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Portal Steel Frame Buildings is the most common structure system. It widely used in light steel buildings such as warehouses, workshops, sheds, garages.
The portal steel frame is a lightweight building system; it is a solid web or lattice portal frame as the main load-bearing framework, composed of welded H-shaped (equal or variable cross-section), hot-rolled H-shaped steel, or cold-formed thin-walled steel. Cold-formed thin-wall steel (C or Z-shaped) use as purlins and wall girt; corrugated metal sheets are used for roofs and walls; use polystyrene foam, rigid polyurethane foam, rock wool, mineral wool, glass wool, etc., as heat preservation and heat insulation materials, and appropriately set up bracing system.
Compared with the reinforced concrete structure. The portal steel frame has the advantages of a lightweight, high rigidity, flexible design, reasonable force, and convenient construction.
Form of Rigid Frame
The portal frame structure has various forms: single-span, double-span, high-low-span, and multi-span, a single ridge, multiple ridge, or single slope, double slope, multiple slopes, flat slope, etc. Single-span rigid frames are mostly used in buildings with low lateral space requirements. Generally, the span is 18-36m. Welded or rolled H-shaped sections are often used. The position and height of variable-section beams and columns are determined according to the bending distance diagram and span. The multi-span rigid frame is suitable for large buildings, and its cross-section is similar to a single-span rigid frame, but the center column generally adopts equal cross-section. At present, the maximum span of the portal frame has reached 72m.
The composition of Portal Steel Frame Buildings
1. 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 .;
Steel columns, roof beams, and bracing systems constitute the primary force skeleton of the portal steel frame. It is the primary framing.
Roof purlins and wall girt not only support the roof and walls but also provide lateral support for the main structural beams and columns, which form the secondary framing of portal steel frame buildings.
The roof and wall panel serves as envelope and closure of the entire structure, while also increasing the overall stiffness of the steel buildings.
The span of the portal frame usually is 9～36m. When the widths of the side columns are not equal, their outer sides should be aligned.
The average height of the portal frame should be 4.5～9.0mm. When there is an overhead crane, it should not be greater than 12m.
The spacing of the portal frame, that is, the longitudinal distance between the axis of the column grid, should be 6-9m.
The length of the overhang can be determined according to the application requirements and should be 0.5~1.2m.
The structural system of the portal steel frame buildings:
1.Transverse load-bearing structure:
1)The composition of the transverse load-bearing structure includes steel roof beams, steel columns, and foundation);
2)Transverse load-bearing structure supports and transfers vertical and horizontal loads.
2. Longitudinal Frame Structure :
1)Composition of longitudinal frame structure includes longitudinal columns, crane beams, wall bracing, rigid tie beam and foundation)
2)Ensure the longitudinal rigidity and stability of the buildings;
3)Transmit and bear the longitudinal wind load, the longitudinal horizontal loading of the crane, the thermal stress, and the seismic acting on the gable at the end of the building and roof.
3. Roof structure
1)Roof panel: It can bear the vertical load and horizontal wind load acting on the roof panel. Generally, it uses a single color metal sheet or sandwich panel.
2)Purlin: Support structure of roof panel, which can bear the vertical load and horizontal wind load transmitted from the roof panel. 3)Rigid frame beam: The primary load-bearing members mainly bear the self-weight of the roof structure and the live load transmitted from the roof panel.
1)Exterior wall panels: vertical and gable wall. Mainly bear wind load. It uses a single color metal sheet or sandwich panel.
2)Wall girt: bearing the vertical and horizontal wind load transmitted by the wall panel.
Type: Roof horizontal bracing, wall bracing.
1)Roof horizontal bracing: enhance the overall rigidity of the roof. It consists of a roof cross bracing, tie beam, and fly bracing.
2)Wall bracing: It uses to improve the stability of the wall frame structure
1)Enhancing the spatial rigidity of the building structure
2)Guaranteed structural stability
3)Transmit wind load, crane brake load and seismic load to the load-bearing members
External loads act directly on the envelope — vertical and lateral loads transmitted to the lateral portal frame of the primary structure through the secondary structure. The portal frame relies on its stiffness to resist external effects. Longitudinal wind loads transferred to the foundation through roof and wall bracing.
The structural layout of Portal Steel Frame Buildings
The span and column spacing of the portal steel frame mainly determined according to the building requirements. The main issues to consider in the architectural design are the determination of the temperature interval and the layout of the bracing system.
Considering the temperature effect, the length of the longitudinal temperature section of the portal steel frame buildings should not exceed 300m, and the transverse temperature section not exceed 150m. If the size exceeds the temperature section, it should arrange the temperature expansion joint. Temperature expansion joints can realize by setting double columns or adjusting the secondary framing.
The main principles of the Bracing arrangement are as follows:
1. 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 uses to ensure the formation of a geometrically unchanged system and improve the overall stiffness of the building structure;
3. If the roof bracing arranged between the second columns, rigid tie bars should arrange 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 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 at 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 provided when the stiffness or bearing capacity is insufficient.
Installation sequence of portal steel structure:
1. 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, and fly bracing, etc.
Starting from the two rigid frames, install them in sequence toward the other end of the house.
Factors of Portal Steel Frame Buildings that consider in the design:
1.Regulations on the load value
The Design software generates the self-weight of the portal steel frame buildings. A load of roof, purlin, bracing, and other loads added to the steel frame was calculated according to the actual design. Corrugated single color sheet and sandwich panel could use as the roof or wall panel. The Insulation materials of the sandwich panel include polystyrene foam, polyurethane, rock wool, glass wool, etc.
The design should combine with specific materials to determine the roof and wall load.
Variable loads include roof live load, ash load, crane load, seismic action, wind load, etc. The “Technical Specification for Portal Steel Frame Light Steel Structures” (CECS102: 2002) stipulates that the roof’s live load is 0.5 kN/M2. If the load area is greater than 60M2, the reduction factor can multiply 0.6. So when calculating steel frame usually uses 0.3kn/m2.
2.Minimize the amount of steel
In the portal steel frame buildings, the steel consumption of primary steel frame and the purlin account for more than 90%. Under the same load conditions, the column spacing arrangement has a great influence on steel consumption.
Several statistical analyses show that the recommended column distance is 6-8m, and the span should not be greater than 36m. Purlins should be thin-walled C and Z type steel, while steel frames generally use H-shaped sections.