Yosemite National Park

Wawona Covered Bridge

Structure Assessment and Engineering for Rehabilitation

 

Reinhard Ludke, S.E.              Creegan + D'Angelo Consulting Engineers

Olivier Ruhlmann, P.E.           Creegan  + D'Angelo Consulting Engineers

Craig Struble               Yosemite National Park Preservationist

Seth Bergstein             Architectural Resources Group Architect

 

Rehabilitation of bridges in a historically sensitive manner is a difficult task, which requires a team effort between preservationists and bridge engineers. Successful restorations can only be accomplished when each side realistically looks at and accepts the balance between historical significance and sound bridge engineering principles.

 

This project is an example of bringing preservationists and bridge engineers into the same classroom and teaching them about the others concerns and problems. Engineers need to know what details are historically significant and how to test and retrofit them. Preservationists need to know what an engineers can and cannot do with a bridge and be ready to accept compromises when appropriate.

 

File written by Adobe Photoshop® 4.0

 

At the request of National Park Service at Yosemite National Park, Creegan + D'Angelo Consulting Engineers assessed the structural condition and load capacity of the Wawona Covered Bridge and prepared recommendations for its rehabilitation.  Concerns about the structural integrity and capacity of the bridge rose because of severe deterioration of main timber framing, failure of members, temporary repairs, and the bridge had a progressive increasing deflections that reached eight-inch sag at mid-span under the bridge's own weight.  Through out its past, the bridge has had a history of developing permanent deflection, decaying members, including major flood damage in 1955.  In 2001, the park took measures to provide temporary strength of transverse beams by bolting sister beams and by tightening the metal rods of the truss.  We made the following observations during our field inspection and testing.

 

- Permanent deflection of bridge along its length

- The upstream truss was deflected more than the downstream truss

- Most main truss members were cambered, indicating that they were heavily loaded in bending

- Many truss web members were not bearing on the top and/or bottom truss chords

- Some structural members were missing

- Some structural members had advanced decay and failure

 

 

The structure engineering assessment and rehabilitation includes three parts:

 

Part I:              Historical Significance and Landmark Status

Part II:             Assessment of the Bridge Structure

Part III:           Rehabilitation Recommendations and Construction Documents

 

                       

Engineering focuses mainly on three topics: the vertical load demand and capacity of the bridge, the lateral load demand and capacity of the bridge, and the integrity of the structure material.  A primary objective of the rehabilitation is Architectural and Historic Preservation. 

 

Bridge History - Description of the Wawona Covered Bridge

 

 

The Wawona Covered Bridge spans the South Fork of the Merced River at the Pioneer Yosemite History Center in Wawona, Yosemite National Park.  The structure is a modified Queen-post truss constructed with native woods braced by steel tie rods.  The overall length of the structure is 130 feet, 4 inches; overall width is 16 feet, 6 inches.  There is a pair of longitudinal stringers parallel to the trusses.  The trusses and stringers are supported by a system of seven transverse floor beams.  The covering structure, added in 1878, a decade after construction of the original bridge, has an internal frame with timber siding.  The total height of the bridge is 30 feet, measured from the peak of the gable-roofed covering structure to the top of the transverse floor beams.

 

The principal truss members are made of hand-hewn Ponderosa Pine; the vertical and diagonal elements of the truss are made of plain-sawn Douglas fir.  The internal frame covering structure is made of plain-sawn Douglas fir with a roof consisting of hand-split sugar pine shakes.  The piers and abutments are composed of concrete with stone facing.  The property was meticulously rehabilitated in 1956, reusing all salvageable materials and replacing deteriorated materials in-kind, regarding species, dimension, and hand-hewn method of preparation.  The bridge remains at its original location in the Wawona region.  Because of this accurate approach to rehabilitation, the bridge retains integrity of design, location, workmanship, feeling, and association.  The structure is currently in fair to poor condition, with deterioration in numerous structural elements.  The covering structure is in good condition, with many original siding boards extant.

 

Construction Evolution of the Wawona Covered Bridge

 

Historic photographs of the bridge taken in the 1880s indicate that the appearance of the structure is essentially the same as the present structure.  Galen Clark, original steward of the Yosemite Grant, built the bridge as an open-deck structure in 1868.  The bridge used a modified Queen-post truss with main members composed of hand-hewn Ponderosa Pine, reinforced with vertical iron tie rods.  Following acquisition of the bridge by the Washburn Group (consisting of Albert Henry Washburn, William F. Coffman, and Emery W. Chapman), the bridge was covered in 1878, using lumber prepared in the Washburn Group's nearby sawmill.  Anecdotal evidence found in the National Park Service archives suggests that the bridge was covered to emulate popular covered bridges in Henry Washburn's native Vermont.  More likely, the bridge was covered to protect its timber elements from the heavy rains and snows found in its mountainous location.  The covering structure was of internal-frame design with plain-sawn Douglas fir structural elements and siding and a gable roof finished with hand-cut sugar pine shakes.  The original foundation for the bridge consisted of timber cribs filled with stone.

 

 

The bridge has undergone a series of alterations since the early Twentieth Century.  In 1900, both approaches were lengthened approximately 13 feet, with new abutments installed.  Running boards for the handling of automobiles were added in the 1930s.  The bridge, which served the original Wawona Stage Road to Yosemite Valley, was taken out of automobile service in 1937, as the Wawona Road was routed over a new bridge located approximately 200 yards southwest of the original bridge.  In 1937, repair work to several transverse floor beams, replacement of some of the wood siding, and the addition of stone to the timber crib foundations was performed by the Civilian Conservation Corps.

 

The bridge suffered severe damage from the floods of 1955 that ravaged the Yosemite region.  Following inspections by Park engineers, the bridge was determined to be in poor condition and replacement was recommended.  Park engineers evaluated several options, but Park Superintendent John Preston recognized the historical significance of the structure and pursued methods to save it.  Along with landscape architect Thomas Vint, who was head of the Park Service's Office of Design and Construction, the two officials successfully lobbied for rehabilitation of the bridge, which began in 1956.  The bridge was pulled to one shore using an ingenious system of rails, cables and pulleys.  The entire structure was field surveyed, with careful measurements of all timber elements taken.  Photographs of this process exist in the records of the Yosemite Research Library.  Severely damaged elements were replaced using the same wood species, dimension and historical method of preparation.  The bulk of the original siding was reused; the sugar pine shake roof was replaced.  The following structural elements were replaced:  both truss lower chords, all transverse floor beams, and all knee braces.  The timber crib substructure was replaced with concrete piers and abutments faced with stone.  Articles appearing in local newspapers and journals, hailed the rehabilitation for its historical accuracy.

 

Various structural elements have been repaired since 1956.  In 1972, Yosemite Park crews replaced five transverse floor beams.  Five transverse floor beams were replaced in 1978 as well, with three of the 1972 transverse floor beams replaced.  The above dates are approximate, as there is some contradiction in actual dates of replacements in the archives.  In 1983, replaced elements included the stringers of the approach spans, several transverse floor beams, several truss elements, and all knee braces and the wood decking.  The 1983 restoration work also included replacement of the original iron tie rods with steel tie rods.  In 2001, 5 transverse floor beams were repaired with sistered wood bolted to the existing fabric.  The two inner stringers that span between the south abutment and south pier were replaced at this time.

