In this article, we will talk about the basic principles of classification of welded metal structures, what they are, where they are used and their features.
Metal structures are used everywhere: industrial and civil buildings, production facilities and equipment, parts of transport routes, etc. There are different ways to connect metal structures, they are distinguished:
- Rivet connections;
- bolted connections;
- welded joints;
Welding is becoming a common solution when choosing a method for connecting metal elements, since a welded structure has a number of advantages:
- Relative ease of manufacture;
- High speed of work;
- Large selection of materials and equipment for welding;
- Ability to create structures of complex configuration;
- Creation of an equally strong hermetic connection;
- Connection of metal meshes and reinforcement cages.
Let's talk in more detail about welded metal structures.
Classification
Classification of welded metal structures is divided into:
According to the materials of the workpieces from which they are made:
- Pipe
- Leafy
- Profile
- Rod
According to the material from which the structure is made:
Metal:
- Made of black steel;
- Made from low, medium and high alloy steel;
- From non-ferrous metals;
- From alloys;
Non-metallic:
- Polyethylene;
- Polypropylene;
- Composite;
According to the type of industrial facility on which they are mounted:
- Oil;
- Gas;
- Thermal power;
- Nuclear power structures;
- Ship;
- Aviation;
- lifting structures.
Types of welded metal structures
Metal structures are something without which it is very difficult to imagine the most modern construction industry. Along with reinforced concrete, they are the most common solution for industrial, public and civil buildings and structures, mechanical engineering and other industries. The most common types (groups) of building metal welded structures are steel:
- Columns.
- Beams.
- Connections
- Shells.
- Cases.
- Rod.
- Lattice.
- Farms.
- Welded supports (for pipelines or equipment).
- Leafy.
Let's talk in more detail about some welded structures, their features during manufacturing and installation. Their production is carried out at enterprises that manufacture metal structures and directly at the construction and repair site.
Metal welded trusses
Welded steel trusses are a flat lattice system of metal elements that allows you to span large spans of buildings and structures. A truss is a structural element of a building that serves to support the roofing system (purlins, decking, roofing pie) and transfer the load from it to columns or walls.
Metal trusses are often used in the construction of industrial buildings, since the technological process often does not allow the installation of intermediate racks. Also, steel trusses are most widely used in the construction of social and administrative buildings. Such as: shopping centers, cinemas, stadiums, swimming pools and many others.
It is such buildings that are characterized by the presence of large free internal space. The metal welded truss consists of:
- Knots of the upper belt.
- Raskosov.
- Stoeck.
Braces, in turn, are divided into supporting and ordinary. Support braces are located at the point where the truss is supported. They carry heavy loads, so their cross-section is often larger than that of ordinary braces (or the grade of steel is higher). Farms can be homogeneous or combined. Because they can combine elements made from different grades of steel.
It is combined metal structures that are an effective solution; their use is most rational in cases where the loads on the elements are significantly different. The braces, as well as the truss posts, are connected to the lower as well as the upper chord directly or using gussets. Their elements can be made from:
- Square pipe.
- Rectangular pipe.
- Round pipe.
- Corner.
- Various profiles.
Steel trusses are the most common solution. Trusses are developed for a specific construction project or selected according to a series, having performed the appropriate calculations.
Truss drawings are developed in the KM and KMD section, where basic instructions for the production and installation of structures are also prescribed.
How to weld metal trusses
Welding is carried out either in the factory or at construction and repair sites. In the factory, production of critical structures, such as trusses for covering public and industrial buildings, as well as objects.
As a rule, large-span metal trusses are divided into shipping marks (KM section), which are delivered to the construction and repair site. There they carry out enlarged assembly of shipping stamps, as well as installation according to the developed technology. The length of shipping stamps usually does not exceed 12 meters; this is necessary for the safe transportation of the shipping stamp by road or otherwise.
Less critical ones, such as trusses for entrance canopies and awnings, are made at the construction site. You can make a simple farm with your own hands, for example for a garage, or for a greenhouse. On the website mrmetall.ru you will find useful information on the choice of steel, selection of electrodes, as well as all the necessary equipment.
Defects and sewing defects
The most common defect in a beginner’s work is a crooked seam with uneven filling. This picture is the result of uneven guidance of the electrode; it literally dances in the hands of the young master. Here you will need perseverance and work: with experience, all this passes without a trace. The second most common mistake is the incorrect choice of current strength or arc length, which leaves “undercuts” or uneven filling. With some defects, aesthetics suffer more, with others - strength.
Lack of penetration - insufficient filling of the joint of parts with metal. It needs to be corrected, since we are talking about the strength of the connection.
In what cases does lack of penetration appear:
- Poor quality processing (or lack thereof) of surface edges;
- The current is too weak;
- Electrode movement too fast.
An undercut is an unnecessary groove along a seam. The diagnosis is simple: choosing an arc that is too long. The treatment is also clear: either a shorter arc, or a higher current intensity.
