At present, the widely used high-strength bolts connecting the standard section of the top-rotating tower crane and the horizontal boom trolley mainly bear tension. In this case, there is a separation trend between the contact surfaces of joints, and the horizontal force between standard joints is borne by the friction force generated by the pre-tightening force of bolts on the joint surfaces. Theoretically speaking, bolts can only bear tension, but not resist shear. If the bolt is not tightened, it is easy to loosen. At this time, the bolt is subjected to both tension and shear, alternately impacting, and the stress deteriorates, which is easy to cause stress fracture. This is the most common failure form of high-strength bolts and one of the common accidents of tower cranes.
Article 5.5.2.2 of Code for Design of Tower Crane (GB/T 13753-92) (hereinafter referred to as the code) requires: "The joint design shall ensure that there is a certain compressive force between the joint surfaces under the action of external tension, and there shall be no separation phenomenon." This can be understood as: under the maximum external force, the compressive force between the bonding surfaces is greater than zero, and the friction force generated by the compressive force should be able to resist the horizontal force between the standard bonding surfaces, that is, meet the requirements.
The formula for calculating the pre-tightening force of high-strength bolt F 1 given in the specification is:
F 1 = k 1 FN
Where: f1-pretension of high-strength bolt, n
K1-coefficient, which is related to load combination, k1=1.1~1.65.
FN —— external tension on a single bolt, n
Take QTZ3 15 tower crane (main parameters: maximum lifting capacity of 30kn；; ; The working range is 3 ~ 41.8m; ; Independent lifting height is 30m；; Take the balance weight of 60 kN) as an example. Under the worst working conditions (sudden unloading in working state or sudden drop of spreader), the maximum bending moment borne by the tower is Mmax = 352 kN·m, which is calculated by the section size of the tower:
f′N = = = 47 1.2 kN
FN=== 1 17.8 kN
The load combination is considered in the calculation of Mmax, and 1 17.8 kN can be used as the pre-tightening force required for the tensile force of high-strength bolts, that is, f1=17.8kn. ..
Under the combined load, the friction force of the joint is generated by the residual pre-tightening force f ′ 0 of the bolt.
f′0 = f 1-( 1-KC)FN
Where: f' 0-residual pre-tightening force of bolt, n
F1-bolt pre-tightening force, n
KC- coefficient, generally 0.25.
Fn-external tension of bolt, n
Therefore: f ′ 0 = f1-0.75fn has f1> 0.75fn = 0.75×117.8 = 88.35kn;
When considering the appropriate safety margin,
The national standard GB38 1 1 takes f ′ 0 = f1-1.4fn and f1> 1.4 FN = 1.4× 1 17.8 = 164.9 kN；
The European standard FEM takes f ′ 0 = f1-fn and f1>; FN = 1 17.8 kN；
The national standard GBJ 17-88 takes f ′ 0 = f1-1.25fn and f1> 1.25 FN = 1.25× 1 17.8 = 147.25 kN；
It is pointed out in the specification that GB/T 13752- 1992 adopts the values in GBJ 17-88, and the pre-tightening force of M24 high-strength bolt with performance grade of 8.8 is 155 kN.
Then press the following formula to calculate the pre-tightening torque t:
T = 0.2 F 1 d
Where: f 1- preload
D- diameter of high strength bolt, where d = 24 mm.
The following data can be obtained separately:
The pre-tightening torque value of M24 high-strength bolt of grade 8.8 given in the specification is: theoretical pre-tightening torque 710n m; The actual pre-tightening torque is 640 n m.
Comparing the above data, it can be concluded that it is wrong and unsafe to discount the value given by the standard in the actual pre-tightening process. At present, there is a view and practice that "the actual pre-tightening torque of high-strength bolts of tower crane can be reduced to 65% ~ 80% of the given value in the code", which still has a certain market. After the example mentioned in the preface appeared, we have been paying attention to the present situation of tower high-strength bolt pre-tightening. We have made several oral surveys at the technical exchange meeting of tower cranes at home and in the province. In China, only a few well-known enterprises, such as Zhonglian and Beijian Engineering, are basically controlled near the given value of the specification, and almost all small and medium-sized tower crane enterprises discount the given value of the specification. They usually divide the actual pre-tightening torque of high-strength bolts of different tower cranes given in the specification by 1.4 as the actual control in construction. Considering that there are many factors that affect the control of pretension value in construction, such as tools, personnel quality, physical strength, environmental conditions and different tower crane models, the interval coefficient of "65% ~ 80%" is given. This view and practice can be said to have a long history, because it is difficult to predict the given value by traditional manual methods, and the control result of 65% ~ 80% of the given value did not directly lead to tower crane accidents. Over time, people think this view and practice is correct. Indeed, we have consulted several accident analysis papers, appraisal reports and books on crane accident analysis, and have never seen such a record of "directly causing an accident". "Direct accidents" are usually considered as accidents in which high-strength bolts break in tension and shear, resulting in machine failure and death, but we believe that some accidents in which the tower collapses due to buckling are indirectly caused by insufficient pre-tightening force of high-strength bolts. If the pre-tightening force is not enough, the bolt will loosen easily. Even if there is a slight gap in the contact surface of the standard section of the tower, the center of gravity of the tower crane will move forward, and the bending moment of the tower will increase greatly, which may lead to accidents. As the saying goes, a little difference is a thousand miles away. The independent lifting height of QTZ3 15 tower crane is 30 m. When there is a gap of 0. 1 mm in the contact surface of the third standard section at the lower center of the high-strength bolt, the upper part of the tower moves forward by 3.2 mm, and the maximum bending moment of the tower increases by1.13kn m under the worst working conditions.