Safety precautions when constructing monolithic foundations. Safety requirements for laying underground communications
Safety measures must ensure the safe conduct of work in the specific conditions of the construction site. They are developed in accordance with .
Concrete and reinforced concrete works
Formwork used for the construction of monolithic reinforced concrete structures must be manufactured and used in accordance with the work plan approved in the prescribed manner.
When installing formwork elements in several tiers, each subsequent tier should be installed only after the lower tier has been secured.
The placement of equipment and materials on the formwork that are not provided for in the work plan, as well as the presence of people not directly involved in the work on the formwork flooring, is not permitted.
Dismantling of the formwork should be carried out (after the concrete reaches the specified strength) with the permission of the work manufacturer, and of especially critical structures (according to the list established by the project) - with the permission of the chief engineer.
The preparation and processing of reinforcement must be carried out in specially designated and appropriately equipped places.
When performing work on the preparation of reinforcement, you must:
- - fence off areas intended for unwinding coils (coils) and straightening the reinforcement;
- - when cutting reinforcement bars with machines into sections less than 0.3 m long, use devices that prevent them from scattering;
- - put the prepared reinforcement in specially designated places;
- - cover the end parts of the reinforcement bars with shields in places of common passages having a width of less than 1 m.
Elements of reinforcement frames must be packaged taking into account the conditions for their lifting, storage and transportation to the installation site.
When preparing a concrete mixture using chemical additives, it is necessary to take measures to prevent skin burns and damage to the eyes of workers.
Installation, dismantling and repair of concrete pipelines, as well as removal of retained concrete (plugs) from them is allowed only after the pressure has been reduced to atmospheric pressure.
During cleaning (testing, blowing) of concrete pipelines with compressed air, workers not directly involved in these operations must be removed from the concrete pipeline at a distance of at least 10 m.
Every day, before starting to lay concrete in the formwork, it is necessary to check the condition of the formwork and scaffolding. Detected malfunctions should be corrected immediately.
When compacting a concrete mixture with electric vibrators, it is not allowed to move the vibrator by the current-carrying hoses, and during breaks in work and when moving from one place to another, the electric vibrators must be turned off.
When the concrete pump is running
Persons who know the rules of operation of this concrete pump and have completed practical work on this concrete pump under the guidance of a qualified operator for a month and have a driver's license are allowed to work on a concrete pump.
The driver’s knowledge of the operating instructions for this type of concrete pump, occupational safety and health requirements and the requirements of this “Instruction” is tested during exams by a qualification commission. Persons who successfully pass exams and undergo practical training receive a (driver) certificate for the right to operate this concrete pump. Examinations must be carried out annually before using the concrete pump truck. It is prohibited to operate a concrete pump without the specified license.
Persons at least 18 years of age who have passed a special medical examination are allowed to operate a concrete pump. A medical examination must be carried out at least once a year.
The factory operating instructions for the concrete pump must always be in the cab of the concrete pump.
It is prohibited to work on a faulty concrete pump or concrete mixer truck.
In the absence of an engineer supervising the work, it is prohibited to perform concrete work using a concrete pump, as well as other work that does not correspond to the purpose and technical characteristics of the machine (for example, pumping solution).
The operator is prohibited from moving more than 2 m away from the concrete pump controls when the pump is running, without having a remote control with him. Do not leave the remote control unattended.
If the driver is not feeling well, operating the concrete pump is prohibited.
It is prohibited to operate the concrete pump without an external inspection.
Members of the team allocated by the construction organization to work with a set of machines must undergo course training and instructions on safe methods of performing auxiliary work. Workers servicing a set of machines must have a certificate for the right to work with a concrete pump.
The team allocated by the construction organization to work with a concrete pump, performing plumbing and installation work, as well as work on accepting and laying concrete mixture into structures and other work related to the operation of concrete pumps, is obliged to comply with the current safety and labor protection regulations.
Pumping concrete mixture is only possible with a concrete pump installed and leveled using outriggers.
Drivers and machine workers must work in special clothing, safety helmets and goggles.
Open parking lots for concrete pumps are arranged at construction sites and equipped in accordance with the recommendations.
When supplying concrete using a concrete pump, the operating positions of the distribution boom must be observed. Outside these areas, work on the distribution boom is prohibited.
Operation of a concrete pump in the security zone of an existing power line with a voltage of more than 42 V should be carried out under the direct supervision of a person responsible for the safety of work, with written permission from the organization that owns the line and a work permit for work in places where hazardous or harmful factors are present, issued to the immediate supervisor work, and a permit issued in the appropriate manner for work near an overhead power line, issued to the driver. When installing a concrete pump in the security zone of an overhead power line, it is necessary to remove the voltage from it.
Before flushing the concrete pipeline, unauthorized persons must be removed from the working area defined in the PPR at a distance of at least 10 m.
Safety requirements when carrying out piling work
The site where piling work is carried out must be fenced in accordance with GOST 23407-78.
Piling work is carried out only under the guidance of a foreman or mechanic. When lifting a pile driver, the danger zone is determined by a radius equal to the length of the structure being lifted plus 5 m. Before starting work, check the serviceability of the mechanisms and working tools, the condition of the rigging, the operation of the winch brakes, sound and signaling devices. It is allowed to drag piles to the pile driver only with a rope through the outlet block at the base of the pile driver and only in a straight line. When lifting a pile, it must be kept from swinging and torsion using braces. Only the foreman or foreman has the right to give the command to pull and lift the pile, as well as the hammer. The weight of the pile being lifted should not exceed the lifting capacity of the winch.
It is prohibited to climb the piledriver mast without a safety belt, since installation and dismantling work is considered steeplejack.
The cutting of piles at a level of more than 1 m from the ground surface is carried out from a scaffold. Only the feller should be in the area of possible scattering of concrete fragments (5 - 6 m).
The foundations are constructed in pre-dug and prepared pits and trenches, which can be made with slopes or vertical walls, both with and without fastenings.
In modern construction, foundations are made of monolithic concrete and reinforced concrete, prefabricated concrete and reinforced concrete blocks, brick or rubble stone.
Labor safety when constructing monolithic foundations is ensured by the following labor protection solutions:
· determination of mechanization means for preparing, transporting, supplying and laying concrete;
· determination of the load-bearing capacity and development of a formwork design, as well as the sequence of its installation and disassembly order;
· development of measures and a list of means to ensure workplace safety at heights;
· development of measures and a list of concrete care products in the cold and warm seasons.
· When constructing prefabricated concrete and reinforced concrete foundations, labor safety issues are ensured by the following solutions:
· determination of the crane brand, installation location and hazardous areas during its operation;
· ensuring the safety of workplaces at height and passages to them;
· determination of the sequence of installation of structures.