A burn-through is a banal hole in a seam for the following reasons:
- Wide gap between edges;
- The current is too high;
- Low electrode speed
And here we are looking for the optimal ratio of three components: current, gap width, electrode movement.
Pores and nodules are multiple small holes. This is a critical defect that affects the strength of the connection.
- Dirt and rust on metal;
- Oxygen reaching the molten metal (in a draft);
- Poor quality edge processing;
- Low quality electrodes;
- Use of filler wires;
Cracks are serious violations of the integrity of the seams. They appear after the metal has cooled and are essentially harbingers of the destruction of the seam itself. In this case, only new welding or complete removal of the old seam and re-applying a new one will save.
Sequences of operations when welding a metal truss
As when welding any other product, before starting work it is necessary to familiarize yourself with the regulatory documentation for manufacturing and the developed technology in full. With drawings of the KM, KMD section, technological maps, as well as prepare the necessary materials, tools, and organize the workplace.
The connection of individual nodes (made from pipes or profiles) leads from the middle node elements to the supporting ones. First, the existing butt joints of the parts are welded, then they move on to T-joints and corner joints. Next, seams with greater thickness are welded. Assembly is carried out using tacks 25-40 mm long. Seams located next to each other must be made after a technological pause.
Fulfillment of this condition is necessary to reduce deformations. Only after the metal has completely cooled down, where a closely spaced seam will be applied. The end of each longitudinal connection (when connecting braces, racks or other units with a gusset) must be brought out to the end of the welded unit by an amount of 2 cm.
Compare, evaluate
Of the above options, butt welding is considered the most reliable and economical. In terms of current loads, they are almost equal to whole elements that were not welded, in other words, to the base material. Naturally, such strength is achieved only with adequate quality of work.
At the same time, it must be remembered that the reliability and efficiency of the method does not mean that it is easy to implement. Requirements for edge processing, adjustment of many factors to the conditions of a specific welding, certain restrictions in application due to the shape - all this requires strict professional discipline.
T-joints (including corner ones) are also quite popular. They are especially often used when welding massive structures.
The simplest ones to perform are overlapping joints. They do not require edge processing, and general preparation is also much simpler. Very popular in welding sheets of small thickness (thickness up to 60 mm is allowed). Simplicity does not mean efficiency: excessive consumption of deposited and base metals is a common situation for such options.
Metal sheet structures.
Metal sheet welded structures are most often used in industry, these include: reservoirs, tanks, bunkers, gas tanks, decks, etc. Sheet metal structures are characterized by a combination of load-bearing and enclosing functions.
Automatic, semi-automatic, electroslag welding is often used for sheets. The parts are longer and are subject to more stringent requirements for joint strength and tightness. You can see the sequence of seams in the image.
We cook pipelines, special requirements
Only experienced, certified and highly qualified craftsmen are allowed to work with industrial pipelines.
Pipe connections belong to the vertical method with all the “vertical” nuances. The peculiarity lies in the angle at which the electrode is held, this is an angle of 45 degrees. The width of the pipe seam can reach 4 cm, it depends on the thickness of the pipe itself. There are separate standards for this type of welding, for example, GOST 16037-80 describes the dimensions of seams for various connections of pipeline structures.
Welding methods
Another classification is based on the welding method. There are many welding methods, differing in performance conditions. In production (factory), as a rule, they use:
- Semi-automatic welding in CO2 environment.
- Automatic electric welding in a shielding gas environment (for electric current welding of thick metal, submerged arc welding).
- In installation work, use: Manual arc welding. Automatic and semi-automatic (rare); Manual welding in argon with a non-consumable electrode (mainly used for aluminum and stainless steel).
What types of welded joints exist
A seam formed as a result of crystallization or plastic deformation (pressure welding) of the material of the joined elements. It represents a space at the junction. Welds can be divided into:
- Factory ones.
- Assembly.
They have different designations on the drawings, so carefully study the design documentation before starting work on the construction of this welding structure. Regardless of whether the seam is factory or assembly, it must meet the requirements and ensure equal strength of the connection. There are many types of welded joints, we suggest you visually familiarize yourself with the most used and common:
- Butt joint.
- Gusset.
- T-joint.
- Lap connection
Tolerances are indicated in regulatory documentation, namely in GOST R 13920-2017.
Type of connections os (bp) os (sp) ds (bz) ds (zk)
Welded joints are divided into the following types:
welded connections made on one side (one-sided welding) - os (ss) and on both sides (double-sided welding) - ds (bs);
welded joints made on a removable or remaining lining, backing ring - sp (mb) and without a lining (in weight) - bp (nb);
welded joints made with stripping the weld root - zk (gg), without stripping the weld root - bz (ng);
welded joints made with gas protection of the weld root (gas injection) - gz (gb);
Types of connections os, ds
How to make welds correctly
To perform a weld correctly, firstly, you need to familiarize yourself with the regulatory welding and design documentation. The necessary design documentation is presented in the form of a basic set of drawings of the KM brand, and KMD (metal structures, metal detailing structures). In the album of the KM section you will receive general information about the metal welded structures of the construction site (or part of it):
- General form;
- composition of structures (how many beams, columns, trusses, braces, etc. parts);
- main connection points;
- instructions for performing work;
- information about loads on structures;
- specification of rolled metal.
