S.P. No. 3806-70 · 2019-05-29 · MHD concurred with Gould’s recommendation for the bridge design, although not for the three-pipe railing, and he was soon drafting plans and considering

Rehabilitation Report:

Bridge 9395 over the Beaver River

Beaver Bay, Minnesota

S.P. No. 3806-70

Prepared by

Edward Lutgen, State Bridge Construction and Maintenance Engineer

Paul Stenberg, Principal Engineer, Final Design Unit

Minnesota Department of Transportation

and

Charlene Roise, Historical Consultant

Hess, Roise and Company

August 11, 2015

Rehabilitation Report:

Bridge 9395 over the Beaver River

Beaver Bay, Minnesota

Table of Contents

Project Background ......................................................................................................................... 1

Introduction ................................................................................................................................. 1

Historical Overview .................................................................................................................... 4

Objectives ................................................................................................................................... 6

Deficiencies................................................................................................................................. 6

Character-defining Features ............................................................................................................ 8

Bridge Nomenclature ...................................................................................................................... 9

Proposed Cross-Section Geometry ............................................................................................... 10

Proposed Rehabilitation ................................................................................................................ 11

Overview ................................................................................................................................... 11

Girders....................................................................................................................................... 11

Wind Bracing ............................................................................................................................ 12

Bearings .................................................................................................................................... 13

Intermediate Diaphragms .......................................................................................................... 16

End Diaphragms........................................................................................................................ 18

Deck .......................................................................................................................................... 19

Approach Slab ........................................................................................................................... 20

Barriers ...................................................................................................................................... 21

Surface Coatings and Treatments ............................................................................................. 27

Conclusion .................................................................................................................................... 29

Attachments .................................................................................................................................. 30

Rehabilitation Report—Bridge 9395 over the Beaver River, Beaver Bay—Page 1

Project Background

Introduction

Bridge 9395 (SHPO No. LA-BBC-005) carries

Minnesota Trunk Highway 61 over the deep gorge

of the Beaver River at the northeast edge of the

small community of Beaver Bay in Lake County

(MnDOT District 1). TH 61, a scenic drive along

the north shore of Lake Superior, attracts tourists,

which are a major source of income for the region.

The route is also critically important for long-haul

commercial transport and local traffic.1

Erected in 1958-1959, the bridge runs on a

northeast-southwest alignment. The total length of

the three-span, continuous, deck-girder bridge is

309′. The eight steel plate girders in each span are

made of non-weathering steel (MN spec 3306)

and have diaphragms (cross-bracing) at regular

intervals. Going west to east, the haunched steel

spans measure 65′, 135′, and 85′. The concrete,

voided abutments are each 12′, including a 7′ slab

span between the back wall of the abutment and a

pile-supported bent. Two hammerhead concrete

piers support the steel superstructure spans. The

slope below the bridge is more gradual on the

southwestern end, so the west pier is shorter than

the east pier.

Built in two stages, the bridge essentially

comprises two mirror-image structures, separated

lengthwise by a 1″ gap. While sharing abutments

and piers, these superstructures are not physically

linked but effectively function as a single

structure with a 66′-4″-wide deck. The deck holds

a 6′-wide median (half on each side of the gap), a

26′ roadway. Although designed to carry two lanes of traffic in each direction, it has apparently

never carried more than one, allowing for generous shoulders.

There is a 3′-curb next to the barrier along both sides. The curb and sidewalk support a two-pipe

aluminum railing with slightly curved, aluminum posts that rest on a concrete base. The base is

solid at the posts but is open below between the posts. Each end of the barrier terminates at a

monolithic concrete post. The posts at the north and south ends have bronze plaques that read, in

1 Descriptive information about the bridge has been obtained from MnDOT staff; current structural assessment

reports; “Preliminary Recommendations for Bridge Improvement” prepared by MnDOT, ca. 1986; “General Plan

and Elevation” for the bridge, January 29, 1958; and a site visit by Charlene Roise on March 28, 2015.

Above: Location map.

Below: Aerial view of bridge, with

intersection of Country Road 4 to left. New

wayside rest area is not yet developed on

open land north of intersection. (Google images)


part “Minnesota Highway Dept.” The lower part of each plaques is obscured by a newer

guardrail that has been bolted into the concrete.

The deck received a bituminous overlay around 1980. As part of a preservation project in 1988,

the bituminous overlay and the top 1/2″ of concrete were removed and replaced with a 2″ low

slump concrete overlay. The contract allowed a 1.5" latex modified concrete overlay, but based

on aggregate size, color, and look, engineers today believe that the 2" concrete layer was chosen

by the contractor. The cork that originally filled the 1″ gap between the structures was failing, so

a recess, about 2.5″ wide and 4″ deep, was cut in the adjacent concrete and a foam filler was

glued into the void. The black-bar reinforcement was left in place. “Due to geometric changes in

the area, the medians [were] replaced.” The median, originally 4′-wide, was expanded to 6′ and

the roadways narrowed from 27′ to 26′. A total of sixteen drains in the end spans were

permanently plugged, and 1″ to 1.5″ metal risers were installed under the grates of the other

drains. Modifications were made at the approaches to address reduce erosion from runoff. The

existing sliding plate expansion joints near the abutments were replaced with strip seals. These

types of preservation repairs were very common in this era as MnDOT developed better details

for corrosion protection.

