Lower estimate of the fundamental frequency of natural oscillations of a truss with an arbitrary number of panels

Abstract

Introduction: the paper deals with oscillations of a statically definable plane, truss with a double lattice of racks and descending braces with massive loads in the nodes of the lower chord. The weight of the truss rods is not taken into account. It is assumed that the freights are moved only vertically. The fundamental frequency of natural oscillations is estimated from the Dunkerley formula by the values of partial frequencies. Materials and methods: an analytical estimate is obtained by generalizing formulas obtained from a series of estimates for trusses with a consistently increasing number of panels. The stiffness of the truss was determined using the Mohr’s integral. The double lattice of the truss does not allow using the cross-section method; therefore, the forces in the rods were calculated (or estimated) in an analytical form using the method of cutting nodes with the compilation of a system of equilibrium equations simultaneously for all rods and three support reactions. The matrix of equilibrium equations was compiled in a software program written in the language of the Maple computer mathematics system based on the coordinates of the nodes and the values of the direction cosines of the forces. For a sequence of coefficients of the desired formula, linear homogeneous recurrent equations were found and solved by means of special operators of the Maple system. Results: the resulting formula estimating the relationship between the fundamental frequency and the panels number has the form of a sixth degree polynomial with coefficients depending on the parity of the number of panels. The analytical result is compared with the smallest frequency obtained numerically from the solution of the problem of oscillation of the cargo system. It is shown that the main frequency, depending on the truss height, has an extremum. Conclusions: the method of generalizing particular solutions using the Maple system operators allowed authors to obtain and analyze a formula for a lower estimate of the fundamental frequency of oscillation of a truss model with an arbitrary number of panels. The resulting estimate can be used as a test for numerically obtained solutions. The formula is especially efficient for systems with a large number of panels; as numerical methods for their calculation are time-consuming require considerable time and have a tendency for accumulating rounding errors.

Analytical evaluation of the fundamental frequency of natural vibrations of the spatial coverage

“…4), asymptotically approaching 40 %. Note that Dunkerley's estimate for planar trusses [25] behaves oppositely. With an increase in the number of panels, its accuracy increases.…”

Rigid Connection With Granite Chips in the Timber-Concrete Composite

“…Semi-rigid and rigid once are two types of timber to concrete connections in hybrid timber-concrete composite structural members, which are used now in the practice [1], [2]. Studies shows that the adhesive composite connection of the timber-concrete structural members is more effective in comparison with the semi-rigid one so as provides higher bending stiffness and leads to smaller deflections and a better structural members behaviour, including higher natural frequency, which is a very important characteristic of the floor structures [3] - [10]. Timber component in timber-concrete panels very often is presented by the cross-laminated timber (CLT).…”