Design Requirements, Structural Standards and Engineering Considerations

Light steel portal frame buildings are widely used in industrial workshops, warehouses, logistics centers, agricultural buildings, and commercial facilities due to their lightweight structure, fast installation, cost efficiency, and excellent seismic performance.

A typical portal frame steel building consists of main structural components including tapered steel columns, tapered rafters, steel beams, bracing systems, purlins, wall girts, and metal cladding systems. Proper structural design is essential to ensure safety, durability, and long-term performance.

1. Structural Forms and Dimensional Requirements

Light steel portal frame buildings generally adopt:

• Single-story structures

• Single-span or multi-span layouts

• Double slope, single slope, or multi-slope roof designs

The common roof slope is less than 10°, and roofing systems normally use profiled steel sheets or insulated metal panels.

Typical Design Parameters

Building Span:

• Common span range: 9m–36m

• Large-span structures can reach approximately 72m depending on project requirements

Building Height:

• Normally ≤12m

• Maximum recommended height: 18m

Column Spacing:
Common column spacing:

• 6m

• 7.5m

• 9m

Typical Portal Frame Dimensions

Single-span Portal Frames

• Haunch depth near the column: approximately L/30

• Haunch length: 15%–25% of span length

• Mid-span rafter depth: approximately L/50–L/60

• Web depth-to-width ratio generally controlled within reasonable limits

Multi-span Portal Frames

• Intermediate column haunch depth: approximately L/25

• Haunch length: approximately L/45–L/55

For transportation efficiency, the maximum length of individual steel members is usually controlled within 12m.

2. Applicable Scope of Light Steel Portal Frame Structures

Light steel portal frame buildings are mainly suitable for:

Crane Applications

Applicable for:

• Light and medium-duty overhead cranes (A1–A5 class)

• Bridge cranes with lifting capacity ≤20 tons

• Suspended cranes ≤3 tons

With special engineering measures, cranes up to 5 tons may also be considered.

Environmental Limitations

Portal frame structures are generally not recommended for environments with:

• Strong corrosive chemical media

• Severe industrial corrosion conditions

For multi-story steel buildings where the top floor adopts portal frame structures, overall structural analysis and seismic design should be carried out according to relevant standards.

3. Seismic Design Requirements

Due to their lightweight characteristics and low-rise configuration, portal frame steel buildings usually demonstrate good seismic performance.

However, seismic design should consider:

• Building span-to-height ratio

• Crane loads

• Regional seismic intensity

• Structural stiffness requirements

Key Seismic Design Principles

• Transverse and longitudinal frames should be analyzed separately.

• Bottom shear force method is commonly recommended for seismic calculation.

• Structural damping ratio is generally considered as 0.05.

• Large-span structures should consider vertical seismic effects.

When seismic action controls the design, additional measures should be adopted:

• Increase connection reliability

• Use bolted connections where possible

• Strengthen haunch areas near rafter-column joints

• Improve column bracing connections

• Ensure anchor bolts meet tension and shear requirements

4. Steel Material Requirements

The main load-bearing components should generally use:

• Q235B steel or higher-grade steel

• Q345B steel or higher-grade steel

Secondary members under static loads may use Q345A steel where permitted.

For structural connections:

• High-strength bolts should mainly adopt friction-type connections.

• Bearing-type high-strength bolts are rarely recommended for portal frame structures.

5. Roof Live Load Requirements

The standard roof live load value has been adjusted:

• Standard roof live load: 0.5 kN/m²

• For load areas exceeding 60m²: 0.3 kN/m² may be adopted

Different load values are considered according to roof area and structural requirements.

6. Wind Load Considerations

Wind load is a critical factor for light steel buildings.

Design should consider:

• Building height

• Building enclosure condition

• Wind pressure distribution

• Roof and wall suction effects

Wind load calculations should distinguish between:

• Fully enclosed buildings

• Partially enclosed buildings

• Open buildings

For purlins and cladding systems, local wind suction effects, especially at corners and edges, should receive special attention.