These alterations did not damage the bridge’s historic integrity. It qualifies for the National

Register under Criterion C for its engineering significance as the state’s first all-welded girder

bridge. It was not associated with the development of TH 61 in the early twentieth century,

though, so it is not eligible for the National Register under Criterion A in that or other contexts,

and it cannot claim a significant association with a significant person under Criterion B.

Looking

northeast.


Top: From southwest end looking northeast.

Middle left: From northeast end looking southwest. New wayside rest area is visible to right in

background.

Middle right: New wayside rest area at southwest end of bridge, with bridge in background left.

Bottom: Railing details.


Historical Overview

In 1957, the Minnesota Highway Department (MHD) hired Edward C. Gould, a Minneapolis

consulting engineer, to prepare plans and a cost estimate for a bridge to replace an older structure

(Bridge 3458) carrying TH 61 over the gorge of Beaver River. On July 31 of that year, Gould

issued his preliminary report: “The new bridge is to have two 27 foot roadways with a 4 foot

island and 2-3 foot sidewalks. The centerline will . . . be 10 inches from the old bridge. This will

allow one half of the bridge to be built without disturbing the traffic. Then a temporary rail will

be placed on the completed portion and traffic will use this portion while the remainder is being

constructed.” He observed that “this requires the new structure to be practically two complete

bridges.” 2

Gould presented two options for the bridge, both continuous deck structures with three spans

measuring 85 feet, 135 feet, and 65 feet. The first was a variable-depth continuous girder, made

from slicing off the bottom flange of a 36-inch steel beam at the fillet. “The flange is then shaped

and positioned to give the proper section and a plate equal to the web thickness is welded into

place.” Most of the welds would be in compression and be in areas where shear stress was low.

The second option was a more traditional, variable-depth, riveted truss. For reasons of economy,

Gould recommended the first option. For both designs, “there is a solid approach slab,

cantilevered over the abutment parapet and supported at the rear by a beam. It is felt that at this

site something more open than the standard end post would be appropriate.” The railing “is the

standard double aluminum rail on a concrete base.” He noted that “possibly a panel rail of 3

pipes would interfere less with the view from the bridge. In order to line up the rail posts with the

stiffeners, a railing of 9'-0" panels will be required. Alcoa-Type C is suggested.”3

MHD concurred with Gould’s recommendation for the bridge design, although not for the three-

pipe railing, and he was soon drafting plans and considering details. In September 10, he sent a

drawing to MHD Bridge Engineer A. E. LaBonte that included a proposed bearing: “We can find

no bearing in the Lubrite literature of outside lubricated bearings with high strength bronze. We

therefore believe it will be necessary to use split bearings.” Gould also worte: “You will recall

that you requested rain water leaders from the floor drain to below the bottom of the girders. If

you have such a detail that has been satisfactory we would appreciate it. Otherwise, we will

develop one.”4

In November, Gould delivered to MHD the final plans and estimates for the welded-girder

structure. MHD issued a request for proposals on January 29, 1958. On March 8, MHD notified

Asbach Construction Company of Saint Paul that is was receiving the contract with its bid of

$345,372.53 for the bridge and $28,936.02 for the approaches, a total of $374,308.55. On March

19, Asbach notified MHD that orders “for structural steel for welding, structural steel and all

2 Edward C. Gould, “Preliminary Report, Br. 9395,” July 31, 1957, in Bridge 9395 files, Minnesota Department of

Transportation, Saint Paul (hereafter MnDOT).The Minnesota Highway Department was the predecessor to

MnDOT. 3 Ibid. Gould added: “It has been observed that at the sides of abutments of bridges on the North Shore, trails

develop down to the canyon. These trails are precipitous and dangerous. Erosion makes the trails into gullies. A stair

outside of the abutments on at least two corners of a bridge on a site like this seems desirable.” There is no evidence

that this suggestion was considered. 4 Edward Gould to A. E. LaBonte, letter, September 10, 1957, in Bridge 9395 files, MnDOT.


bearing assemblies . . . have been placed with [the] St. Paul Structural Steel Company of St.

Paul.” Steel piling would be obtained from the U.S. Steel Supply Company in Saint Paul.5

Being a local innovator for this bridge type presented some problems. MHD had difficulty lining

up an experienced company to conduct “radiographic inspection of all butt welds and certain

fillet welds” as required by the specifications. Bridge Engineer LaBonte wrote: “We have been

unable to locate any firm in this area with the suitable equipment required to do this type of X-

Ray investigation. Larpen Industries in Milwaukee, to our knowledge, is the closest firm doing

this type of work.”6

It managed to cross that hurdle, and by mid-August was touting the fabrication of its first all-

welded steel highway bridge with a press conference at Saint Paul Structural Steel’s shop.

Minnesota Highways documented the event with a photograph on its cover depicting officials

from MHD, including Commissioner Zimmerman and Bridge Engineer A. E. LaBonte,

examining a girder. “After the inspection,” an associated article explained, “the girders were dis-

assembled for train and truck shipment to the bridge site.” The department had chosen this bridge

type, according to the press release for a number of reasons, including the savings in weight and

cost, reduced maintenance needs, and the “cleaner appearance” of the structure.7

The northwestern (westbound) half of the bridge was erected in 1958, while traffic continued to

use the existing bridge. When the new span was ready, traffic was shifted to it and the older

bridge was demolished. In 1959, the southeastern (eastbound) half was installed and the whole

new bridge was placed in service.8

Maintenance for the bridge was routine until it was “identified as a good candidate for a concrete

overlay” in the mid-1980s. “There is a median and 4 foot width with 3 foot safety curbs.”

