The Philosophy of Importance Factors

[DaveAtkins]

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?

DaveAtkins

 

[JAE]

I think this is simply a separation of risk parameters.

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.

 

[JStephen]

Farmer Brown doesn't live in the pole structure- that's where the cows are.

 

[UcfSE]

JStephen - indeed.

 

[SlideRuleEra]

Also, there are people other than engineers who need to know and make decisions on the level of confidence that is designed into a structure.

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.

http://www.SlideRuleEra.net

 

[JAE]

Jstephen...I tried real hard not to go there, and offer a legitimate response....but I guess someone had to do it.

 

[HgTX]

sounded like a legitimate response to me...

Hg

Eng-Tips policies: faq731-376

 

[connect2]

odd enough about the 'cows', we used to design large dairy facilities, between a million and two million dollars. Don't have any reference here but seems to me the wording surrounding the 0.8 importance factor includes mention of property in addition to lives (low human occupancy). so if you have two large silos containing lets say coal, who's potential collapse based on occupant load represents an acceptable probability based risk against human injury or loss of life, would you use an importance factor of 0.8?
Codes are minimum standards at the end of the day.

 

[RARWOOD]

JAE response I think pretty well sums it up. Many of the provisions of the building code governing number of exists, size of door ways, wall fire ratings, ect are based on the occupancy load and the ability of the occupants to exist the building.

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

 

[youngstructural]

I really hate to be a synic, but I have to point out that, from a design code development standpoint, Farmer Brown's life is NOT as valuable as the 300 people in the school gym. In fact, it is a simple statistical analysis which we use to set the threshold between two curves (probability of load at level X and a second curve of probability of reliable material resistence - Any area under the curves in an intersection represents collapse) which sets the "acceptable propability of an individual dying in a structural failure". This is done in order to maximize efficiency: The least amount of money is spent building a structure to a standard set at an "acceptable" probability of failure. We use importance factors to claw-back this economy in cases where it would become a false economy (ie: lots of people dying would mean that they are not spending money, or making money, or producing labour for the economy). Similarly, we claw back (quite rightly) for schools and other educational institutions in recognition that we need to ensure the safety of both innocents and individuals who will be contributing to our future.

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...

 

[csd72]

Australia has a similar clause in the wind code.

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.