Bowstring Truss: Curved Steel Truss Design for Wide Span Roofs

A bowstring truss is a curved roof truss system used when a building needs a wide span, open interior space, and efficient roof load support. Unlike many triangular roof trusses, this system uses a curved upper chord and a lower chord that ties the ends together. The result is a roof structure that can cover large areas while reducing the need for many internal columns.

This type of truss is often used in warehouses, aircraft hangars, sports halls, exhibition buildings, industrial workshops, transportation facilities, and other steel buildings where interior flexibility matters. The curved profile can also create a stronger architectural identity, especially when the roof shape is part of the building’s visual design.

However, the value of a bowstring truss does not come from the curve alone. Its performance depends on the span, roof load, bottom chord design, web member layout, bracing system, connection details, fabrication method, transportation plan, and erection sequence. When these factors are properly coordinated, a bowstring truss can become a practical solution for wide span steel roofs.

What Is a Bowstring Truss?

A bowstring truss is a roof truss with a curved or arched upper chord and a lower chord that acts as a tie. The upper chord resembles the curved shape of a bow, while the lower chord works like the string that connects the two ends. This is where the name “bowstring” comes from.

In structural terms, the curved upper chord helps carry roof loads along an arch-like path. At the same time, the bottom chord helps resist the outward thrust that can develop at the supports. Internal web members connect the top and bottom chords, divide the span into smaller structural zones, and help transfer forces through the truss.

Bowstring trusses can be made from timber, steel, or other materials. For modern industrial and commercial buildings, steel is often preferred because it provides high strength, fabrication accuracy, durable performance, and good long-span capability. Steel members can also be cut, welded, bolted, coated, marked, and transported with better dimensional control.

Why the Curved Shape Matters

The curved shape is not only an architectural feature. It also affects how forces move through the roof system. A straight beam spanning a wide distance may need to become very deep or heavy to control bending and deflection. A properly designed curved truss can distribute load more efficiently through axial forces in the chords and web members.

This does not mean every curved roof should use a bowstring truss. The shape must match the building’s span, loading condition, roof system, support arrangement, and installation method. If the curve is selected only for appearance without proper structural planning, the truss may become expensive or difficult to fabricate.

Common Steel Building Uses

A bowstring truss is useful when the building requires large interior areas without many columns. This makes it suitable for logistics buildings, wide industrial workshops, hangars, sports facilities, markets, transit buildings, storage halls, and large public spaces.

For example, an aircraft hangar needs a wide clear opening and open internal movement. A warehouse may need uninterrupted storage or racking space. A sports hall may need a large floor area without internal obstructions. In these cases, the curved truss roof can help provide both structural support and usable interior space.

How a Bowstring Truss Works

A bowstring truss works by combining the curved action of the upper chord with the tying action of the lower chord. Roof loads are not carried by one solid beam. Instead, the loads are transferred through a system of connected steel members.

In a typical roof structure, the load path begins at the roof panels. The roof panels transfer load to the purlins. The purlins transfer load into the curved top chord of the truss. From there, forces move through the web members and bottom chord before reaching the building supports, columns, or main frame system.

This load path must be clear. If purlins, bracing, web members, or connections are not properly coordinated, the truss may not perform as intended. A wide span roof system needs every part to work together, not as isolated pieces.

Curved Top Chord

The top chord is the most recognizable part of a bowstring truss. It forms the curved upper profile and receives loads from the roof purlins. Depending on the loading condition and geometry, the top chord often works mainly in compression.

Because compression members can buckle, the top chord needs proper lateral restraint. Roof purlins, bracing members, and connection details all help control stability. If the top chord is not restrained correctly, the truss may have strength on paper but still face instability problems in real construction.

Bottom Chord as a Tie

The bottom chord is a critical part of the system. It helps tie the ends of the truss together and resist outward spreading forces. Without an effective bottom chord, the curved top chord may push outward at the supports and create additional demand on columns, walls, or foundations.

In many bowstring truss roofs, the bottom chord works mainly in tension. However, the exact force depends on the truss geometry, support condition, load combination, and bracing arrangement. If the bottom chord also supports suspended services such as lighting, ducts, pipes, or ceiling systems, those loads must be included in the design from the beginning.

Web Members

Web members connect the curved top chord and the bottom chord. They divide the wide span into smaller structural zones and transfer forces between the chords. Some web members may work in tension, while others may work in compression, depending on the load case.