“Actually the bridge was four lanes but it is striped for just two lanes or one lane operation in

each direction which is probably good, since it is only 27 foot roadways.” “This was originally

designed for 4 twelve foot lanes but the bridge is striped for just 2 and it has a width which gives

you a seven foot shoulder on the median and the outside shoulder areas.”9

“Preliminary Recommendations for Bridge Improvement” prepared by Martin Herbers, regional

bridge construction engineer, dated August 6, 1986, call for “scarifying ½” concrete” (17,922 sf),

“removal to top of rebars” (3,200 sf), and “full depth removal (700 sf). “Dut to geometric

changes in the area, the medians are being replaced. (See approved layout).”

The fact that MHD held the August press conference indicates how important an

accomplishment this was for the department. As the first example of its type constructed by

MHD, Bridge 9395 appears to be eligible for the National Register under Criterion C and exempt

from the Program Comment under Section IV(C).

5 Edward Gould to A. E. LaBone, letter and attachments, November 19, 1957, in Bridge 9395 files, MnDOT. 6 A. E. LaBonte to C. K. Preus, July 8, 1958, in Bridge 9395 files, MnDOT. 7 “Memorandum to All News Outlets,” press release, prepared by John Withy and Associates, Saint Paul, for Saint

Paul Structural Steel Company, Saint Paul, [August 12, 1958], in Bridge 9395 files, MnDOT; “Welded Girders,”

Minnesota Highways 7 (September 1958): 2. 8 “Welded Girders.” 9 Jerry Belfangs, draft of report on field inspection, November 8, 1985, in Bridge 9395 file, MnDOT.


Objectives

Bridge 9395 exhibits structural deterioration, particularly on the deck. If allowed to progress, the

deterioration will accelerate to the point where it is not economically or physically feasible to

repair the bridge. The problems have resulted in part from original design features, particularly

wind bracing, that have had unanticipated negative effects.

Due to satisfactory substructure condition and little remaining service life in the deck, barriers,

and joints, the Bridge Office recommends a rehabilitation project to cost-effectively manage the

structure. The bridge is eligible for the National Register, so the Bridge Office intends to

conform to the Secretary of the Interior’s (SOI) Rehabilitation Standards and Guidelines—

specifically, the format adapted for historic bridges and included in the January 2015 update of

MnDOT’s “Management Plan for Historic Bridges in Minnesota.”10 Keeping the bridge in use

for its original function meets SOI Bridge Rehabilitation Standard 1: “Every reasonable effort

shall be made to continue an historic bridge in useful transportation service. Primary

consideration shall be given to rehabilitation of the bridge on site.” The proposed project should

provide Bridge 9395 with a remaining service life of fifty years, with timely preventative

maintenance.

Deficiencies

MnDOT uses a risk-based analysis known as Bridge Replacement Improvement Management

(BRIM) to evaluate the future needs of bridges. The bridge is ranked using the Bridge Planning

Index (BPI), a standard rating ranging from 0 (highest) to 100 (lowest) priority for repair. The

MnDOT Bridge Office and the appropriate District office use the BPI to identify and program

projects. Based upon specific conditions, the Bridge Office recommends repair options to

address deficiencies. In a recent evaluation, the BPI for Bridge 9395 was found to be 58. The

conclusion of the BRIM analysis was that the bridge should be replaced or rehabilitated in the

first period beyond the State Transportation Improvement Program (STIP), which was 2019-

2024.

Key issues that must be addressed include:

Fatigue cracks on girders at abutments: There are numerous fatigue cracks at the

transition by the abutment and the rocker bottom flange caused by wind-bracing cycles.

The wind bracing at the abutment has been removed and arrestor holes drilled in areas

with cracking. There are three diagonal wind-bracing members bent from careless rip-rap

placement near the north abutment. The safety inspection report observed some beam

section loss.

Bracing: Original wind bracing is in place in most locations. The design of the bracing is

flawed and will produce fatigue cracks throughout the structure if left as is. The original

design did not account for another type of stress, namely lateral torsional buckling of

compression elements, and this must be remedied with additional diaphragms near the

piers.

10 “Management Plan for Historic Bridges in Minnesota,” January 2015, Appendix B, prepared for MnDOT by

Mead and Hunt and LHB.


Deck deterioration: The deck has numerous areas of patching, delamination, and spalling

on top and underneath. According to the annual safety inspection report prepared in

accordance with the National Bridge Inspection Standards (NBIS), 12 percent of the

bottom of deck is delaminated. Deck chloride-level profiles taken in 1986 indicated

conditions near the corrosion threshold at a depth of 1.5″.

Bearings: The original bearings are subject to seizing and are not effective dealing with

transverse stresses, which could lead to stress fractures.

Barriers: The barriers and endposts do not meet current safety requirements for the

posted speed of 35 mph.

Substructure: There is some minor concrete deterioration on the abutments and piers.

