WITH AN EMPHASIS ON I AND II
Wind and snow loads typically have the greatest impact on truss design, therefore classifying the way a building is being used into categories is very important; the more people are at potential risk to be harmed due to failure of a building (mainly due to wind, snow, and natural disasters), the more important it is to consider which category should be used for the trusses for that building.
“Buildings and other structures shall be classified, based on the nature of occupancy, for the purpose of applying flood, wind, snow, earthquake and ice provisions. The categories range from I to IV, where category I represents buildings and other structures with a low hazard to human life in the event of failure and category IV represents essential facilities.
The classification for each independent structural system of a multiple use building/structural system of a multiple use shall be that of the highest usage group in any part of the building or other structure that is dependent on that basic structural system.” (ASCE 7-05 section 1.5)
Category I is intended for buildings where there’s no human occupancy, and if so, only for a very short time, mainly just long enough to store things or to tend to livestock. Category I is for buildings used for storage of non-hazardous materials and if they are hazardous materials, they pose no threat to the general public. Examples are agricultural facilities, certain temporary facilities, and minor storage facilities.
Category III is intended for “buildings that represent a substantial hazard to human life in the event of failure.” These include buildings where more than 300 people congregate, day care facilities with a human capacity of more than 150, schools with a human capacity of more than 250, colleges or adult education facilities with a human capacity greater than 500, health care facilities with 50 or more resident patients (health care facilities without surgery and emergency treatment taking place), jails and detention facilities, power generating stations and other public utility facilities not included in Category IV. Also, buildings containing hazardous materials, such as fuels, hazardous chemicals, hazardous waste or explosives – sufficient enough quantities to be a danger to the public if released.
Category IV Buildings are for essential facilities; these buildings include hospitals and other health care facilities where surgery and/or emergency treatment is available, water storage and ancillary buildings, fire, rescue, ambulance, and police stations and garages, designated earthquake, hurricane, or other emergency shelters - basically all buildings critical for emergencies and defense. Also, buildings containing extremely hazardous materials – where the quantity of the material(s) exceeds a threshold quantity established by the authority having jurisdiction – should be analyzed with Category IV. (ASCE 7-05)
Category II is the most commonly used – it is for all buildings other than those listed in Category I, III, and IV. Most residential buildings will be ran Category II. Also, “buildings containing hazardous materials are eligible for Category II if it can be demonstrated to the authority having jurisdiction that the hazardous material poses no threat to the general public”. (ASCE -7-05)
There are many factors that go into the equations for the wind and snow loads applied to a truss. It is important to note that as the categories increase, one of those factors (the importance factor [I]) increases as well, resulting in more conservative wind and snow loading being applied to the truss design. For Category I, the importance factor is less than 1.0 (for both wind and snow), where as for Category II the importance factor is 1.0 (for both wind and snow); therefore, the wind and snow loading will be less conservative or the same for category I and II, respectively. Once you have Category III and IV, the importance factor is over 1 for both wind and snow, causing the wind and snow loads to be increased.
A barn for livestock would be an example of a Category I building, since it’s just storage for animals. A horse riding arena, for example, should be Category II, since it’s a building that has the potential for many people to occupy it for activities, therefore there’s more potential for people to get hurt in the event of structural failure. In some cases it may be a judgment call whether to use category I vs. category II, but if the intended use of a building is for a people to use the building for anything more than storage, even if it will not actually be used a lot, it should be category II, simply because it has the potential to be used for activities where people will be present for an extended period of time.
It is the building designer’s responsibility to specify which category to use, and the truss company will take on that risk if they make that decision themselves. MiTek’s position on which category should be used is that we leave it up to the customer to specify the category the building designer specified, especially since we usually don’t know what type of building a truss will be used for. If the truss has a label that clearly states it’s a horse riding arena, for example, we would know that it should probably be ran category II instead of I and advise the customer on it. If a truss design is designated to be analyzed with category I, we would assume it’s probably a barn or storage facility.
There may be a gray area on when to analyze a truss design using category I vs. category II, and that is another reason why it is MiTek’s position that a truss company should use the category specified by the building designer, and not make that decision on their own to avoid taking on that risk.