Portal Frame Design in SCIA Engineer: The Complete Engineering Guide

Designing portal frames in SCIA Engineer requires careful attention to geometry, restraints, and analysis methods. This comprehensive guide walks through the complete design process, from initial modeling to final code checks, with emphasis on correct restraint modeling and real-world considerations.

🏗️ Overview of Portal Frame Design

Portal frames are widely used for industrial buildings, warehouses, and sports facilities due to their efficiency in spanning large distances with minimal intermediate supports. The key to successful portal frame design lies in understanding the structural behavior and correctly modeling all constraints.

Key Design Principles

Rigid connections at the knee joint transfer moments between column and rafter
Pinned bases are typically used for economy and constructability
Lateral restraint systems prevent out-of-plane buckling
Second-order effects must be considered for slender frames

SCIA Engineer Advantages

SCIA Engineer provides integrated tools for portal frame design including automated geometry generation, comprehensive load application, advanced stability analysis, and code-compliant design checks.

📋 Step 1: Project Setup and Initial Configuration

Creating a New Portal Frame Project

Start SCIA Engineer and create a new project. Navigate to File > New Project and select the appropriate template. For portal frames, choose either “Steel Structure” or use the “Blank Project” option for maximum flexibility.

Project Configuration

Set up your project parameters including:

Design standard:     Eurocode (EN 1990-1998)
Steel grade:         S355 (or S275 for economy)
Units:              Metric (kN, m, MPa)
National Annex:     Select appropriate country
Analysis type:      3D with 2nd order effects

Material Definition

Define your steel material properties. For S355 steel, SCIA typically includes:

Yield strength: 355 N/mm²
Ultimate strength: 510 N/mm²
Young’s modulus: 210,000 N/mm²
Poisson’s ratio: 0.3

⚖️ Step 2: Portal Frame Geometry Modeling

Example Building: Industrial Warehouse

Let’s design a typical single-bay portal frame with these parameters:

Span:               30.0 m
Column height:      8.0 m
Roof pitch:         5° (1:11.4)
Bay spacing:        6.0 m
Frame spacing:      6.0 m
Eaves height:       8.0 m to underside of haunch

Using Catalogue Blocks

SCIA Engineer provides pre-defined portal frame templates through Structure > Catalogue Blocks > Portal Frames. Select the appropriate configuration:

Access Catalogue Blocks: Structure menu > Catalogue Blocks
Select Portal Frame Type: Choose symmetric pitched portal
Input Dimensions: Span, height, pitch angle
Define Cross-Sections: Initial estimates for columns and rafters

Manual Geometry Creation

Alternatively, create geometry manually using Structure > Nodes and Beams:

Key Points (Nodes):

Base nodes: (0, 0, 0) and (30, 0, 0)
Eaves nodes: (0, 8, 0) and (30, 8, 0)
Apex node: (15, 9.31, 0) [accounting for 5° pitch]

Beam Elements:

Left column: Node 1 to Node 3
Right column: Node 2 to Node 4
Left rafter: Node 3 to Node 5
Right rafter: Node 4 to Node 5

📐 Step 3: Cross-Section Assignment

Initial Section Selection

For a 30m span portal frame, typical starting sections might be:

Columns:

HEA 300 or HEB 300 for standard heights
Consider larger sections (HEA 400) for tall columns

Rafters:

IPE 550 or IPE 600 for 30m spans
May require plate girders for longer spans
Haunch details at knee connections

Haunch Modeling

Haunches are critical for portal frame efficiency. In SCIA Engineer:

Create Haunch Profile: Cross-section > Variable Cross-section
Define Geometry: Linear variation from knee to point of contraflexure
Typical Haunch Length: 10-15% of span (3-4m for 30m span)
Depth Variation: Start depth = rafter depth + haunch depth

Example haunch definition:

Start section:      IPE 600 + 400mm plate (total depth 1000mm)
End section:        IPE 600 (600mm depth)
Haunch length:      3.5m from knee joint
Variation:          Linear taper

🔍 Step 4: Critical Restraint Modeling

Understanding Restraint Requirements

Portal frames are susceptible to lateral-torsional buckling and in-plane instability. Correct restraint modeling is essential for:

Preventing out-of-plane buckling of rafters
Stabilizing columns against weak-axis buckling
Providing overall frame stability

Roof Restraint Systems

Purlin Restraints: Purlins provide lateral restraint to rafter compression flanges. Model as:

Point Restraints: At each purlin location (typically 1.5-2.0m spacing)
Restraint Type: Lateral restraint (Y-direction) and twist restraint
Spring Stiffness: Use CD = 60 N/mm/mm for typical purlin/sheeting systems

