Hi Structures Class:I have taken Amanda’s assignment as an example solution to the

terminology assignment. I have added some notes and some corrections to her assignment to give you all the best information

possible. My interventions are printed in bold Italic. I have included a few images as well.

Good luck!

Pieter Sijpkes

Assignment 2: Terminology in StructuresProfessor Sijpkes

Due on the 30th September, 2005

Student Name: Amanda SmithStudent Number: 260170386

1. Steel. (Steel is a iron alloy with very low carbon content, often with some other metals included such as chrome and vanadium; it is made on an industrial scale since 1856 , when Bessemer patented his “Bessemer” process. It revolutionized engineering and architecture by its high strength in both tension and compression, by its elasticityand by its relatively low price. Later on in the 19th century it made the invention of reinforced concrete possible- a composite material that also played a revolutionary role in engineering and architecture ) Steel is mainly made of iron which, in turn, makes it brittle, although it remains stronger and harder than pure iron. Unlike concrete which is better in compression, steel responds in the same way whether it be in compression of tension. This is why it is a widely used material in bridges, which, as we have seen, have components which are in tension and others in compression. In compression, the “plasticity” of steel augments, a property which engineers often take advantage of since this is especially useful in areas where earthquakes and other such events are frequent.

2. Wrought iron. (Wrought iron is an age old product, made since about 1500 BC in small quantities by mixing iron ore and charcoal. The temperatures reached with charcoal as fuel did not allow the iron to melt, it would just turn to a spongy mass that could be ‘wrought’ (hammered) and folded many times to squeeze out the slag. The product was relatively resistant to rust because of the inclusion of slag particles and it was tough because of the layering resulting from the folding and hammering) Wrought iron was the precursor to steel. It is a very pure form of iron with only very little carbon in it (the amount of carbon in iron determines its level of purity). It is very tough, malleable, and ductile and resists corrosion. Furthermore, these characteristics and the fact that the wrought iron melts at high temperatures make it all the more interesting for welding purposes. In the bridges visited, it was mainly used for rivets, bolts or other joints, however it is also possible to use as the main or core material for

the bridge. To sum up: wrought iron has a low compressive strength as well as a high tensile strength.

3. Cast Iron. (Cast iron came into use in England around 1750 when Abraham Darby found a way to make iron using coal instead of charcoal. (the charcoal industry had denuded most forests) The use of coal allowed higher temperatures to be achieved in a blast furnace, and the iron would become liquid in the process; it could thus be easily poured into molds. However the proces mixed in a high percentage of carbon into the iron, which rendered the iron very hard, but also very brittle. It also was resistant to rusting) Various types of cast iron exist, the most commonly used being grey cast iron. In general, cast iron is made up of “pig iron” along with various other remelted pieces of steel (which tend to affect the strength of the material). The fact that cast iron has a lower melting temperature than pure iron also makes it a more brittle type of iron. Just as wrought iron, cast iron is also resistant to corrosion. As well as this, the high thermal conductivity allows the material to “damp mechanical vibrations” (source: www.wikipedia.com). In the case of a bridge, the vibrations caused by heavy vehicles or trains would be lessened. This also allows for sound to be dampened. Its ability to concentrate stress is both a pro and a con: it allows to make repairs more easily (in the case of a bridge), yet reduces shock resistance, tensile strength (maximum amount of stress before it breaks)and the ability to weld it. To sum up in two words: high compressive strength and low tensile strength.4. Cast concrete. Cast concrete is made by combining cement, water and rock. If the structure that is cast is too big or the temperature varies then cracking can occur. Furthermore, concrete is much sturdier than concrete (I guess stone is meant here) and less expensive to make (since it is a combination of elements). Portland cement (versus asphalt cement for example) is used for bridges as we saw during the field trip. The water to cement ratio determines the strength and workability of the final concrete mixture (so too for the various percentage of the lime, alumina and silica contained in it that is, the aggregate). There is very little maintenance and it can be cast into any shape or size as well as concrete is very fire resistant and watertight. Concrete is a material which is very good in compression.

5. Pre-cast concrete. Pre cast concrete is often more desirable as a construction material as the number of environmental elements (temperature, humidity, craftsmanship) which might affect the concrete can be avoided. Furthermore, in the case of bridges, installation time and cost is decreased: the pre cast concrete can simply be put in place with a crane rather than have to be poured into a cast on site (which involves more labor and thus more expense to the builder). Unlike cast in place concrete, there is no need for reinforcing steel for installation. (the steel reinforcing is cast into the precast elements when they are being cast in the factory though..)

