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A shallow foundation is often selected when the structural load will not cause excessive settlement of the underlying soil layers.


In general, shallow foundations are more economical to construct than deep foundations. Common types of shallow foundations are listed in Table 1 and described later:

These types of shallow foundations are probably the most common types of building foundations. Figure 1 shows various types of shallow foundations.

Figure 2 shows various types of mat foundations. Based on economic considerations, mat foundations are often constructed for the following reasons (NAVFAC DM-7.2, 1982):

i. Large individual footings. A mat foundation is often constructed when the sum of individual footing areas exceeds about one-half of the total foundation area.

ii. Cavities or compressible lenses. A mat foundation can be used when the subsurface exploration indicates that there will be unequal settlement caused by small cavities or compressible lenses below the foundation. A mat foundation would tend to span over the small cavities or weak lenses and create a more uniform settlement condition.

iii. Shallow settlements. A mat foundation can be recommended when shallow settlements predominate and the mat foundation would minimize differential settlements.

iv. Unequal distribution of loads. For some structures, there can be a large difference in building loads acting on different areas of the foundation. Conventional spread footings could be subjected to excessive differential settlement, but a mat foundation would tend to distribute the unequal building loads and reduce the differential settlements.

v. Hydrostatic uplift. When the foundation will be subjected to hydrostatic uplift due to a high groundwater table, a mat foundation could be used to resist the uplift forces.

Posttensioned slab-on-grade is common in southern California and other parts of the United States. The most common uses of posttensioned slab-on-grade are to resist expansive soil forces or when the projected differential settlement exceeds the tolerable value for a conventional (lightly reinforced) slab-on-grade. For example, posttensioned slabs-on-grade are frequently recommended if the projected differential settlement is expected to exceed 0.75 in. (2 cm).

The Post-Tensioning Institute (1996) has prepared installation and field inspection procedures for posttensioned slab-on-grade. Posttensioned slab-on-grade consists of concrete with embedded steel tendons that are encased in thick plastic sheaths. The plastic sheath prevents the tendon from coming in contact with the concrete and permits the tendon to slide within the hardened concrete during the tensioning operations. Usually tendons have a dead end (anchoring plate) in the perimeter (edge) beam and a stressing end at the opposite perimeter beam to enable the tendons to be stressed from one end. However, it is often recommend that the tendons in excess of 100 ft (30 m) b stressed from both ends.

Because posttensioned slab-on-grade perform better (i.e., less shrinkage related concrete cracking) than conventional slab-on-grade, they are more popular even for situations where low levels of settlement are expected. Posttensioned slab-on-grade has become common for situations where it is desirable to limit the amount and width of concrete shrinkage cracks.

If the expected settlement for a proposed shallow foundation is too large, then other options for foundation support or soil stabilization must be evaluated. Some commonly used alternatives are as follows:

i. Grading. Grading operations can be used to remove the compressible soil layer and replace it with structural fill. Usually the grading option is only economical if the compressible soil layer is near the ground surface and the groundwater table is below the compressible soil layer or the groundwater table can be economically lowered.

ii. Surcharge. If the site contains an underlying compressible cohesive soil layer, the site can be surcharged with a fill layer placed at the ground surface. Vertical drains (such as wick drains or sand drains) can be installed in the compressible soil layer to reduce the drainage paths and speedup the consolidation process. Once the compressible cohesive soil layer has had sufficient consolidation, the fill surcharge layer is removed and the building is constructed.

iii. Densification of soil. There are many different methods that can be used to densify loose or soft soil. For example, vibro-flotation and dynamic compaction are often effective at increasing the density of loose sand deposits. Another option is compaction grouting, which consists of intruding a mass of very thick consistency grout into the soil, which both displaces and compacts the loose soil.

iv. Floating foundation. A floating foundation is a special type of deep foundation where the weight of the structure is balanced by the removal of soil and construction of an underground basement.

Selection of the most suitable and effficient type of foundation for a particular structure is a tricky step in the whole structural design process. A well designed super structure will be a waste of time, money and efforts if due attention is not give to the choice of right type of sub structure. This brief article enlists some the most important deciding factors during the process. 