The layout of web members affects material use, connection force, fabrication effort, and visual appearance. A clean web arrangement can improve structural efficiency and simplify manufacturing. Poor web alignment can create eccentric forces, difficult welding, and complicated site assembly.

Connections and Nodes

The nodes of a bowstring truss are where members meet and transfer forces. These points are often more complex than they appear. Gusset plates, bolts, welds, splice plates, and hole patterns must be designed according to actual member forces.

A strong steel member cannot perform well if the connection is weak. For wide span trusses, connection design is especially important because forces can be large and repeated across many nodes. Fabrication accuracy, bolt alignment, weld quality, and inspection access should all be considered during the design stage.

Bowstring Truss Design for Wide Span Roofs

A bowstring truss is often selected for wide span roofs because it can create a large covered area with fewer internal supports. The curved geometry allows the roof structure to distribute loads across the span while maintaining open space below.

This is useful for buildings where columns would interfere with operations. In a logistics center, fewer columns can improve forklift movement and storage layout. In a manufacturing workshop, open space can support flexible production lines. In a hangar, wide clear span is essential for aircraft movement. In a sports hall, unobstructed interior space improves usability.

Span Range Considerations

The suitable span for a bowstring truss depends on many factors. Building width, roof load, wind condition, snow or rain load, purlin spacing, steel grade, truss depth, fabrication capacity, and installation access all influence the final design.

A moderate span may be easy to fabricate and install. A very long span may require deeper truss geometry, larger chord members, stronger connections, segmented transport, and heavier crane planning. For this reason, span should not be decided only by architectural preference. It should be reviewed together with fabrication, shipping, erection, and long-term roof performance.

Roof Height and Interior Clearance

The curved form of a bowstring truss can create more roof volume than some flat or low-slope systems. This can be useful when the building needs ventilation space, higher storage, equipment clearance, or a more open interior feel.

However, extra roof height also affects wind exposure, cladding area, transportation dimensions, and installation planning. The final roof height should be practical for the building use and economical for construction. A roof that looks impressive may still create unnecessary cost if the height is not needed.

Deflection Control

Wide span roof structures must control deflection carefully. A truss can be strong enough to resist failure but still deflect too much for the roof system. Excessive deflection can affect roof panels, purlin alignment, drainage, ceiling systems, doors, cladding, and long-term maintenance.

Deflection control depends on truss depth, chord size, web layout, connection stiffness, load combinations, and support conditions. For large buildings, serviceability is just as important as strength. A good design should check both structural safety and practical roof performance.

Key Components of a Bowstring Truss

A bowstring truss roof system includes more than the curved truss itself. The top chord, bottom chord, web members, gusset plates, purlins, roof bracing, temporary bracing, and support system must all be coordinated. If one component is weak or poorly detailed, the whole roof structure can become less reliable.

Top Chord

The top chord creates the curved roof profile and carries loads from the purlins. It usually requires careful compression design and lateral restraint. For steel construction, the top chord may be fabricated from curved members, segmented straight members, or built-up sections depending on the project requirement and fabrication method.

The top chord must also match the purlin layout. If purlins do not land at suitable points, the chord may receive unexpected bending or eccentric load. Coordinating the roof secondary framing with the truss geometry is therefore essential.

Bottom Chord

The bottom chord ties the ends of the truss together. It helps resist horizontal thrust and completes the main force system. Its design should consider tension force, splice details, support connection, corrosion protection, and any suspended loads.

If lighting, ducts, pipes, cable trays, or fire protection systems will hang from the roof, the allowed connection points should be defined clearly. Randomly attaching services to the bottom chord after installation can create unplanned stresses.

Web Members

Web members transfer forces between the top and bottom chords. Their size and arrangement depend on the span, load, truss depth, and connection layout. In wide span roofs, even small changes in web geometry can influence steel tonnage, fabrication labor, and connection demand.

Clear member marking is important during fabrication and site assembly. Many web members can look similar but have different lengths, angles, or connection details. Accurate shop drawings, CNC processing, and proper packing sequence help reduce installation mistakes.

Gusset Plates and Bolted or Welded Connections

Gusset plates and connections are critical in bowstring truss design. They transfer force between members and maintain the intended truss geometry. Bolted connections can make site assembly easier, while welded connections can be efficient for shop fabrication.

The best connection method depends on member size, transportation limits, erection sequence, coating system, inspection requirements, and local construction practice. For large trusses, connection details should be simple enough to fabricate accurately and strong enough to transfer real design forces.