6. Pin joints. A pin joint allows the attached arms to move and rotate however, the force can only push or pull in the direction of the member. Unfortunately, the holes made in the materials in order to put in the pins weaken the materials. Pin joints are usually used for truss structures. (pin joints cannot transmit moments)

7. Rigid joints. Rigid joints do not allow (restrict) movement (unlike pin joints). They neither allow translation nor rotation (movements). They are usually used for the frame of the structure as they provide more stability in the structure. (Rigid joints deform and do transmit moments)

8. Rivet joints. Rivet joints are often used in bridges, however they are a costly way of putting bridges together because of the labor needed: the two holes used for the river must be drilled, aligned and then the rivet must be put in place, hammered in etc. Although the rivet makes a firm and fix hold, the great number needed is also costly (the number of rivets required to hold up a member is proportional to the amount of force or stress put onto it).

9. Bolt connection. The bolt connection is one of the most widely used fasteners in large structures. Unfortunately, when subjected to vibrations or impacts, the connections often loosen, or worse yet, fail, and can sometimes cause the collapse of the structure. One of the pro’s of this type of connection versus a welded connection or rivet connection is that it is reversible (which will be an issue in the destruction of the structure and the removal of the materials).

10. Welded connection. The types of weld connections depend on the type of load. In the case of a ‘lighter load”, a fillet weld is used. If the structure is heavier (for example, a bridge) a groove weld is used. In structures, the metals are joined by heating them and/or applying pressure to them.( I don’t know what the pressure is for) It is also optional but useful to add argon to the weld which increases drying time and prevents slag. (Welding ‘under Argon gas’ is sometimes used to keep the very hot welding bead from reacting with oxygen in the air. A better quality weld results) Unfortunately, welding is very energy consuming (I think that the advantages outweigh the energy cost by far..)(although various methods can be used they are all high energy methods: laser welding, flame welding, electric arc welding). Furthermore, welding can sometimes weaken the beams or the structure (an example of this would be welded heat treated steel which weakens after the welding, and can sometimes collapse).

11. Splice plate. (A splice plate is a simple method to fix two flat elements together by using a third flat element, the splice plate.(in sewing it is analogous to a patch) It is used in steel construction, but also in wood construction. The plate links the two parts together by being welded, bolted, riveted or glued) A splice plate is a plate which joins two girders, usually riveted or bolted together. Spliced bridges can span up to 250’ using four or more splices. An unspliced bridge is typically under 80’ long. According to Continental company’s design for splice plates: “the nuts are tack welded to the inside of the splice plates and the splice plates slide inside the adjoining tube, bolts are inserted and tightened accordingly”.

12. Hot rolled profile steel – this type of steel is made by using extreme forces to force a solid piece of steel into an I-beam, C-channel or other form. The steel is literally compressed and rolled by a mill.

13. I-beams. I-beams are used in large structures but not in wood structures where rectangular beams are used and are less costly. They are made of two flanges. In general, it is a beam or girder with an I shaped cross section. (We DO make wood I beams these days, using high quality glues):

14. C-channels. A Channel is a type of beam (in the shape of a C by profile). The internal stress is increased in the C channel versus

the T channel where it is decreased. (A crosssection of a typical Cchannel is shown below)

15. Angles (even and un-even flanges). Flanges are used in structures for strength. Even angle flanges ensure increased strength and are usually water resistant depending on the material used.

16. T-sections. T sections are a type of beam (in the shape of a T by profile or cross section). (the next two statements is not necessarily true:) The internal stress is decreased in the t-section unlike the C-Channel. Depending on the structure of the bridge, T-sections avoid overloading beams or members.

17. Composite sections. In composite sections there should be a shear transfer between the different materials (steel to concrete for example). In this case the cross sectional properties can be calculated. This shear transfer can occur through the connections, welds etc. of the structure. A composite section is built of several materials (an example of this was the concrete flat bridge with steel beams undersneath). In most cases, one material is good in tension whereas the other material is good in compression.Ex:

18. Diagonal bracing. Diagonal bracing is used in structures in order to prevent “racking” of the structure. It increases the strength of the structure and increases the lateral and overall stability of the structure.

