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Research on anti-floating reinforcement of an existing project

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Due to the lack of structural anti-floating, a certain part of the main structure has cracks, and the structure needs to be reinforced by buoyancy to make the project meet the requirements.After research, the Negative second floor of the original structure is reinforced with add the anti-floating anchors, apply pressure grouting for cracks in the plate, Paste carbon fiber cloth for the bottom of the plate and the damaged shear wall, externally-paste profile steel for the damaged column, and paste Carbon fiber sheet attached to damaged beam.the calculated of shear bearing and flexural capacity show that the board meets the requirements.
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Research on anti-floating reinforcement of an existing project
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ICMEMSCE 2019
IOP Conf. Series: Materials Science and Engineering 758 (2020) 012008
IOP Publishing
doi:10.1088/1757-899X/758/1/012008
1
Research on anti-floating reinforcement of an existing project
Yu Han Huang1, Zheng Bin PAN2,Guo Bo Luo1 And Long Chen1
1Guizhou Construction Science Research and Design Institute of CSCEC,
Guiyang,550000,China
2 Graduate Student, Guizhou University, Guiyang,550000,China.
E-mail:386389880@qq.com
Abstract. Due to the lack of structural anti-floating, a certain part of the main structure has
cracks, and the structure needs to be reinforced by buoyancy to make the project meet the
requirements.After research, the Negative second floor of the original structure is reinforced
with add the anti-floating anchors, apply pressure grouting for cracks in the plate, Paste carbon
fiber cloth for the bottom of the plate and the damaged shear wall, externally-paste profile steel
for the damaged column, and paste Carbon fiber sheet attached to damaged beam.the
calculated of shear bearing and flexural capacity show that the board meets the requirements.
1. Introduction
With the continuous development of the construction industry, more and more buildings are standing
in the crowd, and problems are emerging.Especially in Guizhou Province, the geological environment
and hydrological conditions are complicated, and it is difficult to obtain accurate geological surveys.
Therefore, some structural bearing capacity is likely to occur during the design and construction
process, and reinforcement measures are required for buildings with insufficient structural capacity.An
existing project in Guizhou has found that cracks have occurred in structural components due to
insufficient structural anti-floating, and the structure needs to be anti-floating. For the reinforcement
measures, this paper has made corresponding research.
2. Project Overview
The construction area of the project is 25,808.73m2, and the building height is 8.55m. The basic form
is pile foundation plus waterproof board, independent column base plus waterproof board and raft
foundation.The foundation bearing layer is a medium-grade limestone, and the main structure is a
two-layer underground frame structure. Because of insufficient anti-floating , Some frame columns,
frame beams and bottom plates in the range of (1)~(10)/(A)~(M) axis of the negative one floor and
Negative second floor of the project are deformed and cracked. Therefore, the damage status of
ICMEMSCE 2019
IOP Conf. Series: Materials Science and Engineering 758 (2020) 012008
IOP Publishing
doi:10.1088/1757-899X/758/1/012008
2
concrete members in (1) ~(10)/(A)~(M) axis of the negativ one floor and Negative second floor of the
project is detected.According to the test, the damage range of the structure is (1)~(10)/(A)~(M) axis;
some column has cracking phenomenon, and there are mutation and dislocations in the cross section of
individual frame columns; the negative second floor shear wall (1)~(7)/(A),and negative one floor of
shear wall (1)/(A)~(B) has cracking phenomenon; a few beams have cracking phenomenon; negative
second floor (1)~(2)/ (A)~(C), (3)~(4)/(J)~(K), (6)~(7)/(E)~(G) Plate has being crack.
3. Take reinforcement measures
In view of the engineering problems, the following reinforcement measures are proposed. pressure
grouting is applied to the bottom and basis of the plate before reinforcement. anti-floating anchors are
added to the negative second floor, and the head height of the water for the anti-floating is 4.3
m.Pressure grouting treatment for the board of crack, and the carbon fiber cloth is reinforced on the
bottom of the board.the damaged column is reinforcement by externally pasted steel, externally
attached carbon fiber sheet, and the enlarged section method. Before the reinforcement of damaged
column, the pressing should be removed.The carbon fiber cloth method is used for reinforcement for
the damaged beam. Before the reinforcement of damaged beam, the crack on the beam should be
pressure grouted. the damaged shear wall should be reinforced with carbon fiber cloth.
