The construction of prefabricated buildings cannot be imagined without the use of modern enclosing structures, among which three-layer composite materials occupy a leading place. The basis for the durability and integrity of any industrial or commercial facility is a well-designed and installed metal frame for sandwich panels. It is the supporting frame that absorbs all wind and snow loads, transferring them to the foundation, so mistakes at this stage are unacceptable.
Modern construction technologies dictate their own rules, requiring high accuracy of geometric parameters when assembling racks and purlins. If wall or the roofing panels will be mounted on a skewed structure, problems will arise with the tightness of the joints and the appearance of the facade. In this article we will analyze all the nuances of creating reliable support for lightweight enclosing structures.
The choice of the type of metal structures directly affects the speed of construction and the final cost of the project. Design engineers must take into account many factors: from the climatic characteristics of the region to the specifics of the technological process inside the future building. Understanding of operating principles load-bearing frame necessary not only for designers, but also for customers who want to control the quality of work.
Types of frame systems and their features
The basis of the building can be either a light steel frame (LSFK) or more massive structures made of hot-rolled steel. LSTK frame (light steel thin-walled structures) is gaining popularity due to its low weight and the ability to quickly assemble without the use of heavy lifting equipment. Such systems are ideal for objects with spans of up to 12-15 meters and minimize the load on the foundation.
For larger industrial hangars, warehouses and workshops with large spans it is traditionally used hot rolled steel. I-beams, channels and angles provide the highest strength and rigidity, allowing spans of tens of meters to be covered without intermediate supports. The choice between these two options often becomes a matter of economic feasibility and requirements for the internal space of the building.
⚠️ Attention: When choosing the type of frame, it is critical to take into account the aggressiveness of the internal environment. For workshops with high humidity or chemical exposure, ordinary black steel requires enhanced anti-corrosion protection, while a galvanized LSTK profile may be a more durable solution.
Hybrid solutions are also finding their way into modern construction. For example, the main load-bearing frames can be made of heavy rolled steel, and secondary elements (purlins, crossbars) can be made of cold-formed profiles. This allows you to optimize the weight of the structure and reduce metal consumption, while maintaining the necessary load-bearing capacity.
An important aspect is the method of connecting the elements. Bolted connections allow you to quickly install and, if necessary, dismantle structures, while welded joints provide solidity, but require highly qualified performers and time to control the quality of the seams.
Design and calculation of loads
Designing a metal frame is a complex engineering process that cannot be done by eye. Specialists must calculate all possible loads acting on the building during its entire service life. Snow load varies depending on the geographical area of construction and can reach hundreds of kilograms per square meter, which significantly affects the cross-section of beams.
Wind pressure also plays a key role, especially for buildings of great height or located in open steppe zones. The wind creates not only horizontal pressure on the walls, but also a tearing force on the roof, therefore attachment points must be calculated with a safety margin. Software systems allow you to simulate the behavior of a structure during extreme weather events.
- 🏗️ Constant loads: dead weight of metal structures, weight of sandwich panels, weight of engineering equipment suspended from the frame.
- ❄️ Live loads: snow, rain, temperature expansion, impact of people and machinery during maintenance.
- 🌪️ Special loads: wind pressure, seismic impacts (in earthquake-prone areas), possible explosive loads.
Particular attention is paid to deformations in calculations. The deflection of the beams should not exceed standard values, so as not to compromise the integrity of the cladding and not cause cracks in the panels. The maximum permissible deflection for purlins under sandwich panels is usually 1/200–1/250 of the span length, which is a more stringent requirement than for conventional roofing coverings.
The result of the design is working documentation, including drawings KM (metal structures) and KMD (metal detailing structures). It is from these drawings that manufacturing plants produce elements, and builders carry out installation. Errors in calculations at this stage can lead to catastrophic consequences or, at best, to a significant increase in construction costs.
Foundation preparation and foundation work
The quality of installation of a metal frame directly depends on the preparation of the base. The foundation must be made strictly in accordance with the design elevations, since differences in height of the supporting surfaces of the columns are unacceptable. Before starting assembly, it is necessary to carry out careful geodetic check anchor bolts and embedded parts.
Anchor groups are installed with high precision, usually with a tolerance of no more than 2-3 mm along the axes. To fix the position of the bolts, a special template (jig) is used, which prevents them from moving during concreting. After the concrete has gained strength, a re-inspection is carried out, the results of which are entered into executive documentation.
An important step is the installation of a blind area and the preparation of sites for storing structures. Heavy construction equipment should not drive over the newly laid foundation or disturb the waterproofing. The foundation surface is cleaned of dirt, oil and construction debris before installing the base plates.
⚠️ Attention: Installation of columns on the foundation is permitted only after the concrete has reached 100% of its design strength and the hidden work acceptance certificate has been signed. Ignoring this rule can lead to the destruction of the foundation under the weight of the structures.
To compensate for unevenness and ensure a tight fit of the base plates, the method of grouting or installing shims is often used. This process is called gravy under the stove and requires the use of non-shrinking mixtures of high strength. The quality of this operation affects the uniformity of load transfer from the building to the ground.