19. Triangulation. Triangulation is most often used in bridges to make the structure more stable in all situations, that is to say, they are used to increase stability in the structure and avoid tipping, twisting or other likewise movements.

20. Overall stability (swing bridges and lift bridges)a. Swing bridges.

Concrete (good in compression)

Steel (good in Tension)

A swing bridge is a cantilever bright which “pivots in a horizontal plane” and opens a passage on each side of the central support (http://encyclopedia.laborlawtalk.com/Bridge#Swing_bridge)

b. Lift bridgeLift bridges cost less to build than lift bridges. The stability is assured by the

counterweights contained in the vertical lift. This type of bridge is especially used for heavy railroad use. However, there are disadvantages to this: the height restriction due to the fact that the deck is ABOVE the passageway.

(a good description of the difference between swing bridges and lift bridges! The issue of ‘overall stability was brought up because these bridges MOVE , and during the move they should not change their geometry because of twisting or sagging, because excessive deformation may lead to collapse )

21. Lateral stability (wind bracing). Lateral stability is usually ensured by using trusses and diagonal braces. The members are therefore subject to tensile and compressive forces. While the tension forces pull the member apart, the compression forces push the member together. Should the compression forces become too big, buckling can occur.

22. Compression members. Compression members are usually wider members in the structure. These members are usually subject to axial forces. Depending on the length of the member the result of a failure varies. In the case of a short member, a failure results in a crack (for brittle material) or expansion (ductile material). On the other hand, in long members, the compression member tends to buckle when subjected to too much force. Kneeling occurs in “intermediate’ size members. ( I don’t know what kneeling means)

23. Tension members. Tension members are most often used for lateral bracing and as trusses in bridges. In truss bridges, these tension members are subject to forces which pull outward at the ends of the member. The various tension members form a truss which in turn allows it to distribute the stress throughout the structure, thus allowing the bridge to carry its own weight. (tension members can be very slender, because they are not subject to buckling)

24. Reversible-load members – one of the main problems with reversible load members is that the pin joint at the end of each member “gives a little” when put in tension. If all the members are tensed, then the structure can “move” a few inches. This is an example of the group jumping on the bridge, which then moved. (No! Reversible load members are members that may be subjected to tensile OR compressive forces. For instance in a swing bridge, top members maybe subjected to tension and bottom members to compression when the bridge swings over the water, while when it is back in place on its original foundation the top members have to carry compression and the bottom members tension under loads crossing the bridge. These members thus have to be designed for both conditions. Reversible load conditions also occur when a roof structure is subject to strong winds: the roof normally weighs down on the structural members supporting it, but in cases of high wind the roof may actually tend to lift up and put reverse forces on the structure)

25. Cantilevers. A cantilever is a horizontal member which is free to move at one end but fixed at the other. Cantilevers are most often used in bridges in pairs. The pair

of cantilevers is in turn used to support a central truss. In general, the cantilever carries the load to a strong “mounting point” where the load is turned into torque. The purpose of a cantilever is to avoid having to use external bracing (you mean SCAFFOLDING) on long bridges or structures. Unlike suspension bridges, the strength of the bridge comes from the middle of the structure rather than at the end. In order to support the load, the cantilever is in tension in its upper beams whereas it is in compression in the lower beams. The tension is then distributed to the shore or the foundations.

The arrows point to the wrong places: Tension is on the upper edge, and compression on the lower edge: The arms are in tension, the sticks and upper bodies of the guys are in compression! Look below:

26. Shear. Shear is a force which acts across a beam or structural unit. This type of force causes the various parts of the structure to move in opposite directions. Shearing forces often lead to cracking (usually 45 degree cracking) in the structure. These cracks are often very costly to fix and is therefore preferable to avoid when engineering a bridge.

