the central supports in the direction of the extreme. When a load closes to
collapse, there was a crack in
the middle of the span in the
transverse direction. On the sole ground beam observed a series of cracks along the reinforcing
bars in the central part. In the
area of the most distant piles
no cracks (Fig. 3
(photo)).
Character cracking ÐÊ-5 in
many respects similar to the ÐÊ-2. The distinguishing feature is that all
cracks appeared almost simultaneously.
This sample turned out to be stronger
than 1.17 times compared with the ÐÊ-2, the
reinforced grid (Fig. 2 - 4).
Fig.2. The character of crack formation and failure
mode
of the sample ÐÊ-2 (failure load of 48 tons)
Fig.3. The character of crack formation and failure
mode
of the sample ÐÊ-5 (failure load of 54 tons)
Fig.4. Character of cracking
on the sole samples ÐÊ-5, ÐÊ-2
Scheme of
destruction the ÐÊ-2, ÐÊ-5
The destruction of the samples occurred on the tension
zone with the active development
of cracks on the sole of grills in the central span.
Sloping cracks formed
on the outside of the central supports and evolved grillage
height towards the
extreme supports. The
concentration of reinforcement does
not affect the process crack
formation, but has contributed to an increase in the strength of the sample.