Purlins and Roof Secondary Framing

Purlins carry roof panel loads and transfer them to the truss. They may also help restrain the top chord when properly connected. Purlin spacing should be coordinated with the truss node layout, roof panel system, insulation, wind uplift demand, and maintenance access.

Secondary framing may also include edge members, eave supports, wall supports, gutters, bracing members, and service supports. These elements should not be designed separately from the main truss system. They are part of the overall roof performance.

Bracing System

Bracing is essential for both construction safety and long-term stability. Permanent bracing helps the truss system resist lateral movement, buckling, and wind effects during building service. Temporary bracing keeps the truss stable during lifting and erection before the permanent roof system is complete.

For wide span curved trusses, temporary bracing is especially important. A truss may be stable after all purlins and roof bracing are installed, but unstable during the early erection stage. The installation sequence should clearly define when temporary supports, purlins, and permanent bracing are added.
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Advantages of Bowstring Truss Roof Systems

A bowstring truss roof system offers several advantages when the building requires a wide span, a curved roof profile, and open usable space below. Its curved top chord and tied bottom chord can create a strong structural system while still allowing the roof to remain visually distinctive.

One of the biggest benefits is interior flexibility. In buildings such as warehouses, aircraft hangars, sports halls, and industrial workshops, internal columns can interrupt movement, storage, equipment placement, and production flow. A properly designed bowstring truss can reduce the need for intermediate supports and create a more open working area.

The curved geometry can also improve the way roof loads are distributed. Instead of relying on a single heavy beam, the system transfers loads through chords, web members, and connections. When the load path is clear, this can create an efficient balance between strength, steel use, and roof coverage.

Key advantages include:

• Wide open interior space with fewer internal supports
• Efficient load distribution through curved truss geometry
• Strong architectural appearance for large roof structures
• Good suitability for warehouses, hangars, halls, and workshops
• Compatibility with steel fabrication and segmented transport
• Potential for practical roof height and better interior clearance
• Flexible integration with purlins, roof panels, and bracing systems

Bowstring Truss vs Other Roof Truss Types

A bowstring truss is not the only option for steel roof framing. Its value depends on the building span, roof shape, load condition, budget, fabrication method, and installation plan. In some projects, another roof system may be simpler or more economical. In other projects, the curved geometry of a bowstring truss may provide the best balance between open space and structural performance.

Bowstring Truss vs Fink Truss

A bowstring truss uses a curved upper chord and a bottom tie chord. A Fink truss usually uses a pitched triangular shape with repeated internal web members. The bowstring form is often selected for wide curved roofs, while the Fink form is often practical for pitched steel roofs with efficient triangular load paths.

Compared with the curved profile of a bowstring truss, the repeated triangular layout and practical roof framing logic behind Fink truss advantages may be more suitable for certain pitched roof steel buildings. The best choice depends on span, roof slope, interior clearance, roof load, fabrication cost, and the required appearance of the building.

Bowstring Truss vs Pratt Truss

A Pratt truss usually has straight chords and diagonal web members arranged to create clear tension and compression behavior. It is often used where straight structural geometry is preferred. A bowstring truss, by contrast, is defined by its curved top chord and tied lower chord.

For projects that need a curved roof profile or a large open roof volume, a bowstring truss may be more suitable. For projects that need a simple straight-span truss with predictable diagonal force paths, a Pratt truss may be easier to fabricate and install.

Bowstring Truss vs Warren Truss

A Warren truss uses repeated triangular patterns, often without many vertical members. It can provide efficient force distribution with a relatively simple and repetitive layout. A bowstring truss uses a curved upper profile, so fabrication and connection planning may be more complex.

The Warren layout may be practical for straight truss applications, bridges, and certain roof systems. The bowstring layout may be chosen when the roof shape itself is part of the required structure or architecture.

Bowstring Truss vs Portal Frame

A portal frame is often the simplest and most economical option for many standard steel warehouses and factories. It uses rigid frames, rafters, columns, purlins, and bracing to form the main structure. For many rectangular industrial buildings, a portal frame can be faster and easier to build.

A bowstring truss may become more useful when the project needs a wider open span, a curved roof profile, or a roof form that cannot be achieved efficiently with a standard portal frame. The decision should compare structural efficiency, fabrication complexity, installation cost, roof height, and long-term building use.

Applications of Bowstring Truss in Steel Buildings

A bowstring truss can be used in different types of steel buildings where large roof coverage and open interiors are important. Although the same basic truss concept can be applied across many projects, each building type has different design priorities.

