ASD Stress vs ASD Strength vs LRFD All are LSD

ASD

Allowable Stress Design (1989 9th Edition AISC Manual) or Allowable Strength Design (2005 13th Edition AISC Manual). Both use service level loads and a safety factor to member strength.

WSD

Working Stress Design (not used in design anymore). Uses services level loads and a safety factor to member strength.

LRFD

Load and Resistance Factor Design. Uses factored loads and applies a reduction factor to member strength.

LSD

Limit States Design. A design methodology where different failure mechanisms or states are checked and allowable strengths for each failure mechanism or state are determined. The controlling limit state is normally the one that results in the least available strength. This is more of a general term and includes ASD ’89, ASD 2005 and LRFD.

Strength Design = Generally refers to LRFD however the most new manuals which include ASD could be considered strength design methods as well. Meaning stresses are typically not calculated anymore…well they are but the end result is usually in terms of a members strength. In concrete you may also hear the term Ultimate Strength Design (where the old ’63 code used Working Stress Design) which is referring to LRFD.

Ultimate or Strength Level = Generally strength or ultimate level loads refers to Factored Loads in LRFD design. Ultimate capacity is generally the Factored Resistance or Capacity of the member being designed with LRFD.

Service Level = Generally service level loads are used with ASD methods. They are also used when checking deflection for serviceability.

Nominal Strength = This is the strength of the member for a given limit state before any safety factor or reduction factor is applied to the member. This is used with ASD or LRFD and is normal given in manuals that present a “Unified Approach” aka they give you a nominal capacity then  the user applies a safety factor or resistance factor.

Available Strength = This is the strength of the member based on the nominal strength reduced by the applicable safety factor or reduction factor. In LRFD it is common to refer to this as the Ultimate Strength. In ASD it is commonly referred to as the Allowable Strength.

Required Strength = This is the strength required based on the applicable ASD or LRFD combination. The required strength should always be less than the available strength.

Resistance Factor = The reduction factor applied to the nominal strength as used in LRFD.

Safety Factor = This is the factor which reduces the nominal strength as used in ASD.

These terms can be confusing when your fresh out of school. Most likely in school you predominantly used LRFD design. However when you show up to work you may find some who still use a lot of ASD. Or you may see alot of old ASD example problems or even need or want to use it in your design. I will try to clear some of this up for you.

ASD can mean either Allowable Stress Design or Allowable Strength Design. The Allowable Stress Design is the older or original designation which was used in the 9th Edition of the AISC Steel Construction Manual (1989 AISC) and the old ACI Concrete code (called Working Stress Design. Side note: working stress design can be helpful in reducing cracks and crack size. Therefore the method is sometimes still used in water applications). In these codes service level loads where applied to members. The stresses in the members where found and then checked against an allowable stress value which had a safety factor incorporated into it. Many ‘old timers’ will say that this used to give you more of a feel for the design as you better understood how the material and members where stressed. Allowable Strength Design (2005 AISC) – was mostly developed so that engineers who did not want to use LRFD could still use ASD and service level loads therefore both the ’89 ASD and ’05 ASD both use the same load combinations. It differs from the allowable stress design in that it is a ‘Strength Design’ methodology. The ’05 ASD uses safety factors on the nominal strength of the member based the particular limit state. The 05′ ASD allowable strength values maybe transformed into 89′ ASD stress values by factoring out the appropriate section property. Both ASD methods utilize Limit States Design however they are ‘hidden’ in the ’89 ASD code. Meaning that in the ’05 ASD each limit state is checked (i.e. yeilding, local buckling, lateral-torsional buckling, etc.). In the ’89 ASD code the allowable stress is reduced to the lowest applicable limit state. They also both take advantage of inelastic behavior in some limit states.

LRFD refers to Load and Resistance Factor Design which is also a Limit States Design methodology. This method uses a load factor to ‘factor up or down’ service level loads and also reduce member strength based on reliability and statistical data. When using LRFD you must design the strength based on the LRFD load combinations and factors however deflection should be based on service level loads, so you must keep track of your loads!

In the 2005 AISC both the ASD and LRFD methods for determining nominal strengths are presented side by side. The nominal strength will be the same for both methods and only the allowable strength will differ due to the fact that the safety factor applied for ASD and the reduction factor applied for LRFD will be different.

So why the switch, whats behind it? LRFD is a more reliable and statistical based method for predicting loads and material strengths. Whereas the allowable stress saftey factors where based on engineering judgement and past experiences. It is debated which will give you a more efficient design however it seems in most situations LRFD will produce a smaller sized beam based on strength but not always. Also serviceability and deflection control many designs, in which case both methods will yield the same result as the design is not base on strength at that point.

