RETROFIT RAILINGS FOR THROUGH TRUSS BRIDGES

a

Report on

Contract No. DTFH61-84-C-00052

Prepared for

Office of Implementation Federal Highway Administration

U.S. Department of Transportation

by

J. R. Morgan Assistant Research Engineer

C. E. Buth Research Engineer

R. M. 01 son Research Engineer

W. L. Campise Research Associate

Texas Transportation Institute The Texas A&M University System

March 1986

NOTICE

This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof.

The contents of this report reflect the views of the Texas Transportation Institute, which is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official policy of the Department of Transportation.

This report does not constitute a standard, specification, or regulation.

The United States Government does not endorse products or manufacturers. Trade or manufacturers 1 names appear herein only because they are considered essential to the object of this document.

Technical Report Documentation Page

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle 5. Report Date

RETROFIT RAILINGS FOR THROUGH-TRUSS BRIDGES March 1986 6. Performing Organization Code

f-::--:---:-~c-----:-:---------------------! 8. Performing Organization Report No. 7 · Author's> J. R. Morgan, C. E. Buth, R. ~1. Olson, and RF 7015

W. L. Campise 9. Performing Organization Name and Address

Texas Transportation Institute Texas A&M Research Foundation Texas A&M University System

10. Work Unit No. (TRAIS}

11. Contract or Grant No.

DTFH6l-84-C-00052 t--:-:---=C:-.::o...:..l..:..l..::.e.iil...ge=--:S:..:t::.:a:...:t:...:i-=o.:..:n2., _:_:TX:...:...,...,-:7,....:7....:.8:....:4-=3:.._ ___________ -J 1 3. Type 0 f Report and period Covered

12. Sponsoring Agency Name and Address Fi na 1 Report Federal Highway Administration August 1984 -Research, Development & Technology November 1985 6300 Georgetown Pike 14. Sponsoring Agency Code

Mclean, Virginia 22101 15. Supplementary Notes

FHWA contract manager: James A. Wentworth (HRT-20)

16. Abstract

Through-truss bridges that have remained in service are required to handle traffic having characteristics that are greatly different from those that existed when they were constructed. Today's traffic is composed of heavier vehicles which travel faster and have different operating character­istics, making it desirable to have wide lanes, longer curves, and more sight distance. Some older through-truss bridges may have adequate load­carrying capacity, ·but are considered functionally obsolete because they are too narrow or have inadequate railings. Retrofit bridge rails can make it possible to leave many of these bridges in service until their replacement is economically feasible.

17. Key Words 18. Oi stri but ion Statement

Truck-mounted attenuators, Shadow vehicles, Maintenance crews

No restrictions. This document is availableto the public through the National Technical Information Service, Springfield, Virginia 22161

19. Security Classif. (af this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price

Unclassified Unclassified 23

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

METRIC CONVERSION FACTORS

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TABLE OF CONTENTS

Section Page INTRODUCTION ...•••.••••••.•••.•••.•.•.••.•••.•.••••..••.••.••....•.•.• 1

Through-Truss Bridges ............................................ 1 Current Situation ................................................ 1 Designing For Reduced Collision Severity ......................... 2 A 1 ternati ves ..................................................... 2

EVALUATION OF ALTERNATIVES .....................•.....•...........•.... 4 RETROFIT RAILING SYSTEMS .............................................. 8

High Performance Ra i 1 i ng ( HPR).. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 8 Lower Service Retrofit Railing (LSRR) ............•............... 8 Retrofit Railing Installations ................................... 14

REFERENCES.. . • . • • • • • . • • • . . . • • • • • • . . . . • . . • • • . . . . . • . • • • . . . . • • . . . . . . . . . • . 19

i i i

Figure

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

LIST OF FIGURES

Typical tubular thrie~beam retrofit design (courtesy SwRI) .............................................. 6

Self-restoring barrier (SERB) guardrail ..................... 7 Through-truss retrofit railing estimated material costs ....... · ....................................... 9

Post attachment to floor beam flange .•...................... 11 Suggested intermediate post attachment (concrete deck) ............................................. 12

Alternate intermediate post attachment for wood decks .............................................. 13

North Carolina tubular thrie-beam on collapsing tube blackouts .....•..........................•.. 15 New York box beam retrofit .................................. 16 New York retrofit ......................•....•............... 17 North Carolina interim thrie-beam retrofit .....•............ 18

iv

INTRODUCTION

Through-Truss Bridges

A large number of the through-truss bridges in service were designed and constructed many years ago to meet the service requirements and practices in effect at the time. Many were designed with low strength pedestrian railings or with rails and posts which can snag colliding vehicles.

