Making an Informed Decision--Part 6
10/11/2011 9:59 AM
Part Five explained how factors related to snow and wind loads affect the design and strength of a building from one manufacturer to another.
In this post I will explain how the use and occupancy ratings used during the design process can affect strength from one manufacturer to another.
Most building codes use the level of human occupancy as a key factor in determining the strength requirements in the design of the building. In structural engineering terms, human occupancy means the more human use within the structure, the higher the strength requirements. The level of strength required is determined by selecting a hazard rating. The higher the hazard rating the higher the engineering requirements. So the choice of hazard ratings used in the design can have a dramatic impact on the overall strength, and therefore safety of the structure.
To reduce costs, some manufacturers use a Low Hazard importance factor for all buildings. This allows a building to be termed "engineered", yet designed to the most minimal requirements, which means less materials. The end result is a cheaper product for the customer, but a building that may only be safe under certain weather conditions. This may be fine if the customer chose this knowingly. The confusion can come with comparing "engineered" buildings and thinking that the building that costs less is the same quality as the more expensive one because they are both engineered.
A good guideline to consider is that a Low Hazard rating is used for only low occupancy buildings such as cold storage facilities and agricultural storage buildings. Most other buildings would fall under the Normal importance category. A Normal importance factor of 1.0 provides 25 percent more structural capacity than Low Hazard importance of 0.8. It is important to note if you are unsure about the classification your building should meet, Low Hazard should not be the default choice. Even though Low Hazard ratings include general agricultural buildings, this reduces the risk to livestock and to the humans caring for the livestock.
When comparing manufacturers, it is not enough to ask if the building is engineered. Be sure to ask exactly what engineering requirements have been met. What Hazard rating was used?