The original structure is reinforced and the reinforcement measures are shown in the figure below.
Figure 1. Uncasting concrete screed
Figure 2. Pour screed concrete
anchor practice
Figure5.Poured water-stop
ICMEMSCE 2019
IOP Conf. Series: Materials Science and Engineering 758 (2020) 012008
IOP Publishing
doi:10.1088/1757-899X/758/1/012008
3
Figure 6. Anti-shearing steel bar
arrangement in the board
Figure 7. Locator
Figure 8. Bonded steel reinforcement column practice
Figure 9. Increase section reinforcement
Figure 10. Carbon fiber cloth reinforcement
Hear wall practice
ICMEMSCE 2019
IOP Conf. Series: Materials Science and Engineering 758 (2020) 012008
IOP Publishing
doi:10.1088/1757-899X/758/1/012008
4
(a)
(b)
Figure 11. Paste fiber cloth
Figure 12. Bottom plate carbon fiber layout method
4. Structural reinforcement check
4.1. Anti-floating design water level calculation
According to the geological survey report, the anti-floating level is H=1469.4m, and The basement
floor bottom level of the two-layer part of 1-32 is 1465.1m (the lane is also considered to be within
this part), and the head height is 4.3m. The 32-42 axis basement floor has a bottom elevation of
1467.6m and a head height of 1.8m.
Upper load statistics:
(1)1-32 two-layer part
This load calculation is calculated by taking the 8.1×8.1 plate as an example.
Covering soil: 18×0.7=12.6 kN/m2.
Reinforced concrete slab weight: 0.71×25=17.75 kN/m2.
The thickness of the bottom plate is 350mm, the thickness of the negative two-layer top plate is 200,
and the thickness of the negative one is 160mm.
Concrete plain screed, 7~15cm on site, calculated according to 5cm: 0.05×20=1 kN/m2.
Beam and column calculation:
0.3×0.6-0.2×2+0.4×0.9-0.2×2+0.6×0.6×8.1×25/(8.1×8.1)=3.0 kN/m2.
Dead load calculation (favorable load): 12.6+17.75+1+3.0=34.35 kN/m2.
So 34.35 kN/m243.0×1.05=45.15 kN/m2
The area's anti-floating does not meet the requirements.
(2)32-42of One layer part
This load calculation is calculated by taking the 8.1×8.1 plate as an example.
Covering soil: 18×0.7=12.6 kN/m2.
ICMEMSCE 2019
IOP Conf. Series: Materials Science and Engineering 758 (2020) 012008
IOP Publishing
doi:10.1088/1757-899X/758/1/012008
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Reinforced concrete slab weight: 0.05×20=1 kN/m2
The thickness of the bottom plate is 350mm, and the thickness of the negative layer is 160mm.
Concrete plain screed, 7~15cm on site, calculated according to 5cm: 0.05×20=1 kN/m2
Beam and column calculation:
0.3×0.6-0.2×4+0.4×0.9-0.2×2+0.6×0.6×4.0×25/(8.1×8.1)=1.5 kN/m2.
Dead load calculation (favorable load): 12.6+12.75+1+1.5=27.85 kN/m2.
So 27.85 kN/m218×1.05=18.9kN/m2
The area is resistant to floats.
4.2. Anti-floating anchor calculation
(1) Buoyancy calculation of anchor
The original structure has a dead weight of 34.35 kN/m2.The head height of groundwater buoyancy
of 4.3m is 4.3×10=43 kN/m2.
(The anti-buoyancy (kN) of the anchor rod) =1.05×( the groundwater buoyancy-the original
structure is dead load)=10.8kN/m2.
(2)Calculation of tensile bearing capacity of single anchor
Using a bolt with a diameter of 200mm, the circumference of the bolt is Ur(m)=0.628m.The
effective anchorage length hr(m)=2.6 m in the anchorage section embedded in the rock
stratum.Empirical coefficient ξ = 0.8.The bond strength characteristic value of cement mortar and
concrete to rock is f(MPa)=0.4 (less than the value of the geological survey report).
