From Destruction Comes Knowledge
Two days of expected work turned into a week; one
equipment breakdown cascaded into another; a 30-minute delay became 24
hours. A documentation project that was scheduled to happen in June did
not begin until September. The challenges of keeping a bridge demolition
project on schedule are not unique, but the requirement for historical
documentation of a 1912 reinforced concrete bridge by historians and
engineers added another layer of complexity to a highway widening
project. However, this documentation effort ultimately provided
interesting information about the early development of reinforced
concrete flat slab design.
The historians’ involvement was prompted by a
routine set of circumstances. The structure in question, Bridge No.
92297 -- enumerated as part of a statewide inventory of highway bridges
-- was being demolished in order to facilitate a joint Minnesota
Department of Transportation (MnDOT) and Federal Highway Administration
(FHWA) project to reconstruct and widen a section of the adjacent
Interstate Highway I-35E in St. Paul. The FHWA provided federal dollars,
which triggered the process known as a "Section 106 review." Passed in
1966, the National Historic Preservation Act (NHPA) created the National
Register of Historic Places and requires all federal agencies to take
historic resources "into account" when funding, permitting, or licensing
undertakings. Section 106 of the NHPA describes a process of planning
for preservation in advance of construction.
For this project, MnDOT retained Summit
Envirosolutions, Inc. as the cultural resource consultant to complete
the initial portion of the Section 106 review: identifying historic or
potentially historic resources by researching properties and structures
in the area that would be affected by the highway expansion. Through
this process, the consultants determined that Bridge No. 92297 was
historically significant. In instances when a federally funded project
affects a historic resource, the project agency must work with the State
Historic Preservation Office (SHPO) to determine how best to mitigate
the impact. Options can range from major changes, such as re-routing a
proposed road, to documenting the historic structure prior to
demolition, as was the case with Bridge No. 92297. The pending
demolition of the bridge presented a unique opportunity to investigate
the steel reinforcement concealed within the structure. The team
conducting the sequenced research, documentation and demolition included
Summit Envirosolutions, Preservation Design Works (PVN), a
photographer, MnDOT engineers, and the contractor.
Bridge No. 92297 was a monolithic, single-span,
reinforced concrete flat slab deck with vertical abutments supported on
reinforced concrete strip footings, constructed in 1912 (Figure 1). It
was oriented on a 35-degree skew, measured 49 feet in total length, and
had a clear span of 41 feet with a 60-foot-wide deck. Without any
background about its history, the bridge would have appeared rather
unremarkable. However, research on the bridge revealed that it was an
innovative design for its time. Its documentation shed more light on the
work of the bridge’s designer, and also created a record available for
future study.
C.A.P. Turner and the Flat Slab
Claude Allen Porter (C.A.P.) Turner, a
Minneapolis-based structural engineer, was a pioneer in the development
of the reinforced concrete flat slab and designed bridge No. 92297.
According to several articles by Dario Gasparini, Turner was born in
Lincoln, Rhode Island in 1869, and graduated from Lehigh University in
1890. He subsequently worked for various bridge companies until 1901,
when he began his own consulting firm with the Minneapolis, St. Paul and
Sault Ste. Marie Railroad (the "Soo Line") as a principal client
(Gasparini, 2002). As Turner progressed in his career, he expanded his
practice to the design of buildings, including the first one in
Minneapolis with reinforced concrete floors and columns in 1904. His
major breakthrough in concrete design would be realized two years later:
in 1906, Turner designed his first building with the "mushroom" system
of flat slab floors, the Johnson-Bovey building in Minneapolis (now
demolished).
In the next few years, implementation of Turner’s
proprietary flat slab floor system grew at a furious pace. His design
consisted of floors with four-way reinforcement supported directly on
reinforced concrete columns, each with a distinctive flared capital.
Between 1906 and 1910, Turner claimed that buildings constructed with
his system were "rapidly approaching a thousand acres of floor" (Turner,
1910; 7-12). This growth can be attributed in part to his extensive
publication of designs and load test results for his flooring system in
nationally prominent engineering journals, which proved their
reliability and cost-effectiveness. However, a series of patent lawsuits
and countersuits beginning in 1911 resulted in a dramatic downturn in
the use of Turner’s flat slab system. Nevertheless, he substantially
contributed to the acceptance of reinforced concrete flat slab
technology among practicing engineers (Gasparini, et al., 2001; 17-21).
