Category: Tension and compression in truss bridges

Partial Design Process These resources engage students in some of the steps in the engineering design process, but do not have them complete the full process. While some of these resources may focus heavily on the brainstorm and design steps, others may emphasize the testing and analysis phases. Although no charge or fee is required for using TeachEngineering curricular materials in your classroom, the lessons and activities often require material supplies.

The expendable cost is the estimated cost of supplies needed for each group of students involved in the activity. Most curricular materials in TeachEngineering are hierarchically organized; i. Some activities or lessons, however, were developed to stand alone, and hence, they might not conform to this strict hierarchy. Related Curriculum shows how the document you are currently viewing fits into this hierarchy of curricular materials.

Students model different bridge types such as a cable-stay bridge. Using the countless design possibilities of beam, truss, arch and suspension bridges, civil and structural engineers create the bridges that are essential to the infrastructure of our world. To design bridges of any type, engineers must understand the forces that act on every bridge: compression and tension, and then design bridges to handle these forces without breaking or failing.

Teams of engineers decide on the bridge type, design and materials to best distribute the load across an obstacle, and draw detailed design plans, specifying materials, measurements, shapes and angles for construction of the bridge.

Each TeachEngineering lesson or activity is correlated to one or more K science, technology, engineering or math STEM educational standards. In the ASN, standards are hierarchically structured: first by source; e. Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem. Grades 6 - 8. Do you agree with this alignment? Thanks for your feedback! Alignment agreement: Thanks for your feedback! View aligned curriculum.

Students are presented with a brief history of bridges as they learn about the three main bridge types: beam, arch and suspension. They are introduced to two natural forces — tension and compression — common to all bridges and structures. Working as engineering teams, students design and create model beam bridges using plastic drinking straws and tape as their construction materials. Their goal is to build the strongest bridge with a truss pattern of their own design, while meeting the design criteria and constraints.

Students learn about the variety of materials used by engineers in the design and construction of modern bridges. They also find out about the material properties important to bridge construction and consider the advantages and disadvantages of steel and concrete as common bridge-building materials Students learn about the types of possible loads, how to calculate ultimate load combinations, and investigate the different sizes for the beams girders and columns piers of simple bridge design.

Additionally, they learn the steps that engineers use to design bridges. Optional: Provide each student with a copy of the Bridge Notes Worksheet to fill-in what they know about bridges before the activity and take notes on during the introduction portion of the activity. What impacts do bridges have on our communities and cities?Truss bridgebridge with its load-bearing structures composed of a series of wooden or metal triangles, known as trusses. Given that a triangle cannot be distorted by stress, a truss gives a stable form capable of supporting considerable external loads over a large span.

Trusses are popular for bridge building because they use a relatively small amount of material for the amount of weight they can support.

They commonly are used in covered bridges, railroad bridges, and military bridges. The individual pieces of a truss bridge intersect at truss joints, or panel points. The connected pieces forming the top and bottom of the truss are referred to respectively as the top and bottom chords. The sloping and vertical pieces connecting the chords are collectively referred to as the web of the truss.

The component parts of a truss bridge are stressed primarily in axial tension or compression. A single-span truss bridge is like a simply supported beam because it carries vertical loads by bending. Bending leads to compression, in the top chords or horizontal memberstension in the bottom chords, and either tension or compression in the vertical and diagonal members, depending on their orientation.

tension and compression in truss bridges

Early trusses were built without precise knowledge of how the loads are carried by each part of the truss. The first engineer to analyze correctly the stresses in a truss was Squire Whipplean American who designed hundreds of small truss bridges and published his theories in Understanding precisely how loads were carried led to a reduction in materials, which by then were shifting from wood and stone to iron and steel. Several notable covered bridgeswhich are enclosed truss bridges, were constructed in Switzerland.

The Kappel Bridge of Luzern has been decorated since with paintings in the triangular spaces between the roof and the crossbeams, depicting the history of the town and the lives of its two patron saints. In the 18th century, designs with timber trusses reached new span lengths. In a Swiss builder, Hans Grubenmannused trusses to support a covered timber bridge with spans of 51 and 58 metres and feet over the Rhine at Schaffhausen.

