Load Combinations for NBCC

[ToadJones]

I am relatively new to using National Building Code of Canada an have a few basic questions regarding loads and load combinations.

1). Are Ultimate Limit States (ULS) loads and Serviceability Limit States (SLS) loads determined using different importance factors? These loads are then used with different companion load factors in the combinations?

2). Are there different load combinations used to check different serviceability limit states; i.e. different load combinations for checking deflections than say for for overturning or sliding?

 

[msquared48]

I have an idea how to answer your first question but do not have the Canadian code, so I will leave that to others to answer.

As for the second question, in general the answer is yes (and remember that all generalizations are not worth a d)(*, including this one). Combinations such as dead + wind or seismic will generally give the maximum uplift and lateral deflections for certain members. Other combinations will do the same for other members. It depends on the relative values of the numbers when combined with the load factors as what will control. Some rarely control, some most of the time as mentioed above. Just depends on your project.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[LOKSTR]

NBCC loads are similar to ASCE7 chapter 2 load combinations except for different coefficients for different loads types( e.g. 1.2D in ASCE and 1.25D in NBCC etc.) NBCC gives separate "Principal loads" and "Companion loads which you have to combine while ASCE gives you already combined loads.

FOR ULS and SLS , NBCC gives you different Importance factors for wind, snow and seismic.

Hope this helps.

 

[Klunker]

Toad,
ULS and SLS are equivelant to Strength Design and Allowable Stress design respectively here in the States. Canada, as far as I'm aware, no longer accepts allowable stress design (not that many trades still use this). There is some discussion in the code regarding which State applies to which area ie SLS to check deflection, etc. but in my somewhat limited experience stick with ULS (Strength Design) and you will be fine. Importance factors do not determine which analysis method to use.

 

[SkiisAndBikes]

Quote from the OBC (likely the same in the NBCC). "Those limit states concerning safety are called ultimate limit states and include exceeding the load-carrying capacity, overturning, sliding and fracture, those limit states that restrict the intended use and occupancy of the building are called serviceability limit states and include deflection, vibration, permanent deformation and local structural damage such as cracking; and those limit states that represent failure under repeated loading are called fatigue limit states.)"

ULS loads are factored and combined in accordance with clause 4.1.3.2 - Strength and Stability.
For a sample strength calculation of a beam, a common load combination might be 1.25D + 1.50L + 0.50S. This values of S (snow), W (wind) and E (earthquake), if in your load combination would have to be multiplied by the appropriate importance factor before inserting into the equation, which could vary from 0.8 for a low importance structure (think farm building) up to 1.25 for a post-disaster building (think hospital). ULS importance factors vary depending on whether you are dealing with wind, snow or earthquake.

SLS loads are not factored. If one was to check a beam for deflection under dead, live and snow load, one would apply the importance factor to the unfactored environmental loads, i.e. for a 'normal' category building 0.9S where the importance factor for snow loading is 0.9 for a normal building. The importance factors for Dead and Live loads is 1.0.

Refer to Section 4.1 of the OBC/NBCC, or Commentary A of the NBCC Users Guide for more information.

 

[NS4U]

to answer your first question the importance factor is different. for the albert bldg code, where I=1 for ULS, I=0.8 for SLS