Research On Local Stability Of The Lattice Boom For Cranes

Mobile cranes,as a class of lifting equipment,are widely used in ports,utilities,railways,petrochemical industries and other field,which play an important role in the national economic construction.Lattice boom is the main boom form of wheel crane and crawler cranes,whose carrying capacity decides the lifting capacity of the cranes directly,and local stability of the chord is a main mechanical problem of the lattice boom.At present,in actual engineering,calculation about the local stability of the lattice boom structure is mainly calculated with the overall stability formula of the axial compression member in the specification.Lifting capacity obtained with the calculation method is rather low,and the competitiveness of cranes is limited seriously.Therefore,the local stability of the lattice boom is an urgent problem to be solved in engineering.Calculation formula of effective length factor was proposed by analyzing the chord's buckling load of the cross type lattice boom,behavior of buckling was obtained.The simplified mechanical model and stability equation was established with classical beam-column theory.Buckling loads and modes of the mechanical model and the actual model with buckling eigenvalue are compared to verify the correctness of the simplified mechanical model.By comparing with buckling load of simply supported beam at both ends,influence of braces is very obvious and should not be ignored.The stability equation was further simplified by analyzing the influence of braces in two planes and omitting the minor braces,calculation formula of effective length factor was proposed.Meanwhile,the relation between effective length factor,ratio of chord and brace' moment of inertia,angle of chord and brace was discussed.A theoretical basis was provided for design of the cross type lattice boom.Since braces in the two planes of the point-to-point type lattice boom have similar effect on the chord,and the buckling plane is uncertain,so the mechanical model and stability equation was established with classical beam-column theory in the same way.Buckling loads and modes of the mechanical model and the actual model with buckling eigenvalue are compared to verify the correctness of the simplified mechanical model.By comparing with buckling load of simply supported beam at both ends and buckling load considerging braces in a single plane,influence of braces in both two planes was verified very obvious and buckling load of point-to-point type lattice boom is higher than those of considerging braces in a single plane.In spite of this,calculation formula of effective length factor for point-to-point type lattice boom follows the method of the cross type lattice boom from safety and convenience.Stability factor curve of high strength steel tubes was proposed by the axial compression stability test of high strength steel tubes.The axial compression stability test of total 39 steel tube specimens of 20Mn2 and S890 were carried out by using knife edge device.Comparing with the carrying capacity calculated according to GB50017 and Eurocode 3,it was found that the current specifications underestimate the carrying capacity of high strength steel tubes.In order to facilitate the engineering application of the test results,new factor were obtained by curve fitting in accordance with the stability factor formula of GB50017,and stability factor curve of high strength steel tubes was proposed.In order to comprehensively reflect the influence of various structural forms,loading mode,layout density,specifications of braces on the carrying capacity of lattice boom,the 6m intermediate section of the fixed jib of QUY220 crawler crane and the 6m minor intermediate section of fixed jib of QAY300 all terrain crane were selected as the prototype,and 16 lattice boom specimens in 7 groups were determined.Then,the carrying capacity of lattice boom specimens was calculated using the specification and the method in this paper(calculation formula of effective length factor and stability factor curve of high strength steel tubes).The carrying capacity of lattice boom specimens was analyzed with geometric and material nonlinear method(GMNIA)under different initial imperfection factor,which provides the data basis for the test of the lattice boom specimens.Using the lattice boom loading test platform,the loading test of 16 lattice boom specimens in 7 groups were carried out,and failure loads and failure modes of each specimen were obtained.Comparing with the carrying capacity calculated by specification,the method in this paper and GMNIA show that the specification seriously underestimates the carrying capacity of the lattice boom,and carrying capacity calculated with the method in this paper is more in line with the actual carrying capacity.It is also verified that the initial imperfection factor of 2 ‰ is in accordance with the actual initial imperfection level of the lattice boom.According to the initial imperfection factor,the nonlinear finite element method is also effective for calculating the ultimate load of the lattice boom.The effective length factor and the fitting stability curve in this paper provided a convenient,accurate,and reliable calculation basis of the local stability of the lattice boom.According to the effective length factor calculation method,size limits of chords and braces in the related CIDECT literatures on the lattice structure,optimization and matching method about the chords and braces is proposed.Optimization and matching program about the chords and braces was developed independently.The matching results of two examples show that the method not only provides a way for the design of chords and braces in lattice boom,but also can be used to optimize the structure and achieve light weight of the boom under the premise of satisfying the carrying capacity.