· The safety of masonry work must be ensured by implementing the following labor protection decisions:
· organization of workplaces indicating the location of installation of the necessary scaffolding equipment, load-handling devices, containerization equipment and packaging;
· sequence of work, taking into account ensuring the stability of erected structures;
Organization and technology of performing work on the installation of prefabricated strip foundations
Instructions for the installation of prefabricated strip foundations
Requirements for the quality of work performed
Material and technical resources necessary for
Installation of prefabricated strip foundations
Environmental protection and safety regulations when installing prefabricated strip foundations
General provisions and main elements of prefabricated strip foundations
There are shallow and buried strip foundations.
Prefabricated strip foundations consist of prefabricated foundation pads, reinforced according to calculations, above which walls are built.
The recessed strip foundation structure is installed in buildings with heavy walls or floors on heaving soil. This type is arranged if the project includes a basement. The laying depth should be 200-300 mm below the freezing depth. Recessed strip foundations require large amounts of material. For walls located inside the building, shallower ones of 400-600 mm are arranged.
Recessed strip foundations, in comparison with shallow ones, are stronger and more stable due to their lower part, which is located below the freezing depth of groundwater. But in this case, material consumption and labor costs increase sharply.
Scheme of a reinforced concrete prefabricated strip foundation.
On dry or sandy soil, strip foundations are laid above the freezing level, but not less than 500-600 mm from the ground level.
It is rarely laid on heavily heaving and freezing soil.
Prefabricated ones are often used not only in industrial and civil construction, but also in the construction of cottages and individual residential buildings.
The positive side of the construction of this foundation is a sharp reduction in the time of construction work; it becomes possible to increase the load on the structure after it has been kept for a short time at the final stage of installation. Do not forget that the construction of this type of foundation will be more expensive than a monolithic one and will require the help of lifting equipment.
The negative side is the strength indicator, since the prefabricated one is 20-30% worse than the monolithic one. Prefabricated types cannot be strengthened with additional reinforcement, because the blocks are produced according to a standard design. Strengthening such foundations can be achieved using a mesh laid between rows of blocks, but even this method does not give the desired result, as is done with the spatial reinforcement of monolithic foundations.
Reinforced concrete foundation slabs and concrete wall blocks are unified; the nomenclature provides for their division into four groups, each of which differs in the perceived load. To increase the rigidity of the structure, to level out settlements during construction on soft soils and as anti-seismic measures, prefabricated foundations are reinforced with reinforced seams or reinforced concrete belts placed on top of foundation pads or the last row of wall foundation blocks along the entire perimeter of the building at the same level.
For sandy soils, foundation blocks are laid directly on a leveled base, for other soils - on a sand cushion 10 cm thick. Bulk or loose soil cannot be left under the base of the foundations; it must be removed and sand or crushed stone filled in instead. Depressions in the soil foundation more than 10 cm high are filled with monolithic concrete. The width and length of the sand base are made 20...30 cm larger than the dimensions of the foundation so that the blocks do not hang from the sand cushion.
The foundation blocks are laid according to their layout in accordance with the design (Fig. 1) in order to provide breaks for laying water supply, sewerage and other inputs.
Fig.1. Installation of prefabricated strip foundations:
1 - foundation cushion; 2 - wall block; 3 - sand preparation; 4 - reinforcing belt; 5 - bed made of mortar; 6 - sealing the joint with monolithic concrete; 7 - slinging the block
Installation begins with the installation of lighthouse blocks in the corners and at the intersections of walls. The foundation block is transported by crane to the installation site, pointed and lowered onto the base; minor deviations from the design position are eliminated by moving the block with a mounting crowbar while the slings are tensioned. In this case, the surface of the base should not be disturbed. The slings are removed after the block takes the correct position in plan and height. During the installation process, the gaps between the strip foundation blocks and the side cavities are filled with sand or sandy soil and compacted.
When installing foundations for columns, carefully control the position of the installed blocks relative to the main axes. Using levels, the position of the blocks in height is controlled; for glass-type blocks, the mark of the bottom of the glass is checked; for others, the upper plane of the block is checked.
The installation of basement walls (wall blocks) begins after checking the position of the laid foundation blocks (pillows) and the waterproofing device. If there are no special instructions in the project, then a layer of mortar 2...3 cm thick is spread over the cleaned surface of the foundations as insulation; the solution simultaneously serves as a leveling layer.
In accordance with the installation diagram, the position of the wall blocks of the first (bottom row) is marked on the foundations, noting the locations of the vertical seams. Installation begins with the installation of lighthouse blocks in corners and places where walls intersect at a distance of 20...30 m from each other. After installing the lighthouse blocks at the level of their top, a cord is pulled - a mooring, along which ordinary blocks are installed.
Before starting the construction of foundations, all workers involved in installation must undergo special safety training. Knowledge of safety rules by workers and engineers must be checked at least once a year.
Basic safety provisions must be reflected in the project for organizing work on the construction of the facility. All personnel involved in installation work should be familiarized with these provisions. To do this, before starting work, you need to hang up posters indicating safe installation techniques and warning signs; mark storage locations for items. Areas dangerous to the movement of people and machinery must be fenced off or equipped with warning signals.
Installation work at night is allowed only under good artificial lighting. It is necessary to illuminate not only the installation sites of elements, but also on-site warehouses, as well as areas of movement of structures.
It is prohibited to move prefabricated elements above workplaces.
The blocks should be slinged only using the mounting loops embedded in the blocks, and they should be lifted with special traverses or slings, the strength and reliability of which should be periodically checked.
Before lifting the block, the worker must make sure that it is slinged correctly, after which he should lift the block to a height of no more than 30 cm and make sure that it is securely fastened. The block should be raised and lowered smoothly, without jerking or swinging, strictly vertically. During lifting and lowering, the crane cable must not be twisted. To avoid this, you should hold the block in a certain position using guy ropes. Do not pull or push elements while lifting or lowering them.
If the need arises, then pulling up can be allowed when the boom or crane and cable are stationary in the case when the block is located at a distance of no more than 50 cm vertically from the installation site. While lifting the block and delivering it to the place of laying, no people should be in the area of its movement.
Before installing the block, it must be lowered above the installation site approximately 0.5 m from the ground surface, after which the block is centered and installed in its working position. Removal of hooks from the hinges of the block is permitted after complete alignment and installation of the element in its place.
Leaving raised blocks during a break in work is not allowed. When moving a lifted element horizontally, it must pass at a height of at least 1 m from the top of the tallest object in its path.
Particular attention should be paid to the reliability of the crane installation. Tower cranes are allowed to work after inspection of their tracks. You cannot start work if the crane tracks deviate from their normal position. During the period of soil thawing, crane tracks are checked 2 times a day.
Self-propelled cranes installed on the edge of the pit must be located at a distance from the edge of the slope that ensures its stability. In winter, workplaces, passages, ladders, etc. must be cleared of snow, ice, debris and sprinkled with sand. It is not allowed to lift elements that are frozen to the ground or to each other.