In other aspects, the bridge condition is satisfactory. The current load capacities are

HS20.0/HS33.4 inventory and operating rating, respectively, with a standard C restriction of 2.

Preliminary analysis indicates a post-rehab HL-93 rating factor of 0.84 and 1.05 for the inventory

and operating rating, respectively, with a class C restriction.11 The Bridge Preservation and

Improvement Guidelines (BPIG) provide guidance on repair projects and minimum requirements

for safety and capacity. The BPIG requires an inventory HL-93 rating factor greater than 0.9 and

no permit restrictions for standard overweight trucks. The load capacity is lower than minimum

standards for a rehabilitation project, especially for an interregional corridor, so a design

exception for reduced load capacity will be required. The project is increasing load capacity by

adding shear studs, adding additional diaphragms, using the actual girder mill strength capacities

from original construction, and decreasing deck dead loads, but not enough to meet standards.

The scour rating is “L,” which indicates that conditions are stable and low risk.

The bridge also has the potential to provide an important crossing for the Gitchi-Gami State

Trail, which is under development. This non-motorized, paved recreational trail will extend for

88 miles from Two Harbors to Grand Marais. The Minnesota Department of Natural Resources

(DNR) has asked MnDOT to include a sidewalk in the Bridge 9395 rehabilitation project to

serve the trail. Options for alternative crossings are limited. Because the river is in a deep gorge

in this area, the cost to construct a bridge and approaches specifically for the trail is estimated to

exceed $2 million. In addition, local landowners are unwilling to provide the required right of

way and the DNR does not have authority for imminent domain.

The consideration of possible detours is also a factor in evaluating alternatives. Route 61 is

heavily traveled. It is an essential route for a variety of users: long-haul commercial traffic

between Duluth, Minnesota, and Thunder Bay, Ontario, and the Trans-Canada Highway; lumber,

ore, and other regional industries; local residents; and tourists, who are crucial to the area’s

economy. There are no options southeast of TH 61 because of Lake Superior. To the northwest,

the most feasible detour route involves 7 miles on roads that on local roads that are designed to

carry less traffic. Trucks and oversize and weight permit loads would need to be detoured 22

miles. Hence, MnDOT prefers to rehabilitate the bridge in stages so that traffic on TH 61 can be

maintained without a detour.

11 HL-93 is a standard AASHTO category of analysis for Load and Resistance Factor Design (LRFD) with live

loads. It replaced an earlier standard, HS-20, in AASHTO’s Standard Specifications for Highway Bridges.


Character-defining Features

Bridge 9395 was found to be eligible for the National Register under Criterion C as the first all-

welded girder bridge in Minnesota.

The primary character-defining features are the three welded-girder spans. Secondary features

that are also significant components of the historic fabric of Bridge 9395 and contribute to the

bridge’s historic character include:

Other structural elements associated with the girders: These include wind bracing and

bearings.

Barriers: The historic barriers are in place; they are a standard design from the bridge’s

period of construction.

Concrete substructure: In designing the abutments and hammerhead piers, the engineers

considered the bridge’s picturesque setting—hence, aesthetics are an important

consideration for these features.

In the National Register bulletin How to Apply the National Register Criteria or Evaluation,

seven aspects of integrity are identified: location, design, setting, materials, workmanship,

feeling, association. Bridge 9395 retains most of these. The bridge is in its original location, and

the three-span welded-girder structure remains essentially unaltered from the time of its

construction, maintaining integrity of design, material, and workmanship. The deck was

rehabilitated in 1988 and therefore has poor integrity of material, but this does not significantly

impair the overall integrity of the bridge. The setting, while recently altered by the construction

of a wayside rest at the southwestern approach, remains picturesque—a fact that is

acknowledged by the wayside development. The bridge’s mid-twentieth-century feeling and

association are also intact. All in all, the historic integrity of the bridge is very good.

To maintain the historic character of the bridge while making it functional for the twenty-first

century, the Secretary of the Interior’s Rehabilitation Treatment is an appropriate approach to the

overall project, with Preservation and Restoration applicable in some locations. In general,

modifications to secondary features should be allowed to improve functionality for conditions

today and in the foreseeable future if the modifications are compatible with the historic features.

Damage and deterioration should be repaired when possible, with minor modifications to

features (e.g., wind bracing) where appropriate to facilitate the long-term preservation of the

bridge. Where secondary features are no longer in place (e.g., some sections of the wind

bracing), restoration of these features is not necessary if they are not needed for structural

reasons. Where aesthetics are a factor (e.g., abutments and piers), the original appearance should

be restored to the extent possible.

Replacement of secondary features is justified when essential to meet modern safety standards

(railings) or if modern components will improve the longevity of the structure and will have

minimal visual impact (bearings).

Additions required to improve structural performance (diaphragms) should be obscured from

public vantage points when possible. New components (bolt heads for diaphragms) should be

visible only when there are no reasonable alternatives. In those situations, the new components


should be compatible with historic components but of modern design, featuring the technology

that will best enhance the bridge’s long-term preservation.

Bridge Nomenclature

Approach slab: A short span on either end of the main span(s) that typically is only a reinforced

deck section with no beams.

Barrier: The element above the deck that protects vehicles or pedestrians from falling off the

edge.

