I have learned a lot about loads on steel beams and deflections and where to find basic facts thanks to the great help of knowledgeable people who are members of byggahus.se. One question remains for me as a question mark, and that is the load reduction factor psi. What or when should I use this factor? If I am only going to use a beam to support a garage roof without any other load, should I then multiply the snow load by the load reduction factor? Why is this factor not already included in the value for snow load and safety factor? Can someone explain the relationship snow load - safety factor - load reduction factor in a simple way, i.e., when to use it?
Hello. I will see if I can sort this out for you. However, I am not familiar with Eurocode, only trained according to the Swedish regulations.
Snow Load - Usually the largest snow load that occurs in a 50-year interval. The magnitude of this depends on where in the country you live. Don't forget to consider the shape factor of the roof's form (another factor to multiply in).
Safety Factor - The size of the safety factor depends on how high the risk is for serious personal injury. In garages, you can count on a safety factor of 1.0.
Load Reduction Factor - The load reduction factor depends on which load combination you are going to calculate.
When calculating the ultimate limit state (i.e., when the beam reaches failure) with both snow load and wind at the same time, it is assumed that the 50-year load for both does not occur simultaneously. The load that has the greatest impact on the beam should be set as the main load (increase the load by a factor of 1.3) and the other variable loads are multiplied by a load reduction factor. Note that the self-weight never changes and should thus not be multiplied by a load reduction factor or be the main load. If you can't immediately tell which load has the greatest impact on the beam, it's common to do several parallel calculations with different loads as the main load.
On the other hand, when calculating the serviceability limit state (deflection), long-term loads apply as deflection has a tendency to increase over time even if the load remains the same. Here, you do not need to multiply in a factor because of the main load but rather calculate with a load reduction factor for both wind load and snow load.
Depending on how your garage is designed, wind load can contribute to a compressive normal force in the roof beam, so don't forget about it, even if it is often negligible in this context.
Regards,
G
Snow Load - Usually the largest snow load that occurs in a 50-year interval. The magnitude of this depends on where in the country you live. Don't forget to consider the shape factor of the roof's form (another factor to multiply in).
Safety Factor - The size of the safety factor depends on how high the risk is for serious personal injury. In garages, you can count on a safety factor of 1.0.
Load Reduction Factor - The load reduction factor depends on which load combination you are going to calculate.
When calculating the ultimate limit state (i.e., when the beam reaches failure) with both snow load and wind at the same time, it is assumed that the 50-year load for both does not occur simultaneously. The load that has the greatest impact on the beam should be set as the main load (increase the load by a factor of 1.3) and the other variable loads are multiplied by a load reduction factor. Note that the self-weight never changes and should thus not be multiplied by a load reduction factor or be the main load. If you can't immediately tell which load has the greatest impact on the beam, it's common to do several parallel calculations with different loads as the main load.
On the other hand, when calculating the serviceability limit state (deflection), long-term loads apply as deflection has a tendency to increase over time even if the load remains the same. Here, you do not need to multiply in a factor because of the main load but rather calculate with a load reduction factor for both wind load and snow load.
Depending on how your garage is designed, wind load can contribute to a compressive normal force in the roof beam, so don't forget about it, even if it is often negligible in this context.
Regards,
G