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9.7. Labor protection during installation work
Installation work is the most dangerous of the entire complex of construction and installation work, as it is associated with the movement and installation of heavy structural elements and usually at high altitudes.
At the construction site, the technological installation area must be marked with signs, i.e. the work area, storage areas, pre-assembly and transportation of elements from the ground to the installation site. Particular attention should be paid to the high-risk area - the operation of several installation mechanisms in adjacent installation areas, at the same or different levels of vertical work.
Workers are allowed to install and carry out auxiliary work on unloading, storing and slinging prefabricated elements only after induction training. Installers of at least 4th category, over 18 years of age and with at least two years of work experience are allowed to carry out steeplejack work. To obtain a permit, you must complete a safety training course and pass the necessary tests. Knowledge is checked at least once a year, medical examination is carried out at least twice a year.
Lifting devices, slings and other equipment must be equipped with tags indicating the load capacity. They are tested for double load at least twice a year, and based on the results of the examination, special passports are issued.
When working at height, installers must wear mounting belts and, using a chain with a fastening device, hook themselves to the loops of mounted structures or to tensioned and secured cables. Working tools should be kept in drawers or bags to prevent falls. When lifting elements, to prevent them from swinging or twisting, they must be held on guy wires. Raised elements must not be left suspended during breaks in work. Lifting of any loads is permitted only with the assembly crane's pulley in a vertical position, i.e., without tightening the element being lifted. The load to be lifted must be less than or equal to the lifting capacity of the erection crane at a given boom radius; the corresponding table of the relationship between reach and load capacity should be posted at the driver’s workplace.
Passages and passages are arranged at the construction site, and signs of dangerous and prohibited areas are fixed in visible places. At night, the construction site must be illuminated. Installation with tower cranes is prohibited when the wind speed is 10..L2 m/s; the crane on rails is secured with anti-theft devices; At higher wind speeds, the crane is held on guy wires.
After each repair, lifting devices must be tested to a load 1.25 times their normal load capacity with a holding time of 10 minutes. The results of inspections of load-handling devices are recorded in the logbook. Inspections are carried out: for traverses every 6 months; for slings and containers - every 10 days; for other seizures - in a month.
It is not allowed to perform installation and post-installation work on the same section, but at different horizons. In some cases, an exception is made, but the gap in levels should not be less than three floors.
The boundary of the danger zone is determined by the horizontal distance from the possible place where the load may fall when it is moved by a crane. This distance at a maximum lifting height of up to 20 m must be at least 7 m, at a height up to 100 m - at least 10 m, at a higher height its size is established in the work design.
Installed interfloor floors and coverings must be fenced before the next work begins. This requirement is not met when installing large-panel and large-block buildings, but installers working on the last installed floor are required to attach safety belts to reliable structural elements of the building
Special precautions should be taken when weather conditions change. It is not allowed to carry out installation work at height in open areas with a wind speed of 15 m/s or more, during icy conditions, thunderstorms and fog. Work on moving and installing large-sized wall panels and similar structures with large windage should be stopped when the wind speed is 10 m/s or more.
During installation, great attention should be paid to safe welding techniques to prevent electric shock and fire hazards. It is prohibited to carry out welding work in the rain, during a thunderstorm, heavy snowfall and wind speed of more than 5 m/s.
To lift and lower workers when installing buildings higher than 30 m, it is necessary to install lifts or elevators.
9.5. Technology of installation of building structures in extreme conditions
9.5.1. Features of installation technology in winter conditions
Carrying out installation work in winter conditions is difficult. The cost of work increases and, depending on the temperature zone, the increase ranges from 1.2 to 6% of the total construction cost. Prefabricated reinforced concrete structures are installed in winter using the same methods as in summer. Instructions and recommendations are given in projects, especially in technological maps and work execution plans (PPR), to carry out additional measures to ensure the successful completion of work and the stability of structures erected at subzero temperatures. The grades and composition of mortar and concrete, which are necessary for the installation of prefabricated structures, are also indicated in the projects.
The winter period has less influence on the technology of installation of metal structures than reinforced concrete ones. Basically, the installation of metal structures in winter is carried out using the same machines, devices and methods as in the summer. The main specific feature of the arrangement of joints is the imposition of restrictions on welding work - welding cannot be carried out at temperatures below - 30°C.
Labor productivity during installation work decreases in winter. Correction factors depending on the outside temperature are:
Prefabricated reinforced concrete elements are supplied for installation cleared of snow, ice and dirt. During transportation and in storage they are protected from rain and snow. To a greater extent, this is necessary for parts and structures made of lightweight concrete, open areas of insulating layers of panels, and joining surfaces of elements of prefabricated structures. This is due to the fact that saturation of lightweight concrete or insulation with water worsens the thermal properties of enclosing structures.
If necessary, ice is removed not only with scrapers and brushes, but also by heating the icy areas until traces of ice completely disappear. Gas and other burners are used for heating if the prefabricated elements do not have liners made of combustible materials. It is prohibited to use salt, hot water or steam to remove ice, but it is allowed to use hot air from electric blowers.
It is necessary to take measures to prevent freezing of concrete at the joint until it reaches the specified strength.
In winter conditions it is necessary:
■ warm the joining surfaces to a positive temperature of + 5...8°C;
■ lay the concrete mixture into the structure heated to +30...40°C;
■ maintain or heat the laid mixture at a positive temperature until the concrete gains at least 70% of its design strength.
When installing structures installed on mortar without salt additives, its temperature at the time of installation should be, as for winter masonry, within the following limits:
It is recommended to use equipment adapted for winter work that protects the mortar and concrete mixture from cooling quickly. The solution is spread on the bed immediately before installing the elements in order to obtain a good compression of the solution in the seam. The thickness of the installation joints is strictly controlled, since their increase reduces the strength of the structure and creates the danger of uneven settlement of structures when the mortar thaws in the spring and their deformation.
To work at subzero temperatures, installers use non-slip shoes; they must clear inventory scaffolds, stepladders and platforms from snow and ice. Installation work during icy conditions or heavy snowfall is not allowed. At the installation site, all passages are cleared of snow, ice and sprinkled with sand. One of the most important measures taken with the onset of negative temperatures is to protect the base of foundations from freezing. The presence of frozen soil under foundation pads, especially clayey and wet soil, causes heaving and possible damage to structures. The base and installed foundations are insulated with soil and slag. In basements and technical undergrounds of buildings, all openings and openings in ceilings, basement panels and other places are closed.
The planned sequence of work is disrupted due to downtime of installation, primarily tower cranes, they are stopped at a wind speed of 10... 12 m/s.
For high-quality sealing of joints and seams in conditions of negative temperatures, special auxiliary measures are provided.