Bearing: Elements that the girders rest on rest on the substructure. here are expansion and fixed

bearings depending on the expansion movement design.

Diaphragm: Bracing member perpendicular to the beam that prevents the girders from buckling.

Fatigue crack: A crack that develops from a combination of the stress range magnitude and

number of cycles. Typically happens at poor weld details, stress concentrations or

differential displacements.

Flange: Part of the girder that is a steel plate on the bottom and top of the web. The flange plate

is perpendicular to the web plate.

Haunched: Varying depth of web along the span. The depth is deepest at the piers.

Shear Studs: Small steel elements on top of the girder that is encased in the concrete that allows

deck to carry load. The girder can be designed as composite with concrete deck.

Stool height: The distance between the top flange and the bottom of the deck. This allows for

camber.

Web: Part of a girder that is a thin steel plate in between the top and bottom flanges.

Wind bracing: Secondary frame elements that braces the girder from lateral loads.


Proposed Cross-Section Geometry

Existing (top) and proposed (bottom) cross section.


Proposed Rehabilitation

Overview

Due to poor condition, the deck will be completely replaced. The layout of the new deck will

accommodate a 12' walkway/bikeway for the Gitchi-Gami Trail. A new barrier similar in form to

the original will be installed on both sides of the deck, with a plain concrete barrier separating

the walkway/bikeway from traffic. Beneath the deck, modifications will be made to the original

wind bracing, and new bearings and diaphragms will be introduced to enhance the structure’s

long-term preservation. Repairs to concrete abutments and piers will match the appearance of the

existing concrete.

The following section describes issues with individual bridge elements, discusses proposed

solutions, and provides related illustrations.

Girders

Welded angles on the top flange of the beams are encased in the concrete of the existing deck to

make the beams composite with the concrete. This significantly increases the load capacity of the

bridge to safely carry heavier traffic. The current design code requires additional composite

action elements to transfer the load. Field-welding new angles onto the flange to match the

existing elements would not meet fatigue policy or bridge design code because poor welding

workmanship might cause fatigue cracking of the weld, which could migrate to the top flange

and cause failure of the beam. Instead, shear studs (sometimes called nelson studs) will be

installed on the top flange with a preapproved stud-gun welder. Approximately 400 new shear

studs are needed per beam. See the attached shop drawing beam elevation sheet for the location

of the new shear studs.

To prevent future top flange corrosion the contractor will paint the top of top flange. An

approved intermediate epoxy that is surface tolerant will be used as a primer without zinc.

The steel girders have poor weld details that could cause fatigue cracks and could propagate to

base metal causing fracture and eventual collapse. To lower the risk of developing cracks in the

welds, some areas will get an ultrasonic impact treatment that is sometimes called peening. The

exact locations that will benefit from peening are being determined, but it is typically done for

the ends of welded cover plates in tension areas. The peening changes the profile of the weld to a

defined concave shape, a change that would not be noticeable to most people. Peening extends

the life of welds by allowing them to tolerate a higher stress range and endure more stress cycles

prior to cracking.

The superstructure still has the original paint system, which consists of a lead primer and

phenolic resin aluminum top coat. The paint is failing in many areas of the bottom flange and

beam ends allowing corrosion of the steel plates. Because the paint contains lead and, possibly,

PCBs, its removal will be covered by special provisions, which will include cleaning and

priming the top of top flange of the steel beams after the deck is removed (see Deck section

below). As one of the final tasks of the rehabilitation, all structural steel will receive a new

three-coat paint system with cathodic protection of zinc in the primer. The color will match the

original as closely as possible; the selection of color will be determined at a later date.


Wind Bracing

Wind bracing is located between the girders, above the bottom flange. The wind bracing is

riveted to gusset plates that are riveted to angles that are riveted the girder webs. While this rigid

connection is the original design, subsequent information about this type of connection indicates

that it can be harmful to the structure, producing axial forces that are likely to cause fatigue

cracking in the fillet welds and web. In 2009, District 1 bridge maintenance staff observed

fatigue cracking in the northeast end of the northeast span, by a rocker bearing at the abutment

(see bearings section below and Attachment A: “Repair Recommendations Based on the

November 2008 Annual Bridge Inspection,” September 28, 2009). To deter additional cracking,

a crew removed the end bay wind bracing.

To prevent fatigue cracking throughout the structure, the axial forces in the wind bracing must be

eliminated. Two options were considered: 1) remove the wind bracing, and 2) cut the wind

bracing near the ends and bolt a splice plate with non-tightened bolts with slotted holes. The

holes will allow movement and prevent loads from transferring to the web. To minimize the

visibility of this alternative, button head bolts could be used, with the nut facing up, so the nut

would be virtually invisible from any public vantage point. For either option, the wind bracing

that was removed at the northeast end would not be replaced.

While the wind bracing is a secondary feature rather than a primary feature, Option 2 is preferred

because it meets the SOI Bridge Rehabilitation Standard 2: “The original character-defining

qualities or elements of a bridge, its site, and its environment should be respected. The removal,

concealment, or alteration of any historic material or distinctive engineering or architectural

feature should be avoided.”

Beam framing plan showing proposed location of nelson studs on top of beam and bolt heads on

fascia beam (see discussion of intermediate diaphragms below).