Anchor rod pullout bearing characteristic value Rt(kN)=Ur hrξf X1000= 522.496 kN
Required bolt root n=the anti-buoyancy/anchor pull-out bearing capacity characteristic value of the
anchor= 10.8 × 8.1 × 8.1 / 522.496 = 1.36.
The actual 8.1×8.1 unit takes 2 anchors (pile base parts), and the 1-11 axis independent base part
spans 4 anchors.
The bearing capacity of the overall anti-floating single anchor is: 10.8×8.1×8.1/2=354.294 kN.
Review the anti-buoyancy of a single anchor: the head area of the anchorage of the 8.1m×8.1m
plate is 8.1×8.1-2×2.7×2.7=51.03m2, and the standard of anti-floating force of each anchor is: (43-
0.35×0.25-0.05×20)×51.03/4=424 kN.Then the actual bearing capacity of each anchor is 424 kN.
(3)Calculation of cross-sectional area of anchor bar
Anchor rod body tensile safety factor K b=1.8.Anchorage tensile strength design value fy=360
N/mm2.
The total area of the anchor steel bar section:
As≥K b×NaK/ fy(mm2)=1000×1.8×424/360=2120 mm2
Actual matching steel area As (332) = 2413mm2.
(4)Calculation of the anchor length of the anchor anchor and the formation
Anchor diameter D = 200 mm.Standard value of ultimate bond strength of rock and soil layer and
anchor frb (kPa) = 1800kPa.Bolt anchor solid pullout safety factor K=2.2.
Bolt anchor and length of anchorage section of the formation
La1≥K Nak/(πDfrb) =2.2×397.3/(3.14×0.2×1800)=0.77m.
Calculation of anchorage length between anchor rod body and anchor mortar
Anchor bar diameter d=32mm.Anchor rod number n=3 (root).Design value of bond strength
ICMEMSCE 2019
IOP Conf. Series: Materials Science and Engineering 758 (2020) 012008
IOP Publishing
doi:10.1088/1757-899X/758/1/012008
6
between steel bar and anchor mortar fb=2.1 (KPa).
Anchorage length between anchor rod body and anchoring mortar
La2≥K Nak/(nπd fb )=2.2×424/(0.032×3.14×2.4×1000) =1.28m.
The bolt insertion length is 3.5m.
4.3. Base plate punching calculation
Under the action of concentrated reaction, the punching capacity of the plate without the stirrups or the
bent steel bars shall comply with the following provisions:
F L ≤ (0.7βhft + 0.25 бpc, m) ηumh0
Section height influence coefficient βh=1.0.The weighted average of the effective preloading stress
of the critical section concrete by length бpc, m=1.0N/mm2.Concrete axial tensile strength design
value ft=1.43N/mm2.Section effective height h0=300-50=250(mm).
Critical section perimeter um=3.14×2 (100+250/2)=1413mm.
η=min{η1,η2}
η1=0.4+1.2/βs βs=2
η1=0.4+1.2/βs=0.4+1.2/2=1.0
η2=0.5+αsh0/4um αs=40
η2=0.5+αsh0/4um = 0.5+40×250/(4×1413)=2.269
η=min{η1,η2}=1.0
(0.7βhft+0.25бpcm)ηumh0
=(0.7×1.0×1.43+0.25×1.0) ×1.0×1413×250
=441.916kN
Concentrated reaction force design value = actual single anchor bolt tensile capacity = 397.3
<441.916KN.Therefore, the punching resistance meets the requirements, and it is not necessary to
configure the stirrups or bend the reinforcing bars.
4.4. Floor bearing capacity calculation
Take the 8.1m×8.1m meter span for calculation. After the anti-floating anchor is added to the bottom
plate, the plate span is calculated according to 4.05m, and the one-meter wide strip is taken for
calculation.
The height of the water head is 4.3m, the weight of the plate is 0.350×25×1=8.75 kN/m, and the
water buoyancy is 43×1=43kN/m. The force of the plate is calculated according to the basic
combination: 1.4×43-8.75=51.45
The maximum shear force of a 1 meter wide strip is: ql/2=51.45×4.05/2=104.2 kN.
