How does we calculate load factor?

[Alaazaki]

When we use plastic design approach to design a structure we multiplies the working loads by (load factor) to get the failure condition of loads then we choose the members which can resist this failure under this loads,my question is: How do we know that this load factor-which is unified- will cause the failure for any member I will choose?

 

[KootK]

We don't know that factored loads will cause failure in our members. More troublesome, we can't guarantee that our load factors will prevent our members from failing. The determination of the factors comes down to structural reliability and statistics. I've seen it represented graphically as a normal distribution for load placed over a normal distribution for resistance. The high load tail of the load distribution overlaps the low load tail of the resistance. The overlap represents the -- hopefully unlikely -- outcome of load exceeding resistance.

Based on your mention of plastic design and your choice of wording -- looking to guarantee failure rather than prevent it -- I speculate that you may be asking a more involved question. Capacity design principles for seismic design for example. Can you elaborate on the context of your question?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[BAretired]

The title of the thread "How do we calculate load factor?" is asking a different question than the first post. The answer to the title question is: We don't calculate load factor. Load factors for various types of load are specified in the code. The higher values indicate loads which are more likely to vary. So live load factor is larger than dead load factor because we can be more confident that the calculated dead load is close to the actual dead load whereas the actual live load may vary considerably from the calculated value.

The question in the text is "How do we know that this load factor-which is unified- will cause the failure for any member I will choose?". Not sure what is meant by "unified", but in general, a member will not fail at the factored load because there are additional "resistance" factors accounting for variability of material strength. In the Canadian code, these are called Φ factors. The value of Φ depends on the material and the type of failure. So if you select a member which is rated to resist a factored load, the failure load will be higher than the factored load unless the material strength is below the minimum strength expected.

BA