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Preparation of construction scheme of formwork engineering

(author: Sun Jiang, Meng fan, just met Mengliang)

I. importance of preparation of construction scheme of formwork engineering:

Article 4.1.1 of code for acceptance of construction quality of concrete structures (GB) stipulates that the formwork and its support shall be designed according to the form of engineering structure, load size, foundation soil category, construction equipment and material supply and other conditions. The formwork and its support shall have sufficient bearing capacity, stiffness and stability, and can reliably bear the pouring concrete quality, lateral pressure and construction load. Article 4.1.3 stipulates that the removal sequence and safety measures of formwork and its support shall be implemented in accordance with the construction technical scheme. These two articles are mandatory

according to the scoring table 3.0.6 of the formwork engineering inspection in the standard for safety inspection of building construction (JGJ), 10 points will be deducted if there is no construction scheme for the formwork engineering, and 6 points will be deducted if there is no design calculation for the support system

the formwork construction scheme is involved in the above specifications and standards. When pouring concrete, formwork and its support sometimes expand (deform), run (displace) and even collapse under the action of concrete weight, lateral pressure and construction load. In order to avoid accidents and ensure project quality and construction safety, a formwork construction technical scheme must be formulated

II. Main principles of template design

1 The formwork and its support shall have sufficient bearing capacity, stiffness and stability, and can reliably bear the quality and lateral pressure of the poured concrete, as well as the construction load, so as to ensure the correct beginning size and mutual position of the members

2. The structure of the formwork and its support is simple and easy to support and disassemble

3. The joint is tight without mortar leakage

4 Try to use new formwork, so that the concrete can reach the quality of fair faced concrete after formwork removal

III. template scheme preparation sequence

1 Formwork erection sequence

the connection between beam opening and column head formwork is generally supported by beam formwork, and the connection between cast-in-place slab and beam formwork is generally pressed by cast-in-place formwork against beam side formwork

2. The order of formwork design: first design and calculate the column wall formwork, then design and calculate the cast-in-place slab formwork, and finally design and calculate the beam formwork. This is because the data in the design and calculation of beam formwork needs to use the data in the design and calculation of cast-in-place slab formwork

IV. value of load

1 Concrete self weight 24kn/m3

2 Reinforced frame beam 1.5kn/m3 floor slab 1.1kn/m3

3 Construction load 3.0 kn/m3

4 The load generated during concrete vibration is 2.0 kn/m3 for horizontal formwork and 4.0 kn/m3 for vertical formwork

5 The standard value of pouring concrete on the side of the formwork

where:

γ C - is the gravity density of concrete

h - the height from the calculation position of concrete lateral pressure to the top surface of newly poured concrete

to - the initial setting time (H) of newly poured concrete, which can be determined according to the actual measurement. In the absence of test data, to=200/(t+15) can be used for calculation (t is the temperature of concrete ℃)

v - it can be clearly seen from the concrete that rigid PVC foamed plastic products are widely used, with large market demand, and the pouring speed (m/h)

β 1 - correction coefficient of additive influence, taken as 1.0 when no additive is added; 1.2 for admixtures with retarding effect

β 2. Concrete is the correction coefficient for the influence of concrete slump without strong technical support. When the slump is less than 30mm, take 0.85; At 50~90 mm, take 1 to drive the sustainable development of future electric vehicle schemes through supply chain adjustment 0 When 110~150mm, take 1.15. When the slump is in the middle of two different ranges, the interpolation method can be used to calculate the correction coefficient. If the slump exceeds 150mm, it can be increased appropriately

6. The design value of the anti sliding bearing capacity of the fastener is 8kn/fastener according to the technical specifications

v. examples

a teaching building project adopts a cast-in-place frame structure, with a floor height of 37.5m, a clear height of 3.0m, and a column section size of 500 × 500； The size of the main beam is 300 × 750； The maximum size of the plate is 8.05 × 9.54m, 300mm thick beamless floor, equipped with BDF high-strength thin-walled pipe with d=180mm, and the pipe wheelbase is 260mm. After calculation, the hollow rate is 32%. A centralized mixing plant is set on site, high-efficiency pumping agent is added to the concrete, and the column, beam and slab concrete are poured continuously; The column, beam and slab formworks are assembled with film covered bamboo plywood

(I) design calculation of column formwork

the reinforcement of each side of column formwork is shown in Figure 1. The spacing of column hoops at the lower part of column formwork is 500mm, the spacing of column hoops at the upper part of column formwork is 600mm, and the spacing of the first column hoop from the ground is 100mm