The technology for embedding joints is determined in accordance with the instructions of the work project. The concrete mixture (mortar) for grouting is prepared using thawed and heated aggregates and heated water. The temperature of the mixture without additives at the time of exit from the mixer must be such that its temperature at the time of laying is not lower than +15°C. When antifreeze additives are introduced into the concrete mixture, the temperature at the moment of exit from the mixer should be:
■ for mixtures with the addition of chloride salts and potash at least +5°C;
■ for mixtures with the addition of calcium nitrite and urea +10°C;
■ with the addition of sodium nitrite, as for mixtures without antifreeze additives +15°C.
The concrete mixture must be transported in insulated bins, boxes or vehicles with equipment for heating with exhaust gases. When stored on site, the concrete mixture is protected from wind and precipitation. It is forbidden to place frozen or frozen mixture into the joint cavity, or to add hot water to it.
Joints are sealed using one of the following three methods: unheated - concrete with antifreeze additives, heated - conventional concrete with heat treatment, combined - concrete with antifreeze additives followed by heat treatment.
In addition, the choice of joint sealing method is significantly influenced by specific weather conditions during the work.
The joints of precast reinforced concrete elements are sealed taking into account the load they will bear. Joints that do not have design forces are sealed with a solution of a grade of at least 50 or with concrete, which can be prepared with the addition of potash or other antifreeze additives specified in the PPR. The method of insulating joints, the mode, timing and procedure for curing concrete or mortar are also indicated in the PPR.
Joints that absorb design forces are sealed with mortar or concrete of the composition specified in the project (their class is not lower than the class of structures), with preliminary heating of the joint with hot air and subsequent curing of the concrete using a thermos method or artificial heating (most often electric heating). If permitted by the design, the joints are sealed with a concrete mixture (mortar) with anti-frost additives.
When cementing joints with a concrete mixture without anti-frost additives, it is necessary to pre-heat the mating elements of the joint and warm up the concrete until it acquires the required strength. The strength of concrete prepared with Portland cement, depending on the temperature and heating time, can be approximately determined using special graphs - dependencies.
To preheat grouted joints, blowers are used to pump hot air into the joint cavity. After heating, the inventory formwork is secured on the side of the joint where the blower was, and the joint cavity is immediately filled with heated concrete mixture. Next, the mixture is artificially heated.
Joints, the concrete of which does not withstand the design forces, at an outside air temperature of up to -15 ° C can be monolid with a concrete mixture only with anti-frost additives, since such a mixture hardens even at sub-zero temperatures; Moreover, after laying in the joint, the mixture does not need to be heated; in the event of a sharp drop in the outside air temperature, it is enough to install insulated formwork.
Most often, heating is carried out with electric current, less often with steam. For electrical heating, electrodes, tubular electric heaters, thermoactive and heating formwork are used.
9.5.2. Non-heating method of making joints
The use of mortars and concrete with anti-frost additives is a non-heating method for making joints.
Solutions of calcium chloride salts, table salt (sodium chloride), sodium nitrite, potash, etc. are recommended as antifreeze additives. The use of antifreeze chemical additives of chloride salts when sealing joints with metal embedded parts and fittings is prohibited. Potash and sodium nitrite are not recommended for embedded parts made of aluminum and its alloys, parts with a protective coating of zinc or aluminum. The amount of antifreeze additives taken is the same as when working with monolithic concrete in winter conditions.
To increase the plasticity and water resistance of concrete at the joint, sulfite-alcohol stillage is added to the concrete mixture with anti-frost additives in an amount of up to 0.15% by weight of cement. If it is necessary to obtain high strength embedding in a short time (within a day), concrete prepared with antifreeze additives can be subjected to artificial heating.
9.5.3. Heating methods for making joints
Often, the concrete mixture is heated at the joint of prefabricated elements after installing the inventory formwork and filling the joint with heated concrete mixture. Sewn-on electrodes can also be attached to the inside of the formwork.
Conductive heating is based on the use of heating formwork (Fig. 9.43). Heating formwork is usually used to preheat the joint of structures and warm up the laid concrete. It is installed in the design position and connected to the network for 2...8 hours to heat the joined elements to a temperature of 15...20°C. Then the joint cavity is concreted, after which the cemented joint continues to be heated.
Rice. 9.43. Scheme of contact heating of monolithic structures:
1 - prefabricated reinforced concrete structure; 2 - heating element; 3-heating formwork
To embed vertical joints of columns, universal heating formwork with automatic control of the heat treatment mode is used. It consists of a metal case, heating cassettes, power supplies and control units. The formwork body is used for laying concrete in a joint and is made of two parts, fastened together with bolts. These elements are interchangeable and each has a loading window. Heating cassettes are flat metal heat-insulating boxes with built-in autonomous electric heaters in the form of nichrome spirals, heating wires and low-temperature heating elements, usually with a power of 0.5 kW at a voltage of 220 V. The operating temperature of the heater surface is 600...700 ° C. There is an air gap between the heating element and the wall adjacent to the concrete, and behind the heater there is a reflector made of tinplate, which leads to the combined action of convective and infrared heating. Heating cassettes in various combinations provide heat treatment of the joint of any column section. A set of heating cassettes is inserted along the guides of the metal formwork; the cassettes cover the joint on four sides.
Installation of the heating formwork at the joint of the column is done manually; heating cassettes are fixed to the formwork, which are connected to the network before concreting the joint. After 2 hours of heating the joint cavity, the cassettes are turned off for laying concrete. Subsequent heat treatment is heating to 50°C and isothermal heating at this temperature until the required concrete strength is obtained. The temperature at the joint is controlled with a thermometer, which is inserted into the hole provided in the formwork and cassette.
It is advisable to warm up and warm up the joints of multi-tiered columns, beams and crossbars using thermoactive formwork. Nichrome wire is placed inside the electrical insulating material into the cavity of the double formwork, consisting of inner and outer steel sheets, with the insulated wires being brought out beyond the dimensions of the formwork for connection to the electrical network. The formwork is placed on the area to be joined and held in place with special clamps. The concrete mixture is loaded into the joint through a funnel built into the formwork.
Warming up with infrared heaters (Fig. 9.44) or their main components, tubular electric heaters (TEHs), is widely used for many types of joints, both directly and as heating elements of thermal shields. The infrared method of heat treatment of embedded concrete is based on the use of infrared radiation energy supplied to the open formwork surfaces of heated joints of structures and converted into thermal energy on these surfaces.
Rice. 9.44. Scheme of infrared heating of monolithic structures:
I - prefabricated structure; 2 - tripod with infrared heater in the reflector
Since the depth of penetration of infrared rays into concrete does not exceed 2 mm, radiant energy is converted into thermal energy in thin surface layers of concrete, while the rest of the structure slowly warms up due to heat transfer from these layers and cement exothermy. For these reasons, when grouting joints, the infrared method is recommended to be used for pre-heating the joint zone of prefabricated reinforced concrete structures and accelerating the hardening of concrete or embedding mortar.
A tubular electric heater (TEH) is a hollow metal tube into which a spiral of nichrome wire is pressed; the filler is fused magnesium oxide or quartz sand. The filler acts as electrical insulation. The joint is heated with a heating element placed in an anodized reflector, or the heating zone is covered with a tarpaulin.
