Derivation of factor loading combinations. 1.4DL + 1.7LL

[DennCy]

We all know that in allowable stress design we use ultimate load = D + L only..
While using Ultimate Strength Design we are using Ultimate load = 1.4DL + 1.7LL

my question is is there a derivation for thos factors. 1.4 and 1.7 or its just a probability..

i think there should be a derivation accoording to principles. anyone knows?

this is the link where someone tries to derive it.

http://www.eng-tips.com/viewthread.cfm?qid=325368

 

[JAE]

You need to look up papers and articles done by Bruce Ellingwood (Ellingwood Wikipedia link.

He and others over the years have developed statistical procedures to weigh the variabilities of various kinds of loads, the variabilities of construction (phi factors) and the basic, desired probability of failure to come up with these load factors.

The 1.4D and 1.7L were earlier numbers used which were later revised to the 1.2/1.6 factors on dead and live.

There are numerous papers written by him and others like Galambos. The one's I read were in the ASCE Structural Journal but I'm sure they show up elsewhere.



Check out Eng-Tips Forum's Policies here:
faq731-376

 

[DennCy]

yes in our code 2001 we use 1.4DL + 1.4LL then on our latest published code it is 1.2DL + 1.6LL i wonder how they get the factors for each dead load and live load..

 

[dcarr82775]

DennCy. The answer to your question is Statistics. Like JAE said, some really smart folks analyzed lots of data and came up with those answers. If you want a more detailed answer, you will have to read some journals.

 

[JAE]

Yes - it is all statistics - variability of loads are estimated and weighed against the desired probability of failure. The probability of failure is based on other design systems in the past.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[hokie66]

For concrete, the 1963 ACI Code used 1.5D + 1.8L, but when that was revised, the capacity reduction factors were adjusted as well. The intention has been to keep the margin of safety more or less the same.