Bowstring Truss Roof System Design and Issues

A bowstring truss roof system is a type of roof design that uses a unique combination of triangular and curved elements to provide structural support.

The bowstring truss roof system design is typically used in large, open spaces such as warehouses, arenas, and airports.

This design is particularly useful for areas with high ceilings and open floor plans.

The bowstring truss system is made up of a series of triangular trusses that are connected by curved bowstrings, which provide additional support and stability.

The bowstring truss roof system has been used in various applications, including industrial and commercial buildings.

One of the main issues with the bowstring truss roof system is its susceptibility to damage from natural disasters such as earthquakes and hurricanes.

The bowstring truss system can also be prone to structural issues if not properly designed or maintained.

Regular inspections and maintenance are crucial to prevent potential problems with the bowstring truss roof system.

Bowstring trusses have a unique shape, resembling an upturned bow with a curved top chord and a straight bottom chord. The top and bottom chords are made of heavy wood timber boards, or "plies", that are interconnected using steel bolt fasteners and splice plates.

The combination of chord members and web framing allows bowstring trusses to effectively resist flexural and shear loading. This makes them a reliable option for large buildings that require a lot of open space, like warehouses and aircraft hangars.

Bowstring truss roofs are typically large and arched, providing ample support for heavy loads. They don't need a ridge cap, and water can easily slip off the roof, making them a practical choice for certain building types.

Wood is a naturally occurring and often abundant material that's been fundamental to construction since pre-historic times.

Wood is valued for its high strength and lightweight properties, making it a popular choice for construction. Wood can be effectively worked with simple hand tools, even in remote locations where power tools would be impractical.

Recommended read: Wood for Garden Shed Roof

The history of wood construction is nearly tantamount to the history of construction itself, with wood being used for heavy timber framing, modern stick framing, and single-family home construction throughout the United States.

Wood has been used to create long-span structures, such as bridges, warehouses, and airport hangers, including one such modality, the bowstring truss.

For another approach, see: How to Frame a Lean to Shed Roof

Bowstring trusses have a distinctive shape, with a curved top chord and a straight bottom chord. The top and bottom chords are made of heavy wood timber boards, or "plies", that are interconnected with steel bolt fasteners and splice plates.

Individual trusses are commonly spaced 20 to 25 feet apart, and they're connected with wooden lateral bracing members. This allows for the creation of large open span spaces without the need for internal columns.

The top and bottom chords of a bowstring truss are interconnected by vertical or diagonally oriented web framing. This web framing can take the form of truss framing or lattice framing.

Bowstring trusses are often used to support large buildings like warehouses and aircraft hangars, which require lots of open space.

For your interest: Shed Roof Overhang Framing

Bowstring trusses are susceptible to fire damage, which can reduce their load-carrying capacity. This means that even a small fire can cause significant damage.

Historical wooden bowstring trusses often don't meet modern building code requirements for snow and wind loading. This can lead to over-stressing and even collapse.

Unbalanced snow loading is a common issue that occurs when heavy snowfall combines with strong winds, pushing the snow to one side of the structure. This can cause fractured members and roof sagging.

Wood rot and insect attack can also cause long-term deterioration, especially in areas of elevated moisture. Maintaining an effective roofing membrane above wooden framing is crucial to prevent this.

A detailed structural analysis can determine if an existing bowstring truss is adequate for modern loading. If it's found to be deficient, additional wooden plies or steel reinforcement can be installed to increase its load capacity.

Forensic Issues in Structures can be a major concern for historic buildings. Many wooden construction elements, including bowstring trusses, are susceptible to fire damage.

Wooden bowstring trusses can be severely impacted by fire damage, reducing their load-carrying capacity. Repairs may involve reinforcing or replacing individual plies of a truss.

Historical wooden bowstring trusses often don't meet modern building code requirements for snow and wind loading. This can lead to over-stressing of the trusses, particularly under unbalanced snow loading conditions.

Unbalanced snow loading occurs when heavy snow builds up on one side of a roof structure, often due to strong winds. This can cause fractured members and sagging or collapse of the roof structure.

Wooden bowstring trusses can also experience long-term deterioration due to rot or insect attack. Moisture accumulation in these areas is a major contributor to wood rot.

A detailed structural analysis model can help determine if an existing bowstring truss is adequate for modern loading. If found deficient, additional wooden plies or reinforcement can be installed to increase the truss's load capacity.

Wood is susceptible to fungal deterioration when consistently or repeatedly exposed to moisture.

Moisture content of 20% or higher can lead to decay, which can easily occur near roof leaks, especially around gutters or skylights.

The ends of trusses embedded in masonry walls are particularly vulnerable to moisture exposure.

Once exposed to moisture, these areas can't dry out, making them prone to decay.

Corrosion of U-shaped steel straps can also occur when water is present at truss ends.

Embedded bearing ends should be investigated through probing, visual observations, surface pick tests, and resistograph tests.

Performing a thorough visual inspection is paramount when evaluating bowstring trusses due to the lack of available documentation. This inspection involves careful documentation of the general truss configuration, member sizes, and connection details.

A combination of small aerial lifts and ladders is often necessary to achieve sufficient access to all truss members and their connections, which can be challenging due to existing obstructions and concealed conditions. The general roof characteristics, such as truss spacing and roofing construction, should also be documented during this phase.

Observation of readily visible distress, such as full member splits and shakes, is critical during the general visual inspection.

Historic structures like bowstring trusses require specialized knowledge for inspection.

Inspecting these structures needs the expertise of experienced forensic experts.

Bowstring trusses were widely used in the United States from around 1900 to 1950.

They allowed for large open span spaces without internal columns, a desirable condition for many applications.

Individual trusses are commonly spaced at approximately 20 to 25 feet apart.

These trusses are interconnected with wooden lateral bracing members.

The open span spaces afforded by bowstring construction were commonly used in manufacturing and warehousing applications.

Existing wooden bowstring truss buildings remain in service, and have become desirable as public gathering spaces.

Their visible wooden trusses overhead are considered a desirable aesthetic feature.

The distinct barrel shaped roofs of bowstring truss buildings can often identify them from the exterior.

Related reading: Life Span of a Roof

Field investigation of bowstring trusses can be quite challenging due to lack of documentation and restricted access.

Drawings of existing trusses are rarely available, making field documentation and visual inspection crucial.

A thorough visual inspection is necessary to evaluate the truss's structural capacity, but accessing all truss members and their connections can be difficult due to existing obstructions and concealed conditions.

Small aerial lifts and ladders are often used to navigate through equipment and hung features to achieve sufficient access.

Each structure presents unique features, but once access is achieved, most field investigations follow the same methods.

A general visual inspection includes documenting the truss configuration, member sizes, and connection details.