Bearings

There are eight original expansion rocker bearings at each abutment. As part of the 1988 project,

the pins were replaced, probably because they were seized up and were not rotating. , They were

replaced with new pins with zerks to allow the bearings to get greased by maintenance. Pier 1

also has expansion rocker bearings. Pier 2 has fixed bearings; thus all longitudinal thermal

movement expands from this support.

The existing abutment bearing rocker detail allows for fatigue cracks to form in the web that

could propagate through the web and top flange, causing collapse. As noted above, District 1

bridge maintenance staff removed the northeast end bay wind bracing after observing cracks in

2009. To prevent risk of future cracking, the 16 rocker bearings will be removed and replaced

with an elastomeric bearing (concept sketch follows). Stiffened angle assemblies will be attached

near the bottom of the girder, in the location of the rocker bearing pin, to secure a sole plate and

curved plate. The curved plate will rest on a bearing plate, which will be supported by an

elastomeric bearing pad located on the existing abutment seat.

The existing fixed and expansion fascia-beam bearings at both piers also need to be replaced to

allow for transverse thermal expansion, which is not accommodated by the existing rocker and

fixed bearings (concept sketches follow). A total of 4 pier bearings (2 rocker and 2 fixed) will be

replaced. A spacer plate and sole plate will be bolted to the bottom of the existing flange, with a

curved plate attached to the sole plate. The curved plate will rest on a bearing plate above the

elastomeric bearing pad, which will allow for transverse thermal expansion. The distance from

the bottom of the flange to the top of the pier cap is 2'-2" at Pier 1 and 1'-7" at Pier 2. Because

the height of the new assembly is less than the existing bearings, the elastomeric bearing pad will

sit on a concrete pedestal that will fill the space between the pad and the pier cap.

All the original bearings that are replaced will be salvaged, the lead paint will be removed from

them, and they will be stored at local District maintenance facility. Given the rationale for

replacing the bearings, it is unlikely that they would ever be reinstalled on the bridge, but they

could be retained at the facility to allow for that. Alternatively, some or all could be made

available: 1) for use on another historic bridge that was being rehabilitated and needed

replacement rocker bearings; 2) as an interpretive artifact for installation at the new roadside rest

area southwest of the bridge; or 3) for accession into the collection of an appropriate historical

organization.


Photos of existing bearings (above) and drawing of proposed abutment bearing (below).


Proposed pier bearings


Intermediate Diaphragms

Stress calculations for the design bridge code that was in place in the 1950s did not consider

lateral torsional buckling of compression elements. Based on new research, the current design

code has been updated to reflect the actual beam capacity. For safety, the beams of Bridge 9395

need to be reinforced to increase their capacity and prevent buckling. This will require the

installation of additional intermediate diaphragms between all of the beams near the piers, but

only in the bays with existing diaphragms—a total of 36 new diaphragms (a line of 6 diaphragms

in 6 locations). Drawings on the following page show the design and location of the proposed

new intermediate diaphragms.

The new intermediate diaphragms will comprise steel angles bolted to steel gusset plates, then

bolted to angle stiffeners. The angle will be shop-welded. It is difficult, however, to control the

quality of field welding, particularly in a very challenging location like that of Bridge 9395

where height and wind are concerns. Because of this, the angle stiffeners will be bolted to the

girders. The gusset plates and angles will be beneath the deck and visible only from the gorge

below, so their visual impact will be minimal. The heads of the bolts fastening the angle

stiffeners to the girders will be visible in vertical rows on the fascia beams. As part of the

project, the girders and the heads will be painted, which will help to minimize the visibility of

the bolts.

SOI Bridge Rehabilitation Standards could justify two alternatives for the visible bolt heads.

Modern hex-head bolts would be appropriate under Standard 3, which states that “All bridges

shall be recognized as products of their own time. Alterations that have no historical basis and

that seek to create a false historical appearance shall not be undertaken.” Hex-head or button-

head bolts could be appropriate under Standard 9: “New additions, exterior alterations, structural

reinforcements, or related new construction shall not destroy historic materials that characterize

the property. The new work shall be differentiated from the old and shall be compatible with the

massing, size, scale, and architectural features to protect the historic integrity of the property and

its environment.” Button-head bolts would be the most visually compatible, but could be

confused for original rivets. For this reason, and because the new diaphragms will be similar in

appearance to the historic diaphragms, hex-head bolts are recommended to differentiate the new

diaphragms from the original.


Red lines show locations of proposed new intermediate diaphragms (36)

and end diaphragms (4; see following section).


End Diaphragms

With the new configuration proposed for the deck (see Deck section below), vehicles will drive

on an area of the deck that was previously covered by a median. This area is cantilevered beyond

the inside girders flanking the gap between the two sections. While this is not a structural

concern on most of the deck, the unsupported corners of the deck by the abutments will be

susceptible to fluctuation as vehicular loads transfer between the abutments and the bridge. To

support these wheel loads, a total of one end diaphragm at each abutment will be added in the

center bay. The end diaphragms could either be reinforced concrete or a new steel beam bolted

with new stiffeners. The sketches below show the concrete and steel options for the new end

diaphragms. Their location at the center of the ends of the bridge, beneath the deck, will make

them almost invisible.

