UP Boone River Bridge (Webster City, North)

Located on the north side of Webster City, this unusual truss bridge carries a former Chicago & North Western Railway line over the Boone River. The first bridge at this location consisted of a combination through truss bridge, with iron and timber members and covered by sheet metal. This bridge was approached by timber pile trestle spans, and set onto timber pile piers. During the early years of the 20th Century, the C&NW invested significant capital into this line, replacing timber bridges with heavier stone and steel structures. Between 1901 and 1902, two new steel truss spans on stone piers were constructed to replace the wooden truss spans. Timber pile trestle spans were retained on the south end, possibly to allow approach embankments to settle. Between 1909 and 1917, the C&NW made a second round of improvements along this line, replacing most of the remaining timber bridges. The most recent alteration to the bridge came in 1914, when the south approach trestle was replaced with a steel deck girder span and a concrete abutment. Currently, the bridge consists of two 120-foot, riveted quadrangular lattice through truss spans, set onto stone piers and a stone north abutment. The south approach consists of an 80-foot deck plate girder span, set onto a concrete abutment. Stone for the piers and north abutment consists of a yellow limestone, likely quarried at Mankato, Minnesota. An unknown contractor fabricated the truss spans, while another unknown contractor fabricated the approach span. Two separate unknown contractors also constructed the piers and abutments. It is possible that the stone substructures were constructed by the Widell Company of Mankato, Minnesota; which reportedly constructed most of the stone bridges and culverts for the C&NW in Iowa between 1901 and 1902.

The truss spans follow a somewhat unusual design, with heavily laced members and an unusual M-frame portal bracing. The top chords use a built-up design, with V-lacing on both sides and thin plates on either side of the connections. The endposts are also composed of built-up beams, with X-lacing on the front and rear and thin plates at strategic points. The bottom chords are constructed of two U-shaped channels, which are joined by V-lacing. The compression members are constructed of built-up tightly V-laced beams, while the tension members are composed of L-shaped bars connected by thin plates. The floor system follows a standard design, with two shallow plate girder stringers and plate girder floorbeams spaced at panel points. The portal bracing follows an M-frame design, which is constructed using V-laced beams. V-laced beams are also used for the sway bracing. T-shaped steel bars are used for both the upper lateral and lower lateral bracing. The deck plate girder span follows a standard design, with two heavy plate girders and an open deck. Both piers use an identical design, with a diamond shaped structure founded on timber piles. The north abutment uses stepped wing walls, which extend from the bridge at an angle. The south abutment uses sloped wing walls, which also extend from the bridge at an angle.

The quadrangular lattice through truss design is relatively uncommon throughout the United States. However, a few railroads preferred the design, such as the Chicago, Rock Island & Pacific Railway (Rock Island), the Chicago & North Western Railway (C&NW) and the Chicago, St. Paul, Minneapolis & Omaha Railway (Omaha Road). Spans constructed in the late 1870s and early 1880s for the C&NW featured an arched and pedimented lattice portal bracing, light sway bracing and laced members. The second generation was nearly exclusively constructed by Lassig Bridge & Iron Works between 1884 and 1900, and featured heavier members and a decorative pedimented portal bracing. While the design fell out of favor for the Rock Island and Omaha Road around the turn of the 20th Century, the design remained popular with the C&NW into the 1920s. 20th Century versions of this design used significantly heavier members, were taller and contained no decorative features. 19th Century versions of this design were primarily constructed out of wrought iron, while 20th Century versions of this design were constructed of steel. The particular sub-design used here is unusual for the C&NW, and was only used at a handful of locations during 1901-02. The unusual design could possibly indicate that this span was fabricated by a company other than American Bridge Company. The C&NW preferred lattice truss designs, as they were both strong and demonstrated great resilience in case of a derailment. Since the 1914 reconstruction, the bridge has seen no significant alterations, and remains in use. Overall, the bridge appears to be in fair to good condition, with no significant deterioration noted. The author has ranked this bridge as being moderately significant, due to the truss design.