Many buildings place mechanical equipment—such as air conditioning units, heaters, or ventilation systems—on the roof. These are known as Roof Top Units (RTUs). Installing this equipment on the roof helps save valuable floor space inside the building.
RTUs come in a wide range of sizes and weights. Some are small, only a few square feet in size, while others can cover large areas. Their weight can vary from a few hundred pounds to several thousand pounds. If an RTU sits directly on top of roof trusses, those trusses must be able to safely support the added weight.
RTUs are typically applied as concentrated or partially distributed loads rather than uniform loads. These loads must be evaluated in combination with:
- Dead load of roofing materials
- Snow load accumulation, especially drift loads around RTUs
- Wind loads which can create overturning forces
- Seismic forces (in applicable regions).
All these loads must be specified by the Engineer of Record (EOR).
These combined effects can create complex load cases that require careful consideration. We will review only how to apply the weight of the RTU on the trusses below. The first step is determining the RTU’s footprint relative to trusses below and the maximum load of the RTU. So, the exact dimensions of the unit, the direction of those dimensions, and the exact locations of these dimensions are required.
Let’s review an example of how to determine the loading from a 1,200 pound (lbs), 6 foot by 6 foot RTU placed over roof trusses spaced 24 inches on center, as shown. [For both images, See PDF or View in Full Issue.]
In this example, the RTU is supported by four trusses and has eight connection points. Based on tributary area principles, the exterior trusses carry only half the load supported by the interior trusses. The RTU weight is distributed to the connection points as illustrated.
It is important to remember that mechanical units should be treated as dead loads. Because of the various load combinations required by building codes, the design live load cannot be used as a direct substitute for the mechanical load unless the Engineer of Record (EOR) confirms that the live load adequately covers the RTU weight.
Since the interior trusses typically carry the highest portion of the load, MiTek engineers recommend designing all trusses supporting the RTU for the same higher load to ensure consistency in truss design.
For additional information, or if you have questions, please contact the MiTek Engineering department.