Current Situation

Through-truss bridges that have remained in service are required to handle traffic having characteristics that are greatly different from those that existed when they were constructed. Today•s traffic is composed of heavier vehicles which travel faster and have different operating characteristics, making it desirable to have wider lanes, longer curves, and more sight distance. Some older through-truss bridges may have adequate load-carrying capacity, but are considered functionally obsolete because they are too narrow or have inadequate railings. Retrofit bridge rails can make. it possible to leave many of these bridges in service until their replacement is economically feasible.

Warrants for retrofit railings and bridge replacement should consider a number of items which will contribute to the benefit/cost analysis. Among these, economic considerations wi 11 usually override other considerations. However, aesthetic and other benefits derived from a lower service level bridge with retrofits will sometimes outweigh the benefit provided by a full service replacement structure.

Some consideration should be given to costs of retrofitting and compared with bridge replacement. These comparisons should consider:

1

1 Cost of retrofit, including rehabili~ation, if required, and anticipated maintenance costs.

1 Cost of replacement, including anticipated maintenance costs.

In addition to costs, consideration should be given to level of service of retrofitted bridge rails compared with level of service of a new structure.

Designing For Reduced Collision Severity

When existing through-truss bridges are eva 1 uated by today 1 s design requirements, needs for improvements in terms of reducing collision severity are sometimes identified. Many of these structures were originally designed with railings not adequate for redirecting a colliding vehicle in a safe manner. An impact by an errant vehicle, if it occurs at a primary structura 1 member, ·can cause co 11 apse of the structure, or at 1 east 1 oss of capacity of the bridge to carry traffic. In addition, little protection is provided to the occupants of a colliding vehicle even in a collision which does little damage to the bridge.

An additional difficulty arises from the design width of many of these bridges (whether through-truss, girder, or other). Original construction did not provide width for future installation of a retrofit railing. Some of the bridges did provide for pedestrian traffic, and this space can be used to install a retrofit rail if pedestrian traffic is restricted or if pedestrian traffic is accommodated on cantilevered walkways constructed outboard from the bridge.

Alternatives

Some of the old through-truss bridges can be treated effectively to enhance safety and extend the useful life of the structure. Each

2

structure should be evaluated to determine whether shortcomings exist and whether they are treatable. Two questions should be asked concerning the consequences of a vehicle collision when evaluating a bridge for possible installation of a retrofit railing system. (1) In the event of a collision, what severity of consequences would be expected in terms of injury to occupants of the vehicle, and (2) what severity of consequences would be expected in terms of damage to the bridge structure? After an evaluation it might be decided that the structure and railing system are adequate, as it is, and no action is needed. If shortcomings are identified, then one must determine whether they can be effectively treated. It may be that the expense of possible treatments would indicate that the structure should be taken out of service. However, some structures may be treated and remain in service, although possibly at a lower level of service.

3

EVALUATION OF ALTERNATIVES

Once it has been decided that the structure in question has shortcomings that may be treated with a retrofit railing system, several designs with a variety of features are available. All of these improve impact performance in terms of reduced damage to a colliding vehicle and less severe injuries to occupants of the vehicle. Retrofit railings also provide various degrees of improvement in protecting the structure against damage when a collision occurs. In general, retrofit railings that offer the greatest improvement also require the most space. However, space is usually not available on bridges such as those being considered here. Many of these structures are currently too narrow and further reductions in width may require that the number of traffic lanes be reduced.

Very little space on one-lane bridges and narrow two-lane bridges is available for installation of a retrofit railing. In order to maintain the existing width of the travelled way, one could choose to install a lower service level retrofit railing such as a single thrie-beam mounted on a post connected to the deck. Such a ra i 1 i ng wou 1 d offer reasonably good performance in terms of smoothly redirecting vehicles and would offer improved protection against damage to the truss structure. The approach angle at which vehicles can collide with the railing is significantly 1 imi ted because of narrowness of the structure and frequently traffic speed is slow. In such situations, a lower performance retrofit railing is entirely adequate.

