LRFD vs LFD

[PenPal98]

What is the difference between LRFD (load and resistance factor design) and LFD (load factor design)?

For loads (dead, live, etc.) apply a "load factor" such as:
1.4D
1.2D + 1.6L

The "resistance factor" is the phi for bending, compression, tension, etc.

Does LFD simply not use the "reduction factor" and LRFD does?

 

[DaveAtkins]

I have never heard of "LFD." I have heard of "LSD," which stands for "limit state design."

DaveAtkins

 

[mrwhiteOH]

In General:

LFD is load factor design - meaning all safety factors are applied to the load. The factored load can be as high as the ultimate load.

LFRD is load factor resistance design - meaning safety factors are applied to both the load (demand) and to the materials (resistance). In theory, the factors (load and resistance) should be based on statistical analysis to determine the possible range of loads and possible range of resistance offered by the materials coupled with appropriate factors of safety.

Hope this is helpful

KEW

 

[PenPal98]

Thank you very much.

Then, out of curiousity, the ACI 318-02 is all based on LRFD. To modify it for LFD, should I just ignore all the phi factors?

 

[miecz]

LFD and LRFD are AASHTO terms that don't appear in ACI318-02. ACI318-02 uses the "Strength Design Method" that is more closely related to AASHTO's LFD than LRFD. Both approaches have factors related to material strength. LFD calls them strength reduction factors (see ACI 9.3.2). LRFD calls them resistance factors (see AASHTO 5.5.4.2.1). To design according to ACI318, use the factored loads and the strength reduction factors.

 

[PenPal98]

If the ACI has strength reduction factors, wouldn't it be more closely related to LRFD?

 

[miecz]

You're right, LRFD and ACI "Strength Design Method" are very similar. LRFD doesn't have "Strength Reduction Factors." LRFD has "Resistance Factors". These work the same, so this is not a major difference. ACI318-02's "Strength Design Method" designs to Required Strength (9.2). LRFD designs to "Limit States". Some of the limit states are "Strength" limit states, and some are "Service" limit states. LRFD's "Strength" limit states work similarly to ACI's Article 9.2. LRFD's "Service" limit states correspond to ACI Article 9.5.

 

[crossframe]

"The relationship between LRFD and LFD is somewhat analogous to that of PhD and PE."

 

[LowLax]

I've never heard of LFD, but if you wanted to apply all saftey factors to the load factors, then you would need to divide your load factors by the strength reduction factor. Using LRFD it would go like:

1.2DL+1.6LL<phi*N

where phi is the reduction factor and N is the nominal resistance. So you'd just divide both sides of the equation by phi to get the same factor of safety as using LRFD.

 

[HgTX]

Why would you want to modify ACI 318 to be more like LFD?

Hg


Eng-Tips guidelines: faq731-376

 

[atomicman]

I read a short article on this from AISC. If you go to

http://www.aisc.org

and search "LFD", an article called "Bridge Crossings No. 5" is listed. It's from March 1997 and is entitled "LFD vs. LRFD - What's up with the Letter "R" anyway?"

 

[connect2]

Wow, you can't use factored loads without the strength reduction factors, sort of like using WSD without any consideration for yield points and FS. Ultimate limit states, serviceability limit states, don't confuse the calibaration thats been done..... 2005 huh?