 

Current Appearance of the Wawona Covered Bridge

 

In form, materials, and many details, the Wawona Covered Bridge retains its historic appearance.  The 1956 rehabilitation duplicated the original wood structural elements in-kind, using measurements taken from the original structure. The dimensions of the principal structural elements are as follows:  top chord 13" x 14"; bottom chord and stringers 14" x 16"; transverse floor beams 14" x 14; knee braces 4" x 6." Wherever possible, the 1956 rehabilitation reused sound timber structural elements. However, due to subsequent replacements, it appears that the clear-span interior stringers are the only principal supporting elements that are original. It also appears that greater than 50% of the original siding for the covering structure is extant.

 

Several alteration campaigns have compromised the original appearance of the bridge.  The 1956 rehabilitation changed the appearance of the abutments, which are presently composed of concrete faced with stone.  The stringers replaced in 1983 duplicated the original dimension accurately, but used a faux method of wood preparation, in which the timbers were sawn first and then dressed on the surface to give an appearance of hand hewing.  The 1983 structural strengthening campaign is the only work documented by existing construction drawings.  These drawings indicate that the grade of steel used in the existing tensioning rods could not be verified, thus the steel reinforcing rods were replaced with sizes from 1¼" to 1¾" in diameter, tied to the wood structural elements using 1" bearing plates and hex nuts.  It is not clear from historic photographs of the first bridge if the reinforcing rods used a similar system of attachment to the wood structure.

 

Integrity of the Wawona Covered Bridge

 

The Wawona Covered Bridge retains a substantial degree of integrity according to the seven aspects of integrity defined by the National Register: location, design, setting, materials workmanship, feeling and association.  The original design of the bridge was meticulously documented and duplicated during the 1956 rehabilitation.  The workmanship of the building is still evident in the remaining original siding, the hand-hewn preparation of the original and replaced timber structural elements, and the hand-split shake roof.  Although the substructure has been replaced, the existing stone-faced abutments relate the bridge to its surroundings and represent an appearance common to other structures in the Yosemite area. 
 

The Wawona Covered Bridge retains integrity of setting, feeling and association.  The structure remains at its historic location.  The setting has been somewhat altered by the addition of buildings moved to the north of the bridge to create the Pioneer Yosemite History Center.  However, many original buildings remain at Wawona, including the wagon shop constructed by the Washburn Group, located immediately south of the covered bridge.  The existence of the bridge and wagon shop at their historic locations, as well as the Wawona Hotel and associated buildings are all contributing structures to a probable nomination of the Wawona district as a cultural landscape.  The Wawona Hotel, adjacent buildings and over 16 acres of the hotel grounds were placed on the National Register in 1975.  The Wawona Hotel was listed as a National Historic Landmark in 1987.

 

Development of Stage Routes

 

Following completion of the Wawona stage road in 1875 (discussed in the next section), Washburn dissolved his partnership with William Coffman and Emery Chapman, but remained intent on opening stage routes from the towns of Merced, Berenda and Madera.  In 1877, Washburn incorporated the Yosemite Stage & Turnpike Company, forming partnerships with prominent Merced businessmen, which would enable the new company to operate a stage line from Merced to Wawona, via Mariposa.  With this first line accomplished, Washburn's new stage company sought to increase their control of southern routes into Yosemite and looked to Madera, another stop along the railroad.[1]  On April 1, 1879, Washburn awaited his first passengers along the newly completed Madera route as they arrived at Wawona.[2]  To reduce the length of stage travel and offer a more direct route into Wawona via the railroad, the Yosemite Stage & Turnpike Company persuaded the Southern Pacific Railroad to build a spur line from Berenda (seven miles north of Madera) north to Raymond in the Sierra foothills.  This third stage route was completed in 1886 and was luxurious, with daily trains from Los Angeles and San Francisco delivering eager tourists to Wawona.[3]  By the late 1880s, these three routes increased tourist travel via Wawona substantially, bringing travelers from the East and the major cities of the West by rail and stage to Yosemite Valley.

 

In 1886, James Mason Hutchings one of the first travelers to Yosemite and later an innkeeper in the valley, wrote his noteworthy guide to the Yosemite region.  In his book, In the Heart of the Sierras, Hutchings described Wawona as follows:   

 

Wawona (the Indian name for Big Tree), formerly called ŚClark's,' is the great central stage station where the Berenda, Madera and Mariposa routes all come together; and which also forms the starting point for the Mariposa Big Tree Grove.  The very instant the bridge is crossed, on the way to the hotel, the whole place seems bustling with business, and business energy.  Conveyances of all kinds, from a sulky to whole rows of passenger coaches, capable of carrying from one to eighteen or twenty persons each, at a load, come into sight. Hay and grain wagons; freight teams coming and going; horses with or without harness; stables for a hundred animals; blacksmiths shops, carriage and paint shops; laundries and other buildings look at us from as many different stand-points (sic)[4]

 

Hutchings describes a bustling Wawona, containing shops and services of a typical pioneer town and filled with tourists arriving from the various stage lines, eagerly anticipating their visits to the Yosemite Valley or the Mariposa Grove of Big Trees; or relaxing on the generous veranda of the Wawona Hotel, rebuilt by Henry Washburn in 1879.  For those traveling to the valley from Wawona, the first landmark they encountered was the Wawona Covered Bridge.

 

The Wawona Road

 

Following establishment of the Yosemite Grant in 1864, Commissioners granted private companies or individuals franchise rights to construct access roads into Yosemite Valley.  The owners could collect tolls for road use as compensation for their construction costs.  A stipulation of the agreement was the right of the Commission to purchase the road back from the owners for the cost of construction.[5]  Galen Clark used this incentive to attempt construction of a stage road from Wawona to Yosemite Valley.  However, he proved unable to complete the road due to financial and construction problems and sold his Wawona holdings to the Washburn Group in 1875.  A section of the deed is reprinted and appears in the discussion regarding National Register Criterion C.

 

After their purchase of Clark's Station from Galen Clark, the Washburn Group renamed Wawona "Big Tree Station."  The Group proceeded to contract with builder John Conway to construct a road from Wawona to Yosemite Valley, which was completed in June 1875.  The event was cause for much celebration and a great party was held in the valley, which included the Yosemite Band, speeches, poetry recitals, numerous tourists, and throngs of locals who made the trip from nearby towns.  The Mariposa Gazette noted, "The celebration on the 22nd at Yosemite of the completion of the wagon road, has greatly depopulated our population."[6]  Following this landmark event, the Washburn Group completed a road from Wawona to the Mariposa Grove of Big Trees in 1878, enabling tourists to view the splendor of the Giant Sequoias from the relative comforts of a stagecoach.  Galen Clark's Wawona Bridge was covered in 1878 as well.[7]

 

Tourism to Yosemite increased substantially because of the Wawona Road, for it was regarded as the most scenic route into the famed valley.  A Congressional Report on the state of the Wawona Road published in 1900 noted the road's significant impact on tourist travel to the park:

 

Since the construction of this road, a large majority of the tourists visiting the Yosemite Valley have selected it as their route of travel, induced so to do by the location of this road into the Mariposa Big Tree Grove and the points of interest on the road, including Inspiration Point and Glacier Point; also because this road traverses the National Park and enters the valley on the south side thereof, where the most comprehensive views of the wonders of the valley are obtained.[8]

 

The Wawona Covered Bridge also carried the first automobile to travel to Yosemite, which entered the park via the Wawona road in 1900.  Known as a "Locomobile," and powered by steam, the vehicle was driven by photographer and promoter Oliver Lippincott.  On June 22, 1900, Lippincott arrived at the Wawona Hotel amidst a throng of eager onlookers.  He proceeded to cross the Wawona Covered Bridge and journey to Yosemite Valley via the Wawona Road.  The trip took three hours.[9]

 

Wawona as Gateway and Destination

 

The covered bridge was an important component of the Wawona Tourist experience.  The bridge was a vital link in local pioneer trade, carrying horses, stock and people on business in Wawona or Yosemite Valley.  By the late 1870s, stages regularly used the bridge; automobiles followed until the bridge was taken out of vehicular service in 1937.  A diverse and important list of travelers has taken the bridge, including Teddy Roosevelt and Ralph Waldo Emerson.  For politicians and scholars, as well as typical tourists seeking the grandeur of Yosemite, the bridge was a vital link between Wawona and Yosemite Valley.   However, Wawona itself was a destination, with Galen Clark's Mariposa Grove of Big Trees a short stage ride from the Wawona Hotel and the covered bridge.

