Mastering Portal Frames: Types and Design Considerations

Portal frames play a crucial role in the construction of residential and light commercial projects, providing a reliable and cost-effective solution for transferring vertical and lateral loads in buildings.

These structural systems are widely used due to their versatility and ability to span large distances without the need for interior columns.

This article will explore the basics of portal frame structure design, including the different types, factors we need to consider in the design phase, and the applicable codes and standards in the United States.

For more resources on portal frame design, check out these other guides and resources:

• Structural Analysis of Portal Frames: Worked Examples and Case Studies
• How to Analyze a Portal Frame in ClearCalcs
• Worked Example of Portal Frame Analysis in ClearCalcs

Introduction

Portal frames consist of two vertical columns connected by a horizontal or sloped beam at the top, forming a rigid rectangular or trapezoidal shape. The beam acts as a portal, distributing the loads to the columns typically through bolted end plate connections, providing overall stability to the structure. The columns are usually fixed at their bases and may include moment-resisting connections.

Portal frames offer numerous benefits, making them a popular choice in the construction industry. Advantages of portal frames include:

• They provide open and flexible spaces, allowing for expansive floor plans and easy customization.
• The absence of interior columns maximizes the usable area within a building, enabling more efficient utilization of space.
• Provided that the design considerations are observed, portal frame can offer excellent structural performance, stability, and resistance to lateral loads, such as wind and seismic forces.

Types of Portal Frames

Portal frames can be categorized into different types based on the shape and arrangement of their members. Here are some of the different types of portal frames commonly employed in residential and light commercial projects:

Single-Span Portal Frame

The single-span portal frame is the simplest and most common type. It consists of a rigid frame with two columns and a horizontal or sloped beam spanning between them. This frame provides a clear span between the columns, offering flexibility in interior space planning.

It is often used in residential garages, small warehouses, and workshops.

Multi-Span Portal Frame

The multi-span portal frame consists of multiple single-span frames connected together with intermediate columns and beams. This type of frame is used when a larger clear span is required, exceeding the capacity of a single-span frame.

Multi-span portal frames are suitable for light commercial buildings, such as retail spaces, showrooms, and small-scale industrial structures.

Figure 1 Types of portal frames based on their number of spans (Reference)

Tapered Portal Frame

Tapered portal frames are designed with beams and columns of variable sections along the height. The sections gradually decrease in size from the base to the ridge, which ensures the rafters have a larger section area at the locations of the maximum bending moment to maximize efficiency.

Tapered frames are also commonly used in architectural applications where aesthetic considerations are important, such as entry canopies, covered walkways, and grand entrances.

Figure 2: An example of tapered portal frames (Reference)

Curved Portal Frame

Curved portal frames are designed with a curved horizontal beam instead of a straight one. This frame type adds architectural interest and visual appeal to residential and light commercial projects.

Curved portal frames can be used in applications like atriums, roof canopies, and architectural features where a distinctive curved form is desired.

Figure 3: Curved rafters in a curved portal frame (Reference)

Lean-To Portal Frame

A lean-to portal frame consists of a single sloping roof supported by a primary portal frame structure. It is commonly used as an extension or annex to an existing building, providing additional space.

Lean-to frames are often utilized in residential projects for extending living areas, adding carports or covered patios, and creating storage spaces.

Figure 4: An example of lean-to portal frame (Reference)

Canopy Portal Frame

Canopy portal frames are designed specifically to create covered outdoor areas. They feature a single-span frame with a horizontal beam that extends beyond the columns to provide shelter.

Canopy portal frames are commonly used in residential projects to create covered porches, carports, and outdoor living spaces.

Figure 5: An example of canopy portal frames (Reference)

Truss Portal Frame

Trusses can be utilized in the rafters and ridge of a portal frame to achieve the largest spans of any type of portal frame.

They are typically utilized in commercial warehouses.

Figure 6: An example of truss portal frames (Reference)

Applicable Codes & Standards

In the United States, portal frame design is governed by various standards to ensure the safety and performance of structures. The American Institute of Steel Construction (AISC) standards, in conjunction with the International Building Code (IBC) and American Society of Civil Engineers (ASCE) guidelines, form the basis for portal frame design in residential and light commercial projects.

Adhering to these standards is crucial for structural engineers, architects, and building designers to ensure the integrity and safety of the buildings they create.

Here are the key considerations outlined in the AISC standards for portal frame design:

Load Combinations

The AISC standards provide specific load factors to consider for various types of loads, including dead loads, live loads, wind loads, and seismic loads. The load factor design methodology is covered in Chapter B of AISC 360-16.

These load factors account for uncertainties and variations in load magnitudes to ensure the structure can safely support the intended loads.

Material Properties

The properties of various steel materials, including their mechanical properties, are typically covered in Chapter A and Appendix B of the AISC Steel Construction Manual. The AISC standards specify the material properties of structural steel, such as yield strength, tensile strength, and elasticity modulus.

These properties determine the strength and behavior of the steel members in the portal frame. Designers must select appropriate steel sections that meet the required load-carrying capacities and structural performance criteria.

