USD ~ LSD ~ LRFD

Okay, Great theoretical Design Engineering question, and I'll give you an answer as best as I understand the situation...

Pretty much everything used to be USD (Ultimate Stress Design, aka Allowable Stress Design, aka Working Stress Design). There are occationally some minor differences between the three, but essentially they are the same animal. Basically you find the stress in a member (the Ultimate Stress) and check that it is below some "Allowable" stress. The safety factors are in the Allowable stress, although sometimes (and thus the name differences) there are more generous "permissible" stresses and you are to use Safety Factors on the applied loading as well. Geotechnical design is still very much USD, ASD or WSD in most areas.

LRFD (Load and Resistance Factor Design) is a refinement of USD where you separate your safety factors into load applied factors (Typically >=1) and Resistance Factors (Trypically <1). This allows you to modify your total resulting safety factor (Load Factor/Resistance Factor) based on how well you know your loading as well as (distinctly) how well you know your material's reliability (For example resistance factors for steel are typically >=0.85m, whereas for concrete they normally range from 0.55 to 0.8).

Limit States Design (LSD) is a wholly different animal, and generally is viewed as an improcement on theoretical code development. Now you are not simply working on strength of the structure, but applying a wholistic approach to an entire system in an attempt to control two separate limit states, The Ultimate Limit State (ULS; ie: exceeding this limit your structure collapses) and your Serviceability Limit State (SLS; ie: exceeding this limit in some way causes your structure to stop being functional for its intended purpose, however does not necessarily result in collapse). After understanding this distinction, you can see that the codes are developed to limit the theoretical probability that any limit state (most structures have many) would be exceeded. They are normally codified in a manner similar to LRFD codes, however are considered distinct thanks to the underlying theory behind them.

A great example of ULS vs SLS is to consider the Tacoma Narrows Bridge. When constructed the bridge satisfied both, until a built up of occilations started to be felt by users... As soon as the lower fifth percentile of people complained they were uncomfortable with this (typically this is just feeling vibrations, sometimes are subtle as feeling a little nautious) the bridge has exceeded the vibration SLS limit. As the occilations became greater and the structure could be seen to move, it had breached the SLS deflection limit. As the occilations became so severe as to slow traffic accross the bridge, it failed the SLS general serviceability limit. Once the bridge collapsed then, and only then, did it fail the ULS limit.

Many, MANY, more stages are possible with the Tacoma Narrows Bridge, but this just depends on how fine a pen you want to take to the problem. In general these four stages more than suffice. And to be fair, the bridge started to make people feel quesy almost right away, so to say that it ever met SLS is a stretch!

Hope that helps,
Regards,

YS

P.S. The basic difference is important to understand, however it is very much secondary to applying your code correctly. No matter how the development was acheived, you have a resulting recipe you are expected to apply in its entirety. Good Luck! YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

I have never heard of USD explained in terms of Ultimate Stress Design but rather Ultimate Strength Design...and I seem to recall it is noted that way in several codes...

Ultimate Strength Design here in the states is really about limits and is similar to limit state design which was more popular name in the UK for the method. USD comes with the provisions that certain elements need to be checked via service limits states such as cracking in concrete.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

Limit state design (LSD) is a design methodology that incorporates ultimate and serviceability limit states--conditions for which the structural system or element becomes unfit for its intended purpose. AISC 360 implements a LSD methodology and calls it LRFD. ACI 318 implements a LSD methodology and calls it Strength Design Method, formerly known as Ultimate Strength Design (USD) Method.

I am suprised to hear USD as a limit state code... I had thought Ultimate Strength Design was a turn of the 19th century design procedure replaced by ASD & WSD... This should show the danger of international engineers dicussing matters with abbreviations more than anything else!

The core item for limit states, no matter what the name, is probability curves. These methods are based on probability of failure, and/or, the probability of people dying in an ultimate limit state event. If memory serves the National Building Code of Canada has codified 1:50000 as a probability of death in a ULS event.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

YS
My vote also is for USD as a limit state method, based on Aust/Brit experience. You're right about the confusion that can arise due to regional differences.

YS,

Allowable Stress Design and Working Stress Design are the same thing, and was basically the only type design practiced until the middle of the 20th century. Ultimate Strength Design is the method of reinforced concrete design which became the norm with ACI 318-63. Those of us who went to school in the 60's had to be conversant in both. In USD, you factor the loads and use capacity reduction factors for the material, while in WSD, you don't factor the loads and base everything on the allowable stresses in the material.

That seems very basic but interesting discussion.
I would say basically you can divide the three design philosophies this way:

WSD: Working stress design -- you apply a factor on ultimate strength (in terms of stress capacity) of the material and compare it with the actual forces coming onto the structure. If actual<allowable~~design ok.
USD: further development of design approach: multilpy load factor(>gnerally>1)to increase loading so as to cater for uncertainties associated with their calculation and reduce material strength by dividing material factors (>1)--if design effects are <design strength---design ok.
LSD/LRFD: This is basically same as USD but here you have two limit states to satisfy in your design-- serviceability limit state and ultimate limit state. The first referes to basically the concept that in no case structure shall become unfit for its service for example the deflection, vibration etc should be within permissible limits. and the second menas that your structure should not fail in its capacity to carry the design loading.

This is what you all are saying and I just wanted to summarise these according to my experience.

Cheers

Manoj