 

Galen Clark And Early Exploration Of Yosemite National Park

 

In many ways Galen Clark's settlement and career in Yosemite National Park is analogous to the trends of exploration and development that characterized the West.  Clark was one of the first men to explore Yosemite Valley, making the trip from the mining town of Mariposa in 1855.[10]  A late addition to the throngs of Eastern transplants seeking riches in the gold fields of California Clark was captivated by the grandeur of Yosemite's scenery and filed a claim for 160 acres of land in what is now Wawona in 1856.  He built his first structure, a windowless log cabin, in Wawona meadow in 1857.[11]  Sustaining himself entirely on his own wilderness abilities, Clark epitomized the early California Pioneer: "My nearest neighbor was 16 miles distant.  I was entirely alone."[12]

 

The visionary Clark recognized the coming tourist trade in Yosemite and would feed travelers passing by his rustic cabin.  By the 1860s, Clark had developed his primitive lodgings into an inn that was appreciated by visitors for its facilities and for Clark's hospitality.  The inn became known as "Clark's Station," and it was the precursor of the current Wawona Hotel.[13]  In 1855, an employee for the Mariposa Ditch Company, Richard H. Ogg, found three Giant Sequoia trees near the South Fork of the Merced River.  After hearing this description, Clark set out to find the trees.  In the summer of 1857, Clark publicized what is now known as the Mariposa Grove of Big Trees.[14]

 

Galen Clark's name is synonymous with the early history of Yosemite National Park.  He championed the region's glories immediately following his settlement and remained involved with the park for the rest of his life.  Clark's efforts to preserve the region for everyone to share, as well as his discoveries of outstanding scenery in the Wawona area, reportedly aided in the establishment of the Yosemite Grant, the first legislation enacted by Congress to preserve a wilderness area.  The Grant included Yosemite Valley, and Clarke's Mariposa Grove of Big Trees.  Although decided in 1864, the Grant required approval by the State Legislature, which did not reconvene until 1866.  On April 2, 1866, the Grant was ratified by the California legislature.  Clark was one of the first nine commissioners of the Yosemite Grant and was elected the first guardian of what is now Yosemite National Park.[15]  Although Clark left Wawona in 1875 to live permanently in Yosemite Valley, he remained in Yosemite for the rest of his life, fulfilling many roles, including Commissioner, guide, environmentalist, historian and guardian of a landscape that captivated him from his first visit in 1855.

 

Architectural/Engineering Significance Of The Covered Bridge

 

The date of initial construction is 1868.  An article from the Mariposa Gazette states, "We learn from Mr. Galen Clark that by tomorrow he will have his bridge completed across the South Fork of the Merced River."  The date of this article is July 3, 1868.[16]  The original bridge was constructed uncovered by Galen Clark over the South Fork of the Merced River, as Clark desired to collect tolls for local and stock traffic as well as horseman riding the early trail to Yosemite Valley and eventually hoped to develop a stage road to link Wawona to Yosemite Valley.  The following excerpt from Clark's deed of sale to the Washburn Group confirms the existence of the bridge prior to 1875: 

           

Sale of Clark & Moore's to Washburn, Chapman & Coffman recorded on page 467, Book 2 of Deeds, Mariposa County, dated January 6, 1875; includes a hotel, lodging houses, barn, blacksmith shop, sawmill, and bridge across the South Fork of the Merced River, and all other improvements.[17] 

 

After their purchase of Clark's Wawona holdings, the Washburn Group covered the bridge in 1878.  Subsequent structural rehabilitation and strengthening occurred in 1900 and the 1930s, until the bridge was taken out of automobile service in 1937 (see the continuation sheets for Section 7).  The bridge continued to service local pedestrian and stock traffic after 1937.

 

Following the 1955 floods the park staff considered various replacement schemes for the Wawona Covered Bridge.  Letters and memoranda from the National Park Service files and the archives of the Yosemite Research Library indicate a strong sentiment existed in the 1950s to save the bridge.  A resident of Auburn and local naturalist and philanthropist, Wendell Robie, wrote to Congressman Clair Engle noting, "it is of special interest to people of Tuolumne, Mariposa Counties, and those historical interests around the state to see this remaining covered bridge in Yosemite park continued and maintained."[18]  Congressman Engle forwarded Robie's letter to Park leadership, stating "I would like to be brought up to date on the status of the case."[19]  Letters from Park Superintendent John Preston and landscape architect Thomas Vint (located in the archives of the Yosemite Research Library) also discuss the importance of preserving the bridge.  The existence of this correspondence, among Park staff, citizens and governmental representatives, testifies to the historical significance of the Wawona Covered Bridge.  The letters from Park staff, citizens, and governmental officials successfully built a constituency to save the structure.

 

Rehabilitation of the bridge commenced in 1956.  The entire structure was field surveyed, with careful measurements of all timber elements taken.  The process was photographed extensively.  Severely damaged elements were replaced using the same wood species, dimension and historical method of preparation.  The bulk of the original siding was reused; the cedar shake roof was replaced.  The following structural elements were replaced:  both truss lower chords, all transverse floor beams, and all knee braces.  The timber crib substructure was replaced with concrete piers and abutments faced with stone.  Although considerable replacement was necessary, correspondence prior to and during the rehabilitation note the importance of "preserving as many structure members as possible intact."[20]  Articles appearing in local newspapers and Yosemite Nature Notes, hailed the rehabilitation for its historical accuracy.  Examples of these articles are found in the Bibliography.

 

Although thought to be based on a Howe Truss plan, the bridge is a modified Queen Post design, with metal rods tying the truss system.  This design is relatively unique for covered bridges, particularly in the West.[21]

The engineering of this bridge combined elements of the Howe truss design with a more vernacular Queen Post truss system.[22]  The vernacular approach to the bridge's covering structure, which was integrated to the existing truss structural system in 1878, adds to the unique qualities of the Wawona Covered Bridge.  In addition, the surviving interior stringers of the structure are testimony to the skill needed to prepare the 106-foot long timbers in the Nineteenth Century, which included the laborious tasks of felling, transporting and hand-hewing great trees into the required dimensions.

 

The Wawona Covered Bridge is one of only twelve remaining covered bridges in California.  A structure type that continues to vanish from the American cultural landscape, the Wawona Covered Bridge is an extant example of a type of construction that is not considered common in the West.  The Wawona Bridge is the only example of its type found in the western region of the National Park Service.  The bridge has been linked historically to the Wawona Hotel, pioneer transportation, the development of stage roads, and the tourism industry of Yosemite National Park for its entire existence. 