7. Structural Stiffness Requirements

The deformation control requirements include:

Column Top Displacement

Typical limits:

• Buildings without cranes using lightweight wall panels:

• Approximately H/60

• Buildings with bridge cranes:

• More strict displacement control is required

Rafter Deflection

For single-span portal frames:

• Rafter deflection should meet structural serviceability requirements.

• Buildings with spans exceeding 30m should consider structural camber.

Roof slope changes caused by deformation should not exceed one-third of the designed roof slope.

8. Bracing System Requirements

The bracing system ensures overall stability and longitudinal stiffness.

Common Requirements

Round Steel Bracing

Applicable only for buildings without cranes.

Requirements:

• Diameter should be determined by calculation.

• Minimum diameter should not be less than 10mm.

• Proper connection reinforcement is required.

Buildings with Cranes

• Cranes ≥5 tons:

• Use section steel bracing systems.

• Cranes >15 tons:

• Provide longitudinal roof bracing.

• Improve crane beam lateral stiffness.

Column bracing spacing is generally controlled within:

30m–45m

and should align with roof transverse bracing positions.

9. Portal Frame Member Design

Main design principles:

• Elastic design method is adopted.

• Plastic design is generally not considered.

• Web slenderness and flange width-to-thickness ratios must meet design limits.

• Variable-section members are commonly used for weight optimization.

Important design considerations:

• Plane stability of columns

• Out-of-plane stability

• Effective width of webs

• Post-buckling shear strength

• Influence of hinged or rigid column bases

For crane-free buildings:

• Column bases are generally hinged.

For buildings with overhead cranes:

• Rigid column bases are commonly adopted.

10. Purlin Design Requirements

Common purlin types:

• Cold-formed C-shaped purlins

• Cold-formed Z-shaped purlins

• Welded H-section purlins

Purlins must consider:

• Gravity loads

• Wind suction forces

• Roof panel interaction

• Lateral restraint conditions

Continuous Z-purlin systems can improve structural efficiency through overlapping connections.

11. Sag Rod and Knee Brace Design

Knee braces are important stability components used to:

• Reduce lateral buckling length of compression flanges

• Improve portal frame stability

Installation requirements:

• Mainly arranged at compression flange areas

• Required near rafter-column connections

• Spacing should satisfy stability requirements

If knee braces cannot be installed, alternative strengthening measures should be provided.

12. Anchor Bolt and Column Base Design

Column base design should consider:

• Uplift forces

• Horizontal shear forces

• Bracing forces

Common requirements:

• Anchor bolts should not be the only component resisting horizontal shear.

• Shear keys may be required for large horizontal forces.

• Base plates should provide reliable load transfer.

13. End Plate Bolted Connection Design

High-strength bolted end plate connections are widely used because of:

• Fast installation

• Reliable performance

• Easy transportation

Design considerations:

• High-strength bolts must be used.

• End plate thickness should be calculated and generally not less than 16mm.

• Welding quality requirements must be strictly controlled.

14. Installation Requirements

During construction:

Steel Frame Installation

• Install bracing systems in time.

• Follow the correct erection sequence.

• Provide temporary supports when necessary.

• Use wind cables for temporary stability.

Column Base Installation

• Install adjustment nuts under anchor bolts.

• Perform alignment adjustment before secondary grouting.

• Provide grouting holes in base plates.

Metal Cladding Installation

Requirements:

• Use reliable sealing materials.

• Recommended sealant: butyl rubber tape.

• Maximum fastener spacing between panels and secondary structures should generally not exceed 300mm.

• Strengthen fixing at roof edges and corners due to higher wind suction.

Conclusion

Light steel portal frame buildings provide an efficient solution for modern industrial construction. Through optimized structural design, reliable connection systems, proper material selection, and strict installation control, portal frame steel structures can achieve excellent performance in terms of safety, durability, construction speed, and cost efficiency.

ZSJH Steel Group provides customized portal frame steel building solutions, including structural design, steel fabrication, quality inspection, and global delivery services for industrial warehouses, workshops, logistics centers, and commercial steel buildings.