Check the code you are using for ASD safety factors/combinations and LRFD factors/combinations i.e. IBC, ASCE, ACI, etc.

Also see Chapter 2 of the 2005 AISC manual for further discussion.

10 comments for “ASD Stress vs ASD Strength vs LRFD All are LSD

  1. none
    April 4, 2012 at 3:28 am

    You are posting some things that are not factual.
    1989 ASD also incorporated Limit State Design.
    New ASD is also Limit State Design.
    The 1989 version just buried the Limit States into lower allowable stresses.
    Read the general section of the 2005 AISC code….

    • RFreundhte
      April 4, 2012 at 12:01 pm

      Yes you are correct. However I’m not saying that 1989 ASD wasn’t a Limit States Design, I am just saying that it was based on allowable stresses. I appreciate the feedback though and I will update accordingly.
      Thanks!

  2. RFreundhte
    April 4, 2012 at 1:02 pm

    Better? In all honesty I do appreciate you review. This is what it is about. Granted this basic of a topic should not need input but we will post some other details and calcs where we would love feed back.

  3. Brad
    August 28, 2013 at 8:41 am

    I’ve heard that you can use LSD loads and directly apply to LRFD design. For example a 100kN shear load from LSD design and converting it to imperial 22.5kips, That would be sufficient in proceeding to design shear connection using LRFD? Is there anything needed to be added in using LSD loads for the various LRFD connection types?

  4. August 31, 2013 at 10:52 am

    I’m not exactly sure I understand the question. When you say LSD loads -> what code are these loads coming from? In general there are (2) different categories or levels when speaking about loads -> Ultimate or Strength Level and the other Service level (I’ve heard some call this “actual” load but that is not correct). Ultimate or Strength level loads are used with LRFD design and Service level are used with ASD design. LSD which is limit states design, really applies to both LRFD and ASD. Meaning that say your are designing a beam. Then in this case the “limit states” that you are checking maybe yielding, lateral torsional buckling or local buckling (each of these is a limit state). You can check these limit states by using strength level forces against the ‘ultimate’ capacity which would be LRFD or you can check the service level forces against the ‘allowable’ strength of the member using ASD. Note that deflection is normally check using service level loads and thus the name “service level” loads. Hope this helps!

    • Brad
      November 8, 2013 at 8:42 am

      Man rime flies and I didn’t get an email notification.
      I guess my question is a little more simple, using the Canadian S16 and wanting to buy some sort of conn design software that only comes in LRFD or ASD can you use the S16 loads directly into a LRFD design. Say a beam shear value in 100kN in the Canadian design can be input as 22.5kips or 100kN in a LRFD program. Or are there going to be some cases where a conversion factor needs to be introduced between the 2 systems. I’ve heard that a conversion factor wouldn’t be needed.

  5. omid
    March 8, 2015 at 11:53 am

    I am confused about types of design methods???
    how many types of design methods we have?
    plz answer it if any one know?

  6. ryanfreund
    March 8, 2015 at 4:25 pm

    Well there are many design types so that is a difficult question to answer. However I think the main thing to understand is that there are generally two different levels of “design loads”. There are “service level” loads which are generally used to check deflection. This is too make sure the building is comfortable. Then there are “strength level” loads. These are also called “ultimate loads”. We generally check the strength of the members verse these loads and are not so concerned about serviceability, we just want to make sure the structure will not collapse/fail.

  7. June 29, 2016 at 12:16 pm

    In my perception there is a difference between ASD and LSD, since Allowable Stress Design is an elastic design method while Limit State Design is a plastic design method. In ASD the allowable is determined by an not to exceed elastic stress, which is never beyond yield, while in LSD the capacity of the structure is calculated. This can include yield stresses as long as the structure doesn’t collapse. Also in my perception LRFD is the same as LSD. LRFD is the American name for it. I feel AISC is (and was) confusing in this since the 1989 version is an ASD standard.

    • July 1, 2016 at 8:10 pm

      @Edgar – That is a somewhat reasonable explanation but I would not say that ASD never goes beyond yield and would also not say that LSD is plastic design. Here are a couple examples – A compact beam that is continuously braced is allowed to be designed using the plastic section modulus under ASD design. And conversely a slender column or unbraced beam will buckle (or laterally torsionally buckle) elastically in ASD or LRFD or LSD. Limit States Design just means you are checking the capacity of different failure modes. Having said that I do associate an LRFD approach to be more of a “plastic design” or “ultimate” design method.

Thoughts? Comments? Questions?