 

East Elevation Viewed from Northeast

 

Today the Wawona Covered Bridge remains in a prominent tourist region of Yosemite National Park enabling it to be experienced by a variety of tourists.  The bridge currently aids in interpretation of the National Park Service's Mission 66, a plan that included the development of the Pioneer Yosemite History Center located north of the existing bridge.  The Director of the National Park Service, Conrad Wirth, implemented Mission 66 in 1955 as a nationwide plan to improve interpretation of National Park history, protect wild and historic resources and improve visitor services.  At Wawona, the Pioneer History Center was a manifestation of this plan to educate the visiting public about the early history of this vital area of the Yosemite region.  The Wawona Covered Bridge was viewed as a critical component of the Pioneer History Center during its creation and remains a vital link in interpreting the history of the Wawona area today.

 

 

Condition Assessment and Testing and Inspection

 

Field Inspection Team:

 

            Reinhard Ludke, Structural Engineer  , C+D Consulting Engineers

            Olivier Ruhlmann, Bridge Engineer, C+D Consulting Engineers

            Seth Bergstein, Architect, Architectural Resources Group

            Michael Wolsk, Geotechnical Engineer, Condor Earth Technologies

            Craig Struble, NPS Manager, Yosemite National Park

            Mike Pieper, Civil Engineer, Yosemite National Park

            Kevin Flynn, Wood Technologist, Univ. Cal. Forest Products Labs

 

C+D Engineers surveyed the Wawona Covered Bridge to assess its condition on Thursday March 28, 2002.  Some structural members appeared decayed, and a permanent sag of eight inches at mid-span had formed under the bridge's own weight.  The bridge was originally built in 1868 as an open truss structure.  In 1874, a roof structure was added, giving the bridge its basic present appearance.  In 1900 a 10' approach structure was added on each side of the bridge.  

 

This field inspection includes three parts: bridge structure inspection, a geotechnical report, and an architecture and cultural resources report.

 

The bridge structure is in fair condition.   Abutment and pier integrity appears to be sound for all four locations with no evidence of foundation scour at either abutment or pier.  Both abutments and piers were rebuilt in 1956 after a flood nearly destroyed the bridge.  The new abutments and piers are built with reinforced concrete with a stone veneer. 

 

All the timber members, apart from the transverse floor beams (the transverse floor beams), that constitute the main structural system are in good condition.  The outermost portion of the transverse floor beams and the lowermost portion of the exterior posts have undergone most of the significant decay. 

 

Large gaps were observed between the ends of many diagonal compression truss members and the chords on which they are designed to bear.  These gaps prevent the structure to act as a simple truss, significantly weakening the bridge.

 

C+D Engineers also conducted some non-destructive testing of some of the main structural members to assess their degree of decay using a Resistograph.  Resistographs are a recently developed tool having a needle like drill bit that can penetrate timber members while recording the resistance to drill (the torque).  The torque can in turn be used to estimate the density of wood members, and their strength can be assessed.  This is the best way to assess the integrity of timber members since decay starts along the middle of large timber member sections.

 

C+D Engineers took some field measurements and prepared drawings that indicate the general type, size and location of distress of the elements of the bridge.

 

Figure 1: Resistograph Readings

Observations

The main structural system consists of two Ponderosa pine timber trusses, 13'-6" apart, that span 106 feet from pier to pier.  The roof is supported by roof posts of different sizes that either rest on the top chord of the truss or directly on the transverse floor beams. 

 

Truss Condition:  The top chord of the truss is comprised of 3 - 13" x 14" Douglas Fir timber elements: two 42'-6" long members inclined at approximately 22 degrees, and one horizontal 28 foot long member in the middle that connects the two inclined members.  The bottom chord of each truss is a continuous (not spliced) 14" x 16" Douglas Fir member approximately 118 feet long.  Each element of the trusses was inspected, and corresponding levels of distress were recorded.  

 

C+D Engineers observed no major distress in the top chords of the trusses.  All the top chord members are in excellent condition.   C+D tested the bottom truss chords at two different locations using the Resistograph.  The readings indicated that the wood in the bottom chords is also in very good condition. 

 

The vertical truss web members are tension members and are made up of steel rods sandwiched between paired timber posts of varying sizes.  The steel rods connect the top chord to the bottom chord.  All the nuts holding the rods to the chords seemed tight and flush with the backing plates.  It is our understanding that the bridge was jacked up and the rods tightened a year ago to counter excessive deflections.  Marks on rods suggest that the effective length of the rods was decreased by approximately one foot during the tightening process. 

 

Many of the diagonal compression truss web members (of varying sizes) were no longer in contact with the chord members.  Large gaps between the diagonal members and the chord members could be observed at many joint locations.  In some cases, excess debris and difficulty to access the joints may have hidden some existing gaps.  All compression members ought to be made flush with the top and bottom chords such that the frame may behave in a truss-like manner instead of behaving like an arch subjected to large bending loads.  This is likely to significantly reduce the strength and the stiffness of the bridge.

 

Structural timber members:  The 10" x "14 interior stringers that run parallel to the bottom chords were also tested with the Resistograph.  Again, the readings indicated that the members are in good condition.  The transverse floor beams purpose is to bridge the longitudinal stringers to the bottom truss chords, and to support the roof posts and the diagonal roof post braces against lateral load.  The transverse beams are 14 x 14 Ponderosa Pine members and are 30'-2" long.  A few of these braces are missing as they rotted away and ought to be replaced.  While decay in a few transverse floor beams was obvious, it was questionable in others.  Testing and readings from the Resistograph showed that wood in the vicinity of the transverse floor beam/post connection has undergone advanced decay.  However, other readings taken only a few feet away suggest that the decay may well be very localized to near the connections where water infiltrated.  While this information does not support the idea of completely salvaging the transverse floor beams, it certainly suggests that a proper waterproofing detail will be required to extend the life cycle of the bridge. 

 

Deck lateral bracing (made up of 4x8 or 5x10) was in good condition, however, one member (X7 -Y8) was missing.  It was noted that the bracing is built in the direction opposite to the one indicated on the construction drawings.  However, this has no effect on the structure's ability to withstand lateral loads.

 

Deck: The deck planks are 4x6 members and are in good condition.  Looking down along the surface of the deck, one can notice waviness of the deck surface along the longitudinal direction.  There seems to be a bump at each truss joint along the lower chords.  It is very possible that these bumps may have been induced while the bridge was being jacked up and the rods were tightened.  Nevertheless, these planks seem to be in contact with the both chords and both stringers indicating that this waviness is only an aesthetic matter.

 

Utilities: There are three utility lines running below the deck of the bridge:

- Electric power line

- Three-inch diameter water supply line

- Two-inch diameter abandoned line

A drain valve is located behind the North abutment.

 

Geotechnical, Hydraulic Study and Foundation Rehabilitation

 

Condor Earth Technology performed this work.  The abutments and piers are reinforced concrete with a stone and mortar veneer.  It is our understanding that they were constructed in 1956.  The geotechnical team probed the ground around the abutments and piers.   Only minor undermining downstream of the south pier base was observed.  The foundation of the south pier was partially exposed and appeared to extend beyond the scour into the ground.  Also, a piece of the upstream wing wall of the north abutment seems to have failed.