He and his brother also built a notable arch-truss bridge over the Limmat River in Baden with a clear span of 61 metres feet. In North America the covered truss bridges underwent further evolution. From simple king-post trusses, in which the roadway was supported by a pair of heavy timber triangles, American carpenters in the 18th and 19th centuries developed bridges combining simplicity of construction with their other economic advantages. The first long covered bridge in America, with a metre foot centre span, was built by Timothy Palmera Massachusetts millwright, over the Schuylkill River at Philadelphia in Another highly successful type was designed by Theodore Burrof Torrington, Connecticut, combining a Palladio truss with an arch.

Numerous Town and Burr designs remained standing throughout North America into the early 21st century, some dating back to the early 19th century.What allows an arch bridge to span greater distances than a beam bridge, or a suspension bridge to stretch over a distance seven times that of an arch bridge? The answer lies in how each bridge type deals with the important forces of compression and tension.

Tension : What happens to a rope during a game of tug-of-war? Correct, it undergoes tension from the two sweaty opposing teams pulling on it. This force also acts on bridge structures, resulting in tensional stress. Compression : What happens when you push down on a spring and collapse it? That's right, you compress it, and by squishing it, you shorten its length.

Compressional stress, therefore, is the opposite of tensional stress.

Truss bridge

Compression and tension are present in all bridges, and as illustrated, they are both capable of damaging part of the bridge as varying load weights and other forces act on the structure. It's the job of the bridge design to handle these forces without buckling or snapping.

Buckling occurs when compression overcomes an object's ability to endure that force. Snapping is what happens when tension surpasses an object's ability to handle the lengthening force.

The best way to deal with these powerful forces is to either dissipate them or transfer them. With dissipation, the design allows the force to be spread out evenly over a greater area, so that no one spot bears the concentrated brunt of it.

It's the difference in, say, eating one chocolate cupcake every day for a week and eating seven cupcakes in a single afternoon. In transferring force, a design moves stress from an area of weakness to an area of strength. As we'll dig into on the upcoming pages, different bridges prefer to handle these stressors in different ways. Prev NEXT.The truss bridges include different types of bridges that were mainly constructed for railroads in the modern era.

Construction of this kind of bridge is based on smart use of compression and tension. Truss bridges are the oldest known bridges of the modern era. Due to their unique structure — in which efficient use of materials is made, truss bridges are considered to be economical from the point of construction. The triangular pattern used in the construction of truss bridges offers the necessary strength for setting up the beams.

A truss is a structure composed of triangular units connected at joints called the nodes. Triangular units which form a truss are slender and straight. Components of a space frame truss attain a 3-dimensional form. Planar truss, on the other hand has a 2-dimensional design. Truss is a structure which is commonly used in the construction of bridges.

It is assumed that truss bridges are designed with the help of pin joints. Pin joints are points where the straight components of the truss structure meet. The above assumption leads to the conclusion that only the forces of compression and tension would act on the components of a truss structure.

Design of a truss bridge consists of vertical, lower horizontal and diagonal elements. Vertical members of the truss bridge face tensile stress while lower horizontal ones are under a stress that results from bending, tension and shear stress. Diagonal members which run outwards are under compression stress while the inner diagonals face tensile stress. There are many types of truss bridges. Some of these designs prove to be advantageous if wood is used in the construction; it is especially useful for protecting the components or truss members that face compression.

Tension And Compression In Truss Bridges

There are many designs used for constructing truss bridges. Designs are chosen in accordance with specific needs of the topography of the region. Information pertaining to different truss bridge designs is presented one-by-one. The Howe truss is made up of diagonal and vertical members. Diagonal members of this kind of bridge slope towards the center in an upward manner. The bridge is named after William Howe, who designed it in The design of Pratt truss is similar to that of Howe truss.

Diagonal members of the Pratt truss slope downwards and away from the center. The Pratt truss was invented by Caleb and Thomas Pratt in The maximum length of a Pratt truss bridge can be feet; this bridge is generally used for railroads. The Baltimore truss bridge is a variant of Pratt truss Bridge. The difference between Pratt truss and Baltimore truss is that the latter is strengthened by providing additional support in the lower section.

Additional support is provided in the form of vertical and diagonal sections which have half the length of main sections of the bridge.

It can be observed in the above diagram. The Pennsylvania or Petit truss bridge, just like Baltimore truss, is a variant of Pratt truss.A Warren truss or equilateral truss [1] is a type of engineering truss employing a weight-saving design based upon equilateral triangles.