In addition to implementing his system in
buildings, Turner designed several reinforced concrete flat slab
bridges, most as adaptations of his mushroom floor system. To date, all
known flat slab bridges in the Twin Cities designed by Turner have been
demolished. The bridge decks were often designed with four-way
reinforcement similar to his floors, with longitudinal, transverse, and
diagonal steel. With the exception of a tunnel originally located not
far from the area studied for this project, Turner’s published examples
of flat slab bridges did not bear much resemblance to Bridge No. 92297
(Gasparini, et al., 2001; 12-27). However, Turner held a number of
related patents for both floor systems and bridges, one of which bears a
striking resemblance to Bridge No. 92297, particularly the
configuration of the abutment reinforcement (Figure 2).
Copies of construction drawings and plans dating
to the erection of the bridge, as well as correspondence between the Soo
Line railroad engineers and the city of Saint Paul engineers, revealed
some insights into the bridge’s design and also raised questions.
Although the discovery of original drawings was fortuitous -- and rare
for a structure of this age -- the copies were of poor quality and only
partially legible (Figure 3). Of the six sheets in the set, one was
stamped with "CAP Turner Consulting Engineer" in the title block, while
the "Chief Engineers Office" of the railroad was stamped on the
remaining sheets. The date of the sheet stamped with Turner’s firm was
illegible, but several of the sheets stamped by the railroad engineers
were clearly dated to 1912. The correspondence between engineers
indicates that plans were originally drawn for the bridge in 1908, and
then were revised in 1912 because the earlier plans did not meet the
standards of the 1907 city ordinance. Summit Envirosolutions postulated
that the drawing sheet stamped by Turner was part of the original 1908
set, and the remaining sheets were a revision of Turner’s design made by
the railroad’s engineers.
Interpretation of the original drawings was also
hampered by their poor legibility and a lack of corresponding notes or
engineering calculations. This was compounded by the fact that changes
had obviously been made to the bridge after its construction, such as
the replacement of the railing and the installation of a new topping
slab, which complicated efforts to differentiate original and more
recently added features. Despite these difficulties, comparison with
observed conditions, the original drawings, and Turner’s patent for a
similar bridge design, led to the conclusion that the structural design
of the bridge can be substantially attributed to C.A.P Turner.
The complications that the team experienced in
reading the Bridge No. 92297 drawings are actually typical obstacles to
understanding historic engineering structures. Any engineer asked to
retrofit an older building can relate to the frustration of not being
able to locate the original engineering design drawings; while
architectural drawings are often kept as much for their visual appeal as
their content, engineering drawings are often inadvertently lost, or
even intentionally destroyed for insurance and liability reasons.
Likewise, details of the construction methods and sequence may never
have been recorded, but rather negotiated in the field by a contractor
or builder. Finally, the structure itself is often concealed, limiting
the ability to measure and record the structural elements. While
deconstruction is not often considered an ideal method of research, the
removal of this 1912 bridge presented an opportunity to gain additional
knowledge of early flat slab bridge design.
Deconstruction and Documentation
Bridge No. 92297 was documented to Minnesota
Historic Property Record (MHPR) standards. MHPR is a modified version of
the national standard Historic American Engineering Record (HAER)
program. The HAER program documents nationally significant historic
mechanical and engineering structures and sites; the extensive
collection is digitized and available to the public on the Library of
Congress website (www.loc.gov/pictures/collection/hh/). Both programs
maintain documentation of historic resources, and have a target archival
life of 500 years. The MHPR materials for Bridge No. 92297 included a
report with a written description, large format photographs, and
measured drawings of selected areas of the bridge highlighting its
design and construction.
Deconstructing and documenting a historic bridge
requires time, care and coordination that is not required with standard
demolition and removal (Figure 4). Determining the configuration of
reinforcement for comparison to the original construction drawings
required investigative openings in areas that would expose
representative samples of reinforcement in the bridge deck, abutments
and footings. Maintaining stability of the bridge to allow for safe
access after its partial demolition, as well as to expose sections of
the abutments and footings, required an extensive amount of earthwork.
