Discussion on the stress calculation of the wind r

2022-08-12
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Discussion on the stress calculation of the cable wind rope before and after the safety enclosure of the steel derrick

discussion on the stress calculation of the cable wind rope before and after the safety enclosure of the steel derrick

○ Liu yinpei, he Shaozhong (Guangdong six construction Group Co., Ltd.)

aluminum is the key basic material for the construction and development of the national economy. Because of its low cost, convenient assembly and disassembly and other advantages, the steel well frame has become the main equipment for the vertical transportation of materials in the construction site and has been widely used. However, according to the statistics of relevant departments, steel derricks and gantry frames account for 50% of the major accidents that kill more than 3 people in the national construction industry every year. One of the main reasons for the accident is the improper selection and use of the wind rope to stabilize the frame

in recent years, with the higher and higher requirements of civilized and safe construction on construction sites, the phenomenon of safety enclosure on three sides of steel derrick frame to prevent materials from rolling off is becoming more and more common. So, what is the stress of the cable wind rope before and after the safety enclosure of the steel derrick frame? Next, we give a general example to illustrate this problem

example: due to construction needs, a steel derrick with a height of 30.6m is to be set up at a construction site. The steel derrick body is provided with two groups of wind ropes and diagonally pulled, as shown in Figure 1

Figure 1 stress diagram of steel derrick

surface D of steel derrick frame is the feeding and discharging working surface, and three sides a, B and C are surrounded for safety. The main technical parameters are as follows:

main leg angle steel of frame ∟ 75 × seventy-five × 7

frame transverse batten ∟ 60 × sixty × 5

diagonal batten of frame ∟ 50 × fifty × 5

frame standard pitch ∟ 1.8m

cable wind rope diameter 13mm, 6x (37)

rocker arm length 8000mm

rocker arm lifting weight

lifting pulley block and lifting appliance weight q=150kg ≈ 1500N

rocker arm dead weight g1=300kg ≈ 3000n

as shown in the above figure, the cable wind rope of steel derrick mainly bears three forces: tension caused by cable wind rope dead weight and cable wind rope.Tension caused by wind load Tension of cable when lifting with rocker arm suspender

1. Tension generated by the dead weight of the cable wind rope

the tension generated by the dead weight of the cable wind rope is calculated according to the following formula:

where, - tension generated by the dead weight of the cable wind rope, n

q - self gravity of the cable wind rope, when the rope diameter is 13mm, q=6.2n/m

- length of the cable wind rope= H/cos α, Where h is the height of derrick 30.6m, α Is the included angle of 45 ° between the cable wind rope and the shaft

frame, then,

f - the sag of the cable wind rope, which is generally controlled at about f=/300. After calculation:

‹ the tension generated by the self weight of the top cable wind rope:

the tension generated by the self weight of the middle cable wind rope:

2 The tension of the cable under wind load is shown in Figure 2. If the wind blows from the diagonal direction of the steel derrick, only one cable bears the wind load, and the other three are not. Therefore, at this time, the wind rope bearing the load is the most stressed. The calculation is as follows:

Figure 2 wind blows along the diagonal of the steel derrick

2.1 calculation of wind load

wind load is calculated according to the following formula:

where: - wind load borne by the steel derrick

- basic wind pressure, which is generally taken as 250n/m2, and 350n/M2 should be taken in coastal areas (this value is too small, which is taken according to the local wind pressure - editor)

- wind load shape coefficient, which is calculated according to the following formula: =1.3 (1+ η)×ψ Where:

- wind proof coefficient of truss, which is the projected area of the frame and the wind-induced contour area of the frame

The projection area of the

steel derrick frame is divided into two parts: one is the projection area of the

frame and the other is the safety projection area of the frame. The calculation is as follows:

the dense mesh safety commonly hung on the steel derrick in Guangdong Province, according to the actual calculation, its projected area per square meter is about 0.43m2

=2[2.1 × 30.6-(1.6 × zero point zero seven five × 2+0.2 × zero point one seven × 2+2.7 × zero point zero five × 2) × 17] × 0.34cos45 ° =26.16m2

=+=16.01+26.16=42.17m2

the wind contour area of the steel derrick body is

=2.1 ×× 30.6=86.5m2

η— The effect coefficient, according to the length width ratio b/h of the frame section, is obtained from table 1-11 of the concise building structure design manual. Here b/h=2.1/2.1=1, check it η= 0.33

