LFD & LRFD 1

[aun]

Could anyone explain me the difference between these 2 methods??

 

[jesosan]

Are you sure is not an ASD vz. LRFD comparison what you are looking for?

ASD Allowable Stres Design = Old method
LRFD Load and Resistence Factor Design = Most Recent Method

 

[aun]

In AASHTO, Chapter 10 steel design, it has LFD,( Load Factor Design). My thought, LFD and LRFD are alike. Is it correct??

 

[Qshake]

No, they are not alike.

In a very small nutshell, Load Factor Design includes factors only on loads, while in LRFD factors are considered on both loads and materials.

AASHTO has an entirely seperate code for LRFD. It is very nearly night and day difference between the LFD code and the LRFD code.

 

[whyun]

Who uses LFD?

Here we use:
wood - ASD
masonry - ASD (LRFD for slender walls)
concrete - LRFD
steel - ASD (old skool both gravity and lateral)
(LRFD/AISC-SPSSB97 (for lateral only on occasion)

Oh, I am a building engineer, not bridges.

Is transportation industry the only one using LFD?

 

[Awe72]

HI, I'm new here...

I'm a canadian engineer (mostly buildings)

Here we always use the LRFD method, which we call Limit States Design. Even for Wood and Masonry we use LRFD.

 

[whyun]

Awe72: I am in California. You guys are few decades ahead of us.

Concrete design here has been strength for quite some time now. For steel, most engineers STILL use ASD. Although there are LRFD methods for masonry and wood, I dont know of anyone using them in practice.

LSD, LRFD and LFD are all based on factored up forces vs. element strength with possible variations. I am not familiar with Canadian LSD and LFD mentioned in this post. I am guessing basic design philosophy is the same.

Qshake: would you mind expanding a little further on your description of LFD and LRFD? Thank you in advance.

 

[Qshake]

Before expanding let me just say that, here in the Midwest, we use mostly LFD for steel and concrete structures. Wood and masonry are ASD. LRFD has not caught on here and of course the developer/owners in the private sector could care less what method is used as long as it is done on time, under budget and no change orders occur. In the public sector, there have been a few states that actually practice LRFD and some who've threatened to implement it but have not done so to date.

 

[whyun]

Qshake: It sounds like you do buildings. For steel design, do you use AISC Manual of Steel Construction LRFD, Third Edition? Reason who I am asking is, here we don't talk about LFD. We call it LRFD. And prior to reading this posting, I thought it was the same thing but apparently not.

 

[Awe72]

Well, from my own experience, LRFD (or LSD here) gives you more 'things' to check compare to ASD (we call that WSD - working stress design here).
We design masonry with ASD before and it is quite simple and fast then LSD method, but most of the time it's always a bit over-design. The new Canadian Masonry code will remove the option of designing with ASD and only consider an LSD design.

Yes but I agree with Qshake that the argument is only between us the engineers anyway. And yes, I never heard of LFD too , I got the impression that it's the same with LRFD. So, Qshake, I'll be interested in reading some info. anything online that I can access (website, etc?). Thanks

 

[whyun]

Hello there awe72.

Basic difference between WSD and LSD is that in WSD, we use the actual force a member sees and factor down the strength to a certain comfortable level; and in LSD, we use the actual strength and factor-up the forces to some comfortable level, which by the way, are purely empirical.

It seems like terminology vary from location to location and material to material.

Where I live, these are how we call them:

Wood design: ASD vs. LRFD
Steel design: ASD vs. LRFD
Masonry design: Working Stress Design vs. Strength Design
Concrete: Strength Design

No matter how they call things, it is basically one of two design philosophies.

Oh, yes... masonry/wood LRFD is more complex. Many engineers here are resisting the change to LRFD here. Just like resisting adoption of metric units. Most importantly, our design fee remains about the same regardless of design methods.

Trend is "Go LRFD" so I forsee that us Americans will one day use only LRFD for all materials. Nice weekend everyone.

 

[KLF]

The other thing to keep in mind, is that the various LRFD/Ultimate Strength design documents do not always have the same load & resistance factors. Thus one may have to compute service combinations along with any number of factored combinations for each material (steel, masonry, RC, timber, etc) in order to complete design. LSD permits a single unified load and material resistance factor set.

 

[whyun]

KLF,

For example, UBC97 1612.2 has 1.2D+1.6L+0.5Lr but for concrete, 1909.2.1 calls for 1.4D+1.7L. Both are at strength level forces.

Using the LSD, are you saying that regardless of material, there is a single set of load factors?

Which code has LSD? Canadian code? Being in California, I am completely ignorant of how things are done outside this state. In fact, 2000 IBC will not be adopted for the next code cycle. We will be stuck in 1997 for another 3 years. Oh well.

 

[JAE]

There has actually been movement towards unifying the various codes/specs in the US - Take a look at the latest ACI code and you will no longer see the familiar 1.4D + 1.7L (its now in the appendix) but rather, 1.2D + 1.6L similar to the AISC LRFD Specification.

 

[Awe72]

To whyun:

Yes it is the Canadian LSD (Limit States Design).

 

[whyun]

JAE thanks for the info. It appears that up until ACI318-99, the old factors still remained but when I checked out the 318-02, sure wnough, the factors are changed to look like the UBC combos.

Strength reduction factors, also, changed to compensate for this. My gut feeling is, the change in these two will NOT affect the end result.

It is good that the code writers are making attempts at unifying the load factors. One complaint I have is, all the phi factors that I've come to memorize by heart has to be erased and re-programmed. That's not easy to do. Thanks JAE!

 

[KLF]

Whyun,

Yes...the National Building Code of Canada (NBCC)governs load combinations and factors used (1.25 Dead, 1.5 Live, etc) and individual design codes define the phi material resistance factors (0.9 Str'l Steel, 0.85 rebar, 0.6 concrete, etc.). I still need to carry service combinations - obviously.

FYI, in some very limited comparisons of designs carried out under UBC & ACI versus NBCC & A23.3 (RC Design Code), the END result was approximately the same for R.C. flexural members.