Implementation in SCIA:

Location: Structure > Restraints > Point Restraints
Apply to: Top flange of rafters at purlin points
Direction: Perpendicular to frame plane (Y-direction)
Type: Spring restraint with appropriate stiffness

Side Cladding Restraints

Column Restraints: Side cladding restrains column compression flanges:

Spacing: Typically at rail locations (1.5-2.0m centers)
Stiffness: CD = 80 N/mm/mm for typical rail/cladding systems
Critical Locations: Always restrain at points of maximum compression

Foundation Restraints

Base Connections: Most portal frames use pinned bases for economy:

Translational restraints:  All directions (Ux, Uy, Uz = 0)
Rotational restraints:     About weak axis only (Rx = 0)
Free rotations:           About strong axis (Ry, Rz = Free)

Fixed Base Alternative: For very tall or lightly loaded frames:

All translations fixed:    Ux, Uy, Uz = 0
All rotations fixed:      Rx, Ry, Rz = 0

💪 Step 5: Load Application and Combinations

Permanent Loads (Dead Load)

Self-Weight: SCIA automatically calculates frame self-weight. Additional permanent loads include:

Purlins and rails: 0.15 kN/m² on roof
Cladding: 0.25 kN/m² on walls and roof
Services allowance: 0.15 kN/m²

Application Method:

Create Load Case: Loads > Load Cases > Add “Dead Load”
Apply Area Loads: Convert to line loads on rafters
Typical Values: 0.5-0.8 kN/m on rafters, 0.4 kN/m on columns

Variable Loads

Imposed Roof Load: According to EN 1991-1-1:

Access category H (roof): 0.4 kN/m²
Maintenance loads: Consider 1.0 kN concentrated load

Snow Load: Calculate according to EN 1991-1-3:

Ground snow load: Varies by location (0.4-1.5 kN/m²)
Shape coefficient: μ₁ = 0.8 for pitched roofs
Thermal coefficient: Ct = 1.0 (unheated buildings)

Example calculation for UK (sk = 0.6 kN/m²):

Snow load = μ₁ × Ct × Ce × sk
Snow load = 0.8 × 1.0 × 1.0 × 0.6 = 0.48 kN/m²

Wind Load Analysis

Pressure Coefficients: EN 1991-1-4 provides external pressure coefficients:

Windward wall: Cpe = +0.8
Leeward wall: Cpe = -0.5
Windward roof: Cpe = -0.9 to -0.5
Leeward roof: Cpe = -0.5

Dynamic Response: Check if dynamic analysis required:

Building height > 15m, or
Natural period > 1.0 second

SCIA Wind Load Wizard:

Access: Loads > Wind Load Generator
Building Geometry: Input dimensions and roof angle
Wind Parameters: Basic wind velocity, terrain category
Generate Loads: Automatic application to frame surfaces

📏 Step 6: Load Combinations and Analysis

Ultimate Limit State Combinations

EN 1990 fundamental combination:

1.35 × Dead + 1.5 × Live + 1.5 × Snow
1.35 × Dead + 1.5 × Live + 1.5 × Wind
1.35 × Dead + 1.5 × Wind + 1.05 × Snow

Serviceability Combinations

For deflection checks:

1.0 × Dead + 1.0 × Live + 0.5 × Snow
1.0 × Dead + 0.6 × Wind

Analysis Settings

Configure analysis parameters:

Analysis Type: 2nd order (P-Delta effects)
Iteration Control: Newton-Raphson method
Convergence Criteria: Force 0.01%, displacement 0.01%
Imperfections: Include frame imperfections per EN 1993-1-1

Frame Imperfections

EN 1993-1-1 requires consideration of:

Bow imperfection: L/300 for frame out-of-plumb
Sway imperfection: φ = 1/200 (basic value)
Application: Apply as equivalent horizontal forces

🔧 Step 7: Stability Analysis and Design Checks

Linear Buckling Analysis

Perform eigenvalue analysis to determine:

Critical load factors: Should exceed 10 for frame stability
Buckling modes: In-plane, out-of-plane, and local buckling
Effective lengths: For member design

SCIA Implementation:

Run Analysis: Calculation > Linear Stability
Review Results: Check first mode load factor
Mode Shapes: Visualize critical buckling patterns

Member Design Checks

Steel Code Checks: SCIA performs comprehensive EN 1993-1-1 checks:

Cross-section resistance: Moment, shear, axial
Member buckling: In-plane and out-of-plane
Lateral-torsional buckling: With proper restraint modeling
Interaction: Combined bending and compression

