The Philosophy of Importance Factors
The Philosophy of Importance Factors
(OP)
I'm currently doing a casino project where the importance factor for wind is 1.15. This got me to thinking about something I've been meaning to post here--
Don't importance factors seem a little odd? In essence, when we INCREASE wind or seismic loads for assembly buildings, hospitals, etc., and DECREASE wind or seismic loads for agricultural structures, we are saying the lives of the large group of people at the assembly building or hospital are more valuable than the life of Farmer Brown, by himself in a pole building. Right?
Don't importance factors seem a little odd? In essence, when we INCREASE wind or seismic loads for assembly buildings, hospitals, etc., and DECREASE wind or seismic loads for agricultural structures, we are saying the lives of the large group of people at the assembly building or hospital are more valuable than the life of Farmer Brown, by himself in a pole building. Right?
DaveAtkins






RE: The Philosophy of Importance Factors
In LRFD:
The statistics can be prepared to evaluate the probabilities of various loads. This separates the loads in to levels of safety factors to reflect their individual characteristics.
The same applies to φ factors in that various material limit states have differing degrees of reliability and thus require different resistance factors.
Likewise, for the Importance factors, this is a statistical measure (attempt to measure anyway) of the risk to life and limb should something happen.
For high occupancies, occupancies where exiting quickly might be difficult (hospital patients or large numbers), and occupancies where hazardous materials exist, the I factor simply assesses the probability of injury or death.
So I don't think they are saying Farmer Brown has less value, they are suggesting that it would be more probable that Farmer Brown could "get out of there" faster than 300 people in a high school gym, thus the risk is different.
RE: The Philosophy of Importance Factors
RE: The Philosophy of Importance Factors
RE: The Philosophy of Importance Factors
For example:
Electric power stations are shown as having either Importance Factor III or IV. When briefing senior management on the design criteria for a new station, it was very helpful be able to avoid technical "jargon" and just explain that a base-load, coal-fired station should be in Cat IV to survive a natural disaster and continue to operate, versus a peaking, natural gas fueled station, in Cat III, that could be allowed to be out of service for a while, after the same disaster.
www.SlideRuleEra.net
RE: The Philosophy of Importance Factors
RE: The Philosophy of Importance Factors
Hg
Eng-Tips policies: FAQ731-376
RE: The Philosophy of Importance Factors
Codes are minimum standards at the end of the day.
RE: The Philosophy of Importance Factors
The use of a higher importance factor for buildings such as hospitals or fire station, is also justfied on the basis of those buildings revieving less damage in case of an earthquake or high wind event. After an earthquake you want to move the injured to a working hospital.
Speaking of cows, when I took a course on plan checking: my instructor referred to the "Dumb Cow" theory when talking about exit requirements. It is based on the idea, that 7 people in a room are much more likely to be able to exit the room in case of an emergency than 100 people.
Farmer Brown's life is just as valuabl as the 300 people in the school gym. Also Farmer Brown SE could chose to design his barn for an importance factor of 1.15.
All engineering design involves balancing risk and the probability of an event occuring. We could require all new homes to resit 300 mph wind loads. However if we did there would be very few people who would own their own homes. I think most people would be willing to except the risk of their home being damaged in a 90 mph wind
RE: The Philosophy of Importance Factors
I really am not this cold-hearted, honestly I'm not. But our profession is. We set thresholds, and our math reflects those levels of probability. It really is as simple as that.
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: The Philosophy of Importance Factors
You have to remember that there are hundreds of houses built every day and the average punter cannot afford to build the whole house into a storm shelter.
You are talking about a new orleans magnitude of event that would exceed the code requirements, and it is unlikely to happen to most houses.