Load and Resistance Factor Design “LRFD” for Shafts

Load and Resistance Factor Design“LRFD” for Shafts Geotechnical PerspectiveJim Cuthbertson

LRFD Shaft Design • First we have to review the basic principals upon which LRFD is based • You Geotechs may recall the following:

 2 x 2 = 5

LRFD Shaft Design • First Step Field Exploration • WSDOT Policy • BR Curb to Curb width < 100 ft wide • one geotechnical boring per pier minimum • BR Curb to Curb width > 100 ft wide • Two geotechnical borings per pier minimum • Depth of investigation should extend below the anticipated shaft tip elevation a minimum of 20 feet, or a minimum of two times the maximum group dimension, whichever is deeper.

LRFD Shaft Design Cohesive Soil • No Change from LFD to LRFD

LRFD Shaft Design Non-Cohesive Soil • No Change from LFD to LRFD

LRFD Shaft Design • So What Changed??  OLD LFD

LRFD Shaft Design • So What Changed??  New LRFD

LRFD Shaft Design

LRFD Shaft Design • If resistance factors were safety factors where would we stand • 0.45  2.22 • 0.50  2.00 • 0.55  1.82 • This seems like we are designing our shafts with less “safety” than we were before with SF of 2.5 to 3. Remember there is a load side of the equation too, and those loads have been increased by Load Factors! Overall, right now we are just as safe as past practice

QUESTIONS

Load and Resistance Factor Design “LRFD” for Shafts