☑️ Checking the foundation before installation
Technology of installation of load-bearing structures
Installation of a metal frame begins with the installation of outer frames (blocks), which are then connected to each other by purlins and stiffening connections. To lift the elements, truck cranes of appropriate lifting capacity are used. Each element is lifted into the design position, temporarily fixed and then finally secured with bolts or welding.
The installation process must be carried out sequentially, ensuring the spatial rigidity of the building at each stage. Immediately after installing a number of columns, it is necessary to install vertical connections, which will prevent the structure from swinging. Without connections, the frame is a mechanism and can lose stability even under the influence of wind.
Bolted connections are tightened with a certain force, controlled by torque wrenches. This ensures that the connection does not become loose during use. Welds, in turn, require cleaning from slag and visual or instrumental quality control, often using ultrasonic flaw detection.
| Frame element | Connection type | Quality control | Installation tolerance |
|---|---|---|---|
| Columns | Anchor bolts | Geodesy, tightening nuts | ± 5 mm axial |
| Crossbars (trusses) | Flange Bolts/Welding | Ultrasonic inspection of seams, tightening torque | ± 10 mm in height |
| Runs | Bolts (brackets) | Visual inspection | ± 15 mm in plan |
| Stiffness connections | Rods with turnbuckles | Tension | No sagging |
After assembling the main frame, they begin to install secondary elements - purlins and half-timbers. It is to them that the sandwich panels will be attached in the future. The distance between the runs must strictly correspond to the design step, which depends on the thickness and load-bearing capacity of the selected panels.
Construction of a system of purlins and fastening units
Purlins are the direct base for sandwich panels, so their geometry and strength are critical. The most commonly used are Z-shaped or C-shaped profiles made of galvanized steel. Z-profiles allow for overlapping joining of elements, which creates the effect of a continuous beam and increases the load-bearing capacity of the system.
The purlins are fastened to the main load-bearing beams using special brackets or directly through the shelf. It is important to ensure the possibility of temperature sliding, since the linear expansion of the metal can be significant. To do this, oval grooves are provided in the fastener holes or sliding joints are used.
- 🔩 Self-tapping screws: To fasten the panels to the purlins, special self-tapping screws with an EPDM washer are used to ensure tightness.
- 🛡️ Thermal break: in some cases, a thermal break gasket is installed between the purlin and the panel to eliminate cold bridges.
- 📏 Installation step: The standard pitch of the purlins is from 1 to 3 meters, depending on the thickness of the panel and the wind load.
Particular attention is paid to corner areas and junction areas, where the wind load is much higher. Here, the pitch of the purlins is often reduced, and the number of attachment points is increased. Additional elements, such as plinth and cornice units, also require reliable support, which is provided by a properly installed purlin system.
⚠️ Attention: When installing purlins (it is strictly prohibited), damage the zinc coating. All scratches and cuts of metal should be immediately treated with an anti-corrosion compound (for example, zinc-containing paint) to prevent rust spots.
The system of fastening units must ensure not only strength, but also tightness. The use of low-quality screws or violation of the technology for tightening them (under-pressing or over-clamping) will lead to leaks and a decrease in the energy efficiency of the building.
Anti-corrosion protection and fire safety
The metal frame is susceptible to corrosion, so rust protection is a must. Depending on the aggressiveness of the environment, structures can be hot galvanized or coated with special paints and varnishes. Galvanizing provides the most durable protection, operating on the principle of cathodic protection even when the coating is damaged.
If ferrous metal is used, a multi-layer protection system is used: primer-enamel, epoxy compounds or fire-retardant paints. The thickness of the dry layer is regulated by the project and is usually at least 100-120 microns for outdoor work. The quality of surface preparation (cleaning to Sa 2.5) directly affects the adhesion of the coating.
Fire safety of metal structures is another critical aspect. At high temperatures, steel loses its load-bearing capacity, so for buildings of a certain hazard class it is required fire retardant treatment. It can be made in the form of intumescent paints, coatings or cladding with non-flammable materials.
Regular maintenance of the frame can extend the life of the building. Visual inspection of components, checking the condition of the paint and varnish coating and timely repair of minor damage are the key to long-term operation of the facility without major repairs.
How often should the condition of metal structures be checked?
It is recommended that the initial inspection be carried out one year after the building is put into operation. In the future, planned inspections are carried out at least once every 5 years, and for objects with an aggressive environment - annually. Particular attention is paid to the junction of panels and gutters.
Can heavy equipment be attached to the frame?
Fastening of equipment weighing more than 50 kg (ventilation, pipelines) must be agreed upon with the designer. The attachment points must be located on the nodes or reinforced areas of the purlins. Arbitrary drilling of shelves can disrupt the static operation of the element.
What is the service life of a metal frame?
With proper anti-corrosion protection and the absence of aggressive influences, the service life of the metal frame is 50 years or more. Modern zinc coatings can maintain protective properties for up to 70-100 years, depending on the thickness of the layer.