Tension (vertical)Compression (hor)

27. Bending. (Bending happens when a load is put on a beam, or when rigid joint is turned. Bending is the result of the MOMENT caused by these loads. The greater the loads, the greater the moments and the greater the stresses and deformations that result. Trusses are designed to avoid bending by having short members that are pinjointed, so that no moments can occur. In normal construction, say of a house, floor joists are always in bending, and lintels over windows are inevitably in bending) The risk of bending increases in the middle of beams. To avoid bending, arches are often used. By doing so, the horizontal forces occur in the bearings of the arch. Bending usually leads to a deformation of the steel of concrete depending on the bridge. This leads to unevenness in the bridge and in some cases in the bridge actually breaking. Of course, it is necessary to avoid bending at all costs (with the exception of swing bridges where bending is natural due to gravity) since failure of the bridge due to the bending involves building a new one therefore increasing the cost of the material and design. The risk of bending increases as the span of the bridge increases. The further the span between the vertical members upholding the structure, the more the possibility of bending increases in the horizontal member. Bending is due to the moments about the ends of the member.

28. Torsion. Torsion is defined as the strain produced by twisting. Torsion is the stress which causes one end of an object to twist while the other end is held still or is twisted in the opposite direction.

29. Buckling. Buckling usually occurs when the members of a bridge are subjected to great amounts of compression. In this case, there are transversal displacements in the bridge. In some cases the buckling is stable and the structure continues to sustain the loads it carries. Unfortunately, in most cases, the buckling becomes unstable and causes the collapse and failure of the bridge which can be both fatal and costly.Two examples of Buckling that are more

common are shown below: The cause of buckling failure is progressive deformation and catastrophic collapse of the structure due to forces becoming increasingly excentricin an instant .

y.

30. Corrosion of steel constructions. The corrosion of the various members in a steel structure vary but are usually associated to atmospheric conditions (humidity, salt etc). One of the main causes of corrosion in colder regions is the fact that salt is used to cover the roads over the bridges. In this case, the mixture of salt and water causes corrosion. Furthermore, snow, ice and other such weather conditions further the corrosion. To avoid corrosion, the structures are usually painted or coated with a protective coating, which is expensive as it involves maintenance. Today, a new material is slowly replacing steel: aluminum. This material is preferable as it does not corrode (it forms a layer of corrosion on the outside but does not continue to corrode) and is much lighter than steel.

31. Corrosion of precast prestressed beams. Corrosion occurs in precast concret beams when cracking develops and the inner steel strands are exposed to the exterior conditions which hasten the corrosion of the materials. This is very costly, maintenance is required and can be very costly. In some cases, this corrosion, if not controlled or repaired, can cause the beam (and thus the structure) to fail.

32. Camber. A “positive” or upward curve which is built to compensate for the vertical loads and which anticipates the future deflection of the structure. In construction, one usually stresses the cables in order to make the camber.

33. Steel-concrete-composite construction – in a steel-concrete composite construction, the concrete acts as the compression member whereas the steel acts as the tension member. This therefore creates the equilibrium necessary for the structure to maintain itself without swaying or even collapsing. (It is a very economical way of construction: the concrete top acts as the roadway, doing double duty)

34. Box beam construction. A beam made of metal plates which forms a structure in box shape. The actual shape of the box is a trapezoid so that the wind can pass below the structure (in the case of a bridge).

35. Orthotropic beam construction. In the case of orthotropic beam construction bridges, the bridge deck (usually made of steel plates) is supported by ribs underneath. In the case of the

bridges we visited, the concrete decks were supported by steel supports. The properties of members in one direction are different to the properties of members in the perpendicular direction. This helps avoid twisting or other unwanted distortions in the steel members. It is stiffer in the direction of the span than it is laterally.

36. Precast, pre tensioned beams. Unlike post tensioned beams, the steel rods/strands are stressed as the concrete is setting. Precast beams are useful in construction since they can easily be transported from the manufacturer to the site where a crane or other device can be used to put it in place. The bridge is basically prefabricated and put together.

37. Precast, post tensioned beams. Concrete beams which have been precast contain steel strands which are tightened after the concrete has hardened.