The induction method (Fig. 9.45) of heat treatment of embedment concrete is based on the use of the magnetic component of an alternating electromagnetic field to heat the reinforcement due to the thermal effect of an electric current induced by electromagnetic induction. With induction heating, the energy of an alternating electromagnetic field is converted into thermal energy in reinforcement or steel formwork and transferred through thermal conductivity to concrete.
Rice. 9.45. Scheme of induction heating of the joint of prefabricated columns:
1 - prefabricated structures; 2 - fittings outlets; 3 - induction winding; 4 - inventory formwork; 5 - layer of thermal insulation; b - contact terminals of the electrical network; 7 - supply wires
The use of induction heating for joints of frame structures saturated with reinforcement makes it possible to easily and quickly, without additional heat sources, warm up reinforcement, rigid frames, metal formwork, and previously laid concrete that needs to be heated. When using induction heating, the following order of work is adopted: installation and insulation of the formwork, installation of an inductor (winding of conductive wires onto the formwork), heating of the reinforcement and previously laid concrete, laying a new portion of the concrete mixture into the structure, heating the structure according to the accepted regime, controlled cooling.
The combined method involves a combination of heating and anti-frost additives, which makes it possible to guarantee the required strength of joints and seams in a shorter time. The method is the heat treatment of concrete containing an antifreeze additive (sodium nitrite), which ensures that the required mobility of the mixture is maintained during the period of its placement in the joint cavity before the start of heat treatment.
The combined method should be used in cases where the outside air temperature is below -25°C, with strong winds of more than 10 m/s, as well as for joints with a high surface modulus. Calculation of electric heating elements when heating a mixture with the addition of sodium nitrite in joints using external heat sources (contact heaters, infrared emitters) and determination of the specific power with the electrode heating method is carried out as for a concrete mixture without the additive.
9.5.4. Sealing joints and seams
Sealing joints and seams at subzero outside temperatures has certain limitations. Sealing of joints between elements of enclosing structures with mastics is carried out at temperatures not lower than -20°C and in compliance with the following requirements. Before sealing, the surfaces of joints and seams are cleaned of mortar, dirt, snow and ice. Before applying sealing mastics, the surfaces of the seams are dried and primed.
When carrying out work, it is necessary to control the quality of surface preparation for sealing, the dosage of components and the temperature of the mastic, the thickness of the layer and the applied strip of sealant, the tightness of the contact of mastics to the joining surfaces and the quality of gluing of sealants to them. For better adhesion (connection) to concrete, polyisobutylene mastic should be preheated to a temperature of 100...120°C.
Otherwise, the process of sealing joints in winter conditions proceeds in the same way as in summer conditions.
9.5.5. Features of installation in hot climates
High ambient temperature conditions impose some restrictions on installation work. To maintain relatively high labor productivity of workers, it is recommended to take a long break from work during the daytime, the hottest time of the day. Breaks from work during the remaining time, with shelter from direct exposure to sunlight, can be arranged more often and for a longer period.
The labor intensity and duration of care for the laid concrete and mortar in the construction of joints increases to protect them from dehydration. In addition, all joints must be abundantly moistened with water before they are sealed.
9.5.6. Features of installation of structures during reconstruction of buildings
Replacement of existing structures precedes or accompanies the installation of new structures. Replacement of structures can be carried out using a separate method, when on a certain area or the building as a whole, all the structures being replaced are first dismantled, and then new ones are installed in their place. Different options for work are possible - one crane first dismantles the old ones, then installs new structures, or two or more cranes are involved, the work of which is organized in a continuous manner. It is important to provide a guarantee against significant overloads of adjacent adjacent elements and the overall stability of the building.
Combined method provides for sequential dismantling and installation of structures in a single flow, with a single set of construction machines. The scope of work with this organization of work is reduced to the size of one or several cells while maintaining the strength, rigidity and stability of adjacent structures. Dismantling of structures can be carried out element by element or in enlarged blocks, depending on the design solution of the dismantled structures and the technological capabilities of the means used for dismantling.
Replacement of structures coverings can be carried out by various self-propelled and tower cranes, depending on the structural design of the building, its space-planning solution and the rationale for the chosen option of the mechanization used. In some cases, when replacing lightweight covering elements, process pipelines and other equipment located between the belts of trusses, you can use a converted truck crane moving along the roof on special riding beams.
In the case of increasing the height of a reconstructed one-story building, it may be rational to initially erect a new covering over the existing one until all work is completely completed, and then dismantle the old covering using winches, overhead cranes and appropriate rigging equipment. In this case, installation and dismantling of structures can be carried out during short-term shutdowns or, without disturbing the production process, in a reconstructed building.
When dismantling roofing elements, measures must be taken to protect against falling down dismantling materials and fire of individual roofing elements during fire cutting of load-bearing structures. If, when removing an individual element, a statically stable equilibrium may be disrupted, it is necessary to strengthen, brace or suspend structures that are dangerous from the point of view of collapse with slings to the crane hook.
Replacement of crane beams. When using crane equipment of appropriate lifting capacity, the replacement process is carried out using traditional methods. If the crane’s lifting capacity is not enough for the required boom reach, and the mass of the beam does not exceed the maximum lifting capacity of the crane, then it is necessary to pre-brace the crane boom with fastening the braces to stable elements of the structure. If it is impossible to use cranes, work is carried out using winches with the use of retaining guys.
Replacement of columns. Replacement without disassembling the covering requires preliminary hanging of the covering structures, i.e. transferring the load from the columns to other auxiliary elements. Hanging can be done by installing temporary supports under the rafter structures. The support points of metal structures on temporary racks must be reinforced. The gap between the temporary posts and the supporting units of the rafter structure (8... 10 mm) is provided with jacks. A steel plate of the required thickness is inserted into the resulting gap and secured against possible displacement. When forces are transferred from the coating to the temporary posts, a gap should appear between them and the column, indicating that the column is completely unloaded from the influence of the structures located above. If the structures are not torn off, then additional jacking of the structures is carried out over temporary supports and the resulting gaps are filled with steel spacers. The clearance during the jacking cycle should not exceed 10 mm.
In some cases, it is difficult or impossible to install support posts directly under the supporting roof structure. In this case, two racks are installed as close to the truss as possible, and a steel beam is placed on them, to which the load from the truss will be transferred.
When dismantling a column, it is initially disconnected from the foundation (by cutting, felling, crushing, removing nuts, etc.). The dismantling itself can be performed by rotating around the hinge using a pulley and a pulling winch. The method is based on slowly lowering the head of the column while resting its heel on the foundation. It is possible to use three winches, during the interconnected operation of which the heel of the column slides from the column towards one of the winches, while others ensure lowering of the column head in the sliding plane.
