Overview:
This 1874 treatise on modern American bridge building was written by G.B.N. Tower, a civil and mechanical engineer, for the purpose of instructing young engineers on the subject. The book covers the basic principles of bridge building, including the different types of strains that bridge components are subjected to, the strengths of various materials, and the methods for calculating the required dimensions of those components.
The book provides detailed explanations and calculations for several common types of bridge structures, including single beam bridges, Howe trusses, Pratt trusses, and arch-brace trusses. It also covers the use of iron in bridge construction, but focuses primarily on wooden structures. The book offers practical advice on building and maintaining bridges, including instructions on constructing falsework, splices, and diagonal bracing, as well as providing tables of dimensions and strengths for different bridge components.
Key Findings:
- The book focuses on wooden bridge construction, providing valuable insight into the building practices of the 19th century.
- It highlights the importance of understanding the different strains on bridge components (tension, compression, and detrusion) and using appropriate materials and dimensions.
- It emphasizes the use of specific formulas and tables for calculating the dimensions and strengths of bridge components.
- The book provides practical examples of different bridge designs and their applications, including Howe trusses, Pratt trusses, and arch-brace trusses.
Learning:
- Understanding bridge design and construction: The reader learns about the fundamental principles of bridge building, including the types of strains on components, the importance of structural integrity, and how to calculate the strength of different materials.
- Different truss designs: The reader learns about the unique characteristics and applications of different types of trusses, such as Howe, Pratt, and arch-brace trusses.
- Calculation methods and formulas: The reader gains practical knowledge on how to apply formulas and tables for determining the appropriate dimensions of bridge components based on specific loads and strains.
- Historical context of bridge building: The reader gains insight into the evolution of bridge design and construction techniques in the 19th century, including the challenges faced by engineers at the time and the common materials and practices used.
Historical Context:
The book was written in 1874, during a period of rapid industrialization and expansion in the United States. This period witnessed the rapid development of railroads and the need for efficient and durable bridges. The book reflects the state of bridge building technology at the time, including the growing adoption of iron in bridge construction.
Facts:
- The simplest bridge is a single beam: This type of bridge is only suitable for spans of 20 feet or less due to the rapid increase in required sectional dimensions with larger spans.
- Wood and iron are the primary materials for bridge construction: The book focuses on wooden bridges, but also touches on the use of iron, particularly for columns and girders.
- Different materials have different strengths: The book provides specific strengths for wood and iron under tension, compression, and detrusion, which are key factors for determining the appropriate materials and dimensions for specific bridge components.
- Transverse strains are critical for beam design: The book highlights the importance of considering the load distribution on beams and how these loads impact the bending and shearing forces within the beam.
- The neutral axis is crucial in understanding beam behavior: This axis represents the point within a beam where there is neither tension nor compression, and its position is determined by the material’s relative resistance to compression and extension.
- The factor of safety is vital for ensuring bridge stability: This factor represents the ratio of the ultimate strength of a material to its working strength, and engineers use it to ensure that the bridge can withstand unforeseen loads and stresses.
- Lateral adhesion is key for secure joints: This force prevents timbers from sliding past each other at joints and is essential for maintaining the integrity of the structure.
- The length of a beam affects its compressive strength: Beams under compression are more susceptible to bending with increasing length, necessitating larger dimensions to compensate for this.
- Cast iron beams are more efficient with an inverted T-shape: This design optimizes the material distribution, enhancing the beam’s resistance to transverse strains.
- Hodgkinson’s girder is a prime example of an efficient cast iron beam: It utilizes a small top flange and a larger bottom flange to maximize the beam’s strength.
- The rise of a truss should be around 1/8 of the span: This ratio balances vertical and horizontal forces, ensuring optimal structural stability.
- The weight of the bridge itself is crucial for long spans: In larger bridges, the self-weight becomes a significant factor in the overall load and should be accounted for in calculations.
- The Howe truss is a reliable design for spans under 150 feet: It utilizes diagonal braces for compression and vertical rods for tension, offering a robust and adaptable design.
- The Pratt truss utilizes diagonal rods for tension and vertical posts for compression: This design allows for easier adjustments and offers a more compact structure compared to the Howe truss.
- The arch-brace truss transfers the load directly to the abutments: This design eliminates the need for multiple web members, making it potentially more efficient and stable for long spans.
- Camber is essential for preventing bridge sag: By slightly curving the chords, engineers ensure that the bridge maintains its intended shape and avoids excessive deflection under load.
- Diagonal bracing is crucial for stiffening bridges: This bracing system connects the trusses together, enhancing stability and reducing vibrations caused by moving loads.
- Pile bridges are ideal for crossing marshy or shallow areas: These bridges utilize piles driven into the ground to support the structure, offering a cost-effective solution for unstable terrain.
- Trestle work provides temporary or permanent support: These structures consist of posts, caps, and braces, offering flexibility for various applications.
- Iron bridges are gaining popularity but wooden bridges remain relevant: The book acknowledges the growing use of iron but recognizes that wood is still a viable option for many bridge construction projects.
