Could someone please explain the basic differences between LRFD (as used in AISC Manual of steel construction, or AASHTO bridge design specifications) and LFD (as used in AASHTO standard specifications)? I have a good working knowledge of LRFD design, but have had much less exposure to LFD design. I’ve noticed that even in LFD design some resistance factors are used, although the absence of the “R” in LFD seems to imply they are not used.