Method of sliding onto old supports. The method of replacing individual structures as a whole involves moving (shifting from the foundation) the old one and sliding a new structure into its place, which can significantly reduce the shutdown period for the enterprise. Two options for movement are possible: pulling - using winches and a pulley system and pushing - using electric or hydraulic jacks. The advantage of the pulling method is the continuity of movement of the object being moved; the second method has the simplicity and compactness of the devices used, which is especially important in cramped conditions for the reconstruction of the object.
The movement is carried out along multi-line rail tracks, on a reinforced concrete base with laid steel plates and cylindrical steel rollers with a diameter of 100...ISO mm.
9.6. Quality control of construction installations
The quality of installation of structures is checked using geodetic instruments and templates using previously applied axial and other marks and marks. Geodetic control of the accuracy of installation of prefabricated elements in the design position consists of a stage-by-stage (by type of mounted elements, sections, floors) carrying out an as-built survey - geodetic verification of the actual position of the mounted structures in plan and height.
When installing foundations, basement walls and walls of the above-ground part of buildings, the correct ligation and thickness of the seams between them, the filling of the seams between blocks and panels, the verticality and straightness of the surfaces and corners of the building, and the quality of anchoring of structures are monitored. When laying the first row of wall blocks, it must not be allowed that the seams between them coincide with the seams of the foundation blocks or foundation pads. The dressing should ensure that the vertical seams in adjacent rows are displaced by “U” of the length of the block.
Basement walls made of concrete blocks must have vertical and horizontal joints 15 mm thick; individual joints can be more than 10 mm and less than 20 mm. The deviation of rows of block masonry from the horizontal along a length of 10 m is allowed within 15 mm, the vertical deviation of surfaces within one floor should not exceed 10 mm. The displacement of the axes of the foundation and wall structures is allowed by ± 12 mm, the deviation of the marks of the supporting surfaces of the foundations from the design ones should not exceed 20 mm, and the surfaces of the wall blocks - 10 mm.
In large-panel buildings, quality control of installation and fastening of prefabricated elements in the design position is ensured by checking the position of elements for axial and installation risks, as well as the quality of sealing joints between elements. The displacement of the axes of wall panels and partitions in the lower section relative to the alignment axes should not exceed 8 mm, in the upper section - 10 mm. The width of vertical and horizontal joints of external wall panels should be within 10...20 mm. For floor panels up to 4 m long, a deviation from the design support value of no more than 8 mm is allowed, for longer slabs - up to 10 mm.
In frame-panel buildings, including one-story industrial buildings, the stability of structures during installation and the reliability of their operation depend on compliance with the technological sequence of assembly of elements, the quality of their installation and fastening, including sealing of joints.
Operational quality control of installation is aimed at preventing the installation of subsequent structural elements if the required accuracy of the position of the previously installed structure is not ensured during alignment. The accuracy of installation before fixing the structural element is confirmed by measurements using a tape measure, templates, plumb lines, levels or geodetic instruments. On each tier, grip, after the installation of frame elements of one type is completed, as-built diagrams are drawn up indicating the actual position of the structures.
Structures mounted in frame one- and multi-story buildings must be securely supported by their ends on the underlying structures. The reduction in the depth of support of elements in the direction of the overlapped span against the design one should not exceed 5 mm for an element length of up to 4 m, and 10 mm for a length of 16 m or more.
The brands of solutions used when installing bed structures must correspond to those specified in the project. It is not allowed to use a solution whose setting process has already begun, or to restore its plasticity by adding water.
If a package of gaskets made of steel sheet is used when aligning crane beams in height, they must be welded together, and the package must be welded to the base plate.
In single- and multi-story frame buildings made of steel structures, maximum deviations of the actual position of mounted structures should not exceed permissible values. Deviation of column support marks from the design ones and displacement of the column axes from the alignment axes - 5 mm; deviation of the column axes from the vertical in the upper section with a column length of up to 8 m - 10 mm, with a length of over 16 and up to 25 m - up to 15 mm. It is allowed to shift trusses and beams from the axes of columns of one-story buildings up to 15 mm, crossbars and beams in multi-story buildings - no more than 8 mm. The following standards have been established for crane beams: the displacement of the crane beam axis from the longitudinal alignment axis is 5 mm, the displacement of the support rib from the column axis is no more than 20 mm.
9.4. Installation of metal structures of one-story industrial buildings
9.4.1. General provisions
The installation elements of industrial buildings with steel frames are columns, crane beams, sub-rafters and roof trusses, half-timbering elements, braces, and steel profiled flooring.
The overall dimensions of structures sent to construction sites depend on the transportation conditions. Often the weight of the structure turns out to be less than the lifting capacity of the installation crane and the structure is enlarged before installation. This allows you to reduce the number of crane lifts, which means faster installation. When installing enlarged structures, the main thing is achieved - reducing the time of work at height, more rational use of installation equipment and improving working conditions.
Steel structures arrive from manufacturing plants in parts (shipping marks). Building structures are divided into component parts if they do not fit on a railway platform or on specially equipped semi-trailers for tractors. To enlarge metal structures into assembly blocks at the construction site, enlargement sites are equipped in the structure warehouse or in the immediate vicinity of the installation area.
Steel trusses, beams and columns, which have assembly holes at the joints that fix the relative position of the parts of the elements being enlarged, are assembled on racks in a horizontal position using bolts and plugs, which fix the relative position of the elements and prevent their shift. If there are no assembly holes at the junctions of structures, then clamps are attached to the racks, which are used to determine the main dimensions of the enlarged element. When the assembled structure has mounting holes in the places where it adjoins the clamps, then holes are also drilled in the clamps and the structures are fastened to the clamps with bolts.
Steel crane beams for the outer rows of columns are enlarged in a vertical position together with brake structures. Simultaneously with the enlarged assembly, the structures are equipped with ladders, cradles, and safety ropes are pulled. The parts necessary for installation and assembly are attached to the structure directly in the design position.
For one-story buildings with a metal frame, complex installation is recommended, when columns, crane beams, sub-rafters and rafter trusses are sequentially installed in a separate assembly cell, and the roof covering is laid.
9.4.2. Installation of columns
Metal columns installed on solid concrete foundations can be supported:
■ on anchor bolts pre-embedded in the foundations with grout at the joints of the cement mortar after alignment of the installed column along two mutually perpendicular axes;
■ directly on the surface of foundations erected to the design level of the milled base of the column without subsequent filling with cement mortar;
■ on pre-installed, calibrated (with a layer of cement mortar if necessary) steel base plates with a top planed surface (no-calibration installation).
When preparing columns for installation, the following marks are applied to them: the longitudinal axis of the column at the level of the bottom of the column and the top of the foundation.
Columns installed on foundations are provided only with anchor bolts if the column has wide shoes and their height is up to 10 m. Higher columns with narrow shoes, in addition to being bolted, are braced in the plane of least rigidity on both sides. The braces are secured to the top of the column before it is raised and, during installation, are secured to anchors or adjacent foundations. After tensioning the braces, the slings can be removed from the column.