- single limb member coefficient when the wind direction blows from the diagonal direction, =1.1

- wind vibration coefficient, which is calculated by the following formula:

where: - pulsation increase coefficient, check table 1-12 of the manual, get =2.1

- pulsation influence coefficient, check table 1-13 of the manual, get =0.79

- Vibration mode coefficient, check table 1-16 of the manual, get =0.73

- wind pressure height change coefficient, According to table 1-10 in the manual, =1.11

‹ wind load

‹ average wind load in the height direction of steel derrick

q=w/h=65631/30.6=2144n/m

2.2 calculation of cable wind rope tension

under the action of wind load, considering that the upper and lower cable wind ropes at the top and middle of the derrick are used, the whole derrick can be approximately calculated as a two span continuous beam, As shown in Figure 3:

horizontal component of the cable at the top of the steel derrick

Figure 3 calculation diagram of the derrick under wind load

tension of the cable at the top

horizontal component of the cable at the middle of the steel derrick

3 Tension of cable wind rope when lifting with rocker arm suspender

when lifting, only one cable wind rope at the top is considered to work. The tension of the cable wind rope is calculated according to the following formula:

, where: K - the dynamic load coefficient of the lifting load, see the table in the manual of calculation of common structures in construction, k=1.2,

- the weight of the lifted materials or components (n), taken =10000n

Q - the weight of the lifting pulley block and lifting appliances (n), taken q=1500n

- the dead weight of the rocker arm (n), taken =3000n

H - the height of the steel derrick, H=30.6m

s - horizontal distance from rocker arm boom to steel derrick body, taking s=6.93m

4 During this period, the company worked with the national composite center of the United Kingdom, Tencent advanced composite materials and Rolls Royce to calculate the wind rope resultant force

the resultant force of the wind rope on the top of the steel derrick

the resultant force of the wind rope in the middle of the steel derrick

5 Calculation of cable wind rope tension when the steel derrick is not hung safely

when the steel derrick is not hung safely, the projected area of the frame is the sum of the projected areas of all components, that is,

truss wind-proof coefficient

also found in the table η= 0.66

wind load

average wind load in the height direction of the frame

‹ tension of the top cable:

tension of the middle cable:

‹ when it is not hung safely, the stress resultant of the top cable

stress resultant of the middle cable

6 The force difference of the wind rope before and after the safety hanging is

7 Analysis of calculation results

from the above calculation, some advanced control systems developed by domestic enterprises at present provide the possibility for intelligent production of high molecular materials. We know that if the steel derrick frame is not set with dense mesh safety, the cable wind rope on the top of the frame is under the maximum force of 26085n. According to the specification, if the safety factor of the cable is 3.5, the allowable breaking force of the cable and its anchor is 3.5 × 26085=91297.5N。 According to the mechanical design manual, the most commonly used specification on the construction site is 6 × 37. The allowable breaking force of 13 round strand steel wire rope (nominal tensile strength is 1400MPa) is 93100n, and 93100n > 91297.5n, which meets the requirements for safe use. When the three sides of the steel derrick are densely covered with safety, the middle cable wind rope bears the maximum force of 64258n. Similarly, according to the specification requirements, the cable wind rope understands from the manufacturer the model and scope of application of the pressure testing machine and the allowable breaking force of its anchor should be greater than 3.5 × 64258=224903n, if the steel wire rope with the above specifications and properties is still used at this time, and the allowable breaking force of the steel wire rope is

93100n < 224903n, it does not meet the requirements for safe use

it can be seen that the steel derrick body is densely covered with safety on three sides, which increases the tension of 38625n to the cable wind rope in the middle of the steel derrick body. If you do not pay attention to the existence of this force and the selected cable wind rope is inappropriate, the steel derrick body will be in danger of overturning, which does not meet the requirements for safe use. In order to make the steel derrick body meet the requirements of safe use, on the one hand, we should not use dense safety to close the frame body, on the other hand, we must select the appropriate cable wind rope according to the stress calculation of the cable wind rope

from the above calculation, we can also know that even if there is no safety hanging, the force of each wind rope may reach more than 20KN (2 tons) at some time. Therefore, in the construction management, we must carefully check the anchor point of the cable wind rope, and absolutely do not allow any falling off and relaxation. Only in this way can the stability and safety of the steel derrick frame be guaranteed

(quoted from building safety 2002.1)

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