What is a Truss Bridge?

The Warren truss consists of longitudinal members joined only by angled cross-members, forming alternately inverted Equilateral triangle -shaped spaces along its length.

This gives a pure truss: each individual strutbeam, or tie is only subject to tension or compression forces, there are no bending or torsional forces on them. Loads on the diagonals alternate between compression and tension approaching the centrewith no vertical elements, while elements near the centre must support both tension and compression in response to live loads. This configuration combines strength with economy of materials and can therefore be relatively light. The girders being of equal length, it is ideal for use in prefabricated modular bridges.

It is an improvement over the Neville truss in which the elements form isosceles triangles. A variant of the Warren truss has additional vertical members within the triangles. The Warren truss is also a prominent structural feature in hundreds of hastily constructed aircraft hangars in WW2.

In the early parts of the war, the British and Canadian government formed an agreement known as the British Commonwealth Air Training Plan which used newly constructed airbases in Canada to train aircrew needed to sustain emerging air forces.

Hundreds of airfields, aprons, taxiways and ground installations were constructed all across Canada. Two characteristic features were a triangle runway layout and hangars built from virgin British Columbia timbers with Warren truss configuration roofs. Warren truss construction has also been used in airframe design and construction, for substantial numbers of aircraft designs.

An early use was for the interplane wing struts on some biplanes. The Warren truss is also sometimes used for fuselage frames, such as in the Piper J-3 Cub. From Wikipedia, the free encyclopedia. For the Australian politician, see Warren Truss.

tension and compression in truss bridges

July Garrett's Bridges. Brunel in South Wales. III: Links with Leviathans.A truss is a series of individual members, acting in tension or compression and performing together as a unit. On truss bridges, a tension member is subject to forces that pull outward at its ends.

Mechanical Engineering: Trusses, Bridges & Other Structures (27 of 34) Tension vs Compression 1

Even on a "wooden" truss bridge, these members are often individual metal pieces such as bars or rods. Compressive forces push or compress together and are heavier. The individual members form a triangular pattern. One bridge historian describes a truss bridge in this manner: "A truss is simply an interconnected framework of beams that holds something up. The beams are usually arranged in a repeated triangular pattern, since a triangle cannot be distorted by stress. In a truss bridge, two long - usually straight members known as chords - form the top and bottom; they are connected by a web of vertical posts and diagonals.

The bridge is supported at the ends by abutments and sometimes in the middle by piers. A properly designed and built truss will distribute stresses throughout its structure, allowing the bridge to safely support its own weight, the weight of vehicles crossing it, and wind loads.

The truss does not support the roadway from above, like a suspension bridge, or from below, like an arch bridge; rather, it makes the roadway stiffer and stronger, helping it hold together against the various loads it encounters. The pattern formed by the members combined with the stress distribution tension and compression creates a specific truss type, such as a Warren or Pratt. Most truss types bear the name of the person s who developed the pattern, such as the Pratt truss that is named for Caleb and Thomas Pratt who patented it in For instance, the configuration or pattern of a Pratt and Howe truss appears identical a series of rectangles with X'sbut a Howe's diagonals are in compression and the verticals in tension.

In a Pratt, the reverse is true. In theory, a truss bridge contained no redundant members. Builders considered each member or element essential to the functioning of the truss, although some were more important than others were.

While most trusses could sustain considerable damage and lose the support of some members without collapsing, severe traffic damage to a member could result in the collapse of the bridge. Kingpost Builders first developed the Kingpost as the most basic and earliest truss type. The outline consisted of two diagonals in compression and a bottom chord in tension that together formed a triangular shape.

A vertical tension rod called a Kingpost and thus the origin of the truss name divided the triangle in half. After the midth century, builders used metal not wood for tension rods. Builders typically used the Kingpost truss for shorter spans, up to about 35 feet. Queenpost The Queenpost, another early and basic truss type, is a variation of the Kingpost truss.

A Queenpost truss contains two vertical members rather than the one in a Kingpost. These vertical members require the use of a top chord to connect them. This arrangement forms a three panel span in which the center rectangular area may or may not have crossed diagonals.