The braces can be removed only after the columns have been secured with permanent elements. The stability of the columns in the direction of the building axis is ensured by crane beams and connections installed after the installation of the first pair of columns and the crane beam connecting them.
Metal columns installed on foundations are secured with anchor bolts during installation (Fig. 9.41). If metal gaskets are placed under the base of the column, they must be welded. The columns of the upper tiers (for example, in a built-in shelf) are secured with high-strength bolts or welded.
Rice. 9.41. Installation diagram (a) and permanent fastening (b) of a metal column on a support:
1- foundation slab; 2 - support plate (shoe); 3 - column; 4 - cap to preserve threads during installation; 5 - anchor; b - nut; 7 - welding
Alignment of frame structures, especially columns, requires a lot of labor. The use of the non-calibration installation method makes it possible to improve the quality of work while simultaneously reducing the construction time of the structure.
For non-calibration installation, appropriate preparation of structures is required at the manufacturing plant and at the construction site. Increased precision in manufacturing structures is ensured by the following:
■ the structures of the column shoe and the shoe base plate are manufactured and delivered to the site separately;
■ the ends of the two branches of the columns must be milled;
■ base plates are made planed.
Each base plate must be welded to 4 strips with threaded holes for installing bolts; Axial marks must be applied to the branches of the columns.
With the non-alignment method of installation, steel columns rest on a steel plate. In this case, the surface of the foundations is concreted below the design mark by 50...60 mm and, after precise installation, the slabs are topped with cement mortar. The base plate is installed with adjusting bolts on the support strips, which must be concreted into the foundation flush with its surface as embedded parts. The reference plane of the slab is set by adjusting the nuts of the set screws using a level. The actual elevation of the base plate should not differ from the design one by more than 1.5 mm.
When installing a column, the axial marks on its branches are combined with the marks marked on the base plates, which ensures the design position of the column, and it can be secured with anchor bolts. In this case, additional displacement of the column for alignment along the axes and height is not required. After installing the braces to the mounted column structures and their tension, the crane beams begin to be installed. Crane beams installed along axial risks do not require additional alignment. After they are secured to the bolts, the braces are removed.
9.4.3. Installation of crane beams
Crane beams are installed immediately after the columns are installed in the assembly cell. When lifting, the crane beam is held in place by two guys. The installers receiving the beam at height are on scaffolding or platforms, on assembly ladders. They keep the structure from contact with previously installed elements and turn it in the desired direction before installation. The correct lowering of the beam is controlled by the coincidence of the marks of the longitudinal axis on the beam and the console, as well as by the mark of the previously installed beam. Deviation from the vertical is eliminated by installing metal pads under the beam. The beam is temporarily secured with anchor bolts.
When installing columns with milled soles on foundations concreted to the design level, or on planed metal slabs, the position of the crane beams is verified only in the direction of the main axis.
9.4.4. Trusses and covering made of steel profiled decking
Preparation of the truss for installation consists of the following operations: enlarged assembly, arrangement of cradles, ladders and braces, slinging, lifting to the installation area, turning using braces across the span, temporary fastening using conductors, braces, struts between trusses and guy wires. The position of the truss is verified by the position of the axial marks at the ends of the truss.
Depending on their weight and length, the trusses are lifted using traverses with one or two cranes. The trusses are slinged only at the nodes of the upper chord, so that bending forces do not arise in the rods; the trusses are slung at four points using traverses with semi-automatic remote-controlled grippers. For large installation loads, the elements are temporarily reinforced with wooden plates or metal pipes. The first truss to be lifted is deployed using guys to the design position at a height of 0.5...0.7 m above the top of the columns, lowered onto mounting tables welded to the columns, temporarily bolted, aligned and final fastening is carried out. When lifting, to avoid swinging, it is supported by four flexible guy ropes.
After installing and securing the first truss and securing it with four braces, a second one is installed, which is connected to the first with the help of girders, ties and struts; they all together form a rigid spatial system. On the columns of the middle rows, the truss is additionally connected with bolts to the trusses next to the mounted span.
For building plans with trusses and sub-trusses, the latter are 11.75 m long and are installed on columns with gaps of 25 cm. In this gap, a supercolumn is installed on which the roof truss will rest.
Coverings made from steel profiled flooring are used in buildings with a metal and reinforced concrete frame to lighten its weight, as well as when installing coverings in large blocks. Factory-made insulated panels of profiled flooring can be supplied for installation.
Steel profiled decking is a panel made of galvanized and then coated with an anti-corrosion layer of steel sheet 3...12 m long, 0.8-1 mm thick with longitudinal corrugations 60, 79 mm or more high. The width of the flooring sheets is 680...845 mm, the length is a multiple of three - 6, 9 and 12 m and is assigned by the project in accordance with the location of the truss runs (Fig. 9.42).
Rice. 9.42. Covering of steel profiled flooring:
b - coating scheme; b - connection of flooring sheets with a combined rivet; c - sequence of installation of the rivet, d - fastening the decking with a self-tapping screw; d - fastening the flooring with a dowel: e - dowel; I - steel purlin; 2 - flooring; 3 - connection of the flooring to the purlin with a self-tapping screw at the junction; 4 - the same, in the gaps (grooves) of the flooring; 5 - rivet made of aluminum alloy; 6 - steel rod; 7 - self-tapping screw, 8 - steel washer; 9 sealing washer; 10 - tool for setting rivets; II - dowel; 12 - polyethylene gasket, 13 - polyethylene tip
The sheets are enlarged into cards on horizontal stands equipped with stops adjusted to the size of the cards, and connected to each other with combined rivets or resistance spot welding. After laying out the sheets, use a hand-held electric drill to drill holes for rivets at the joints of the sheets in the overlap wave. Holes are drilled in accordance with the project, usually after 50...60 cm. Rivets are installed in the drilled holes, thus connecting the sheets into a single card of the required size.
It is impractical to install coverings made from profiled flooring in an element-by-element (sheet-by-sheet) manner due to the high labor intensity - the entire volume of work must be performed at height. Coverings are often installed using cards of the sizes indicated above. The assembled cards are mounted during the installation of the coating structures (following the installation of columns and crane beams). The stand on which coverage maps are collected is moved, if necessary, by crane to new sites.
The card is slung according to the slinging diagram and, depending on the size of the card, is lifted by a crane and delivered to the installation site. Flooring in the form of sheets or pre-enlarged cards measuring 6 x 6, 6 x 12, 12 x 12 m is laid on the purlins of the roof or roofing block. The covering purlins are installed along the truss nodes, and when using trusses made of rectangular closed profiles - directly on the upper chords of the trusses. The position of the profiled flooring cards is adjusted according to the marks of marking the installation sites.
