barriers, parapets, and railings - minnesota department … · barriers, parapets, and railings ......

Barriers, Parapets, and Railings

Arielle Ehrlich | State Bridge Design Engineer

May 17, 2017

Bridge Office | mndot.gov/bridge

MnDOT Vocabulary

Is it a barrier, a parapet or a railing?

5/17/2017 Bridge Office | mndot.gov/bridge 2

BARRIER. It is concrete and does not have a vertical front face.

MnDOT Vocabulary



PARAPET. It is concrete and has a vertical front face.

MnDOT Vocabulary



RAILING. It is steel.

MnDOT Vocabulary

• Barriers: Concrete, sloped or safety shape

• Parapets: Concrete, vertical face

• Railings: Steel (ornamental, fencing, structural)

• AASHTO calls these all bridge rails!


Vocabulary



RAILING and PARAPET. This is the T‐1 railing on the P‐2 parapet.

Why Barriers?


Purpose of Crash Testing

• Crashworthiness is determined by:

• Strength – ability to contain a vehicle

• Geometry – ability to redirect a vehicle

• Occupant safety –ability to minimize risk to vehicle’s occupants


Crash Testing


History of Crash Testing Standards

• Highway Research Board (HRB) Circular 482 (1962) – only one vehicle!

• Several other documents produced:

• NCHRP Report 153 (1974)

• TRB Circular 191 (1978)

• NCHRP Report 230 (1981)

• Publications refined the process and the vehicles



NCHRP 350 – “Recommended Procedures for the Safety Performance Evaluation of Highway Features”

• Publication in 1993

• Implementation in 1998



Manual for Assessing Safety Hardware (MASH)

• Originally published in 2009

• Second edition published in 2016

• All new hardware must be tested to MASH (not NCHRP 350) starting January 1, 2011


MASH Implementation

FHWA/AASHTO Implementation Plan

• January 7, 2016: Bridge barriers must be tested to MASH (2016)

• On National Highway System (NHS)

• Projects let after December 31, 2019

• New installation or full replacement


MASH Implementation

FHWA/AASHTO Implementation Plan

• January 7, 2016: Bridge barriers must be tested to MASH (2016)

• Existing systems do not need to be replaced

• Agencies are encouraged to develop a policy for non‐NHS routes and other types of projects (such as mill and overlay projects)

• Full testing is required; finite element analysis alone is insufficient to validate NCHRP 350 devices meets MASH


MASH Implementation

Deadlines for other safety devices:

• December 31, 2017: W‐beam barriers, CIP concrete barriers

• June 30, 2018: W‐beam terminals

• December 31, 2018: Cable barriers and terminals, crash cushions

• December 31, 2019: Transitions, all other longitudinal barriers (including portable barriers), terminals , sign supports, breakaway hardware


MASH Implementation

• NCHRP 20‐07/Task 395• Work being done by Texas Transportation Institute (TTI)

• Determining commonly used barriers around the country

• Trying to determine which rails need to be retested to MASH and which can be approved based on previous evaluation

• Maintaining a database of MASH tested hardware and wish list for hardware to be tested

• https://www.roadsidepooledfund.org/mash‐implementation/

• NCHRP is considering a new project in FY 18 to continue the work of this project


Test Levels

Test Level AASHTO Article 13.7.2 Description MnDOT Usage


TL‐1 Work zones with low posted speeds, very low ADT, low speed local streets

N/A

TL‐2 Work zones and local collector roads with favorable site conditions, small number of heavy vehicles, posted speeds are reduced

Low speed

TL‐3 High speed arterial highways with low mixtures of heavy vehicles and with favorable site conditions

High speed on approach panels; guardrail

Test Levels

Test Level AASHTO Article 13.7.2 Description MnDOT Usage


TL‐4 High speed highways, freeways, expressways, and interstates with a mixture of trucks and heavy vehicles

High speed on bridges

TL‐5 Similar to TL‐4, but where large trucks are a significant portion of ADT

Pier protection; historically on the outside of curved decks

TL‐6 Applications where tanker‐type trucks or other high center of gravity vehicles are anticipate, especially with unfavorable site conditions

N/A

NCHRP 350 vs. MASH

• Test vehicles are updated to reflect the 85th percentile of the United States’ passenger vehicle fleet.

