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Portal Frame Steel Buildings use the portal frame as the force carrier, and the enclosure structure is a system of thin-walled C/Z-shaped steel purlins and colored profiled steel plates. It is a new type of building structure system widely used. It has the advantages of being lightweight, having a high degree of industrialization, having a short construction period, having high comprehensive economic benefits, and having a flexible column network layout.
The Portal Steel Frame is a lightweight construction system that utilizes a sturdy web or lattice portal frame as the primary support structure. The frame is made from either welded H-shaped steel with equal or variable cross-sections, hot-rolled H-shaped steel, or cold-formed thin-walled steel. To complete the building, cold-formed thin-walled steel in the shape of C or Z is used for the purlins and wall girts, while corrugated metal sheets serve as the roof and walls. For insulation, various materials, such as polystyrene foam, rigid polyurethane foam, rock wool, mineral wool, and glass wool, can be utilized to keep the interior temperature regulated. Additionally, a proper bracing system is installed to reinforce the structure further.
Compared to a reinforced concrete structure, a portal steel frame boasts several advantages, such as being lightweight, having high rigidity, allowing for a flexible design, exhibiting reasonable force, and offering convenient construction.
Form of Rigid Frame
The portal frame structure comes in several shapes, including single-span, double-span, high-low-span, and multi-span, with options such as a single ridge, multiple ridges, single slope, double slope, multiple slopes, and a flat slope. Single-span rigid frames are commonly used in buildings that don’t need much lateral space and have spans ranging from 18 to 36 meters. The beams and columns of these frames are usually made of welded or rolled H-shaped sections and are positioned and adjusted in height based on the span and the bending distance diagram. The multi-span rigid frame is ideal for large buildings, with a similar cross-section to the single-span rigid frame but with a center column that typically has an equal cross-section. Currently, the maximum span of a portal frame has reached 72 meters.
The composition of Portal Steel Frame Buildings
- Primary framing: transverse rigid frames (including middle and end rigid framing), floor beams, crane beams, support systems, etc.
2. Secondary framing: roof purlin and wall girt, etc.
3. Envelope structure: roof and wall panels;
4. Auxiliary structures: stairs, platforms, handrails, etc .;
The primary force skeleton of portal frame steel buildings are composed of steel columns, roof beams, and bracing systems. This is considered to be the primary framing. The roof and wall panels act as the envelope and closure for the structure, enhancing its overall stiffness. Additionally, the roof purlins and wall girts provide not only support for the roof and walls but also lateral support for the main structural beams and columns, forming the secondary framing.
The primary force skeleton of portal frame steel buildings consists of steel columns, roof beams, and bracing systems. This is the primary framing. The roof and wall panels act as the structure’s envelope and closure, increasing overall stiffness. The roof purlins and wall girts not only support the roof and walls but also provide lateral support for the main structural beams and columns, forming the secondary framing. The portal frame typically spans 9 to 36 meters, so if the side column widths differ, their outer sides should be aligned.
The portal frame’s recommended height ranges from 4.5 to 9.0mm, with a maximum height of 12m if an overhead crane is present.
To ensure proper structural integrity, the spacing between the axis of the column grid in portal frame steel buildings should fall between 6 and 9 meters. Additionally, the length of the overhang should be determined based on the specific application requirements, typically ranging from 0.5 to 1.2 meters.
The structural system of the portal steel frame buildings:
1. Transverse load-bearing structure:
- The transverse load-bearing structure comprises steel roof beams, columns, and foundations.
- This structure serves to support and transfer both vertical and horizontal loads.
2. Longitudinal Frame Structure :
The longitudinal frame structure comprises of longitudinal columns, crane beams, wall bracing, rigid tie beams, and foundation. Its purpose is to maintain the building’s stability and rigidity along its length and to support and withstand various forces, such as longitudinal wind loads, horizontal crane loads, thermal stresses, and seismic effects acting on the gable and roof.
3. Roof structure
1). The roof panel is designed to withstand both horizontal and vertical wind loads on the roof. It is commonly made using a single-color metal sheet or sandwich panel.
2). A purlin serves as a support structure for the roof panel and can bear both vertical and horizontal wind loads transmitted from the roof panel.
3). The rigid frame beam is a critical load-bearing component responsible for carrying the weight of the roof structure and any live load transmitted from the roof panel.
4. Wall structure
1). The exterior wall panels, which include the vertical and gable walls, are primarily responsible for withstanding wind loads. These panels are commonly made from single-color metal sheets or sandwich panels.
2). A wall girt is designed to bear both vertical and horizontal wind loads that are transmitted by the wall panel.
Type: Roof horizontal bracing, wall bracing.
1). The roof horizontal bracing comprises roof cross bracing, tie beam, and fly bracing. Its primary function is to increase the overall rigidity of the roof.
2). Wall bracing is implemented to enhance the stability of the wall frame structure.
1). Improving the structural rigidity of the building’s interior space.
2). Guaranteed structural stability.