The cards are secured to the purlins with self-tapping galvanized screws, less often with dowels and electric rivets. To attach the covering decking to the purlin, through holes with a diameter of 5.5 mm are first drilled into them using a power tool, then self-tapping screws with a diameter of 6 mm are screwed into these holes using a wrench with a plastic or steel washer placed under the head.
For combined rivets (which are used to connect coating sheets to each other), holes with a diameter of 5 mm are also drilled in the sheets, rivets are placed in the holes, lowering them with the head of the steel rod down and the head of the aluminum rivet up. Riveting is performed with a pneumohydraulic gun or special lever pliers. When riveting, the head of the rivet is pressed down and the captured steel rod is pulled upward with force. When the rod is pulled out, its head crushes the lower cylindrical part of the rivet, thereby forming the lower head of the rivet. As soon as the formation of the lower head of the rivet is completed, the metal rod breaks off in a narrowed section and its upper part is pulled out of the rivet.
Steel profiled flooring is used when installing coatings in large blocks assembled on a conveyor. In this case, when assembled into finished cards, a vapor barrier is applied to the flooring, a layer of insulation is laid, and a waterproofing carpet is glued on.
Prefabricated reinforced concrete is very rarely used for covering. In this case, the covering slabs are laid symmetrically in the direction from the supporting units to the ridge. If there is a lantern, the slab is initially mounted along the truss, and then along the lantern from the ridge to the edges.
9.4.5. Welded joints of metal structures
Assembly connections of steel structures can be welded, bolted, or especially critical - rivet-based. “If necessary, steel structures are connected to reinforced concrete ones by welding the connecting elements to the embedded parts of reinforced concrete structures or the connections are made with bolts.
Welded joints are used for rigid connections of load-bearing structures and, if necessary, to have a tight, water-gas-tight connection of elements. Such structures include sheet structures of blast furnace casings, dust collectors, tanks, and gas tanks. Rigid connections include joints between columns, columns and crane beams, columns and trusses.
Welded connections of mounting elements are initially fastened together with rough mounting bolts, and since the resulting strength is insufficient for strength calculations, the elements are welded together. Depending on the type of structures being connected, elements can be welded directly or using additional butt plates.
Column joints. Columns with a height of 18 m or more are divided into shipping elements before transportation, based on the dimensions of the vehicles. During installation, these parts of the columns are connected together; welding can be performed directly or using steel plates, which are installed on bolts and welded to the elements being connected. Joints of columns of one-story industrial buildings are usually made in the crane part above the crane beams. The milled ends of the crane and main parts of the column are joined together and welded along the joint plane. For greater rigidity, both parts are connected to each other with a butt sheet overlay.
Connection of crane beams with columns. The crane beam rests with the edge of a vertical sheet directly on the column base plate and is connected to it with bolts. Additionally, the crane beam is attached to the over-crane part of the column with brake structures, which are attached to the columns and beams with bolts and are additionally welded with an extended seam.
Connection of trusses with columns. When the truss is hingedly supported on a column, the upper chord of the truss is attached to the column, connecting the gusset with bolts and an assembly weld to the plates welded to the column. In the rigid connection of the truss with the column head, a joint plate is additionally installed in the interface unit, which is connected to the support plate of the column head and the truss belt with bolts and welding. The lower chord of the truss is supported with a gusset on the mounting table and attached to the column with bolts and welding.
Quality control of welded joints. Welds are checked by external inspection, identifying unevenness in height and width, lack of penetration, undercuts, cracks, and large pores. In appearance, welds should have a smooth or finely flaky surface, and the deposited metal should be dense along the entire length of the weld. Permissible deviations in the dimensions of weld sections and welding defects must not exceed the values specified in the relevant standards.
To control the mechanical properties of the deposited metal and the strength of welded joints, test joints are welded, from which test samples are cut out. Tests are carried out for tensile strength, hardness, relative elongation, etc. To check the quality of welding, X-ray and y-radiation is used on film, and ultrasonic flaw detectors are used.
Defects in welds are eliminated in the following ways: seam breaks and craters are welded; seams with cracks, lack of penetration and other defects are removed and welded again; The undercuts of the base metal are cleaned and welded, ensuring a smooth transition from the deposited metal to the base metal.
9.4.6. Bolted connections of metal structures
Bolted connections steel structures, depending on the design solution and the loads taken, are performed on bolts of coarse, normal and high accuracy and on high-strength bolts. Rough and normal precision bolts are not used in shear connections.
Holes for such connections are drilled or pressed. The diameter of the hole is 2...3 mm larger than the diameter of the bolt, which greatly simplifies the assembly of connections. But at the same time, the deformability of the connection increases significantly, so bolts of rough and normal accuracy are used to fix connections of direct support of one element on another, in units of force transmission through a support table, in the form of strips, as well as in flange connections.
Connections with high-precision bolts are used instead of rivets in hard-to-reach places where it is almost impossible to install rivets. The diameter of the hole in the connections on such bolts can be no more than 0.3 mm larger than the diameter of the bolts. Minus tolerance for holes is not allowed. Bolts in such precise holes fit tightly and absorb shear forces well.
Connections with high-strength bolts combine ease of installation, high load-bearing capacity and low deformation. They are shear resistant and can replace heavy duty rivets and bolts in almost all applications.
Assembling bolted connections at the installation site includes the following operations:
■ preparation of joining surfaces;
■ alignment of bolt holes;
■ tie the package of joint elements being connected;
■ drilling holes to the designed diameter and installing permanent bolts.
Preparation of the joining surfaces consists of cleaning them from rust, dirt, oil, dust, and straightening irregularities. They file down or cut off burrs on the edges of parts and holes.
The alignment of the holes of all connected elements is achieved using pass-through mandrels, the diameter of which is slightly smaller than the diameter of the hole. The mandrel is driven into the holes, thanks to which they are aligned. The screed must provide the necessary density of the package of connected elements. The package is tightened with temporary or permanent assembly bolts; After tightening the next bolt, additionally tighten the previous one. The required density of the assembled package can be ensured by installing the bolts in the following order: the first bolt is placed in the center, the subsequent ones - evenly from the middle to the edges of the field.
The installation of permanent bolts begins after the structure has been aligned. The bolts are installed in the same sequence as when tightening the package. The lengths and diameters of the bolts are specified by the project.
The nuts of high-strength bolts are tightened with a calibration wrench, which allows you to control and adjust the tension force of the bolts. In order for the bolts to withstand high tightening forces, they are made of special steels and subjected to heat treatment. Bolts allow you to have a tighter and more monolithic connection. Under the influence of shear forces, friction forces arise between the connected elements, preventing the movement of these elements relative to each other.
Finally, high-strength bolts are tightened to the design force after checking the geometric dimensions of the assembled structures. The specified bolt tension is ensured by one of the following methods of force regulation: by the angle of rotation of the nut; by axial tension of the bolt; by the moment of tightening with an indicator type wrench; by the number of blows of the impact wrench.