In the KMD section you will find:
- detailed drawings of each metal element;
- detailed components for connecting elements;
- updated specification of rolled metal products.
Read the notes and instructions for performing work on the drawings. It may contain important points, without which the manufactured metal structure will not meet the requirements applied to it.
Assembly and welding of elements must be carried out only if there is a documented technology for welding work for this structure.
Documentation is carried out in the form of technological maps and (or) a production project (PPSD) for welding work. They reflect in detail all questions about the process and materials.
Auxiliary signs
Drawings often contain special auxiliary symbols that help briefly indicate the required information. There are not so many of them and all of them, along with the decoding of the meaning, are given below:
| № | Image of the auxiliary sign | Information about this sign | Where should the given auxiliary symbol be located on the leader line? | |
| Top of the line | Bottom of the line | |||
| 1 | Seam reinforcement needs to be removed | |||
| 2 | Existing irregularities and sagging must be processed to create a smooth transition from the weld bead to the base metal | |||
| 3 | The seam must be made during installation of the product, that is, during installation everything is done at the place of use | |||
| 4 | Creating a spot or interrupted seam with a chain seam arrangement. The tilt angle should be 60 degrees | |||
| 5 | Create a spot or interrupted seam with a staggered seam pattern. | |||
| 6 | Creating a seam along a closed line. The diameter of such a sign in the drawing is from 3 to 5 mm. | |||
| 7 | Creating a seam along an open line. This sign is used if the location of the welded joint is clear from the drawing. | |||
Welding sequence
We'll tell you in more detail how to make a seam correctly. To do this, it is important to carry out work in stages:
Stage 1: Familiarization with technical documentation for welding.
Stage 2: Preparation of the workplace, equipment and materials (you can find out which materials to choose for welding on our website mrmetall.ru).
Stage 3: Carrying out the welding work itself.
Stage 4: Non-destructive testing of the resulting seam.
The choice of method depends on many conditions and various factors. The main ones are: the thickness of the elements, the length of the seam made, the type of welding and seam used, the responsibility of the welded structure, etc.
Non-standard cases
How is a weld seam indicated on the drawing if its dimensions do not fit within the GOST standards? In such a situation, the dimensions of all its elements should be included in the design documentation.
Then the performer, in the process of directly solving the problem, will be able to achieve the desired quality of the result, even taking into account the effect of residual stresses. The latter appear due to shortening deformation (which occurs due to uneven heating of surfaces) and are distributed depending on the selected mode, geometry and other factors. The thickness of the workpieces also cannot be discounted, because it can provoke plane or volumetric stresses.
Welding technology for metal structures
For seams of varying lengths, it is necessary to use special welding methods.
Short seams with a length of >250 mm are made “overall”. This is when the welder lights an arc and leads it from one edge of the product to the other without stopping during the welding process.
Seams with a length of 1000 mm are called medium. It is correct to make such seams from the center of the product and move to the edges. The best way to carry out 2 welders in parallel. With this approach, two welders work towards the edge of the seam, trying to do it at the same speed.
Long seams are those whose length exceeds 1000 mm. Several different methods are used to join such long seams. The first is the reverse step method. When using it, the seam is divided into sections, which are welded by welding 150-200 mm of the seam. Stopping and performing the next seam so that its end comes to the beginning of the previous one. Welding goes in one direction, and the location of the new section goes in the opposite direction. This is easy to understand by looking at the picture below.
Similarly, using the reverse-step method, welding can be done from the middle of the product to the edges. Two welders perform work on their half of the product. This method can be used to brew either a thin product in one pass or a thick one in several passes. Each welder's area is divided into smaller areas.
The process, as in the previous case, goes in one direction, and the application of new sutures in the other.
Another way is randomly. The seam is divided into equal sections, 5-6 sections. Next, cook the area in the center. Also applying short-length seams in the opposite direction from the direction of welding. Next, the most distant areas along the edges are welded and the process of the middle sections of the seam is completed. Each of these methods will significantly reduce structural deformation after welding.
GOST butt weld legs
In order for metal structures to withstand the loads placed on them during operation, all seam joints present on them must comply with standard indicators.
The main document regulating the dimensions of welded joints, as well as the types and characteristics of structural elements of metal products is GOST.
It is clearly stated here that if it is necessary to join parts of different thicknesses, they can be joined in the same way as workpieces of equal thickness. Only in this case the difference between the two indicators should not exceed standard values.
One of the GOST clauses stipulates the permissible displacements of the edges relative to each other and establishes clear parameters for the displacements in accordance with the thickness of the workpiece. The appendix to the document describes all possible minimum sizes of legs, which must be taken into account when welding metal structures.