Statistics:
- ⅓ ton per foot run: This is the estimated weight of a single track wooden truss bridge with a span of 120 feet, including floor timbers and bracing.
- 1.33 tons per foot run: This is the estimated maximum load on a single track wooden truss bridge with a span of 120 feet, assuming it’s solely loaded with locomotives.
- 10 tons per panel: This is the estimated load per panel of a single truss bridge with a span of 120 feet and 12 panels, considering both the weight of the bridge and the maximum load.
- 240 tons: This is the estimated total weight of a bridge and its load for a 120-foot span, assuming a load of 1.33 tons per foot run.
- 268800 lbs: This is the estimated tension on one lower chord of a 120-foot bridge, assuming a total weight of 240 tons.
- 134.4 square inches: This is the minimum area required for one lower chord to safely withstand a tension of 268800 lbs, assuming a working strength of 2000 lbs per square inch for wood.
- 288 square inches: This is the actual area of a lower chord composed of three 8″ x 12″ timbers, providing ample strength for both tension and transverse strains.
- 160 square inches: This is the effective area of the lower chord after accounting for allowances for splicing, foot blocks, bolts, and washers.
- 268.8 square inches: This is the minimum area required for one upper chord to safely withstand a compression of 268800 lbs, assuming a working strength of 1000 lbs per square inch for wood.
- 4.48 square inches: This is the area required for a single tie rod to safely withstand a strain of 67200 lbs, assuming a working strength of 15,000 lbs per square inch for wrought iron.
- 2000 lbs per lineal foot: This is the estimated maximum load per lineal foot of a bridge for moving loads, which is used to calculate the strains on counter braces.
- 2,500 lbs at the centre: This is the load supported by a 150-foot arch-brace truss model, demonstrating the strength of this design.
- 150 lbs: This is the movable load used to test the 150-foot arch-brace truss model, highlighting the truss’s stability under dynamic conditions.
- 45000 lbs: This is the weight supported by each set of braces in the 150-foot arch-brace truss model, calculated based on a load of 3000 lbs per foot run.
- 5625 lbs: This is the weight supported by each stick of the brace in the 150-foot arch-brace truss model, assuming 4 sticks per brace.
- 7031 lbs: This is the compressional strain on the first brace of the 150-foot arch-brace truss model, accounting for its inclination.
- 21937 lbs: This is the compressional strain on the last brace of the 150-foot arch-brace truss model, accounting for its inclination.
- 120 lbs per square foot: This is the maximum load that a highway bridge can safely withstand, based on the area of the bridge deck.
- 0.266 tons per foot run: This is the estimated weight of a single track wooden truss bridge with a span of 25 feet.
- 0.792 tons per foot run: This is the estimated weight of a single track wooden truss bridge with a span of 200 feet.
Terms:
- Tension: The force that pulls or stretches a material.
- Compression: The force that pushes or squeezes a material.
- Detrusion: The force that shears or crushes a material against a fixed point.
- Neutral Axis: The point within a beam where there is neither tension nor compression.
- Factor of Safety: The ratio of the ultimate strength of a material to its working strength.
- Lateral Adhesion: The resistance offered by the fibers of a material to sliding past each other.
- Camber: The slight upward curvature of a bridge’s chords, designed to prevent sag.
- Truss: A structural frame made of interconnected members that support loads through a combination of tension and compression.
- Panel: A section of a truss between two adjacent vertical members.
- Diagonal Bracing: A system of diagonal members used to stiffen bridges and reduce vibrations.
Examples:
- Single Beam Bridge: A simple bridge with a single beam spanning the opening between abutments, suitable for spans up to 20 feet.
- Inclined Beam Bridge: A bridge with two inclined braces supporting a single beam, suitable for spans up to 50 feet.
- Howe Truss: A bridge with diagonal braces for compression and vertical rods for tension, widely used for spans under 150 feet.
- Pratt Truss: A bridge with diagonal rods for tension and vertical posts for compression, allowing for adjustments and offering a more compact structure.
- Arch-Brace Truss: A bridge with braces extending directly from the abutments to the head of each vertical, eliminating the need for multiple web members.
- Pile Bridge: A bridge supported by piles driven into the ground, ideal for marshy or shallow areas.
- Trestle Work: A structure of posts, caps, and braces used for temporary or permanent support.
- South Royalton Bridge: A Howe truss bridge on the Vermont Central R.R., showcasing the practical application of the design.
- White River Bridge: A Howe truss bridge on the Passumpsic R.R., strengthened by an arch, highlighting the use of arches for reinforcement.
- Cheshire Bridge: A Howe truss bridge with a span of 175 feet, further illustrating the design’s adaptability for longer spans.
Conclusion:
This 1874 treatise offers a comprehensive and practical guide to building wooden bridges, providing invaluable insights into the design principles, calculation methods, and construction techniques used in the 19th century. It emphasizes the importance of understanding the different strains on bridge components, choosing appropriate materials, and calculating the required dimensions for stability and strength. By exploring various truss designs, including Howe, Pratt, and arch-brace trusses, the book provides a thorough introduction to the world of wooden bridge building, leaving a lasting legacy of knowledge and innovation for future generations of engineers.