Károly Jármai

Cost calculation and optimisation of welded steel structures

Abstract The cost optimisation is important in structural design. The welding times of various welding technologies are different. Using the costcomp program we can calculate the welding times. Adding other times, like flattening plates, surface preparation, cutting, electrode changing, deslagging, painting, etc. we can form a complete cost function. Times are usually general, but costs are different in various countries. Introducing the fabrication and material specific cost ratio of k f / k m , which varies between 0 and 2 kg/min, it is possible to build the cost function from the times and to work out optimisation in different economic conditions. The investigated structures are predominantly welded from plates. Examples are shown of applications for the design of welded box beams and stiffened plates. The fabrication cost percentages for welded box beam and stiffened plate are 29–35 and 46–71% of the total costs, respectively, thus they can have a significant effect on optimum dimensions. The discrete optima depend on the manufacturers, on the k f / k m ratio and the welding technologies.

Optimum Design of a Belt-Conveyor Bridge Constructed as a Welded Ring-Stiffened Cylindrical Shell

In the structural optimization of a ring-stiffened cylindrical shell the unknown variables are the shell thickness as well as the thickness and the number of flat rings. The shell diameter enables to realize a belt-conveyor structure inside of the shell. The uniformly distributed vertical load consists of dead and live load. The design constraints relate to the local shell buckling strength, to the panel ring buckling and to the deflection of the simply supported bridge. The cost function includes the material and fabrication costs. The fabrication cost function is formulated according to the fabrication sequence and includes also the cost of forming of shell elements into the cylindrical shape as well as the cost of cutting of the flat plate ring-stiffeners. Since the shell thickness does not depend on the number of ring-stiff-eners (n), the nopt is calculated for a selected region of n.

Optimum Design and Cost Comparison of a Welded Plate Stiffened on One Side and a Cellular Plate Both Loaded by Uniaxial Compression

Two types of stiffened plates are used in welded structures as follows: plates stiffened on one side and cellular ones, which consist of stiffeners welded between two deck plates. For a realistic cost comparison each type is optimised for minimum cost in the case of axial compression. Both plates are longitudinally stiffened by halved rolled I-section ribs. The deck plate thickness as well as the dimensions and number of stiffeners are sought, which minimise the cost function and fulfil the design and fabrication constraints. The cost function includes the material and fabrication costs. The design constraints relate to the overall and local plate buckling. It is shown that the cellular plate is cheaper than the plate stiffened on one side, since its large torsional stiffness enables us to use smaller plate thicknesses and smaller stiffener height.

Optimum design of steel structures

Experiences with the Optimum Design of Steel Structures.- Newer Mathematical Methods in Structural.- Cost Calculations.- Beams and Columns.- Tubular Trusses.- Frames.- Stiffened Plates.- Cylindrical and Conical Shells.

Optimum Design of a Welded Stringer-Stiffened Steel Cylindrical Shell Subject to Axial Compression and Bending

A column fixed at the bottom and free on the top is made of stringer-stiffened cylindrical shell and loaded by axial compression as well as by a horizontal force acting on the top. Halved rolled I-section stringers are welded outside of the shell by longitudinal fillet welds. The aim is to study the economy of stiffened shells. Therefore both the stiffened and the unstiffened version are optimised and their costs are compared to each other. The stiffening is economic when the shell thickness can be decreased in such a measure that the cost saving caused by this decreasing is higher than the additional cost of stiffening material and welding. The cost function to be minimized includes the costs of material, forming of shell elements into the cylindrical shape, assembly, welding and painting. The constraints relate to the shell buckling, stringer panel buckling and limitation of the horizontal displacement of the column top. The cost comparison shows that the cost of stiffened version is lower than that of the unstiffened one only in those cases, when the constraint on horizontal displacement is active.

Single- and multicriteria optimization as a tool of decision support system

Abstract An interactive decision support system was developed on an IBM PC in the fortran language by connecting six various types of single-criterion and seven various multicriteria optimization methods. Several mathematical test problems were used to determine the effectiveness and the applicability of the algorithms. In the test problems two objective functions, twelve inequality constraints, and two variables were used. Using the system it is possible to make 3D drawings of the objective functions with nets and central projection.

Economic Orthogonally Welded Stiffening of an Uniaxially Compressed Steel Plate

Stiffened plates and shells are the most characteristic structural types for optimization, since the number of stiffeners influences the cost significantly. A previous study has shown that a plate stiffened on one side with open section longitudinal ribs subject to uniaxial compression is not so economic than a cellular one. In the present article a plate orthogonally stiffened on one side is optimized. The orthogonal grid of ribs is more economic, since the transverse stiffeners increase significantly the overall buckling strength of the plate. Constraints on overall buckling and on stiffener induced failure are considered. The cost function includes material, welding and painting costs.

Optimum design and cost calculation of a simple frame with welded or bolted corner joints

A one-bay one-storey steel planar frame is constructed from rolled I-profile elements consisting of universal columns and a universal beam. The buckling strengths of the columns and beam according to Eurocode 3 (2002) constitute the design constraints. The beam and column profiles are optimised to minimise the structural mass subject to the design constraints for both a flush-end-plate bolted (semi-rigid) structure and a welded (rigid) structure. For the specific numerical case considered the structure is subject to a horizontal force and a uniformly distributed vertical load. Comparing the costs of these optimal solutions shows that the bolted frame is 7% or 14% cheaper than the welded frame on the base of British and South African cost data respectively.

Decision support system on IBM PC for design of economic steel structures applied to crane girders

Abstract A decision support system (DSS) was developed on a personal computer for the economic design of steel structures. The DSS contains six various-type single-criterion and seven multicriteria optimization methods. The economic design of the asymmetric main box girders of overhead travelling cranes was realized by considering four objective functions and sixteen nonlinear inequality constraints according to BS 2573 and BS 5400. Constraints on static and fatigue stress, on local buckling of flange and web plates, as well as on static deflection, are considered. Numerical computations show the effects of material, welding, surface preparation and total cost on each other. Use of higher strength steel may result in savings on the cost of materials, depending on the cost factor employed.

Cost Minimization of a Ring-Stiffened Conical Shell Loaded by External Pressure

A ring-stiffened slightly conical shell is optimized for external pressure according to the design rules of Det Norske Veritas. The whole length and the different two end radii are given. The shell is divided into n equidistant segments with one stiffener in each segment. Each segment has a different shell thickness determined using a buckling constraint, and each ring-stiffener of welded square box section is designed by means of the required moment of inertia. The optimal number of shell segments (nopt) is determined by means of costs calculated for a series of n. The cost function includes the material, fabrication and painting costs. The shell normal stresses are determined also by finite element method.