Climatic Design Loads05 May 2015
The design of a pre-engineered steel building must account for climatic loading, such as ground snow loads, wind pressures, and seismic data.
In British Columbia it also must conform to Part 4 of the BCBC—using loads, and deflection and vibration limits from either Part 4 or Part 9. The British Columbia Building code lists loading criteria for over 600 major cities. Interpolation from the values in the Table to other locations is not acceptable and local municipalities, failing that Environment Canada, must be contacted to provide climatic design loads for locations not listed.
Here is a very brief description of what climatic loads are considered in the pre-engineered steel building’s design and how they are considered in the British Columbia Building code.
Ground Snow and Rain Loads
The roof of a pre-engineered steel building has to support the greatest weight of snow that is likely to accumulate on it in 1-50 years. The unit weight of snow (generally 2-5 kn/m3) converts snow depth to load Ss and for rain failing on the snow the same procedure is used to get Sr. These loads are factored under guidelines set out in the building code, an importance factor is applied based on the buildings end use and an overall roof design load is determined.
Pre-engineered steel buildings need to be designed to ensure that the main structural system and all secondary components, such as cladding will withstand the pressures and suctions the wind put on a building. Wind loads are calculated considering environmental data on winds for each area, mean pressures that will act on the building, gust factors, terrain type (exposure class), building openings and importance factors related to the building type (or end use).
Like all structures, the earthquake loads on a pre-engineered steel building is a major factor in the design. Earthquakes can cause damage through ground shaking, soil failures, or surface fault ruptures. This part of the design only considers ground shaking. The other earthquake factors are considered in other areas of the design. The National Building Code, NBC 2005, lists median values for design ground motion, a mean of 2% in 50 year probability, of acceleration for periods 0.1 seconds, 0.5, 1.0 and 2.0. As well the peak ground acceleration is considered in the same mean factor. Like the other climatic design loads, an importance factor is applied to these loads based on the steel buildings end use. The site class (A,B,C,D,E,F) classify the ground type, soft soil, hard soil, rock, etc and this value must be confirmed by a Soils engineer through a geotechnical report.
The collateral load included in the design of a pre-engineered steel building is a dead load to cover the additional loading requirements for mechanical, electrical, or any other services or equipment the building requires.
Limit States Design
Introduced in 1975 into the National building code, limit states design was initially designed for steel building but now is incorporated into the design of all major structures. Limit states design ensures building safety from collapse during construction and safety after completion. The limit states on safety are called ultimate limit states (ULS) and design for maximum capacity, fractures, overturning, etc. SLS, or serviceability limit states factors building deflection during use and ensures required deflection performance from the building while in use. An example of this is a building with a bridge crane and the amount the building will deflect when that crane is carrying its maximum load, or the possibility of damage or vibrations in the building. Excessive deflection in a building can affect building use and can even cause problems opening doors and windows. Limit states design recognizes all categories of loads and stresses a building will experience and allows the designers to plan and size the building members accordingly.