Alternative mounting methods that would offer more advantageous transfer of co 11 is ion forces to the supporting structure are sometimes possible. For example, transverse beams may be added beneath the existing structure and extended laterally. Posts can then be mounted on these beams and 11 blockouts 11 extended inward through the truss to support the rail element just inside the face of the truss.

4

On wider structures, additional alternatives are available and a higher performance railing can be installed more readily. Many structures have space available in the form of shoulders, emergency lanes, sidewalks, emergency walkways; or space occupied by curbs that can be used to accommodate a retrofit railing. (Some nontruss structures have similar characteristics and are candidates for installation of a retrofit railing.)

On structures which have a wide raised sidewalk, a retrofit railing may be installed in alignment with the face of the curb edge of the sidewalk by anchoring posts in the sidewalk. This leaves a clear space behind the new railing that can remain in use as a sidewalk. The old railing on the outside edge would remain in place for a pedestrian railing, as shown in figure 1.

Higher performance retrofit railings which use a tubular thrie-beam rail element such as the one developed by Southwest Research Institute (SwRI) and box beam elements such as those used by New York DOT require from 6 inches to about 2 ft of lateral space. (1) The SwRI design shown in figure 1 and 2 is very suitable for installation on top of a narrow safety wa 1 k. The face of the ra i 1 i ng is a 1 i gned with the curb face. This type of design precludes the continued use of the safety walk by pedestrian. Such designs do not reduce the effective width of the travelled way.

Other situations exist where a higher performance railing is needed and space for the railing is simply not readily available. In such situations it may be necessary to reduce the service level to a one-lane bridge or replace the structure.

5

2 in

36 in

(J)

9 in

60 in

4-3/8 in 5-5/8 ·in

32 in

P."-"" ;,~ .; .

,. # .·0 ,_ ....

Tubular Thrie

Pedestrian Railing (To R..e_main)

.· . ... ~

...... .. ;.

a. Typical Installation b. R(IIIW) -1 Retrofit Design

Figure l. Typical tubular thrie-beam retrofit design (courtesy SwRI).

34 in

*Varies To Maintain Top Of Rail Elevation At 33 In Above Grade

*

33 in

13 in

Varies

Figure 2. Self-Restoring Barrier (SERB) Guardrail.

7

RETROFIT RAILING SYSTEMS

Two railing systems will be discussed in this report: (1) systems which have recently been designed and subjected to full-scale crash tests, and (2) untested systems which have been designed and installed on existing bridges. The former systems have not been implemented on bridges, and the latter systems have not been crash tested.

The SwRI reported a series of tests conducted from 1980 to 1982, and recommended two systems for implementation. These are called High Performance Retrofit and Low Service Retrofit systems. These systems are shown in figure 3.

High Performance Railing (HPR)

This system was designed to contain and redirect a· 20,000-lb school bus colliding at 55 mi/h and a 15 degree impact angle without damaging truss members behind the railing. An HPR was tested and met the design goals. The railing was next struck by a 2,250-lb automobile (58.8 mi/h, 15.8 degrees). The vehicle was smoothly directed. Two 4,500 lb automobiles struck the HPR at 58 mi/h and 28 degrees. These vehicles were smoothly redirected, 11

... but hood snagging on post between rail opening caused windshield penetration... The development test of the approach guardrai 1 showed that a deeper blackout was needed if the guardrai 1 or bridge rail could be impacted at angles greater than 25 degrees.

The HPR is intended for use on trusses having a significant number of vehicles weighing 20,000 lb or less.

Lower Service Retrofit Railing (LSRR)

This system was designed to contain and automobile colliding at 60 mi/h and 15 degrees.

8

redirect a 4,500-lb A 4,466-1 b automobile

1.0

High Performance Retrofit

Est. Cost 1

Item ($/Linear ft)

1. Post - $100 ea.

2. Upper railing & hardware

3. Tubular thrie-beam & hardware

4. Miscellaneous hardware

Total estimated cost

20.70

4.00

19.00

1.00

$44.702

1Anderson Safeway Guard Rail Corp., Flint, MI.

