Why some factors in the formulas of calculating loads are less than 1(in Building Codes)? 2

[ccpe]

Does anyone know why some factors are less than 1 in calculating loads? For example, Table 4.1.3.2. (Part 4 for structural design) in Ontario Building Code shows that some factors are less than 1. My previous understanding is that the factors in structural design, especially in calculating loads, are always greater than 1.

 

[DoubleStud]

Probably it is a load case for with uplift. For example, you put LF less than 1 just in case the DL is not accurate.

 

[driftLimiter]

Yes more load isn't always a good(conservative) assumption.

 

[Tomfh]

For example, Table 4.1.3.2. (Part 4 for structural design) in Ontario Building Code

There are a few reasons.

The main reason (as that code explains) is when the load is used to “resist” other loads . So you treat it conservatively and reduce it, in case you don’t have as much mass helping resist as you’d assumed.

Another reason is when you are combining loads, eg wind, live load, etc. It is statistically far less likely that a building will see 100% wind load and 100% live load at the same time as compared to 100% live load alone, so the code tells you to reduce the loads if they are acting simultaneously. Codes adjust these factors in an attempt to produce consistent building reliability across various load cases.

As for why wind is not above 1.0, loads such as this are at ultimate already. They don’t need factoring, because they are calculated from the outset to match the required level of reliability, eg a 1 in 1000 year wind.

 

[ccpe]

Thank you. Good explanation. When they already set the loads to very high values, we don't need to increase the values of factors. This can make a building safer but cost more money.

 

[rapt]

Sometimes it is also done where there is a combination of variable loads, eg you do not assume full LL and full Wind Load to occur simultaneously.

or when the variable load acts in the reverse direction to the permanent loads.

 

[JStephen]

Another reason that comes up is if you are looking at loads for allowable stress design, but the loads in question were derived based on ultimate design. This may not be included in the Ontario code, but is included in ASCE 7, etc.

 

[ccpe]

Thank you all for your contribution. I understand this question more now.