 

East Elevation View From Southeast

 

Generally, the piers and the abutments are in good condition.  Condor Earth Technology estimated that the 500 years flood level is 2.5 feet below the bottom of the bridge and thus does not directly load the bridge. The use of riprap to protect the abutments is not recommended as it would constrict the channel and create adverse consequences.   The hydraulic study of storm water flow in the surveyed channel suggests deep scour in alluvium materials. 

 

Available evidence and observation indicates that the bridge piers and abutments are founded on bedrock.  The recent winter storm of 97-98 caused severe flooding with water flows approaching the 500-year storm intensity.  Local extreme scour of bridge piers and abutments did not occur.  Based on these facts, the report states that local scour of the piers does not appear to be a significant problem.  The scour and voids noted under the South Pier should be repaired.  The repair plan will include clearing sand and organics and filling the voids with concrete mortar and install a face coarse of mortared stone.  The repair concrete will be keyed into the existing base or use drilled and grouted dowels to tie the repair to the existing structure.                

           

A floor level survey was performed.  The bridge sags approximately 0.80 feet at mid-span.  The upstream truss is approximately 0.25 feet lower than the downstream truss.

 

Bridge Structure Condition Assessment

 

C+D Engineers surveyed the condition of the wood members both visually and with the help of a Resistograph.  All the main truss members were first inspected visually, and only the main members that seemed to show signs of decay were then tested with the equipment.

             

Generally, all the truss members that were protected under the roof structure appeared free of decay and are in very good condition.  Three of the diagonal compression truss members are checked along their full length.  This could become problematic if these members became fully split and were subjected to the worst code-level loading, under which the rest of the bridge would become unsafe. 

 

More serious material problem could be observed outside the roof enclosure.  Most of the main structural members exposed to weather seemed to have undergone some degree of decay.  Decay was particularly obvious where members connect.  Connections often provide an entryway for water to infiltrate deep into the wood fibers.  Transverse floor beams, exterior roof posts, and stringers all seemed to show signs of decay.  C+D Engineers therefore conducted some non-destructive testing of some of the main structural members to assess their degree of decay using a Resistograph. 

 

 

Taking measurements with the Resistograph

 

Resistographs are a recently developed tool having a needle like drill bit that can penetrate timber members while recording the resistance to drill (the torque).  The torque can in turn be used to estimate the density of wood members, and their strength can be assessed.  This is the best way to assess the integrity of timber members since decay starts along the middle of large timber member sections.  Measurements and measurement locations are shown in Figure 17.  Resistograph readings suggest that all stringers and lower truss chords are in good condition.  Lower density toward the middle of some stringers may either suggest that the pith of the tree is located near the middle of the members, or that incipient decay is present.  No decrease in wood density in the stringers that were tested seemed pronounced enough nor spread enough to cause an appreciable decrease in the stringers' capacities.  On the other hand, transverse floor beams were found to have undergone a significant amount of decay locally if not globally.  Decay into the transverse floor beams, braces, and exterior posts seem to have been triggered by water infiltration into the transverse floor beam to post connections and the transverse floor beam to brace connections.  

 

Rotted Transverse Timber Beams

 

No sign of significant decay was found in any truss member or in any stringer.  Some transverse floor beams (transverse floor beams) have undergone extensive decay throughout while others only have significant decay where they connect with the roof posts and post braces.  Many compression truss members have gaps at their ends rendering them useless.  This could significantly affect the behavior of the trusses.  A load rating assessment of the superstructure will be performed to establish load capacity of the existing bridge.

 

Decay in Transverse Floor Beam

 

Bridge Structural Assessment

 

From the site surveillance report written by Condor Geotechnical Engineers, the piers and the abutments of the bridge seem to be in good condition, and to the best of our knowledge, are adequate to carry the loads to which they are subjected.  However, we were unable to find more documentation on the actual shape of the reinforced concrete foundation supporting the piers and the abutments.

 

Assessment and Rehabilitation Design Criteria

 

In California most buildings are designed according to the UBC (Uniform Building Code), and most bridges are designed according to the Caltrans BDS (Bridge Design Specifications), which is a modified AASHTO design specification.  The Wawona Covered Bridge has a roof structure similar to the one of a building, and a truss structure similar to the one of a bridge.  C+D Engineers provides structure engineering for many pedestrian and special non-highway vehicle bridges, so we adapt provisions of both the UBC and Caltrans BDS for the assessment and design of these special bridges.

 

Loads

Dead Load:  Total bridge weight = 115 kip

Live Loads: - UBC Evenly Distributed Pedestrian Live Load = 100 PSF (reducible to 60 PSF)

                   - BDS Equestrian Live Load = H10 (half of H20 loading)

                   - 200 150-LBF people standing on bridge at one time

       - Large Stage Coach:  10.6 Kip (includes 12 riders and 4 horses).  Axles are 7 feet apart

                   - Small Stage Coach:  5 Kip (includes 4 riders and 2 horses).  Axles are 6 feet apart

 

Snow Load: UBC = 60 PSF (reduced to 35 PSF)

Wind Load:  UBC 70 mph wind speed = 20 PSF

Seismic Load: UBC, Zone 3.  Acceleration = 0.147 g

 
Vertical Load Combinations

Combination #1*: Dead Load

Combination #2: Dead Load + 35 PSF Snow Load

Combination #3: Dead Load + 60 PSF Live Load

Combination #4: Dead Load + 200 150-LBF people at a time Live Load

Combination #5*: Dead Load + 60 PSF Live Load + 35 PSF Snow Load

Combination #6: Dead Load + Large Stage Coach Live Load

Combination #7: Dead Load + Small Stage Coach Live Load

 
Lateral Load Combinations

Combination #8*: Dead Load + Wind Load

Combination #9*: Dead Load +75% Large Coach + 75% Wind Load

Combination #10*: Dead Load + 75% Small Coach + 75% Wind Load

Combination #11*: Dead Load + 75% 60 PSF Live Load + 75% Wind Load

 

NOTE: * denotes those combinations that are part of the UBC 97 Basic Load Combinations.  The other combinations were included for their eventual interest for the park.

 

Material Assumptions

Assume that all Ponderosa Pine members are of grade "Select Structural"

 

Vertical Load Structure

 

The Wawona Bridge was constructed with a truss configured structural system.  An ideal truss is a structural system whose members are all experiencing axial forces only.   To meet this criteria, it is necessary that all members be pinned at their supports and that all loads be applied at the member joints only.  Some truss bridges, however, are designed in such a way as to have all their members being pinned, but allowing loads to be applied uniformly along the lower chord of their trusses.  This somewhat lowers the capacity of the bridge because some of the members are subjected to both bending and axial forces simultaneously.  The original construction of the Wawona Bridge used continuous members, not pinned at each joint.  Additionally, the roof structure that was added at a later date is partially supported by posts that rest in between member joints and if snow loads are large, they will induce large bending moments into some of the members.  Figure 2 illustrates how a truss with continuous members and roof loads applied in between joints will deform under its own weight.  All top and bottom chord members are loaded both axially and in bending.

 

 

Figure 2: Truss Behavior

 

 

Many diagonal web compression members were not connected to the top and/or bottom chords of the trusses.  Not having diagonal compression web members in the trusses causes the structure to behave like an arch loaded in bending instead of a truss.  This arch-in-bending behavior lowers the capacity of the bridge even more by inducing even larger bending moments into the members.  Figure 3 illustrates how an arch-in-bending deforms under its own weight.  It can be noticed that each top and bottom chord member bends more than the ones in Figure 2.

