Engineering Criteria for Roof Truss Design

Prefabricated trusses have proven to be a far more efficient alternative than traditional framing for a roof. Often, all trusses can be erected in one day. This relieves the large amount of measuring and skilled labor required to frame a roof. The cost-effective sweet spots are mid-level complexity, where roof designs are more complex than a simple gable roof. Hip roofs, Dutch roofs, and roofs with several dormers lend themselves well to prefabricated trusses. Schools, banks, multi-family housing, housing developments, and churches meet the criteria for cost-saving in mass-producing trusses. With that knowledge, once you have decided to build with LGS trusses, these are the criteria you will have to design and build with.

Governing Engineering Codes

In the USA, commercial construction projects are typically governed by the International Building Code (IBC) and the International Mechanical Code (IMC), and American Society of Civil Engineers (ASCE)

Regarding roofing specifically, the IBC provides guidance on the design and construction of roofs, including requirements for materials, insulation, and drainage. The IBC also includes provisions for wind and fire resistance, as well as requirements for roof ventilation.

In addition to the IBC, other codes and standards may also apply to roofing construction, such as the National Roofing Contractors Association (NRCA) Roofing Manual and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 90.1 for energy efficiency.

Here are engineering loads laid out for a high-end medical clinic on the gulf cost exposed to Hurricane winds. You can see most of the standards are pulled from IBC & ASCE.

Wind Exposure Category

Wind Exposure Category is a way to classify the surrounding environment around a structure based on how much it may affect the wind speed and direction. This classification is then used to calculate the expected wind loads on the structure during a storm or high wind event. The categories range from less obstructed suburban areas to highly obstructed urban or mountainous areas. The design of a structure must take into account the Wind Exposure Category to ensure it can withstand the expected wind loads and remain safe.

• Wind Exposure Category B (WE-CB): Buildings located in urban and suburban areas, wooded areas or other terrain with numerous closely spaced obstructions, such as buildings, trees, and hills.
• Wind Exposure Category C (WE-CC): Buildings located in flat, open areas with scattered obstructions, such as isolated buildings, trees, and other structures.
• Wind Exposure Category D (WE-CD): Buildings located on coastal barrier islands, dunes, and beaches, exposed hilltops, and coastal plains subject to wind loads from hurricane or tropical storms.
• Wind Exposure Category E (WE-CE): Buildings located in areas that have high-velocity winds associated with tornadoes, as well as areas subject to hurricanes with wind speeds greater than 170 mph.
  

Risk Category

The Risk Category is a classification system used in the International Building Code (IBC) for the design of buildings and structures. It is based on the occupancy and use of the building, and the potential consequences of failure, such as loss of life, property damage, or economic impact.

The IBC defines four Risk Categories, with Category I being the lowest and Category IV being the highest:

• Risk Category I: Buildings and structures that represent a low hazard to human life in the event of failure, such as agricultural facilities, storage sheds, or other similar structures.
• Risk Category II: Buildings and structures that represent a substantial hazard to human life in the event of failure, such as schools, offices, and retail stores.
• Risk Category III: Buildings and structures that represent a substantial hazard to human life in the event of failure, and which are also considered essential facilities, such as hospitals, emergency shelters, and power plants.
• Risk Category IV: Buildings and structures that represent a very high hazard to human life in the event of failure, and which are also considered essential facilities, such as nuclear power plants, dams, and other critical infrastructure

Dead Load Design

Dead load design refers to the weight of the permanent materials used in the construction of a building or structure, such as walls, floors, and roof. The International Building Code (IBC) provides minimum dead load design data for various construction materials, which the designer must use to ensure the actual dead load of the structure does not exceed these values.

For example, a typical dead load design for a building may include the weight of the concrete slab floor, which could be 150 pounds per square foot, and the weight of the steel beams supporting the floor. Other dead loads that may be included in the design could be the weight of the roof, walls, and any permanent fixtures like mechanical or electrical systems. For the majority of our projects the dead load is 20 psf.

Truss Spacing, 2′ or 4′

For metal trusses, the only spacing you ever see is 2′ or 4′. This is based on the decking type. If the roof is being decked with plywood, you want to use 2′ truss spacing because plywood structurally should only span a maximum of 2′. These will usually be your flat, low-sloped roofs. 4′ spacing is possible, typically seen with sheet metal decking.

When to add interior load baring walls for truss support

If you can get away with less than a 40-ft span, it makes sense to only have exterior load baring walls as you will save on foundation concrete runners and costly load-bearing framing. However, for spans larger than 40′, like the building shown above, you should have interior load-bearing walls. If not, the trusses will be increasingly heavy and bear a lot of weight on the concrete runners/exterior walls.

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