Consequence classes CC1, CC2, CC3: what they are, how they are determined and what they affect

What a consequence class is

The consequence (responsibility) class of a structure shows how severe the consequences will be if the structure stops being used or loses its integrity. It is set by the level of possible material damage and social losses. There are three classes: CC1 for minor consequences, CC2 for medium and CC3 for major.

It is a characteristic of the structure as a whole, not of a single beam or pole. The same column can work in a CC1 structure and in a CC3 structure, and the requirements for it will differ precisely because of the structure.

How it is determined: the DBN table

Classification is carried out under the Law of Ukraine «On regulation of urban planning activity» and table 1 of DBN V.1.2-14:2018. It has several indicators: how many people may be at risk and how large the losses are. The assessment takes into account the protective measures included in the project (division into independent sections, duplicated elements, warning devices). Building a scenario of a possible accident helps.

How to read the notes to the table

People are «permanently present» on a site if they are there for more than eight hours a day and at least 150 days a year (at least 1200 hours in total). People who visit «periodically» are there for no more than eight hours a day for no more than 150 days a year (from 450 to 1200 hours in total).

A threat to the living conditions of people outside the site means a probable disruption of normal living conditions for more than three days. The amount of economic loss is calculated by the approved method in minimum wages, which the state budget law sets every year.

Social losses are assessed by several factors: danger to life and health, a sharp deterioration of the environment near the site, loss of historical and cultural monuments, stopped communication, power supply, transport and other life-support systems, impossibility of helping victims, a threat to defence capability. Material damage is not only the cost of restoring the structure, but also side losses from stopped production and lost profit.

Examples by number of people

Below is only an illustration of how to read one of the indicators. The class of a structure is set by the full assessment, not by one column of the table.

Consequence class and the responsibility category of a member

These two concepts are mixed up most often. The consequence class belongs to the structure and is CC1, CC2 or CC3. The responsibility category belongs to a member or its element and is A, B or C. Category A is members and elements whose failure may make a building or structure unfit for use. Category B is those whose failure complicates normal use or leads to the failure of other members that are not in category A. Category C is members whose failure does not disturb the operation of the structure as a whole. Category A includes A1: the main load-bearing members that keep the structure from complete collapse in an accident.

If the design standards for a particular type of structure give no specific recommendations on assigning members to categories, they are put into category B.

There is also the complexity category of a structure. It relates to the complexity of construction, not to the consequences of failure, and it should not be replaced by the consequence class.

What the consequence class affects in the calculation

The most noticeable effect is on the reliability factor for responsibility γn. It depends both on the class of the structure and on the responsibility category of the member. For persistent design situations and the first group of limit states the values are as follows.

Additional requirements for higher classes

For the second group of limit states and for transient and accidental situations the values of γn differ; they are taken from the table in the DBN.

For CC3 structures the load-bearing members must be calculated twice, with different design models and in different programs, and the difference in the main results (forces in category A members, natural periods for the first three modes, stability margins) must not exceed 10 %.

For classes CC3 and CC2, where the class is set by the threat to life and health, the main load-bearing members are designed to reduce the probability of progressive collapse in an accident. The disproportionate collapse check is done for such structures as high-rise residential and public buildings, shopping centres and the under-stand structures of stadiums, if no other measures exclude such collapse. CC3 structures whose collapse may lead to catastrophic consequences are equipped with automatic monitoring and control systems.

For mass-produced structures it is recommended to set one value of γn, independent of the class of the structure where they are used, if the actual consequence class and categories do not exceed the design ones.

Where else the consequence class appears

In the design brief and in the design documentation: it is one of the input data. More about the content of a project is in the article on the composition of design documentation under DBN A.2.2-3.

In the requirements for those who carry out a technical survey: the certificate of the responsible executor is issued for a stated consequence class, and it must match your structure. See the article Who is entitled to carry out a technical survey.

In the calculations that our articles refer to: beam, column and truss. In the examples class CC2 is assumed conditionally, and for a real structure it is set by the designer.

What this means for ordering steel structures

The consequence class of a structure is given by the project or the customer; we do not set it. If requirements for the structures, the documents or the survey depend on it, send the class and, if available, the responsibility categories of the members we fabricate. We will take them into account in the KM and KMD design and in steel fabrication.

Standards and validity

The article relies on DBN V.1.2-14:2018 «General principles of reliability and structural safety of buildings and structures» (in force; edition with Amendment No. 1) and on the Law of Ukraine «On regulation of urban planning activity». The design of steel structures is covered by DBN V.2.6-198:2014 «Steel structures. Design standards» (in force; edition with Amendment No. 1). The values in the tables are given for information; the current edition is checked before use in a project.