 

 

 

 

 

Figure 3: Arch Behavior

 

 

Because the structure behavior is very sensitive to the effectiveness of the diagonal compression members, C+D analyzed the bridge structure in two different ways: the "As Designed" model refers to the truss model and the "As Built" model refers to the arch-in-bending model.  At this time we believe that the actual behavior of the structure is somewhere between the behaviors predicted by each model.  We conservatively assume that the actual structure behaves as the arch-in-bending. 

 

The covered bridge is comprised of two individual structures: the truss structure, and the roof structure.   Because of its own weight and because some of the roof posts bear between truss joints, the roof structure addition actually weakens the original truss structure.  

 

Vertical Load Rating

 

BDS (Caltrans's Bridge Design Standards) does not require taking snow loads into account because they are less than traffic load, and full traffic loads will not occur simultaneously with full snow loads on a highway bridges.  However, with a covered bridge it is possible (even though unlikely) to have a high traffic load simultaneous to a snow load on the roof.  Both codes require engineers to design pedestrian bridges with loads that will exceed the loads that are likely to be encountered at the Wawona Covered Bridge.  The controlling UBC '97 vertical load combination is: Self-Weight + Live Load + Snow Load.  The live load is 100 PSF and can be reduced to 60 PSF.  The snow load is 60psf and can be reduced to 35 PSF for design purposes.  While it seems possible to have 60 PSF of snow load, 60 PSF of live load would be equivalent to 800 150lbf-people standing on the bridge at the same time.  C+D Engineers prepared Table-A to show the overload factors for different load combinations.  The "As Built" column refers to the structure behaving like an arch with disconnected diagonal web truss members.  The "As Designed" column refers to the values obtained if the bridge were built or repaired such that the diagonal web truss members bear on the top and bottom truss chords. 

 

 

Figure 4 shows the (Demand/Allowable Capacity) ratios of the upper and lower chord truss members for load combinations.  The web members are never overloaded, so the (Demand/Allowable Capacity) ratios have been ignored for these members. 

 


 


Figure 4: Truss Elevation

 

 

Table-A below summarizes the (Demand/Allowable Capacity) ratios for the chord members.

 

 

 

The transverse floor beams are loaded in shear and bending from the exterior posts vertical reactions.  The allowable capacity of the transverse floor beams is never exceeded for any of the vertical load combinations when the structure behaves as a truss.  However, if the structure behaves as an arch, load combination 3 and 5 yield a (Demand/Allowable Capacity) ratio greater than 1.0.  Table-B summarizes the (Demand/Allowable Capacity) ratios.

 

 

 

The existing bridge vertical load carrying system was designed as a truss.  Because of gaps at the ends of some truss diagonal web members, the as-built bridge is weaker and behaves like an arch.  Some of the existing members demand/capacity ratios exceed 1, which supports strengthening.  The roof structure is supported by posts that bear on the top chord of the truss, which induce bending stresses and permanent deformations with the joint gaps. 

 

 

Lateral Load System

 

Because of the height and lightness of the roof structure, wind loads are much higher than seismic loads, even if accounting for the weight of snow when computing seismic loads. 

Wind load is initially picked up by the wood siding, goes into the roof structure lateral systems, and is then directed to the bridge deck and into the piers.  The roof structure has two types of lateral load resisting systems.  One system uses diagonal braces to transfer lateral load from the exterior roof posts to the transverse floor beams (lateral floor beams).  This system is only used toward the center of the bridge where exterior roof posts are present.  To be effective, this system requires that the exterior posts, the braces, and the transverse floor beams be in good condition.  This is problematic since all three are exposed to weather and appear to be the members most affected by decay.  The second system is a knee braced frame and is used toward the ends of the bridge.  The braced frames consist of interior roof posts braced with diagonal members that connect to the horizontal 4x8 ceiling-level beams. 

 

Figure 5: Braced Outrigger Frame

 

Figure 6: Knee Braced Frame

 

The load in the roof structure lateral systems is transmitted to the deck of the bridge that acts as a horizontal diaphragm.  Each lower chord of the bridge truss additionally becomes a diaphragm chord and is therefore subjected to an additional axial force.  The diagonal bracing of the bridge deck is used to insure that the deck deforms as a diaphragm.  If this bracing were to fail, the truss bottom chords would be heavily loaded in bending, seriously weakening the whole structure's lateral and vertical capacity. 

 

Lateral Load Rating 

 

The governing lateral load combination from UBC '97 for the lateral systems is Dead Load + Wind (combination 8).  Table-C summarizes the overload ratios (Demand/Allowable Capacity) for different members and different load combinations.  The most heavily loaded members are the interior roof posts that are part of the knee-braced frame, with a (Demand/Allowable Capacity) ratio of 2.0.  If these members were to fail, damage would be restricted to the roof structure and the main truss structure would not be affected.  The governing lateral load combination for the truss lower chords is combination # 11:  Dead Load + 75% 60 PSF Live Load + 75% Wind Load.  The overload factor for the truss bottom chord under load combination #11 is 0.76.   The main truss structure is therefore appropriate by today's lateral load code requirements. 

 

The lateral load carrying system of the bridge consists of a series of knee-braced frames, and frames braced to the transverse floor beams (the transverse floor beams).  The "as-built" bridge was found deficient to carry most vertical and lateral load combinations put forth by the 1997 Uniform Building Code.  All transverse floor beams (transverse floor beams), some roof posts, and transverse floor beam braces have undergone advanced decay and need to be replaced.

 

Sliding and Overturning

 

Lightweight and unusual (height/length) ratio of the bridge render the bridge susceptible to sliding and overturning.  Our calculations show that both overturning and sliding are of concern during a windstorm.  The actual (Demand/Capacity) ratio for overturning is 1.23 while the (Demand/Capacity) ratio for sliding is 1.67.

 

 

 

 

Structural Assessment and Load Rating

 

Wood structural members covered by the roof structure are in very good condition and show no sign of decay.  All transverse floor beams ought to be replaced, exterior posts and their braces need to be tested if they are to be preserved.  Preliminary testing with a Resistograph suggests that the truss lower chords and the stringers are still in good condition.

 

The vertical load capacity of the bridge falls short of code requirements.  This shortcoming is not alarming for live loads because codes overestimate the number of people that will be on this isolated bridge at the same time.  The bridge is not able to support its designed snow load and live load simultaneously (Load Combination #5), so posting live load limitations during the winter may be required.  At this time, the bridge is not built like it was designed: the compression diagonal web truss members do not bear on the upper and lower chords, causing the bridge to behave as an arch instead of a truss.  This significantly reduces its capacity and its stiffness. 

 

The lateral load capacity of the roof structure of the bridge is also not code compliant.  Strong windstorms may cause partial or complete failure of the roof structure.  On the other hand, the truss portion of the structure is more adequate to survive a strong windstorm.  The truss portion of the bridge will not collapse during a windstorm if it remains unloaded.

 

 

Recommendations and Conclusions

 

 

Knee Brace - Lateral Load System

 

 

Exterior Roof Post Bracing and Transverse Floor Beam

 

 

Structure Rehabilitation Strategies

 

Historic Preservation And Rehabilitation Guidelines:

 

Generally, ARG recommends that any work proposed preserve the historic integrity of the Wawona Covered Bridge.  Replacement of the bridge or elements of the structure should follow the "Secretary of the Interior Standards for Rehabilitation" using in-kind materials and retention of historic fabric.  For this reason and others, Alternative 1: Rehabilitation of The Existing Structure is recommended for it retains more "original" fabric and historic character of the bridge.  In-kind replacement of materials and replacement members.  Match the old design when replacing members.  Changes to sizes proposed, configuration of cross bracing, and wind anchors are justified and accepted as they protect the bridge from structural failure. 