Figure 1 GUI formwork reinforcement cylinder figure

the allowable anti sliding force of each fastener is 8kN, and the impact resistance of each side of each column hoop is fc=16kn. The maximum height of the column is 3.75m, and the maximum size is 500 × 500； The measured slump of the delivery pump is 150mm, which can be completed in less than one hour, that is, v=3.75m/h; Take the temperature of concrete pouring as 20 ℃, then to=200/(t+15): 200/35=5.71

take β 1=1.2 β 2=1.15

1. Stress calculation of the first column hoop at the bottom of the column formwork

take the small and medium value f1=80.57kn/m2, and the action range of the first column hoop at the bottom of the column formwork is 0.5 × The pressure within 0.35 is (80.57+4) × zero point five × 3.75=14.8knc, meeting the requirements

2. Stress calculation of the second column hoop at the bottom of the column formwork

f1=80.57kn/m f2=24 × 3.15=75.6kn/m2

take the small and medium value f2=75.6kn/m2, which acts on the lowest part of the column formwork and the action range of the second column hoop is 0.5 × 0.4=15.92knc, meeting the requirements

3. Stress calculation of column hoop in the middle of column formwork

f1=80.57kn/m2

f2=24 × 1.5=36kn/m2

take the small and medium value of the two f2=36kn/m2, and the action range of the column hoop in the middle of the column formwork is 0.5 × The pressure within 0.6 is (36+4) × zero point five × 0.6=12knc, meeting the requirements

to sum up, the construction control dimension of column hoop spacing of column formwork is: the spacing of column hoop at the lower part of column formwork is 400mm, the spacing of column hoop at the upper part of column formwork is 600mm, and the spacing of the first column hoop from the ground is 100mm

(II) design calculation of cast-in-place slab formwork (see Figure 2)

the spacing of steel pipe support of cast-in-place slab formwork is 0.8m, the spacing of two rows of support is 1.15m, and the maximum size of the slab is 7.6 × 9.54m (net size) is a calculation unit, and the thickness of cast-in-place slab is 300

then the load in the unit is borne by 8 rows (8.05/1.15=7) of columns, and each row of columns has 13 (9.54/0.8=12) fasteners

Figure 2 cast in situ slab formwork support cylinder figure

concrete weight is 7.6 × nine point five four × zero point three × 0.68=16.27kn

the formwork weight is 7.6 × nine point five four × 0.4=29.0kn

dead load: ngk= beam weight + formwork weight + reinforcement weight =400.25 kn

construction load: nqk=3 × seven point six × 9.54=217.5kn (the standard value of uniformly distributed live load for construction is 3kn/m2)

according to the formula n= 1.2ngk+ 0.85 × 1.4nqk gets n=737.3kn,

the pressure on each fastener is 737.3/(8 × 13) =7.09kngk= beam weight + formwork weight + reinforcement weight =61.61kn

construction load: nqk=3 × nine point five four × 0.30=8.59kn (the standard value of uniformly distributed live load for construction is 3kn/m2)

according to the formula n= 1.2ngk+ 0.85 × 1.4nqk n=84.15kn:

the weight is supported by 9.54/0.8+1 ≈ 13 pieces, that is, by 13 × 2 = 26 fasteners, then the pressure on each fastener is 84.15/26=3.24kn/fastener. At the same time, consider the following two aspects:

1 If the side formwork of the beam is used as the formwork support of the cast-in-place slab, the pressure on each fastener should be added to the pressure from the cast-in-place slab since 1998, which is 3.24+7.09>8kn/fastener, which does not meet the requirements of fastener bearing capacity

2. For example, as a row of supports of the cast-in-place slab formwork, the beam formwork support upright can bear the pressure of the slab, which can meet the requirements of the fastener bearing capacity. The pressure on the upright is f=3.24+7.09=10.33kn

1. Checking calculation of bearing capacity of foundation at the bottom of beam formwork support pole

the special bottom of the bottom pad of a single pole and the pad is 200mm wide and 50mm thick full-length wood board, then the stress area s=0.6 × 0.2=0.12m2

p=f/s=10.33kn/0.12m2=8.16kn/m2

that is, the requirement of the resistance of the plank at the bottom of the pole to the ground is 90 kn/m2. Reaching this data can make the pole not sink and ensure the quality of concrete. Therefore, the supporting site must be compacted, especially the bottom of the beam supporting pole

2. The stability checking calculation of beam formwork support upright and the bending strength checking calculation of beam formwork support cross bar have been described in "building safety" 6/2002, and will not be repeated here

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