Low Service Retrofit

Item

1. Post - $25 ea.

2. Beam - 12 ga thrie-beam & hardware

Total estimated cost

2 Does not include installation and post anchorage costs.

W6x25 Post_......l R 5 ft Spacing ~

Tubular Thrle-Beam__.....(.... (

W6x8.5 Post 5 ft Spacing

5/8 In Dla Button Head Bolt

...- Thrle-Beam ( 12 ga.)

32 In

Figure 3. Through-truss retrofit railing estimated material costs.

Est. Cost 1

($/Linear ft)

5.00

5.00

$10.00 2

struck the railing at 59.3 mi/h and 19.1 degrees. The vehicle was redirected and design goals were met. A 1,750-lb automobile struck the railing· at 57.9 mi/h and 16.9 degrees. The vehicle was smoothly redirectea, and no repairs to the railing were required.

The SwRI Low Service Retrofit Railing is intended for use on through truss bridges having the following characteristics:

• One-lane travelled way. • 20-ft wide two-lane travelled way. • Automobile traffic only. 1 Posted speed limit of 35 mi/h or less that carry truck/bus

traffic.

All of these tests were conducted on bridge slabs having adequate strength to resist the impact forces in a collision incident. However, most through-truss bridge decks do not have the required strength. Therefore SwRI proposed several methods for transfer of the impact forces to the structure.

SwRI engineers proposed severa 1 post anchorage detai 1 s intended to transfer impact forces. One suggested design shown in figure 4 has the post welded to a base plate which is bolted to the existing floor beam flange. Two plate stiffeners are required to strengthen the floor beam.

At locations between floor beams, intermediate posts may be installed by emp 1 oyi ng a knee brace which transmits the overturning moment to the lower side of the bridge deck, as shown in figure 5. This type of bracing can be installed on steel or timber trusses. Details for installations on timber trusses are shown in figure 6.

As noted earlier, installed on bridges. experimental devices.

these new tested designs have not yet been They are recommended for implementation as

10

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Figure 4. Post attachment to floor beam flange.

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Figure 5. Suggested intermediate post attachment (concrete deck).

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Figure 6. Alternate intermediate post attachment for wood decks.

13

/

Retrofit Railing Installations

When adequate bridge deck width is available a blocked out railing can be installed as shown in figure 7. This system, which is based on tests at SwRI, distributes the impact forces a 1 ong the 1 ength of the existing concrete beam and post railing. This retrofit system eliminates snagging on the concrete rail.

A tubular strong box beam can be i nsta 11 ed on a through truss, as shown in figure 8. This box beam is attached directly to the main truss members. In such an installation, care should be taken to insure that the truss members have adequate strength to resist the impact forces.

The most widely suggested guardrail retrofit design consists of a thrie-beam which is attached to the existing bridge. Thrie-beam retrofit railings are shown in figures 9 and 10. In some cases bridge width, deck or rail strength, and structural member capacity are not adequate. The system in figure 10 utilizes outboard posts which minimize the intrusion of the retrofit into the existing travel way. While these posts are attached to the bridge deck, the majority of the load is transmitted to a transverse beam below the bridge stringers.

14

Figure 7. North Carolina tubular thrie-beam on collapsing tube blackouts.

15

_,,..,~-.....~ /•c:

,~~----~ .. -: ~;:~~::::.··~-;.~ _;

Figure 8. New York box beam retrofit.

16

Figure 9. New York retrofit.

17

Figure 10. North Carolina interim thrie-beam retrofit.

18

(1)

(2)

REFERENCES

M. E. Bronstad, Through Truss FHWA/RD-82/099, January 1983.

and C. E., Jr. Kimball, 11 Retrofit Railings for Narrow and Other Obso 1 ete Bridge Structures, 11 Report Southwest Research Institute, San Antonio, TX,

Jarvis D. Michie, and Maurice E. Performance in Retrofitting Traffic FHWA-RD-77-40, Southwest Research September 1976.

Bronstad, 11 Upgrading Safety Ra i 1 i ng Sys terns, 11 Report No.

Institute, San Antonio, TX,

(3) James E. Bryden, 11 Bridge Rail Upgrading A Low-Cost Solution to a Difficult Safety Problem, 11 New York State Department of Transportation, Albany, New York, March 1983.

19