 

Bridge Structure Rehabilitation:

 

C+D Engineers presented three alternatives to rehabilitate the existing structure:

            1) Rehabilitation of The Existing Structure: selective replacement and load posting.

            2) Use of Different Wood Species: total bridge replacement / recycling.

            3) Pinned Truss Solution: total bridge replacement / recycling.

 

Other modifications (such as installing hold-downs) are required for all three alternatives.

 
Truss Rehabilitation Construction

 

Insuring that all truss diagonal members ends are flush with the top and bottom chords will cure the arch behavior of the trusses.  This can be done either by providing whole new members, or by adding shims at the ends of the existing members.  Because the metal rods have been tightened without bearing contact of the diagonal members, closing the gaps will require simultaneous shoring of each of the transverse floor beam to the desired/original elevation.  Once the shoring is set, the rods must be loosened, the diagonal truss members must be adjusted/replaced, and the metal rods retightened (snug tight is sufficient).  Note that after the shoring is removed, the bridge will deflect approximately 3/4 inch at mid span (more if proper truss action is not developed).  Therefore, the bridge would have to be raised 3/4 inch higher than the desired/original elevation at mid span. 

 

Because the bridge is unlikely to ever experience the reduced UBC design live load of 60 PSF (equivalent to 800 150lbf-people), and a 35psf snow load at the same time (load combination 5), C+D Engineers recommends posting a live load limit of no more than 75 people at a time during winter months.  This load limitation does not overstress the bridge and appears to be a reasonable limit for the size and location of the structure. 

 

Because wood creeps (undergoes permanent deformation) when exposed to relatively heavy loads during a sustained time period, it is good practice to avoid loading wood members unnecessarily for a long time.  Looking at a Historic photograph taken in 1956, it was observed that a positive camber was created in the bridge deck.  The only way this crown could have been created was by tightening up the metal rods to "deform" the bridge.  Crowning the bridge therefore resulted in permanent stresses in the chord members.  After a long time, it is likely that these chord members creeped, thus creating gaps at between the diagonal truss web members and the truss chords.  C+D Engineers therefore advises the Park against recreating the camber shown in the 1956 photographs.

 

Lateral Load System

 

The first members susceptible to fail in a windstorm are the knee frame posts.  The (Demand/Allowable Capacity) ratio of posts in a 70 mph windstorm is 2.0 (See Table-C, Part I). The knee frame posts would be safe, with a (Demand/Allowable Capacity) ratio of 1.0, at a maximum wind speed of 50 mph.  C+D Engineers therefore recommend closing the bridge when wind speeds exceed 50 mph and/or using larger posts.

 

Figure8: Replace Transverse Floor Beam

 

Transverse floor beams are important lateral load resisting members, so it is important that they be free of decay.  By code, the transverse floor beams in place are inadequate to resist forces generated by a 70 mph wind. Our second site inspection revealed that all transverse floor beams that have not been sistered are unsafe and must either be replaced or sistered promptly.

 

Overturning and sliding should also be addressed urgently.  Calculations indicate that wind blowing at 54 mph is sufficient to push the bridge off the piers.  While it is straightforward to provide sufficient sliding resistance with hardware, overturning is more problematic.  Developing the full shear strength of the truss lower chords is not sufficient to satisfy the code requirements for overturning, however, it will provide a (Demand/Capacity) ratio of 0.80. 

 

 

 

Adding hold-downs will induce large shear forces in the bridge deck, which must be designed for.  The current deck diaphragm uses a combination of 4x8 and 5x10 members in a crisscross configuration.  In order to transmit the diaphragm loads, all 4x8 members will have to be changed to 5x10 and the single diagonal pattern must be modified to a double crossing bracing framing.

 

 

Figure 9: New Deck Bracing

 

 

Construction Alternatives

 

From the findings, there are two main alternatives to rehabilitating the bridge and one is included 

that would replace the bridge with a true pinned joint truss bridge. 

 

Alternative 1: Rehabilitation of Existing Structure

 

This first alternative consists in accepting the current bridge's structural limitations, and trying to extend its life as much as possible.  This alternative remains viable for as long as the main structural members remain mostly free of decay.  Because some structural members are more critical than others, it is important to develop and follow a timeline of structural repairs.  Table-D lists some repairs organized by order of priority with the corresponding load capacity increase achieved for each repair.  The repairs must be done in the order in which they appear on Table-D in order to obtain the corresponding load capacity.  Also, this alternative requires posting the following load limitations: 

 

 

 

Additional load limitations may also apply.  Refer to Table D to determine the additional load limitations depending on the stage of repair.

 Alternative 2: Replacement Bridge - New Wood Species

 

The second alternative consists in making the bridge completely code compliant by rebuilding it using a stronger and more decay resistant wood species (Redwood, for example).   This would require the complete dismantlement of the existing structure and minor structural changes, such as the addition of hold-downs and increasing the size of some lateral load resisting elements.  From an architectural conservation point of view, using a different wood species and keeping the structural system changes down to a minimum seems like the most appropriate choice.

 

Alternative 3: Replacement Bridge - Pinned Truss Solution

 

This third approach consists in changing the structural behavior of the bridge by forcing it to behave as a real truss (axial loads only in the members).  This could be achieved by substituting each continuous chord members with a series of distinct members that are pin connected using metal inserts at each joints.   Each metal insert could be concealed within the wood members, through bolted, and covered with wood plugs.  Each truss would then require 37 prefabricated wood members and 20 prefabricated steel joints. 

 

 

 

Figure 10: Pinned Truss

 

 

 

This approach would also require the complete dismantlement of the existing structure.  Other minor structural changes such as providing proper anchorage to the piers, and increasing the size of some lateral load resisting members would still be required. 

 

Building a new bridge, using either alternative 2 or alternative 3 would offer longer life and lower maintenance cost at the expense of a higher initial cost.

 

 

Table D:  Structural Repairs Timeline and Load Capacity


 


Wood Condition, Reconstruction Species and Preservatives

The Forest Products Laboratory suggested that it is preferable to use a naturally decay resistant wood species instead of a chemically treated one.  Preservatives, retention levels, and general detailing and flashing guidelines are suggested that will lengthen service life.  Large timber members, requiring incisions for preservative treatment, should be incised and pressure treated green, after being hand hewed.

 

It is our understanding that the top and bottom chords of the trusses and the transverse floor beams (transverse floor beams) are made out of Ponderosa pine while the rest of the bridge utilizes Douglas fir.  Generally, Ponderosa pine is relatively weak and has little decay resistance.  Douglas fir is somewhat stronger than Ponderosa pine and offers more resistance to decay.  From a structural and serviceability point of view, it would make sense to use stronger wood for the most heavily loaded members, and use species with more decay resistance for members that are exposed to the environment. 

 

If the bridge were to be dismantled, using some wood species stronger than Ponderosa Pine for the top and bottom chords could increase the vertical load capacity substantially and help control the sag.  As a comparison, using Redwood instead of Ponderosa pine would increase the structural properties as follows:

 

 

Larger increases are possible by using higher structural grade available with Redwood.

Choosing a different wood species is an easy way to make the bridge be code compliant without modifying the historical dimensions of the sections or the bridge geometry. 