• Impact condition criteria were modified to correct inconsistencies

• Evaluation criteria were modified to be more objective


NCHRP 350 vs. MASH


NCHRP 350 vs. MASH

• Significant differences at TL‐4


350 MASH

NCHRP 350 vs. MASH

• Significant differences at TL‐4

• Single unit truck values have changed substantially


Value NCHRP 350 MASH

W (kips) 18.0 22.0

G (in) 49 63

Speed (mph) 50 55

Minimum Height of TL‐4 Barrier

NCHRP 350 32”

MASH 36”

MnDOT Type S Barriers

• 32” F shape doesn’t meet minimum height for MASH TL‐4

• New series of barriers developed: Type S


MnDOT Type S Barriers

• Minimum height based on crash testing is 36”; 32” F doesn’t meet MASH

• Based on Texas DOT SSTR Rail

• Designed to meet or exceed strength of TxDOT SSTR

• 10.8o slope

• Three heights: 36”, 42”, 54” – All MASH TL‐4

• Working on eligibility letter with TTI


New MnDOT MASH TL‐2 Parapet

• Current parapet (P‐1, Fig. 5‐397.166) doesn’t meet MASH TL‐2 when mounted on a sidewalk

• Developing a replacement based on Caltrans Type 732SW

• Low speed only

• Proposed parapet meets MASH TL‐2 on a sidewalk


Other MnDOT Barriers

• T‐1 railing with P‐2 parapet(Fig. 5‐397.157)

• Not currently tested to MASH

• Meets 36” height, so possible it will meetMASH TL‐4

• May be crash tested in the future

• F shape will remain on repair projects where barrier isn’t being replaced


Barrier Design

Interior Section Yield Line End Section Yield Line


• Yield line does not extend into the deck.• Deck needs adequate strength to force the yield line to stay in the barrier.

Barrier Design

• What if the segment length is less than Lc?

• Diagonal yield line can’t develop

• Design for yield line at the joint between the parapet and the deck.


L

Loads

• Values are for NCHRP 350 only. No MASH values determined yet.

• Recommended values for MASH TL‐4 are too high

• Biggest issue is for overhang design; Barriers can be physically tested.

• MnDOT standards still showing NCHRP 350, although barriers meet MASH and deck is expected to


Pedestrian and Bicycle Railings

• Current MnDOT height requirement: 54”

• Higher than minimum in AASHTO Article 13.8.1

• May need to be higher over interstates and railroads



• Design loads: 50 plf + 200 lbpoint load per AASHTO Article 13.8.2

• For posts, apply the load at the lower of the top horizontal rail or 5 feet above the top of the walkway surface



5/17/2017 Bridge Office | mndot.gov/bridge

• Maximum clear opening size: 4” in lower 27”6” above 27”

• Opening sizes are more restrictive than AASHTO

32

Ornamental Metal Railings

• Two standard ornamental metal rails:• T‐3 (includes chain link fence)

• T‐4 (no chain link fence)

• Curb and parapet mount standards available

• Low speed applications only

• At 40 mph, ornamental metal railsmust be protected by a traffic barrier

• At lower speeds, can be mounted on a sidewalk or behinda traffic barrier


Chain Link Fence

• AASHTO Article 13.8.2 includes 0.015 ksf wind load on chain link fence

• Wind load does NOT need to be applied simultaneously to live load

• Wind load is applied for both design of fencing and for the posts


References

• AASHTO LRFD Bridge Design Specifications – Chapter 13

• MnDOT LRFD Bridge Design Manual – Section 13http://www.dot.state.mn.us/bridge/pdf/lrfdmanual/section13.pdf

• MnDOT Bridge Details Manual Part IIhttp://www.dot.state.mn.us/bridge/bridgedetails2.html


Thank you!

Arielle [email protected]

651.366.4506


barriers, parapets, and railings - minnesota department … · barriers, parapets, and railings ......

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