3). Transmit wind load, crane brake load, and seismic load to the load-bearing members.
External loads exert their impact on the envelope of the structure. The secondary structure carries the vertical and lateral forces to the lateral portal frame of the primary structure. The portal frame’s resistance to external influences is dependent on its stiffness. The roof and wall bracing transmit longitudinal wind loads to the foundation.
The structural layout of Portal Steel Frame Buildings
The building’s requirements dictate the span and column spacing when designing the portal steel frame. Consequently, the critical factors to consider during architectural design are the bracing system layout and determining the temperature range.
To account for the impact of temperature, longitudinal temperature sections of portal steel frame buildings must not exceed 300m, and transverse temperature sections must not exceed 150m. When the size of the temperature section surpasses the limit, temperature expansion joints must be implemented. These joints can be achieved by setting double columns or modifying the secondary framing.
The main principles of the Bracing arrangement are as follows:
- The distance between bracing is generally 30m-40m and should not exceed 60m
2. The horizontal roof bracing and the wall bracing arranged between the same column, it used to ensure the formation of a geometrically unchanged system and improve the overall stiffness of the building structure;
3. If the roof bracing is arranged between the second columns, rigid tie bars should be put between the first columns.
5. The 45 ° inclined bar can most effectively transfer horizontal loads. When the angle of the single-layer bracing member is too large due to the high column, the double-layer or three-layer wall bracing should be set;
6. Rigid tie bars shall provide at the turning points, such as the column tops and roof ridges. Longitudinal rigid tie bars shall provide the structure longitudinally at the bracing truss nodes;
7. The rigid tie bar of the portal steel frame buildings can use the purlin at the corresponding position. The tie bar is provided when the stiffness or bearing capacity is insufficient.
Installation sequence of portal steel structure:
- Install the steel column
First, fix the anchor bolts, and the steel column is set on the foundation by connecting with the anchor bolts.
2. Install the tie beam between the steel columns.
3. Assemble the steel beam
Steel beams should be combined with high-strength bolts on the ground and assembled.
4. Install the purlin between the two roof trusses to form a stable frame system.
Installation sequence: start with the two rigid frames supported between columns near the gable. Install purlin, bracing, fly bracing, etc.
Install them in sequence toward the other end of the house, starting from the two rigid frames.
Factors of Portal Steel Frame Buildings that consider in the design:
1. Regulations on the load value
The design software generates the portal steel frame building’s self-weight, which calculates loads of the roof, purlin, bracing, and other components based on the actual design. The roof and wall panels can be either corrugated single-color sheets or sandwich panels, with insulation materials such as polystyrene foam, polyurethane, rock wool, glass wool, etc. The specific materials used in the design should be combined to determine the load of the roof and walls.
Variable loads encompass live roofs, ash, crane, seismic action, wind, and other factors. According to the “Technical Specification for Portal Steel Frame Light Steel Structures” (CECS102: 2002), the live load on the roof should be 0.5 kN/M2. However, if the load area exceeds 60M2, a reduction factor of 0.6 can be applied, resulting in a typical steel frame calculation of 0.3kn/m2.
2. Minimize the amount of steel
The primary steel frame and purlins constitute over 90% of the steel consumption in portal steel frame buildings. Moreover, the arrangement of column spacing significantly impacts steel consumption under the same load conditions. Statistical analyses suggest that the recommended column distance is between 6-8m, and the span should not exceed 36m. Purlins should be made of thin-walled C and Z-type steel, while H-shaped sections are typically used for steel frames.
Features of the Portal Steel Frame Buildings
Less steel consumption and lightweight
The portal steel frame buildings are composed of lightweight and multi-functional new roof and wall materials integrated with waterproofing and enclosure, which greatly reduces the load weight of the structure itself. The load-bearing components are generally high-strength steel, which can reduce the structure’s weight. Usually, the weight of the portal steel frame is only 1/8-1/10 of the reinforced concrete structure and 1/2-1/3 of the weight of the ordinary steel structure, which fully reflects the advantages of new materials.
The construction speed is fast
The degree of industrialization is high, the construction period is short, and the steel structure is easy to manufacture. Because the essential components, such as beams and columns of the portal steel frame, are shaped and manufactured in the factory, they are assembled on-site according to requirements, which speeds up the construction progress and shortens the construction period. The period of the concrete bent structure is shortened by at least 60%. For generally small-scale industrial plants, the portal steel frame only takes 50 days to 2 months, while the reinforced concrete bent structure takes 10-12 months.
The fire protection solution of the portal steel frame buildings
Although steel is a non-combustible material, it is easy to conduct heat, and its fire resistance limit is only 15-20 minutes. For steel structures without fire protection, when the temperature reaches 350°C, 500°C, and 600°C, its strength will be reduced by 1/3, 1/2, and 2/3, respectively. When the structure’s temperature reaches above 600°C, it will ultimately lose its bearing capacity. . The surface of the steel structure can be covered with fire-resistant paint to prolong the fire-resistant time.