Proposed new end diaphragms


Deck

The original deck was 6.5" thick. The bridge slab, curbs, and railing will be removed. A new 8"

monolithic concrete deck will be poured with a 2.5" top cover, epoxy reinforcement, and

3YLCHPC concrete deck mix. It is MnDOT standard practice for long term durability to

construct a 9" concrete deck with 3" of top cover. Due to girder capacity constraints and

maximizing allowable live load, an 8" monolithic deck with a 2.5" top cover is planned.12

To minimize the stool heights and thus the deck dead load, the cross slopes will match the

existing.

The deck, like the rest of the bridge, was built in two sections. The 1" gap between the sections

was originally filled with cork. During the 1988 rehabilitation, a strip seal was applied, which

has deteriorated. To minimize future damage to deck coping and prevent steel beam corrosion,

the center longitudinal joint and center median will be removed and the deck made continuous.

Finite element analysis has determined that new diaphragms in the center bay at the same

locations as others is not required if the deck is made continuous.

The new deck width will consist of 1'-5" outside barriers, 1'-6" inside barrier, 12' sidewalk, and

50' width roadway, for an overall width that matches existing of 66'-4".

The proposed new deck roadway width of 50' exceeds the 38'-width requirements of the Bridge

Preservation and Improvement Guidelines (BPIG) tech memo and the 40' to 48' width specified

in the Bridge Width Standard for State Highways tech memo, thus no design exception is

required for bridge roadway width.

The 12' sidewalk will accommodate the safety inspection snooper vehicle on the trail. A

narrower width would not allow the snooper to access over the side and down to the bottom of

the pier columns. To tie in with the Gitchi-Gami Trail, the sidewalk will be on northwest side of

the bridge.

A total of 16 drains in the end spans were permanently plugged as part of the 1988 rehabilitation.

Fourteen deck drains remain. To prevent chloride roadway drainage from going to the river, no

drains will be installed on the new deck at the request of the DNR. On the new deck, water will

be carried onto the approaches. The drains are not character-defining features of the structure, so

their elimination will not adversely affect the bridge.

12 3YLCHPC: 3 = type of concrete; Y = grade of concrete; LC = low cement; HPC = high performance concrete.

This is a standard MnDOT concrete mix for a monolithic deck.


Approach Slab

There is a short approach slab span past the abutment. The slab shows the same deterioration

issues as the rest of the deck. The 7'-long slab span carries traffic between the abutment stem

and a pile supported bent. The slab span will be replaced with similar thickness and a little more

rebar to meet current loads but will look exactly the same from the deck surface.

Approach span that will be replaced in kind.


Barriers

The curb and sidewalk support two-pipe aluminum railings with slightly curved, aluminum posts

that rest on a concrete base (see detail from original drawing). The base is solid at the posts but is

open below between the posts. Each end of the barrier terminates at a monolithic concrete end

post. The barrier system does not meet the current FHWA requirements for a crash-tested barrier

for the posted speed of 35 mph as outlined by the National Cooperative Highway Research

Program (NCHRP) Report 350 Test-Level 4 (TL-4) or the AASHTO Manual for Assessing

Safety Hardware (MASH).

The new deck configuration will

require three barriers, and each

presents a different condition.

Barrier 1, replacing the original one

on the northwest edge of the deck,

will restrain pedestrians and

bicyclists rather than vehicles.

Barrier 2, a new barrier on the

northwest side of the roadway, will

separate pedestrians and bicyclists

from vehicles. Barrier 3 will be on a

curb at southeast edge of the bridge

as it was historically, with no

accommodation for pedestrians and bicyclists.

Three options were considered for replacing the barriers (see drawings on the following pages).

Option B includes a crash-tested T1 barrier on the outside of the roadway (Railing 3) with solid

concrete traffic barrier (Railing 2) and an ornamental metal rail that meets pedestrian

requirements on the outside. Option C includes 2 T1 barriers with an ornamental metal on the

trail outside. Other barriers were investigated but did not meet either the pedestrian, traffic safety

requirements or historic needs.

The preferred alternative is Option A, which presents a compromise between historic needs,

traffic, and pedestrian safety requirements. A TL-4 crash-tested design, the Nebraska Department

of Roads Concrete Bridge Rail, topped with the salvaged aluminum railing from the existing

bridge, matches the existing design as closely as possible. MnDOT has proposed the “Nebraska”

railing for the historic Kennedy Bridge (Bridge 9090) in Grand Forks, which opened in 1963 and

featured an original railing of the same design as that on Bridge 9395. For bridge 9090, MnDOT

will have to fabricate a new aluminum rail due to existing poor condition. If the aluminum

railing on Bridge 9395 cannot be salvaged in whole or in part, new sections matching the

original will be fabricated.

The proposed barrier dimensions are 50" high, with no openings lower than 27" and 6.5"

openings above 27". This does not meet MnDOT requirements of minimum 54" high, maximum

4" opening below 27" and 6" above 27". The federal requirement is less restrictive with only

minimum 42" height, maximum 6" opening below 27" and 8" above 27". Since the barrier meets

Proposed cross section

Barrier 1 Barrier 3 Barrier 2

Roadway Trail


FHWA requirements but not MnDOT policy, a design exception is required but has been

approved on other projects. The concrete section of the railings will be cast, not slip-formed.

