Design & Engineering

Bracing Systems in Steel Buildings

May 20, 2026

Introduction

A portal frame provides strong stability in its transverse plane. However, a building also needs stability along its length.

Longitudinal actions can arise from wind, crane operations, erection effects, and other horizontal forces. In a braced steel building, these forces must be transferred through a continuous and adequately designed load path toward the foundations.

Bracing establishes this load path and connects the individual transverse frames into a three-dimensional structural system. This creates an important distinction: portal action provides transverse stability, while longitudinal stability is commonly provided by a separate bracing or other stability system.

This article focuses primarily on bracing systems used in single-storey portal-frame steel buildings, where longitudinal stability is commonly provided by a combination of roof plan bracing and vertical wall bracing.

1. Why Steel Buildings Need Bracing

Portal frames are inherently stable in the plane of the frame. Stability along the building length, however, requires a suitable longitudinal stability system.

Longitudinal forces can come from:

Wind acting on gable ends and longitudinal walls
Crane surge and traction
Erection effects and temporary loads
Other horizontal forces acting parallel to the building length

Bracing serves two essential purposes. First, it resists longitudinal actions and transfers them through the structural system toward the foundations. Second, it provides restraint to structural members that would otherwise be insufficiently supported in the longitudinal direction.

In a typical portal-frame building, the longitudinal stability system may include roof plan bracing, vertical wall bracing, eaves struts, and the secondary steelwork and connections that transfer forces between them.

Where conventional vertical bracing cannot be accommodated, alternative stability systems such as portalised bays or other moment-resisting arrangements may be required.

The final system must therefore be designed as a complete load path rather than as a collection of individual braces.

For further technical background, see the SCI guidance on portal-frame design and longitudinal stability.

2. Roof Plan Bracing

Roof plan bracing is installed within the plane of the roof, typically between purlins and the primary frame.

Its functions can include:

Transferring longitudinal forces through the roof plane
Transferring wind forces from gable-end framing to the vertical bracing system where the arrangement requires it
Transferring relevant roof-level drag forces to the vertical stability system
Providing a stiff anchorage for purlins and other secondary members that contribute to primary-member restraint
Contributing to construction-stage stability

The bracing arrangement must form a continuous and credible load path rather than simply placing X-braces in an arbitrary bay. Discontinuous or poorly connected bracing cannot reliably transfer forces through the intended structural system.

Where roof plan bracing is used to collect forces from the gable-end framing, its connection to the appropriate gable-end structural members is important for efficient force transfer.

Typical bracing members include round rods, angles, flat bars, hollow sections, and other suitable tension or compression members. The selection depends on the structural design, force levels, member behavior, and connection requirements.

3. Vertical Wall Bracing

Vertical bracing is normally located in selected wall bays. Its main role is to transfer longitudinal horizontal forces toward the foundations.

It can also:

Stabilize the building during erection
Provide a stable framework for side rails
Contribute to the overall longitudinal load path
Provide restraint to the primary frame where the structural arrangement relies on it

Bracing may be located in one or more bays depending on building length, openings, crane requirements, architectural constraints, load paths, and structural design.

Portal-frame guidance notes that vertical bracing can be arranged at one or both ends of a building. Alternative portalised bays may be required where conventional bracing cannot be accommodated, such as in locations with large openings or specific architectural requirements.

The final arrangement should provide a clear and continuous load path while remaining compatible with openings, access, crane systems, and the overall building layout.

4. Eaves Struts and the Connection Between Bracing Systems

Roof plan bracing and vertical wall bracing do not always occur in the same building bay. Where their locations differ, an eaves strut or other suitable transfer member may be required to connect the two stability systems.

The eaves strut can transfer forces from the roof bracing into the wall bracing while also helping restrain the tops of the columns. Its exact role depends on the structural arrangement and the forces that need to be transferred.

The eaves strut is therefore more than a simple secondary member. It can form a critical link in the longitudinal load path:

Roof plan bracing → Eaves strut → Vertical wall bracing → Foundations

The connection between these components must be capable of transferring the required forces in the relevant load cases.

The eaves region can also be important during erection. The stability provided by the completed structure should not be assumed to exist before the permanent bracing, eaves members, purlins, and other required components have been installed and connected.

For further guidance on the role of eaves struts in portal-frame stability, see the SCI portal-frame guidance.

5. Bracing Member Selection

Bracing does not have to use one universal section type. The choice depends on whether the member is intended to act primarily in tension or in both tension and compression.

5.1 Round Rods

Round rods are commonly used in tension-bracing systems where the design arrangement allows the member to act primarily in tension.

Advantages:

Lightweight
Simple
Easy to install
Economical for many tension-only applications

Their use should be consistent with the design assumptions for load reversal, connections, and the overall bracing arrangement.

5.2 Angle Sections

Angle sections are useful where greater robustness or compression resistance is required.

Advantages:

Familiar fabrication
Suitable for bolted connections
Can provide compression capacity when designed appropriately
Available in a wide range of structural configurations

Angle sections can also be useful where a bracing member must accommodate both tension and compression actions.

5.3 Hollow Sections

Circular hollow sections (CHS) or rectangular hollow sections (RHS) can provide higher stiffness, good compression performance, and clean structural geometry.

They can also be suitable for robust connection arrangements when appropriately detailed. However, they generally involve greater fabrication and connection requirements than simple rod bracing.

5.4 Flat Bars and Crossed Flats

Flat bars or crossed flats may also be used in some vertical bracing arrangements, particularly where lightweight tension bracing and simple connections are required.

Their suitability depends on the required force capacity, slenderness, connection arrangement, and whether the member is intended to act in tension only or in both tension and compression.

There is no universally best bracing section. Selection should follow the structural design, force levels, member behavior, connections, fabrication requirements, and construction constraints.