 

Transverse floor beams and vertical roof posts are important structural members and are both exposed to weather.  Both have undergone some decay, and advanced decay in some transverse floor beams.  It is likely that all exterior posts need to be replaced (replace all if proper testing is not available), Decay in transverse floor beams is mapped on Figure 18.  Figure 18 suggests that all transverse floor beams have undergone excessive decay and shall be replaced.  A short-term alternate solution to replacing all transverse floor beams consists in sistering all transverse floor beams, (some have already been sistered).  This alternate would allow the park to safely reopen the bridge this summer, until the transverse floor beams are replaced.  Having to replace these members provides the park with a good opportunity to use another wood species that will be more decay resistant.  Table 3-10 of the "Wood Handbook" shows a classification of different wood species according to their decay resistance.  It would be wise to consult this reference prior to selecting a wood species for the transverse floor beams and posts.

 

Even though the truss chords could greatly benefit from the extra strength of a different wood species, their resistance to decay is sufficient given their limited exposure to weather.  Readings from the Resistograph indicated that all chords and stringers had not undergone any significant decay.

 

Conclusion and Recommendation

 

The rehabilitation alternative recommendations and cost estimates support a bridge rehabilitation construction that replaces deteriorated timber structure, strengthens the bridge lateral bracing, provides anchorage at the piers and specifies timber material and flashing that will improve the long term service of the bridge.  An inspection and maintenance plan will be prepared that will be used to preserve and lengthen the service life of the rehabilitated bridge, a historic asset of Yosemite National Park.

 

Cost Estimates

 

Alternative 1: Rehabilitating Existing Bridge



Alternative 2: Replacement Bridge - Different Wood Species



Alternative 3: Replacement Bridge - Truss Alternative


 

 


Bridge Rehabilitation construction commenced in September 2002. The selected structure retrofits are the result of collaboration, the course of action bridge engineers, preservationists and historians agreed on. This project is a case study of successfully completed restoration that demonstrates how the process worked from identification and agreement of the historic significance to the actual testing, design and construction of the work.

 

References

 

Wawona Road, Yosemite National Park, Historic American Engineering Record

            (HAER CA-148).

 

Johnson, Hank, The Yosemite Grant; 1864-1906.

 

HAER, CA.

 

Sargent, Shirley, Yosemite Historic Wawona.

 

Hutchings, James M., In the Heart of the Sierras.

 

Greene, Linda Wedel, Yosemite, The Park and its Resources.

 

Mariposa Gazette, July 24, 1875.

 

Report of the Commission on Roads in Yosemite National Park, California Senate Document       No. 155;  56th Congress, 1st Session, February 8, 1900.

 

Reminiscences of Galen Clark Yosemite Valley 1880, "Yosemite Nature Notes"

            Volume 29, No. 3.

 

Adams, Kramer A., Covered Bridges of the West, 1963.

 

Rempsel, Arthur, "Can the Wawona Covered Bridge be Saved?" Yosemite Nature Notes.,                Volume 35, March 1956.

 

"Saving the Wawona Covered Bridge", Yosemite Nature Notes; Volume 36, Number 11,  

            November 1957.

 

Proceedings of Conference on Nondestructive Evaluation of Bridges, FHWA Report

            FHWA - RD - 040A, Arlington, VA August 1992.

 

Caltrans Bridge Design Specification.

 

Uniform Building Code.

 

Ditton, Richard P. and Donald E, McHenry. Yosemite Road Guide   El Portal, 

            CA: Yosemite Association, 1989.

 

Johnston, Hank.  Yosemite's Yesterdays.  Yosemite, CA: Flying Spur Press, 1989.

 

Menderhausen, Ralph Rene.  Treasures of the South Fork: Trails and History Along the South   Fork of the Merced at Yosemite's Front Door.  Privately Published,  1983.

 

Meyer John C., III.  Yosemite: The Forest Domain of the Pierce-Arrow.  Canoga Park, CA:     Southern California Region of the Pierce-Arrow Society,  1984.

 

Sargent, Shirley.  Galen Clark: Yosemite Guardian.  San Francisco: Sierra Club,  1964.

 

 

 

 

 


To contact the principal author of this paper, please contact Reinhard Ludke, S.E.

 

Creegan + D'Angelo Consulting Civil and Structural Engineers

170 Columbus Avenue, Suite 240

San Francisco, CA  94133

 

Ph)  415-834-2010

FX)  415-834-2011

 

Email: rludke@cdengineers.com

www.cdengineers.com

 

 

Seth Bergstein - Historic Architect

Architectural Resources Group

Ph) 415-421-1680

 

Craig Struble - Park Preservationist

Yosemite National Park

Ph) 209-379-1220



[1] Sargent, Shirley, Yosemite's Historic Wawona, pp. 28-31.

[2] Sargent, p. 33.

[3] Sargent, p. 43.

[4] Hutchings, James M., In the Heart of the Sierras, pp 253-254.

[5] Greene, Linda Wedel.  Yosemite, the Park and Its Resources, p. 91.

[6] Mariposa Gazette, July 24, 1875.  Copy from Wawona Road File, 979.447, Y20-d, Yosemite Research Library

[7] HAER CA-148, p. 6.

[8] Report of the Commission on Roads in Yosemite National Park, California, Senate Document No. 155; 56th Congress, 1st Session, February 8, 1900, p. 5.

[9] Johnston, pp. 214-216.

[10] Johnston, p. 29.

[11] Johnston, p. 89.

[12] "Reminiscences of Galen Clark Yosemite Valley 1880," Yosemite Nature Notes, Vol. 29, No. 3, p. 23.

[13] Sargent, p. 12.

[14] Johnston, pp. 49-50.  Greene, p. 47 also notes others that passed through the Grove, but Galen Clark and Milton Mann (one of the early trail builders of Yosemite) were the first to realize the tourism potential of the grove.

[15] Johnston, pp. 55-56.

[16] Quoted in HAER CA-148, p. 2.

[17] Quoted in Memorandum to Park Superintendent John Preston from Donald McHenry, Park Naturalist, June 29, 1954, found in Wawona Covered Bridge file, National Park Service Western Regional Office, Oakland, California.  The listing of "Moore" in the deed refers to Edwin Moore who purchased a half interest in the Wawona property from Galen Clark to alleviate Clark's financial difficulties in 1869.

[18] Letter from Wendell T. Robie to Congressman Clair Engle, File 979.447, Y-19, Yosemite Research Library; The letter is undated, but was written after the 1955 flood and before rehabilitation commenced in 1956.  Wendell and his wife Inez Robie went on to form the Wendell and Inez Robie Foundation, a non-profit organization dedicated to preserving California's scenic land.  See the website: http://www.robiefoundation.org.

[19] Letter from Congressman Clair Engle to Conrad Wirth, Director of National Park Service, File 979.447, Y-19, Yosemite Research Library. 

[20] Telegram from Associate Director Scoyen, National Park Service to Yosemite National Park Superintendent Preston, dated November 7, 1956, File 979.447, Y-19, Yosemite Research Library. 

[21] Adams, Kramer A., Covered Bridges of the West, published in 1963, lists the covered bridges remaining in Oregon, Washington, and California at that time.  A letter from the author to Richard T. Hart, Yosemite Park naturalist, dated September 25, 1962, also notes that the bridge "is located at the highest elevation of any covered bridge in the nation."

[22] HAER CA-148, p. 5.