While the proposed 50" height is taller than the existing 37"-high existing barrier, this design

will permit the reuse of the historic aluminum components. Another change from the existing

design is that the new concrete barrier will not have openings at the base. This will extend the

life of the bridge by preventing water, salts, and other materials from draining down the side of

the deck and outer girders and causing deterioration. One-inch recessed panels matching the

length of the existing openings will recall the horizontal pattern of those openings. The height of

the panels is dictated by two constraints: at the base, the need to include a projecting course 6″

tall and 1″ deep to keep plows back from the face of the barrier, reducing the risk of damage by

angled plow tips; and at the top, by structural reinforcing requirements. The edges of the panels

will be beveled (rather than at right angles) to avoid the collection of water, salt, and debris on

horizontal surfaces and make the edges less likely to be caught by plows.

To meet standards, the new bridge barrier will need a new end post for attaching the approach

guardrail. The end post on the pedestrian side will need to be moment slab (self-supporting) on a

concrete approach panel. The design is still under development. The two existing bridge plates

will be salvaged and reinstalled; new bridge plates documenting the rehabilitation will also be

installed.

To improve durability of the new concrete barrier, an approved penetrating silane sealant will be

applied. It is MnDOT standard practice to apply a special surface finish, which includes acrylic

paint, for protection to the barrier and outside of deck to the fascia girder.


Various barrier design and cross-section options were evaluated before the proposed barrier and

cross section were selected. The existing cross section and the proposed Option A are above.

None of the options included the 6'-wide raised median that is on the existing deck because of the

need to add a trail on the northwest side of the deck. Trail widths of 8' to 12' were considered; 12'

was chosen for maintenance reasons (this is the minimum width to accommodate a snooper truck,

which required for periodic inspections) and because it was preferred by the DNR to reduce

hazardous conflicts between pedestrians and bicyclists. Because of the trail, all of the options

require that the centerline be moved to the southeast.

Drawings of the proposed barriers are on pages 25-26.

SUPERSTRUCTURE CROSS SECTION (IN PLACE)

PROPOSED CROSS-SECTION BARRIER (OPTION A)


Options B and C were considered but not selected. They show the same cross section as Option A,

but different barriers. Option B has a crash-tested T1 barrier on the outside (left) with a solid

concrete barrier traffic barrier between the road and the trail and an ornamental metal rail, similar

to that used on other bridges along Highway 61, on the right. Option C includes two T1 barriers

(flanking the roadway) and an ornamental metal rail on the right.


Fo

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of

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isto

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left

) and

pro

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Typ

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4 B

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(rig

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appro

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cale


Type T1 Barrier Option—Between

sidewalk/bikeway and roadway


Surface Coatings and Treatments

There are delaminated and spalled areas on the piers and abutments, which will be repaired with

shotcrete. The estimated scope of the repair comprises 5 square feet on Pier 2, 10 square feet on

the south abutment, and 5 square feet on the north abutment. Areas of delamination under the

bearing area of the south abutment will be repaired after the deck is removed, before the new

deck is formed. The finish of the shotcrete will match the surface profile of the surrounding

areas. The shotcrete will be moist-cured with wet burlap and no curing compound. The shotcrete

will be stained to match the color of the surrounding areas. Prior to the stain application, the

contractor will prepare mock-ups of the texture and color for review and approval by the project

historian, in consultation with MnDOT CRU staff.

This approach meets two SOI Bridge Rehabilitation Standards:

5: “Distinctive engineering and stylistic features, finishes, and construction techniques or

examples of craftsmanship that characterize an historic property shall be preserved.”

6: “Deteriorated structural members and architectural features shall be retained and

repaired, rather than replaced. Where the severity of deterioration requires replacement

of a distinctive element, the new element should match the old in design, texture, and

other visual qualities and where possible, materials. Replacement of missing features

shall be substantiated by documentary, physical, or pictorial evidence.”

No special surface treatments will be applied to the repaired substructure areas.


Pavement Marking

To provide a better transition at the intersection of TH 61 and CSAH 4, the permanent pavement

markings will be realigned.

Deck and approach markings today (above) and proposed (below).

Bridge

9395

Bridge

9395


Conclusion

Based on the preliminary design outlined in this report, the project appears likely to conform to

the Secretary of the Interior’s Rehabilitation Treatment Standards and should have no adverse

effects on this historic bridge.


Attachments

A. “Repair Recommendations Based on the November 2008 Annual Bridge Inspection,”

September 28, 2009, memorandum from Dan Organ, State Bridge Engineer, to Duane

Hill, District 1 Maintenance Engineer.

B. Additional Photographs

Rehabilitation Report—Bridge 9395 over the Beaver River, Beaver Bay—Attachment A

Attachment A

“Repair Recommendations Based on the November 2008 Annual Bridge Inspection,” September

28, 2009, memorandum from Dan Organ, State Bridge Engineer, to Duane Hill, District 1

Maintenance Engineer.

Rehabilitation Report—Bridge 9395 over the Beaver River, Beaver Bay—Attachment B

Attachment B

Additional Photographs

Rehabilitation Report—Bridge 9395 over the Beaver River, Beaver Bay—Attachment B

Documents

S.P. No. 3806-70 · 2019-05-29 · MHD concurred with Gould’s recommendation for the bridge design, although not for the three-pipe railing, and he was soon drafting plans and considering