A Beginner's Guide to ASCE 7-10

Chapter 1 - General Information

© 2012, T. Bartlett Quimby

BGASCE7-10 Section 1.4

Structure Classification

Last Revised: 11/04/2014

ASCE 7-10 Section 1.5 provides the criteria for the classification of structures based on the nature of occupancy and consequences of failure on the general public.  The structure classification is used to determine importance factors when computing the magnitude of loads to be considered.  Buildings are primarily classified by the risk they pose to occupants and contents.  For example, the loss of a small outbuilding on a farm may not be as large of a problem as the loss as a hospital, so the design of the hospital will require greater design loads to minimize the risk of loss. If a building creates a potential hazard for a larger number of people than an other then it is designed for larger loads. The potential hazard includes both physical harm of structural failure as well as the impact that the loss of the structure will have on its dependent or adjacent community. The ASCE 7-10 commentary points out that "the acceptable risk for a building or structure is an issue of public policy".

The basic approach of the standard is to vary the magnitude of loads in accordance to a building's acceptable exposure to damage (i.e classification). Four levels of risk are identified and structures are assigned to one of the categories according to the criteria listed in ASCE 7-10 Table 1.5-1. As stated in the commentary  "Classification... reflect(s) a progression of the anticipated seriousness of the consequence of failure from lowest risk to human life... to highest..."

ASCE 7-10 Table 1.5-1 should be examined closely.  This table contains the criteria for the four general structural classifications.  The commentary suggests that "a rational basis should be used to determine the risk category for structural design, which is primarily based on the number of persons whose lives would be endangered or whose welfare  would be affected in the event of failure."  ASCE 7-10 Figure C1-1 presents an approximate relationship between the lives at risk to risk category.  Note that lives at risk may include lives not directly connected with the structure, for example if a hospital fails many lives in the community are at risk since the primary care facility will not be available to deal with trauma resulting from the load event--this would result in a structure such as a hospital being raised to a higher risk category than its occupancy would indicate.

Additionally, ASCE 7-10 Table 1.5-2 contains the associated "Importance" factors associated with the general structural classifications to be used when determining various environmental loads.

Another point made in ASCE 7-10 section 1.5.2 is that a building may be classified differently for different load types.  An example in the commentary uses a shelter in a hurricane region which is also a low seismic area--it would not be fair to classify the structure the same for wind as seismic in this case.  Such multiple cases are rare.

A footnote to ASCE 7-10 Table 1.5-1 indicates that the risk classification can be reduce in some cases if an acceptable risk management plan is in place.  The plan must satisfy the authority having jurisdiction over the plan.  It would appear that reduction of risk category based on an risk management plan presupposes that there are enforcement measure in place to ensure that the plan is fully implemented as long as the risk remains.

For an in-depth discussion of on classifications, their history, and intent refer to the commentary in ASCE 7-10.  The commentary does an excellent job of explaining the concepts behind the classifications and the changes that are manifest in the 2010 version of the Standard.