Deconstructing Bridge No. 92297
This blog post is a condensed and edited version of an article that appeared in the January 2014 issue of STRUCTURE magazine, published by the National Council of Structural Engineers Associations (NCSEA), and is adapted with permission. The original article can be downloaded here.
Documentation during deconstruction is a rare opportunity that can be prompted by a routine set of circumstances. This was the case with Bridge No. 92297 in St. Paul, which was demolished in the summer of 2013 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. The FHWA provided federal dollars, which triggered the process known as a "Section 106 review." The team conducting the sequenced research, documentation and demolition included Summit Envirosolutions, Preservation Design Works (PVN), Daniel Pratt (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 unremarkable. However, research on the bridge, particularly by Andrew Schmidt and Andrea Kampinen of Summit, revealed that it was an innovative design for its time. Documentation of the bridge 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 and a pioneer in the development of the reinforced concrete flat slab, designed bridge No. 92297. Turner is generally, but with some debate, credited with the development of the reinforced concrete flat slab floor system. His proprietary system, called the “Mushroom System,” can be identified by its pattern of four-way slab reinforcement and distinctive flared column capitols. The system was first used in Minneapolis in 1906, but was ultimately exported and implemented nationally and globally.
In addition to implementing his system in buildings, Turner designed several reinforced concrete flat slab bridges, most as adaptations of his 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. But, Turner’s published examples of flat slab bridges did not bear much resemblance to Bridge No. 92297. 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 (lower image in Figure 2).
Bridge No. 92297 was commissioned by the Minneapolis, St. Paul and Sault Ste. Marie Railroad (Soo Line), and 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. 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.
Deconstruction and Documentation
A phased demolition proposed by MnDOT engineers accommodated the documentation process (Figure 4). Reinforcement was revealed at the bridge deck, bridge-to-abutment connection, and the abutment walls. The location (plan and vertical profile) of steel 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 four-way reinforced short-span bridge design (upper image in Figure 2). Rather, the two layers of slab reinforcement responded to the geometry of the road:
- a lower layer of 5/8-in. nominal diameter deformed reinforcement was placed perpendicular to the abutment walls at approximately four inches on center;
- a layer of smooth 5/8-in. nominal diameter reinforcement oriented parallel to the deck edge at approximately four inches on center was placed on top of the lower layer;
- approximately 1/16‐in. diameter wire ties connected the bars of the two layers;
- the two layers were draped similar to Figure 2, with approximately a 12-inch drop from the abutment to the center of the 41-ft. clear span;
- pairs of 1-1/8 in.-diameter smooth bars at 5 ft-6 in. on center provided support for the draped profile and were oriented parallel to the abutment walls.
The profile of the slab and abutment reinforcement correlated well with the design illustrated in Turner’s patent:
- both layers of deck reinforcement hooked into and extended approximately 7 ft. into the abutment wall;
- the vertical abutment steel consisted of 1-inch nominal diameter deformed reinforcement at 24 inches on center;
- the vertical abutment steel followed an S-shaped profile from the deck slab to the back of the abutment wall curving to the exposed face of the abutment wall and ultimately terminating in the footing;
- the abutment steel profile was supported by 3/4 in. square horizontal bars;
- an additional layer of layer of 5/8 in. nominal diameter deformed bars spaced approximately 4 in. on center followed the curvature of the filleted corner and extended about 6 feet into the deck slab. The fillet steel was neither shown in the patent drawing nor the original design documents.
Observed and measured details of the reinforcing included placement and location, diameter, cover, spacing, support method, and lap length of the steel. 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.
Prior to the start of demolition, the condition of the bridge was also documented. As is often the case, graffiti covered most of the walls and significant spalling and efflorescence were observed on the underside of the deck. However, despite decades of use, as well as exposure to deicing salts and weather, the measured vertical deflections of the slab were within the range of likely construction tolerances and there was no appreciable long-term movement. The laudable performance of the bridge indicated 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. Because Bridge No. 92297’s construction and condition were documented prior to demolition, engineers and scholars have the opportunity to continue learning from this historic structure.