38. Cast in place post tensioned structure. Cast in place post tensioned structures do not offer the ease of precast structures. Whereas precast structures are made in factories and manufactured to with great precision and exactitude, the cast in place structures are often subject to outside phenomena’s whilst drying. An example of this would be acid rain, which could easily affect the way the concrete sets.(The post tensioning is achieved by including in the concrete carefully placed metal tubes into which, after the concrete has hardened, high strenght steel cables are inserted. These cables are then stretched by hydraulic jacks, and clamped into place so that they cannot slip back. The space left between the tube walls and the cables is then filled (injected) with mortar, (called grout), to fix the cables to the concrete and to prevent corrosion. This method was used extensively during the construction of the Olympic Stadium)

39. Thin shell concrete construction.(Best studied by looking at an egg shell: very thin, double curved and continuous. The more curved, the stronger the thin shell. Look at a car.. all surfaces are doubly curved to prevent getting a bump ( a bump is local buckling) Buckling is not uncommon in this type of structure as the thickness of the material is much less than the other dimensions of the structure. These structures require structural reinforcements such as steel or wire mesh. The thinner the concrete construction, the more the strength of the structure decreases.(and the more we have to make sure that there is enough (double) curvature)Subject to brazier buckling = local buckling. Therefore must reinforce the thin shell. The material used must be good in compression. The reinforcements should b good in tension. Bulkheads/nodes are used to reinforce. They go around the structure.

40. Arch construction. Arch bridges have incredible strength compared to the many other forms of bridges which exist today. Although today they are generally made of steel or concrete, they were used by the greeks in their architecture long before us. They are the oldest type of bridges and were originally made of brick or stone, which

unfortunately did not allow for a great span. The use of steel has allowed us to build bridges with a greater span. The load of the arch is carried outward along the curve of the arch to the ends of the arch. The weight lies at the ends of the arch (versus pushing downwards in other bridges). (The Greeks are given too much credit here and the Romans too little. The Greeks knew the arch but hardly used it in their buildings; they persisted in using very restrictive post and beam construction, as in the Parthenon. It was the Romans who exploited the arch in many wondrous ways by repetition( aqueducts) sliding horizontally (as in barrel vaults in their huge Bath structures )and by turning around an axis as in the Pantheon, which with 43 meters held the record of the biggest span for over 200o years!)

41. Bailey bridges. Bailey bridges are portable and prefabricated truss bridge which use reversible joints rather than non reversible joints. During the second world war, this was an effective method used in order to save time and energy: instead of building an entirely new bridge, with new material (thus more cost), the soldiers would simply dismount the bridge piece by piece and rebuilt it. Furthermore, one could easily double its length, width or height by taking all of the parts apart and putting them in a different order.

42. Modularity (stackability, doubling-up). Very few bridges are modifiable. Unfortunately, when designed or built the engineers often forget that the materials should be easy to dismantle, so that when the structure is no longer needed, one can easily do so. Bailey bridges and bridges solely made of bolt or reversible connections are the only kind of bridges which can be modified, doubled in length, height etc.

43. Reversible joints vs. non-reversible joints (pro’s and cons). Although reversible joints are good for dismantling the structure, they often cause sag in the structure as these joints are not as rigid as non-reversible joints. In other words, non –reversible joints are more sustainable than reversible joint structures. (non –reversible joints like rivets, welding and gluing are more rigid, can sustain more load, you probably mean)

44. Detail-rich versus detail-poor construction (labor versus material cost, esthetics?). As we saw, in several structures, the cost and labor of the structure were reduced by designing the structure so that the handrail is also the cross beam of the structure. Furthermore, there are attempts to reduce the area exposed to the exterior. This is to avoid corrosion but also to avoid having to use more paint: the less area exposed to the exterior, the less are to paint, the less the cost of the structure is.

As for labor and material cost, the bridges built with rivet joints and welds are the most expensive and labor intensive. Rivets require alignment, drilling and are often very difficult to do as they are around corners or in awkward places. Welding is very effective but very energy consuming. Esthetics are often intertwined with the structure: although in many bridges, the actual structure is visible, it has a certain “elegance”, the trusses, triangles, arches all form a beautiful sustainable structure. In some cases, where the materials are not so elegant (concrete vs. steel), the concrete or steel is painted.

Tensile Strength

Compressive Strength

High melting temperature?

Corrosive?

Steel High High High YesConcrete Low High Variable NoCast Iron Low High High NoWrought Iron High Low Very High NoAluminum High Low Low NoDetail rich versus detail poor can also be a design decision. Architects can often choose between a method that allows lots of details, such as bolts or rivets or mortar joints, or little recesses between bricks, and a method that has a smooth featureless surface, ( such as stucco, or glass). The Victorians were very much into detail-rich architecture (just walk around the McGill Ghetto), while the modern movement was promoting the beauty of ‘sleekness’, of smooth machine made glass and metal surfaces.