LRFD and GDOT

Challenges and Solutions to Implementation

of AASHTO LRFD 5th Edition

Ian Rish, P.E., Environmental Testing Branch

Supervisor

Factor of Safety (OLD)

All the uncertainty of analysis lumped into one number.

Resistance Factor (NEW)

Statistically based for each component of design.

What is different about

Load and Resistance Factor Design (LRFD)

GDOT Work Flow for

Bridge Foundation Investigations

Preliminary Bridge Layout

Geotechnical Investigation

Foundation Selection

Bridge Foundation

Loads LRFD BFI

Final Bridge Plans

AASHTO 17th Edition Design

Geotechnical capacity controlled by factors of safety.

AASHTO LRFD 5th Edition Design

Geotechnical resistances based on resistance factors.

Preliminary Bridge Layout

Geotechnical Investigation

ASD BFI Final Bridge

Plans

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.

Bridge Office Information

Tolerances:

• Vertical Deformation • Settlement

• Axial Resistance

• Horizontal Deformation • Settlement

• Lateral Resistance

Service Limit State

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.

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 Limit State for Piles

Strength?

Scour

• Extreme rain events can scour out bridge piers

Downdrag

• Settlement can induce loads in piles

Lateral Loads

• Horizontal forces could bend a pile

Strength Limit State (cont.)

Resistance Factors for Driving

Resistance Factors (cont.)

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• Driven 1.2 is free but no longer updated (has issues)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

Static Analysis

What does this tells us?

Driven Output

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

Drivability Analysis Output

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)

Extreme Events

Rock

• RMR

• No settlement

Soil

• Soil properties

• How much settlement?

Effective footing size?

Spread Footings

Skin friction (soil/IGM)

• Methods are same as before

• Load tests recommended

End bearing (rock)

• Old practice based RQD and old data

• 5th and 6th edition based on RMR

• 7th edition based on GSI

• Load test?

Drilled Shafts

Old BFI Template

New BFI Template

Questions