Transitions › divisions › engineering › structures › ...How Transitions Function Increase strength as you approach the bridge rail by: Reduced post spacing Larger/longer posts,

Transitions The Good, The Bad and the Ugly

Emmett McDevitt

Transportation Safety Engineer

Federal Highway Administration

Roadside Design Options 1. REMOVE the obstacle 2. REDESIGN the obstacle for safe traversal 3. RELOCATE the obstacle 4. REDUCE severity (make breakaway) 5. SHIELD the obstacle 6. DELINEATE the obstacle

RDG Page 1-4 3-3

Run-Off-the-Road

Problem 1/3 of all Traffic Fatalities 2/3 on 2-lane roads 1/2 at night

Avoidance of obstacle

Run-off-the-Road Reasons

Speeding

Impaired Driving

Inattention

Poor visibility

Utility Pole9%

Embankment10%

Other10%

Sign Post6%

Guiderail/Barrier

14%

Drainage Features

19%

Fence4%Tree

28%

Roadside Hazard Fatalities

Barrier Transitions

Bridge Connections Stiffened Sections Other Transitions

Learning Outcomes

Why Are Transitions Necessary? How Transitions Function Highlight Key Elements of Good

Transitions

Why are transitions Necessary?

Required to account for transitioning from one barrier to another with different dynamic deflections (barrier with one stiffness versus another)

Provides continuity in strength of different barrier systems

Smooth connection allows redirection without snagging on or penetrating the barrier

Maintains vehicle stability to prevent vaulting or rollover

Barriers are Systems Include Standard Sections Terminals Transitions

7-13

It is important to ensure tension continuity throughout the system

Flexible barrier deflection ranges from 2.5 m to 3.4 m (98” to 11’2”).

Needed where a semi rigid system joins a rigid system (Never a flexible directly to a rigid)

Transitions

Semi-rigid barrier deflection ranges from 330 mm to 2.5 m (13” to 8’2”).

No Deflection

No Deflection

Welcome to

Boise

At least it was delineated

How Transitions Function Increase strength as you approach the bridge rail by:

Reduced post spacing Larger/longer posts, or both Soil plates to resist impact forces Nesting the Rail Rigid connections to bridge rail

Nested Guardrail

”Nested”

13-29

Pocketing

Soil Plates

Key Elements of Good Transitions

Crash Tested (Test Level 2-4) Transition Sections should be long enough so that

significant changes in deflection do not occur within a short distance (10-12 times the difference in deflection of the two systems)

Curb/rubrail to prevent wheel snag (especially on W or box beam)

Tapering bridge rail behind transition at connection point Design end connection to minimize snagging by errant

vehicles (both directions on a 2 way facility) Adequate structural connection to the bridge railing

Test Levels

• Test Level 2 4400 lb vehicle @ 43 MPH

• Test Level 3 4400 LB vehicle @ 62 MPH

Don’t do this!

13-36

Structural Connection

13-38

Gradually Increase Stiffness

13-39

Block Outs and Rub Rails

13-42

Curb at Face of Rail

Tapered behind Transition

Concentration Areas

Data that shows run off the road crashes or crash experience Outside of curves Higher speeds Higher volumes

Summary

13-52

• Required for transitioning smoothly from semi-rigid to bridge rail

• Should be crash tested • Strengthening methods • Adequate connection • Curb or rub rail and tapered sections • No snag points or pocketing

• Many retro-fits available for existing rail systems

• Doing something, using these principles, is better than doing nothing.