Comments on: ASD Stress vs ASD Strength vs LRFD All are LSD

By: Ryan Freund

Ryan Freund — Sat, 02 Jul 2016 01:10:05 +0000

@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.

By: Edgar Peetam

Edgar Peetam — Wed, 29 Jun 2016 17:16:54 +0000

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.

By: ryanfreund

ryanfreund — Sun, 08 Mar 2015 21:25:18 +0000

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.

By: omid

omid — Sun, 08 Mar 2015 16:53:26 +0000

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

By: Brad

Brad — Fri, 08 Nov 2013 14:42:34 +0000

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.

By: Ryan Freund

Ryan Freund — Sat, 31 Aug 2013 15:52:23 +0000

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!

By: Brad

Brad — Wed, 28 Aug 2013 13:41:20 +0000

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?

By: RFreundhte

RFreundhte — Wed, 04 Apr 2012 13:02:22 +0000

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.

By: RFreundhte

RFreundhte — Wed, 04 Apr 2012 12:01:36 +0000

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!

By: none

none — Wed, 04 Apr 2012 03:28:09 +0000

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….