In the truss industry, camber refers to the gradual curvature of a chord member either naturally occurring or created to alleviate the natural deflection of a truss structure. That sounds simple enough, but there are key elements about camber that should be considered.
Camber in Action
All structures, regardless of type or form, will sag. To use the engineering term, we say they “deflect” under loading. This is principally a design concern and parameter. If it doesn’t flex, it’s brittle. If it’s brittle, then you have another problem. [For images, See PDF or View in Full Issue.]
Deflection is generally not a problem if the design can accommodate for the deflection. Deflection can exist as an entire structure or can be experienced in segments. An example would be the bow of a wood element between panel points. Common examples of structures that are designed to accommodate potential bowing are flat bed trailers that haul freight around the country or the bridges across which the trucks drive.
Deflection is a problem because it can cause appearance issues or create ponding issues. Deflection also creates movement in structures that can translate into breaching the building envelope. When a board crowns up, it is considered good. Crown down looks like a failure.
Inducing camber means to create a curve and build it into the structure to counteract calculated and anticipated behavior.
Taking Camber into Consideration
Currently, the industry has taken several steps away from camber considerations either because of lack of knowledge or because some products do not allow for camber (such as LVL and wood I-joists).
Fortunately, trusses were originally designed using camber. In fact, trusses need camber to create a better product. (Note that laminated beams also anticipate camber.) Thus, these products that are inherently capable of adding camber produce better results.
Furthermore, camber remains an important design consideration and is required by many specifications and municipalities. Often good engineering practice dictates cambered design.
Most floor truss machines utilize a very specific camber that is not adjustable for the length of the truss. These machines are built with a fixed guide segment of a large radius curve. A few machine designs have adjustable camber which allows the builder to build trusses with zero camber or increased camber to accommodate heavier loads. Lack of floor camber is generally much more noticeable and can create many problems in residential and commercial construction. Slopes in floors due to deflection are very noticeable and create a concern for structural soundness and dependability (particularly in the homeowner’s mind).
Roofs are another matter. Most roofs today are not designed for camber. This is not because our technology is incapable, but because designers are unwilling or are unaware of the benefits or the potential requirements some projects demand. For example, flat roof designs are plagued by ponding effects when not adequately pitched or cambered, creating potential overload situations.
Long roof spans in large buildings will visibly deflect, especially if no camber is employed. Often these situations are magnified by line-of-sight situations, such as walking down steps into a large room where the ceiling lines are illustrated clearly. Driving up to a simple pole barn will also give a clear example of when camber should be employed.
So, how should camber be determined? In most floor systems utilizing automatic cambered equipment, no decision needs to be made. The camber is a set radius. The amount of curve will be a function of the length. In most situations, except for long span trusses, this will be acceptable. For long span trusses, particularly connected or integrated with shorter span trusses, there will be an issue. It’s called deflection differential, but that’s a discussion for another day.
In both roof and floor designs, always read your specifications carefully and follow the architect’s requirements. This may also be mandated by the municipality. When in doubt, analyze your company’s complaints and resolution data. Ask your client. Follow the recommendations from your engineers.
Camber Can Be Accomplished in Two Ways
As far as roof truss manufacturing camber needing to be incorporated into the design parameters, there are ways to adjust cutting, or not adjust cutting, to build camber.
- Incorporate camber by simply employing a gradual curve in the truss without adjusting cutting and adjusting joint locations to close gaps. Warning: this method makes it difficult to follow laser settings for plating.
- The longer the span, the less likely cutting will need to be adjusted.
- Lasers will follow the design, as will puck systems if camber option is selected.
- Design camber into trusses, allowing the software to adjust web lengths and angles to make curves.
- This method is not generally required on long spans.
- This process can negatively impact gable ends that are continuously supported.
- Use this method to make laser projections more accurate.
The Bottom Line
When possible, camber should be adjusted to counter the impact of permanent loads and dead loads. In reference to the tractor trailer, the trailer design considers live load and dead load. Within the truss industry, total load deflection should not be considered when incorporating camber. The reason for this is that permanent and dead loads will be very close to actual load conditions. Live loads can and will vary depending on the building’s use and design, and may never be achieved, resulting in a dramatic camber up in the structure.
An ANSI/TPI 1 3rd Party Quality Assurance Authorized Agent covering the Southeastern United States, Glenn Traylor is an independent consultant with almost four decades of experience in the structural building components industry. Glenn serves as a trainer-evaluator-auditor covering sales, design, PM, QA, customer service, and production elements of the truss industry. He also provides project management specifically pertaining to structural building components, including on-site inspections and ANSI/TPI 1 compliance assessments. Glenn provides new plant and retrofit designs, equipment evaluations, ROI, capacity analysis, and CPM analysis.