Importance Factor

The magnitude of the specified loads you enter in Sizer do not include the importance factor that is ordinarily part of the calculation of loads using the NBC. Importance factors are not included ito allow the program to calculate serviceability limit loads for deflection design and ultimate limit state combinations for strength design using the same set of input loads.

Sizer automatically applies the importance factor when load combinations are constructed, according to the Importance Category you input and the following design code clauses:

NBC

Section 4.1.2.1 .3) and Table 4.1.2.1 p 4-3 – Define Importance Categories

4.1.5.1 2) and 4.1.5.2 2) p 4-7– Live load factor
4.1.6.2, Table 4.1.6.2 p 4–15 - Snow load factors
4.1.7.1 –Table 4.1.7.1 p 4-17– Wind load factors
4.1.8.5, Table 4.1.8.5 – Earthquake load factors

NBC Structural Commentaries

Commentary A 9 gives rationale for changes and states no factor applied to dead load and only Low importance factor for live loads.
Commentary A 10 addresses importance factors for serviceability states.
Table A-2 summarizes factors for Earthquake, snow and wind
A-2 Note 1 gives the Live load factor for low-hazard buildings
A-2 Note 2 refers to Commentary J for Earthquake SLS factors. Commentary J35 and J 115 – 119 are relevant, but neither give an importance factor

Input

The means of entering importance factors is described in Load Settings and Options.

Output

The importance factors for all load types that exist on the member are output at the head of the load combination list in the analysis results as follows:

 LC#  Ultimate Limit States               Serviceability Limit States       Pattern

----  ---------------------               ---------------------------       -------

Importance factors also included:

      Snow: 1.15                          Snow: 0.9

      Wind: 1.15                          Wind: 0.75

      Earthquake: 1.3

Where individual load combinations are output in the Design Check results or Analysis Diagrams, the importance factor appears in brackets within the load combination:

		LC #15 = 1.0D+1.0Ds+0.5L+1.0Ls+(1.15)0.25S+(1.3)1.0E

In both cases, the live load factor is output only for low-hazard buildings, otherwise it is 1.0.

Serviceability Importance Factor for Earthquake Loads

The design codes do not provide a serviceability factor for earthquake loads, and no earthquake combinations are generated for serviceability design. Sizer does not perform deflection design for earthquake load combinations.

Part 9 Snow Loads

For structures meeting the requirements of Part 9 of the NBC, many users design using Part 4 but with snow loads specified using Part 9, In this case, the importance factor is 1.0 for serviceability (deflection) rather than the 0.9 Part 4 importance factor shown in O86 Table 5.2.3.2. For this reason there is an Importance category called Normal, Part 9 snow. When this is selected, the factors are the same as for the Normal importance category, except that for snow loads are selected the serviceability limit state (SLS) factor is rather than 0.9