Critical Design Locations:

Knee joints: High moments and axial forces
Rafter mid-span: Maximum sagging moments
Column bases: High compression with moments

Connection Design

Portal frame connections require special attention:

Knee Joint (Moment Connection):

High moment transfer capability
Often requires stiffeners and haunches
Check bolt shear, bearing, and tension
Verify plate buckling and beam web resistance

Base Connection (Pinned):

Design for vertical reaction only
Include nominal moment resistance (10% of plastic moment)
Check anchor bolt shear and concrete bearing

📐 Step 8: Serviceability Checks

Deflection Limits

Portal frames typically have generous deflection limits:

Vertical Deflection:

Roof deflection: L/200 = 30,000/200 = 150mm
Column sway: H/150 = 8,000/150 = 53mm

Calculation in SCIA:

Apply SLS loads: Use characteristic combinations
Run Analysis: Linear analysis sufficient for serviceability
Check Results: Compare with code limits

Dynamic Considerations

For large spans, check natural frequency:

Target frequency: > 3 Hz to avoid resonance
SCIA Modal Analysis: Calculation > Dynamics > Natural Frequencies

🏗️ Step 9: Advanced Modeling Techniques

Non-Linear Analysis

For accurate frame behavior:

Material non-linearity: Steel yielding and strain hardening
Geometric non-linearity: Large displacement effects
Connection behavior: Semi-rigid joint modeling

Tapered Members

For optimized designs:

Create Variable Section: Cross-section > Variable Cross-section
Define Variation: Linear or parabolic depth variation
Optimization: Minimize material while meeting strength requirements

Multiple Bay Frames

For continuous structures:

Frame Spacing: Typically 6-8m for economic design
Bracing Systems: Plan and elevation bracing
Construction Sequence: Consider erection stability

🛠️ Step 10: Practical Design Considerations

Constructability Issues

Transportation Limits:

Maximum depth: 1.8m for road transport
Maximum length: 18m standard, longer requires permits
Splices: Required for longer rafters

Erection Sequence:

Temporary bracing: Required during construction
Crane capacity: Affects maximum piece sizes
Weather sensitivity: Steel erection limitations

Connection Details

Knee Joint Options:

Welded haunch: Most efficient but requires shop welding
Bolted splice: Field-friendly but requires larger sections
Curved knee: Architectural preference but complex fabrication

Base Detail Considerations:

Anchor bolt layout: Standard patterns for economy
Grout requirements: Level bases and load distribution
Drainage: Prevent water accumulation around bases

📊 Design Example Summary

Final Design: 30m Span Portal Frame

Component	Section	Unity Check	Status
Columns	HEA 300	0.85	✓
Rafters	IPE 600 + Haunch	0.92	✓
Knee Joint	Moment Connection	0.78	✓
Deflection	145mm	150mm limit	✓

Material Requirements

Steel tonnage: Approximately 2.5 tonnes per frame
Connection materials: High-strength bolts, welding consumables
Surface treatment: Hot-dip galvanizing or paint system

🔍 Common Modeling Mistakes to Avoid

Restraint Modeling Errors

Insufficient lateral restraints: Leading to premature buckling
Unrealistic stiffness values: Over-conservative restraint springs
Missing twist restraints: Critical for open sections

Load Application Issues

Incorrect wind directions: Must consider all critical directions
Missing load combinations: Overlooking serviceability cases
Imperfection modeling: Forgetting frame out-of-plumb effects

Analysis Problems

First-order analysis only: Missing P-Delta effects
Inadequate mesh: Poor results near connections
Convergence issues: Non-linear analysis difficulties

📚 Summary and Best Practices

Portal frame design in SCIA Engineer requires systematic attention to:

Accurate geometry modeling with proper haunches and tapers
Comprehensive restraint systems reflecting real construction details
Realistic load application including all relevant load cases
Appropriate analysis methods considering non-linear effects
Thorough design verification at all critical locations

Key Success Factors

Understand structural behavior: Portal frames rely on frame action
Model restraints realistically: Neither too stiff nor too flexible
Consider construction sequence: Temporary stability requirements
Verify all assumptions: Support conditions, material properties, load paths

SCIA Engineer Advantages

Integrated workflow: From modeling to design checking
Code compliance: Automatic EN 1993-1-1 verification
Optimization tools: Automatic section selection
Detailed reporting: Comprehensive calculation documentation

Successful portal frame design combines understanding of structural behavior with proficient use of analysis software. SCIA Engineer provides the tools, but engineering judgment remains essential for safe and economical designs.