Warehouses and Logistics Centers

Warehouses and logistics centers often need large open spaces for racking, forklift movement, loading areas, and storage flexibility. Internal columns can reduce storage efficiency and complicate circulation. A bowstring truss roof can support wide coverage while helping keep the interior layout more flexible.

The roof design must still consider wind uplift, drainage, insulation, suspended lighting, fire protection systems, and possible future changes to storage layout. For logistics buildings, practical installation speed and cost control are also important.

Aircraft Hangars

Aircraft hangars require wide clear spans and large open interiors. Aircraft movement, maintenance access, and large door openings make internal columns undesirable. A bowstring truss can be suitable when the curved roof profile and wide-span capacity match the hangar layout.

For hangars, the design must also consider large wind loads, door-frame coordination, roof bracing, service platforms, lighting, ventilation, and corrosion protection. The truss system should be coordinated with the full building frame, not designed as a separate roof element.

Sports Halls and Gymnasiums

Sports halls, gymnasiums, and indoor activity centers require open floor areas with limited obstruction. A bowstring truss can provide both roof support and an appealing curved interior profile. The roof volume may also help with lighting, ventilation, and acoustic planning.

In these buildings, deflection control and ceiling coordination are important. Suspended lighting, sound systems, insulation, and interior finishes should be planned before the truss design is finalized.

Industrial Workshops

Industrial workshops may need open production areas, machine clearance, ventilation ducts, cable trays, and flexible equipment layouts. A bowstring truss can help provide clear interior space, especially where standard roof framing would require too many internal supports.

If the workshop includes cranes, heavy suspended equipment, or large mechanical systems, these loads must be reviewed carefully. A roof truss should not be assumed to support heavy services unless those loads are included in the design.

Exhibition and Public Buildings

Exhibition halls, markets, transportation terminals, and public buildings may use bowstring trusses for both structural and visual reasons. The curved roof can create a more open and recognizable space. It can also help cover large gathering areas with fewer internal obstructions.

For public buildings, appearance, fire safety, maintenance access, coating durability, and service integration may be just as important as the structural span.

Design Factors for Bowstring Truss Structures

A bowstring truss must be designed according to real project conditions. The curved shape does not automatically guarantee strength or economy. Loads, support conditions, bracing, fabrication, and installation must all be considered together.

Dead Load

Dead load includes roof panels, purlins, insulation, ceiling materials, gutters, fixed equipment, connection plates, and the self-weight of the truss. These loads remain on the structure throughout the life of the building, so they must be calculated accurately.

Live Load and Maintenance Load

Live load and maintenance load may include workers, tools, access systems, and roof maintenance activity. Even if the roof is not intended for regular occupation, maintenance access must still be considered. If walkways or equipment platforms are planned, they should be included early.

Wind Uplift

Wind uplift is a major factor for large steel roofs. Curved roof forms can experience different pressure and suction zones, so local wind conditions should be reviewed carefully. Roof panels, fasteners, purlins, bracing, and truss connections all need to resist uplift forces.

Rain and Snow Load

Rain and snow loads depend on the project location, climate, roof slope, drainage system, and roof shape. Poor drainage can create ponding risk, while snow accumulation can increase roof load in cold regions. The truss design should be coordinated with roof drainage and local code requirements.

Suspended Loads

Suspended loads may include lighting, ducts, cable trays, fire pipes, ventilation systems, signs, platforms, and ceiling systems. These loads should not be added randomly after installation. The design should define where suspended services are allowed and how much load each point can support.

Lateral Stability and Seismic Effects

Wide-span roofs also need lateral stability. Wind, seismic action, frame movement, and construction-stage instability must be transferred through bracing, columns, frames, and foundations. A roof truss cannot perform well if the global building stability system is weak.

Fabrication Considerations

Fabrication has a strong influence on bowstring truss cost and quality. The curved top chord may require bending, segmented fabrication, curved built-up members, or straight members arranged to approximate a curved profile. The best method depends on the span, steel section, equipment, tolerances, and project budget.

Important fabrication considerations include:

• Accuracy of curved or segmented chord fabrication
• Cutting and drilling precision for web members
• Welding sequence and distortion control
• Gusset plate thickness and hole alignment
• Splice details for transportable sections
• Surface preparation, painting, or galvanizing
• Member marking and packing sequence
• Trial assembly for large or complex trusses

For export steel structures, fabrication planning is especially important. If members are mislabeled, holes are misaligned, or segments do not fit on site, correction can be costly. Clear shop drawings, CNC processing, quality inspection, and organized packing help reduce these risks.