Design Methodologies

The AISC standards outline different design methodologies for portal frames, including the Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD) methods. The design methodologies for portal frames can be found in Part 2 of the 15th edition of the AISC Steel Construction Manual. This chapter provides information on different design approaches and methods.

The ASD method allows for a direct comparison of applied loads to the allowable stresses in the members. In contrast, the LRFD method considers load and resistance factors to determine the required member capacities.

Designers must choose the appropriate method based on project requirements and code provisions.

Stability Analysis

Stability is a critical aspect of a portal frame design. The AISC standards provide guidelines for stability analysis, including evaluating the slenderness ratio, effective length, and bracing requirements for columns and beams. The stability analysis of portal frames, including the evaluation of buckling and lateral-torsional buckling, is covered in Part 3 and Part 13 of the AISC Steel Construction Manual.

Designers must ensure that the portal frame components are adequately braced to resist lateral forces, such as wind and seismic loads, and prevent buckling or instability.

Connection Design

The AISC standards provide detailed guidelines for designing moment-resisting connections in portal frames. nformation on connection design, including the design of moment connections, bolted connections, and welded connections, can be found in Chapter 10 of the AISC Steel Construction Manual.

These connections are crucial for transferring loads and moments between the beams and columns. Designers must consider factors such as connection capacity, shear resistance, moment transfer, and bolt design to ensure reliable and efficient connection performance.

Construction Tolerances

The AISC standards also address construction tolerances for portal frame members and connections. These tolerances ensure proper fit-up during fabrication and construction, minimizing alignment issues and potential structural deficiencies.

Designers should specify appropriate tolerances to maintain the structural integrity of the portal frame during the construction process. Guidelines and recommendations for construction tolerances can be found in Chapter 6 of the AISC Steel Construction Manual.

Design Considerations

During the design phase, several factors must be carefully considered to ensure the structural integrity of portal frames:

Load Calculations

Accurate load calculations are essential for a portal frame design. Considerations include dead loads (weight of the structure itself), live loads (occupant and equipment loads), wind loads, seismic loads, and any other relevant loads specific to the project.

Adhering to local building codes and standards is crucial when determining the magnitudes and combinations of these loads.

Load Combinations

Combining different types of loads is vital to accurately assess the portal frame's structural response. Load combinations involve considering the simultaneous effects of multiple loads, such as dead load with live load, wind load, or seismic load.

Combinations should be determined in accordance with applicable design codes, such as the International Building Code (IBC) in the United States.

Material Selection

Choosing the appropriate material for portal frames is critical. Structural steel is commonly used due to its high strength-to-weight ratio and versatility.

The material selection should consider factors such as the required load-carrying capacity, environmental conditions (corrosion resistance), and architectural considerations. Other materials, such as reinforced concrete or timber, may be used for specific applications.

Structural Stability

Ensuring the stability of portal frames is crucial for their performance. Stability considerations include assessing the frame's resistance against buckling, lateral-torsional stability of beams, and stability of columns. Bracing systems, moment connections, and appropriate member sizing are key aspects of maintaining stability.

The outputs of the portal frame analysis wizard can be input into software design tools ClearCalcs calculators to detail the design of the portal frame. ClearCalcs can be utilised for the bracing system design (which often use steel beams) and the rafter design using the steel beam calculator. The moment connections can be designed by iteration using the steel bolt connection and welded connection calculators. The stability of the columns can be assessed using ClearCalcs’ steel column calculator.

Connection Design

The design of connections between beams and columns is critical to portal frames' overall performance and strength.

Moment-resisting connections, such as bolted or welded connections, are commonly used to transfer loads and moments between members.

The design should consider the required connection capacity, shear resistance, moment transfer, and compatibility with the chosen material.

Deflection and Serviceability

Portal frames must be designed to meet serviceability requirements, which include controlling deflections and vibrations.

Excessive deflections can impact the functionality of the structure and its aesthetic appearance. Designers should consider deflection criteria provided by relevant codes and standards to ensure occupant comfort and structural performance.

Tools like Portal Frame Analysis Wizard allows you to perform quick FEA analysis of a 2D portal frame to determine deflection for simple and common applications.

Figure 7: Deflection diagram for Portal Frame Analysis in ClearCalcs (Reference)

Architectural Integration

Portal frames often play a prominent role in the architectural aesthetics of a building. Design considerations should consider the desired architectural features, such as the shape, height, and proportions of the frame.

Integration with other building elements, such as cladding systems and architectural finishes, should also be considered.

Constructability and Cost-Efficiency

Designing portal frames that are constructible and cost-efficient is important. Considering factors such as standard sizes of steel sections, ease of fabrication and erection, and minimizing waste can contribute to a more economical design.

Conclusion

In conclusion, mastering portal frames design involves understanding their types, design considerations, and compliance with applicable building standards and codes. Portal frames offer versatile and cost-effective solutions for residential and light commercial projects, providing open and flexible spaces while maximizing usable area.

For more resources on portal frame design, check out these other guides and resources:

• Structural Analysis of Portal Frames: Worked Examples and Case Studies
• How to Analyze a Portal Frame in ClearCalcs
• Worked Example of Portal Frame Analysis in ClearCalcs