The maximum bending moment of a 1 m wide strip is: ql2/12=51.45×4.052/12=70.32 kN·m.
The 1 m wide strip is subjected to the maximum bending capacity (calculated by the 14@200
board with smaller reinforcement):
'
y s 0 s
( )=77.6k mM f A h a N −
The shear capacity is:
cv t 0
a f bh =315kN
ICMEMSCE 2019
IOP Conf. Series: Materials Science and Engineering 758 (2020) 012008
IOP Publishing
doi:10.1088/1757-899X/758/1/012008
7
It can be seen from the calculation that the shear bearing capacity of the plate is satisfactory.
5. Conclusion and Prospect
the Negative second floor of the original structure is reinforced with add the anti-floating anchors,
apply pressure grouting for cracks in the plate, Paste carbon fiber cloth for the bottom of the plate and
the damaged shear wall, externally-paste profile steel for the damaged column, and paste Carbon fiber
sheet attached to damaged beam.After calculation, the following conclusions are drawn:
(1)1-32 two-layer partial area anti-floating does not meet the requirements, 32-42 layer area
anti-floating meets the requirements.
(2)The actual bearing capacity of each anchor is 424 kN, and the bolt insertion length is 3.5 m.
(3)The punching of the bottom plate meets the requirements, and it is not necessary to configure
the stirrups or bend the steel bars.
(4)The shear bearing capacity of the plate is satisfactory.
References
[1] Yu Y , Gu S , Wu Z , et al. Experimental study of load transfer law of prestressed cables under
loess stratum[J]. Chinese Journal of Rock Mechanics & Engineering, 2010,
29(12):2573-2580.
[2] Xing J Y , Yong Z X , Lin C P . Safety Analysis of Shield Tunnel Segment Lining Based on
Field Test[J]. IOP Conference Series Earth and Environmental Science, 2017, 94(1):012200.
[3] Sun S R , Wu J M , Wei J H . Ground settlement research of excavation for doubled arch tunnels
in expressway[J]. Rock and Soil Mechanics, 2006.
[4] Qingfeng D . Anti-floating Reinforcement Design of Underground Engineering[J]. Chinese and
Overseas Architecture, 2010.
[5] Wang F X , Yuan Z R , Cai H X . The Method Comparison of Anti-Floating of Reinforced
Concrete Slab at Underground Business Street[J]. Key Engineering Materials, 2017,
730:429-434.
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Based on the previous theoretical analysis and the systematic analysis of field pull-out test data, the influence of anchorage length, pull-out load as well as cable diameter on the load transfer law of the prestressed cable under the specific condition of loess stratum are analyzed. The analysis results show that: (1) The most rational effective anchorage length of prestressed cable is 6-8 m. The ultimate bearing capacity of the prestressed cable can not fully be exerted when the anchoring segment is too short; and the prestressed cable is easy to be wasted under the condition that the anchoring segment is too long. (2) The axial force of cable continues to transfer the remote anchorage section getting smaller and smaller whose peak value grows and shifts to the remote with the increase of the pull-out load, which is associated with the fact that the axial force of the front anchorage segment produces local plastic damage when the pull-out load exceeds the anchor's ultimate tensile strengths. (3) Soil mass and anchoring body have significant heterogeneity and nonlinearity, which results in being of the relatively weak or noncontinuous interfaces between grout and soil. The distribution of the anchorage force on the weak plane discretes jump while the pull-out load increases to a certain level. (4) In engineering practice, the starting points of anchoring force distribution curves do not overlap with the port anchor absolutely but deviate from the relative vertical axis to varying degrees, which makes the actual length of anchor is usually shorter than the theoretical design length. (5) Under the loess stratum, the ultimate capacity of some prestressed cables increases linearly with their diameters increase, independent to the effective anchorage length. The growth factor is about 1.1. These conclusions with some theoretical and practical values provide a reference for the design and construction of anchor support engineering in the loess region.
Anti-floating Reinforcement Design of Underground Engineering[J]
  • Qingfeng
Qingfeng D. Anti-floating Reinforcement Design of Underground Engineering[J].