A two-stage demolition process accommodated the
documentation process. Backhoes equipped with hydraulic jackhammers
removed concrete in selected areas of the bridge to expose
reinforcement. Fill placed below the bridge stabilized the abutment
walls during the exposure and removal of the deck. Two full-depth
openings in the bridge deck -- one near the middle, and another along
the edge and the adjoining transition into the top of the abutment --
facilitated its documentation before complete demolition. Next came
excavating soil on both sides of the abutment to the top of the footing,
then removing concrete from the selected area to expose the underlying
reinforcement. The investigation team took measurements and photographs
all along the way.
This investigative process was hampered by poor
accessibility of the machinery, especially after demolition of the
bridge began to compromise its ability to support heavy loads. There
were several equipment breakdowns, and the existing concrete was
stronger than expected in some locations. These issues created
unforeseen delays that impacted the demolition schedule. Despite the
slower than expected progress of the work, careful operation resulted in
exposure of the majority of the reinforcement with minimal changes to
its as-built configuration. The destructive nature of the work resulted
in some deformation or breakage of the reinforcement being recorded. In
these cases, carefully exposing adjacent sections made it possible to
document the typical configuration of reinforcement as originally
placed.
The plan and profile of reinforcement was
generally congruent with the original construction drawings from 1912,
with the exception of minor details and extra reinforcement along the
fillet corner in the deck-to-abutment transition. The skewed geometry of
Bridge No. 92297 was not well-suited to Turner’s patented short-span
bridge design, but the two layers of slab reinforcement in the bridge
were similar to the configuration of diagonal reinforcement in Turner’s
patent. One layer of slab reinforcement was placed parallel to the span
of the bridge, and the other layer was placed perpendicular to the
abutment walls. Some transverse reinforcement was present, which
correlated with the patent, but it was so widely spaced -- over five
feet on center -- that its intended purpose was likely just to support
the draped geometry of the two primary layers of slab reinforcement. The
profile of the slab and abutment reinforcement correlated closely with
the design illustrated in Turner’s patent. Because of the geometry of
the bridge span, the flat slab of Bridge No. 92297 more closely
resembled a one-way structural system, rather than the four-way systems
found in Turner’s published designs.
Considering its age, Bridge No. 92297 was in
remarkably good structural condition and continued to perform as
intended by carrying heavy vehicular traffic even into the start of
demolition. Despite the somewhat deteriorated condition of the bridge,
including concrete spalling and substantial graffiti, its continued use
had demonstrated that the early design was not only adequate for the
streetcar loads at the time of construction, but also remained suited
for the loading demands imposed by modern traffic.
Conclusion
Researching the history of engineering has unique
and persistent challenges: structural details are concealed, drawings
are often not available, and the field is relatively new compared to the
more established scholarship of architectural history. However,
programs such as the MHPR and HAER provide a framework for expanding
this field of study. When demolition of a resource is unavoidable,
documentation can partially mitigate its loss by recording and allowing
for the future study of its features. Understanding the history of a
profession can provide a valuable perspective on how its common
practices and philosophy have evolved.
Likewise, engineers seeking to preserve or
rehabilitate existing structures can benefit from studying previously
documented and demolished examples for the insights that they provide
into design and construction. Bridge No. 92297 offered a unique
opportunity to document the details of the steel reinforcement in a
historic reinforced concrete structure, a task that is -- for obvious
reasons -- generally infeasible for such structures that are to remain
intact.▪
References
Dario A. Gasparini, Contributions of C.A.P.
Turner to Development of Reinforced Concrete Flat labs 1905-1909, J.
Structural Engineering 128 (October 2002).
C.A.P. Turner, The Mushroom System as Applied to Bridges, Cement Age X (January 1910).
Dario A. Gasparini and William Vermes, C.A.P.
Turner and Reinforced Concrete Flat Slab Bridges, Proceedings of the 7th
Historic Bridges Conference, September 19-22, 2001, Cleveland, Ohio.
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Deconstructing Bridge 92297
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