Numerical investigation of die cuts in welded joints in steel structures

Thesis for Master of Civil Engineering - Structures

Abstract

In metal structures, one of the most important places of stress concentration is the joint. In form cutting, the shear force at the connection point is so high that a part of the beam or its reinforcement is broken and separated in pieces, and this is due to negligence in the design of the connection or failure to correctly predict the load. In this research, with the help of software and using theoretical methods, we intend to investigate the behavior of various beam-to-beam connections and its critical conditions in terms of deformation and tension. And in these researches, the behavior of die cuts in welded joints was followed and further, it was sought to provide a solution to strengthen the beam-to-beam connection without the risk of die cuts, and the best and most resistant type of reinforcement is suggested in this connection.

In this research, Catia advanced software (CTIA-R 19) was used to model connections and examine die cuts in welded joints, and finally, finite element analysis was performed in the ABAQUS software environment.

From the results of the research, it has been determined that some methods of strengthening a beam-to-beam connection in molded cuts in welded joints are very efficient and useful, while others are completely useless and unnecessary.

Keywords: molded cut, welded joints, tongued beams, finite element analysis

Chapter 1

Definition of the problem, necessity and objectives of the work

-1 Introduction

In metal structures, beam-to-beam connection is often seen. In many cases, it is necessary to strengthen the beam at the connection point at the discretion of the design engineers. Generally, the reinforcement of the beam is done at the connection point with a plate or with a corner of the beam, and the type of connection is either welding or bolts. The vertical force on the beam at the connection point causes a lot of shear stress, and in a special case, there is a possibility that all or part of the beam will break. Because we know that there is a state of brittleness and residual stress in the welding area, and also near the screw holes, the cross-section is weakened due to the smaller area. Therefore, the vertical force may easily cause the growth of cracks in the weak section and form cutting occurs.

The figure below shows a beam whose upper wing is cut and its core is pierced for bolt and nut connection. The location of the crack growth and rupture of the web is specified in figure [1].

(images are available in the main file)

2 Definition of the problem

The term die cutting in the beam web at the connection point quickly turns the mind to the plastic behavior of the material exposed to severe stresses. Therefore, mathematical relationships may not be compatible with experimental results and software analysis due to their complexity. However, by comparing theoretical and practical information as well as software analysis, useful findings can be obtained, which is expected to lead to better design of beam-to-beam connection.

In this research, the regulations are first examined, and then mathematical relationships and proposed models are examined. These relations are naturally based on practical experiments, the sources of each of which are mentioned. After the preliminary familiarization with the behavior of the structure at the joint, the main task, which is the analysis of reinforced beams with the help of finite element analysis[1], will be carried out, which will lead to more attention to the problem of die cut and beam breaks and to find solutions to prevent it. Obviously, the more advanced the model is assumed, the more complex the mathematical relationships and the more accurate the results, but it is better to consider some simplifications for a better understanding of the subject. In general, the investigations are based on whether the beam is being strengthened or weakened, because the excessive use of screws or welding at the joint may turn the conditions completely upside down and expose the section of the beam to die cutting and the phenomenon of die cutting which is the subject of this research will increase in these joints. or it is done in John, and the type of reinforcement in John is more sensitive because there is a possibility of mold cutting. Usually, cyclic fatigue forces such as earthquakes also initially target weak connections, and this doubles the importance of research in the field of connection and its strengthening.If the corner connection or the reinforcing plate of the beam is made with bolts and nuts, it should be noted that the number of holes and their diameter should not weaken the cross section of the beam. If it is the same type of welded connection, it should be noted that the location and position of each welding line does not damage the strength of the girder.

The goal is to provide a solution to strengthen the girder-to-girder connection without the risk of mold cutting, so that the vertical forces and bending moments in the support are neutralized and serious damage is not caused to the weak girder section. If the reinforced connection does not lose its resistance to external loads in the most critical conditions and remains in the elastic range, it can be assured that the phenomenon of mold cutting will not occur. Perhaps by comparing the analytical results, it is possible to find the best type of reinforcement and simulate its behavior in the case that the beam is subjected to die cut, so that the design of the connection can benefit from the results of this analysis.

Die cut is mainly for screw connections, but the articles presented during several recent experimental investigations have shown that die cut can also occur in welded joints. The fracture mechanics of die cutting in welded joints is different from that of screw joints, so the need to check die cuts in welded joints is justifiable. With the help of finite element analysis, we intend to find the place of stress concentration in the beam-to-beam connection, where yielding is likely to start and the phenomenon of die cut occurs.

1-4 Thesis structure

In this research, the chapters are compiled as follows:

The first chapter is dedicated to generalities and includes the introduction, working methods, necessity and goals of doing the work.

The second chapter is first a definition of Mold cut and basic definitions and regulatory criteria are given, then the works done and the proposed criteria for connection based on mold cut are discussed and finally a conclusion of the theoretical issues is stated.

In the third chapter, the modeling and analysis of several reinforced connection samples is discussed.

The fourth chapter is related to the analysis and review of sample connections, in which 5 types of connections are from analysis number (1), two connections are from analysis number (2) and three connections are from analysis number (3) and 5 connections are given from the analysis number (4) and finally an analysis is given about the effect of the weld line on the formwork cut. Finally, the fifth chapter deals with the conclusion, summary discussion and suggestions. Chapter Two: Formwork cut (theory, relationships and regulatory points) 1 Introduction: Due to tensile or shear force, most of the beams are split [1]. Those connected with bolts and nuts are subject to mold cutting.  In order to connect two beams, a part of the upper and lower wing may be cut so that the beams are closer to each other and the two beams can be connected perpendicularly. This means that removing part of the wing of the beam increases the probability of the occurrence of die cutting in the beam web [2]. 2-2 Definition of die cutting [2] and its causes

The phenomenon of die cutting occurs when a part of the beam or the member involved in the connection is separated from the main body due to severe shear loads. Usually, two factors cause mold cutting.

1- Breaking due to tension and yielding in cutting

2- Breaking in cutting and yielding in tension

The first case is more common. The second case rarely happens because the ductility of steel is less in tension than when cutting occurs. Molded cutting is more common in bolt and nut connections because the holes of the bolts weaken the cross-section of the beam and the beam is prone to breaking apart due to cutting. The time of die cutting does not depend much on the ultimate load limit because the elasticity of the material is not the same in tensile and shear loading and the ultimate rupture limit usually occurs earlier in tension than in shear. Mold shearing may occur in beams with cut wings, ED BEAMs, notched beams (the tension is earlier than the yield strength of the material in tensile and shear loading is not the same, and fracture occurs due to cutting. And gusset plates [3] or wind braces.

Analysis and testing for gusset plates have been done much more than for beams with cut wings. Mold cutting is mostly arranged. And the geometrical conditions of the connection depend on the type of loading and the thickness and height of the beam, the diameter and the number of screws.