Installation and Erection Planning

A bowstring truss requires careful installation planning because wide-span curved members can be large, heavy, and flexible during lifting. A truss that is stable after final installation may still be unstable during the erection stage.

The lifting plan should define lifting points, crane capacity, lifting angle, temporary support, and safe assembly sequence. Long trusses may require spreader beams or multiple lifting points to avoid deformation. If the truss is delivered in segments, site assembly must be planned before lifting.

Temporary bracing should be installed before the truss is exposed to unstable conditions. Purlins, roof bracing, and permanent connections should be added in the correct sequence so the roof system gains stability step by step.

After erection, the inspection should check:

• Truss alignment and final geometry
• Bolt tightening and connection fit-up
• Weld quality where applicable
• Coating damage and repair areas
• Purlin connection and roof bracing completion
• Temporary bracing removal only after permanent stability is achieved

Common Mistakes in Bowstring Truss Projects

Cost Factors of Bowstring Truss Roofs

The cost of a bowstring truss roof should be reviewed as a complete system. Steel weight is important, but it is not the only cost driver. Curved fabrication, connection complexity, transport size, crane planning, temporary bracing, and roof system integration can all affect the final budget.

Major cost factors include:

• Steel tonnage and member sizes
• Curved chord fabrication method
• Number and complexity of web members
• Gusset plates, bolts, welds, and splice details
• Surface treatment and corrosion protection
• Segment size and transportation distance
• Crane capacity and lifting method
• Site assembly and temporary bracing
• Purlins, roof panels, insulation, gutters, and roof bracing
• Inspection, maintenance access, and long-term durability requirements

A slightly heavier but simpler truss may sometimes be cheaper than a lighter truss with difficult connections and complicated installation. The best cost strategy is to coordinate engineering, fabrication, transportation, and erection from the beginning.

When Is a Bowstring Truss a Good Choice?

A bowstring truss is often a good choice when the project needs a wide clear span, a curved roof profile, and a large open interior. It is especially useful when internal columns would interfere with building operations or when the roof shape is part of the architectural concept.

It may be suitable when:

• The building needs a wide open span
• The roof design requires a curved profile
• The interior must remain flexible and unobstructed
• The project needs strong roof load support
• The building is a warehouse, hangar, hall, workshop, or public facility
• Steel fabrication accuracy and planned installation are available
• The project budget can support curved fabrication and careful erection planning

It may not be the best choice when a standard portal frame or a simpler triangular truss can meet the same span and roof requirements at lower cost. The right decision should be based on structural demand, building use, roof appearance, fabrication practicality, and total installed cost.

Conclusion

A bowstring truss is a practical curved steel truss system for wide span roofs when the geometry, load path, bottom chord tie, bracing, connections, fabrication, transport, and installation are planned together. Its curved top chord can create an efficient and visually distinctive roof profile, while the bottom chord helps control outward forces and complete the structural system.

The best results come from treating the truss as part of the full roof and building frame. Purlins, roof panels, bracing, columns, foundations, suspended services, and erection planning all influence performance. When these elements are coordinated from the beginning, a bowstring truss can provide reliable roof support for many wide-span steel buildings.

FAQ About Bowstring Truss

What is a bowstring truss?

A bowstring truss is a roof truss with a curved or arched upper chord and a lower tie chord. The curved top chord carries roof loads, while the bottom chord helps resist outward thrust at the supports.

Why is a bowstring truss used for wide-span roofs?

It is used for wide-span roofs because it can create large open interior spaces with fewer internal supports. This makes it useful for warehouses, hangars, sports halls, workshops, and other large buildings.

Is a bowstring truss suitable for steel buildings?

Yes. A bowstring truss can be suitable for steel buildings when the span, roof load, wind condition, bracing, fabrication method, and installation plan are properly engineered.

What is the difference between a bowstring truss and a Fink truss?

A bowstring truss has a curved top chord and a bottom tie chord. A Fink truss usually has a pitched triangular form with repeated internal web members. Bowstring trusses are often used for curved wide-span roofs, while Fink trusses are often used for practical pitched roof structures.

Is a bowstring truss expensive?

The cost depends on span, steel tonnage, curved member fabrication, connection complexity, transportation, surface treatment, crane requirements, and installation sequence. It can be economical when the design matches the building needs, but unnecessary curvature can increase cost.

What loads should be considered in bowstring truss design?

Dead load, live load, maintenance load, wind uplift, rain load, snow load when applicable, suspended service loads, lateral stability forces, and construction-stage loads should all be considered before the design is finalized.