LRFD and GDOT Challenges and Solutions to Implementation of AASHTO - PowerPoint PPT Presentation

LRFD and GDOT Challenges and Solutions to Implementation of AASHTO LRFD 5 th Edition Ian Rish, P.E., Environmental Testing Branch Supervisor

What is different about Load and Resistance Factor Design (LRFD) Factor of Safety (OLD) All the uncertainty of analysis lumped into one number. Resistance Factor (NEW) Statistically based for each component of design.

GDOT Work Flow for Bridge Foundation Investigations AASHTO 17 th Edition Design Geotechnical capacity controlled by factors of safety. Preliminary Geotechnical Final Bridge ASD BFI Bridge Layout Investigation Plans AASHTO LRFD 5th Edition Design Geotechnical resistances based on resistance factors. Preliminary Bridge Geotechnical Foundation Final Bridge Bridge Foundation LRFD BFI Investigation Selection Plans Layout Loads

Bridge Office Information Provide pile sizes based on their design Assumed Driving Condition (6.5.4.2) • Severe Provide factored and service loads to optimize foundation design. • Old estimated tips for piles based on maximum allowable load instead of specific design load for foundation element.

Service Limit State Tolerances: • Vertical Deformation • Settlement • Axial Resistance • Horizontal Deformation • Settlement • Lateral Resistance

How much design done at service limit? Spread footings • Old method controlled for settlement of 1 inch. • New method - what is tolerable? Pile bents/footings • L-pile analysis infrequent for piles. Drilled shafts • L-pile analysis routinely performed.

Strength Limit State for Piles Piles driven into soil Piles driven to soft rock • Acts like soil Piles driven to hard rock • Loads controlled to reduce damage • Rock is harder than the pile

Strength?

Strength Limit State (cont.) Scour • Extreme rain events can scour out bridge piers Downdrag • Settlement can induce loads in piles Lateral Loads • Horizontal forces could bend a pile

Resistance Factors for Driving

Resistance Factors (cont.)

Static Analysis Old Practice (ASD) • In-house developed method • Source data not available • Not able to come up with resistance factor New Practice (LRFD) • AASHTO approved method • Driven 1.2 is free but no longer updated (has issues) • In process of purchasing new software

Driven Output What does this tells us?

Drivability Analysis Why/when is this needed? What information is needed? • Boring Log • Hammer • Pile Information • Type (H-pile, PSC, MS) • Size • Physical Properties (fy, f’c , etc.) • Length

Drivability Analysis Output

Extreme Events Seismic • Not usually an issue for Georgia Scour • This is our primary concern due to issues in the past. Downdrag • Normally taken care of with consolidation before driving (Design – Bid – Build) • Load included in pile or other options (Design Build)

Spread Footings Rock • RMR • No settlement Soil • Soil properties • How much settlement? Effective footing size?

Drilled Shafts Skin friction (soil/IGM) • Methods are same as before • Load tests recommended End bearing (rock) • Old practice based RQD and old data • 5 th and 6 th edition based on RMR • 7 th edition based on GSI • Load test?

Old BFI Template

New BFI Template

Questions