For European steel structures, member and connection design should be coordinated with the applicable Eurocode requirements. Eurocode 3 includes general rules for steel buildings, provisions for joints, and specific provisions for tension components.

6. Tension-Only vs Compression-Capable Bracing

This distinction is extremely important.

Some bracing systems are intended primarily to resist tension. Round rods are a common example. Other bracing members may need to resist both tension and compression.

This affects:

Member selection
Slenderness
Connection design
Buckling behavior
Erection sequence
Response to load reversal

Wind can reverse the direction of longitudinal loading. Therefore, the designer must understand whether a bracing member is intended to work in tension only or in both tension and compression.

In a crossed tension-only system, load reversal may cause the opposite diagonal to become the active tension member rather than requiring the same member to resist compression.

For compression-capable members, buckling resistance and the associated effective length and restraint conditions also need to be considered.

The intended structural behavior should never be assumed from appearance alone.

7. Bracing Connections

A bracing member is only useful when its connection can transfer the required force.

Typical connection components include:

Gusset plates
Bolts
Cleats
End plates
Brackets
Welded attachments

The gusset plate, bolts, welds, and supporting primary member must be considered as part of the same force-transfer mechanism.

Connection detailing affects:

Force transfer
Fabrication
Installation access
Tolerance
Bolt installation
Adjustment
Erection sequence

A theoretically adequate bracing member can still create construction problems if the connection is difficult to assemble, cannot accommodate reasonable tolerances, or does not provide the required force path into the primary structure.

Good connection design should therefore consider not only resistance but also practical fabrication and installation requirements.

For European projects, the design of steel connections is addressed within EN 1993-1-8, while the overall steel member design is covered by the relevant parts of EN 1993.

8. Bracing and the Complete Load Path

Bracing members should not be considered in isolation. A single brace, no matter how well designed, cannot function without a complete load path.

A typical longitudinal load path may run from:

The point of force application, such as wind on a wall or crane surge
Through the roof or wall bracing system
Through the eaves transfer system where required
To the vertical bracing bays
Through the foundations

The exact load path depends on the structural arrangement.

If any link in this path is missing or inadequately connected, the bracing system cannot perform its intended function.

The purlins, girts, bracing members, restraint systems, connections, cladding, and primary frame should therefore be considered together wherever their structural interaction is relied upon in the design.

For more information on the secondary steelwork that interacts with these stability systems, see Purlins and Girts in Steel Buildings.

9. Temporary Stability During Erection

A building that is stable after completion may not yet be stable during construction.

This is particularly important for:

Roof plan bracing
Wall bracing
Purlins
Side rails
Eaves struts
Temporary restraints

Roof plan bracing and vertical wall bracing can contribute to stability during erection, while purlin and side-rail restraint can also be relevant before the cladding system is fully installed.

The erection sequence should therefore be considered when designing and detailing the bracing system. In some cases, temporary bracing may be required until the permanent system is fully installed and connected.

The permanent bracing arrangement should be checked against the actual erection sequence rather than assuming that the completed structure and the partially erected structure have the same stability system.

This is particularly important where the completed building relies on the interaction between primary frames, secondary steelwork, bracing, and cladding restraint.

For further technical guidance on erection-stage stability in single-storey buildings, see the SCI Engineering Guide to Single-Storey Buildings.

10. Common Bracing Design Mistakes

Several mistakes occur when bracing is treated as a collection of independent members.

Mistake 1: Assuming all bracing members work the same way.
Tension-only and compression-capable systems behave differently and require different design assumptions.

Mistake 2: Ignoring load reversal.
Wind can change the direction of longitudinal forces. The bracing system must be designed for the actual load cases, not just the most obvious one.

Mistake 3: Neglecting connection detailing.
A strong bracing member with an inadequate connection cannot transfer forces effectively.

Mistake 4: Creating discontinuous load paths.
Bracing must form a complete path from the point of load application to the foundation, including any required transfer between roof and wall bracing.

Mistake 5: Assuming roof and wall bracing are automatically connected.
Where the roof plan bracing and vertical wall bracing are located in different bays, an appropriate transfer system such as an eaves strut may be required.

Mistake 6: Overlooking erection-stage stability.
The permanent bracing system may not be in place during early construction stages, and the partially erected structure may have different stability requirements from the completed building.

Conclusion

Bracing systems are an essential part of the three-dimensional stability of many steel buildings. Portal frames provide transverse stability, while longitudinal stability is commonly provided by roof and wall bracing or, where conventional bracing cannot be accommodated, by other suitable stability systems such as portalised bays.

Roof plan bracing, vertical wall bracing, eaves struts, secondary steelwork, connections, and foundations must work together to create a continuous and credible load path.

Member selection, connection detailing, load reversal, restraint assumptions, and erection sequencing all influence whether the bracing system performs as intended. The designer must understand whether members are intended for tension only or for both tension and compression, and must ensure that the connections can transfer the required forces.

A bracing system is therefore not a collection of independent members. It is a coordinated structural system that connects the building's primary and secondary components and transfers longitudinal actions through the building to its foundations.

Key Takeaways

Portal frames provide transverse stability, while longitudinal stability requires a suitable stability system, commonly including roof and wall bracing in single-storey portal-frame buildings.

Roof plan bracing transfers longitudinal forces through the roof plane, while vertical wall bracing transfers these forces toward the foundations.

Eaves struts can form a critical link between roof and wall bracing, particularly when the two systems are located in different bays.

Bracing members may be round rods, angles, flat bars, CHS, or RHS, selected according to force levels, member behavior, connections, and construction requirements.

Bracing systems must be designed as complete load paths, with tension/compression behavior, connections, secondary steelwork, and erection-stage stability considered together.

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