Again, the outer members act in compression and the vertical rods wood or metal act in tension. This truss type can support spans up to about 70 feet.

Howe Truss William Howe patented the Howe truss in End diagonals connect the top and bottom chords, and all wood members act in compression. Each panel has a diagonal timber compression member and a vertical metal tension member, a material that conducts tensile forces better than wood. The metal tension member eliminated a heavy wooden member and reduced the dead load weight, and builders could more easily join the screw and nut connections between iron and wood than between wood members.

Builders could use multiple panels to increase the length of the bridge, typically ranging between and feet. Go to TN. Print This Page. Go to Search. What is a Truss Bridge? Social Media facebook twitter youtube instagram flickr linkedin.A truss bridge is a bridge whose load-bearing superstructure is composed of a trussa structure of connected elements usually forming triangular units. The connected elements typically straight may be stressed from tensioncompressionor sometimes both in response to dynamic loads.

tension and compression in truss bridges

The basic types of truss bridges shown in this article have simple designs which could be easily analyzed by 19th and early 20th-century engineers. A truss bridge is economical to construct because it uses materials efficiently. The nature of a truss allows the analysis of its structure using a few assumptions and the application of Newton's laws of motion according to the branch of physics known as statics.

For purposes of analysis, trusses are assumed to be pin jointed where the straight components meet. This assumption means that members of the truss chords, verticals and diagonals will act only in tension or compression. A more complex analysis is required where rigid joints impose significant bending loads upon the elements, as in a Vierendeel truss. In the bridge illustrated in the infobox at the top, vertical members are in tension, lower horizontal members in tension, shearand bending, outer diagonal and top members are in compression, while the inner diagonals are in tension.

The central vertical member stabilizes the upper compression member, preventing it from buckling. If the top member is sufficiently stiff then this vertical element may be eliminated. If the lower chord a horizontal member of a truss is sufficiently resistant to bending and shear, the outer vertical elements may be eliminated, but with additional strength added to other members in compensation. The ability to distribute the forces in various ways has led to a large variety of truss bridge types.

Some types may be more advantageous when wood is employed for compression elements while other types may be easier to erect in particular site conditions, or when the balance between labor, machinery and material costs have certain favorable proportions. The inclusion of the elements shown is largely an engineering decision based upon economics, being a balance between the costs of raw materials, off-site fabrication, component transportation, on-site erection, the availability of machinery and the cost of labor.

In other cases the appearance of the structure may take on greater importance and so influence the design decisions beyond mere matters of economics. Modern materials such as prestressed concrete and fabrication methods, such as automated weldingand the changing price of steel relative to that of labor have significantly influenced the design of modern bridges. A pure truss can be represented as a pin-jointed structure, one where the only forces on the truss members are tension or compression, not bending.

This is used in the teaching of statics, by the building of model bridges from spaghetti. Spaghetti is brittle and although it can carry a modest tension force, it breaks easily if bent. A model spaghetti bridge thus demonstrates the use of a truss structure to produce a usefully strong complete structure from individually weak elements. Because wood was in abundance, early truss bridges would typically use carefully fitted timbers for members taking compression and iron rods for tension membersusually constructed as a covered bridge to protect the structure.

In a simple form of truss, Town's lattice trusswas patented, and had the advantage of requiring neither high labor skills nor much metal. Few iron truss bridges were built in the United States before Truss bridges became a common type of bridge built from the s through the s. Examples of these bridges still remain across the US, but their numbers are dropping rapidly, as they are demolished and replaced with new structures. As metal slowly started to replace timber, wrought iron bridges in the US started being built on a large scale in the s.

Bowstring truss bridges were a common truss design during this time, with their arched top chords. The bowstring truss design photo fell out of favor due to a lack of durability, and gave way to the Pratt truss design, which was stronger. Again, the bridge companies marketed their designs, with the Wrought Iron Bridge Company in the lead. As the s and s progressed, steel began to replace wrought iron as the preferred material.

Other truss designs were used during this time, including the camel-back. By the s, many states developed standard plan truss bridges, including steel Warren pony truss bridges. As the s and s progressed, some states, such as Pennsylvaniacontinued to build steel truss bridges, including massive steel through-truss bridges for long spans. Other states, such as Michiganused standard plan concrete girder and beam bridges, and only a limited number of truss bridges were built.