The selection of a particular type of foundation is often based on a number of factors, such as:

1. Adequate depth. The foundation must have an adequate depth to prevent frost damage. For suchfoundations as bridge piers, the depth of the foundation must be sufficient to prevent undermining by scour.

2. Bearing capacity failure. The foundation must be safe against a bearing capacity failure.

3. Settlement. The foundation must not settle to such an extent that it damages the structure.

4. Quality. The foundation must be of adequate quality so that it is not subjected to deterioration, such as from sulfate attack.

5. Adequate strength. The foundation must be designed with sufficient strength that it does not fracture or break apart under the applied superstructure loads. The foundation must also be properly constructed in conformance with the design specifications.

6. Adverse soil changes. The foundation must be able to resist long-term adverse soil changes. An example is expansive soil, which could expand or shrink causing movement of the foundation and damage to the structure.

7. Seismic forces. The foundation must be able to support the structure during an earthquake without excessive settlement or lateral movement.

Choosing the right calculator for your needs can be a tough job. But you can compare some of the best calculators in the industry with our brief but useful list below. The features and number of functions vary from calculator to calculator. Some have around a hundred functions while others have several hundred. Some calculators have bright, well lit screens, while others are more difficult to see but users favor the style. Depending upon your needs, you’re sure to find the calculator for you on this list.

1. Texas Instruments TI-84 Plus CE Plum Graphing Calculator

This graphing calculator has a very high resolution backlit display. It’s likely the highest quality display available on any scientific calculator. The calculator is lightweight and available in plum, red, denim, blueberry, gray, gold, silver and white. The battery may get weak quickly if you use it a lot. A huge benefit is the fast processor. It comes with a user guide and double sided cable. The only complaint that some users have is that the font is too small. It’s great for high school students taking advanced math. If you need to easily graph complicated expressions, this is the calculator for you.

2. Casio fx-9750GII Graphing Calculator

Create bar graphs and pie charts with ease. The screen is easy to read in a number of different lighting situations. Enjoy a high speed RAM CPU and USB connectivity, making it easy to share files. Some users have said that it’s not the easiest to program. However, the functions are easy to understand. It comes with a well organized user manual.

3. HP HP50G 50g Graphing Calculator

This calculator has 30% more screen space than the previous version and most other scientific calculators for that matter. It has a better keypad than the last model and is quipped with an SD card slot. You can choose between RPN, Algebraic, and Textbook data entry. Not that this graphing calculator uses a lot of battery power. The advantages of this calculator are the high range of functions and the ease of programmability.

4. Casio fx-9860 GII (Engineersdaily's recommendation)

This versatile programmable calculator is very suir´table for complex jobs especially in exams where lots of calculations are needed under time pressure. Formulae can also be programmed easily. It also features a USB connection and comes handy with a cable to connect to your computer. More specs can be found at the link above.

Fire and other safety features of high rise buildings and structures is essential. Types and concerns related to these features is discussed.

The high-rise building construction concept when compared with other buildings, possess certain features and characteristics that makes them unique and highlighting. The high-rise buildings are considered as the product of the modern evolution. It is filled and composed of sophisticated systems and essential components.

Each of these systems carry out special roles either positive or negative. These elements have an effective role in the overall fire department’s operation.

Most of the components in the high-rise construction focus on safety during emergency or fire risk. They are more focused on fire systems to protect the occupants.

These will hence demand costlier building systems and unique fire safety codes. The fire and safety issues with different features available in the high-rise building is explained hereby.

Over years, the high-rise buildings have garnered significant attention in the fire safety throughout the world. The multiple floors present in the high-rise building makes great number of persons to travel long vertical distances by the stair during an evacuation.

The public regulations, the design of the building, the ownership, code bodies, the regional, local and federal governments are affected by the high-rise building safety.

The high rise buildings are designed to be safe at all undesirable conditions. But when there is a need for a full scale evacuation, it will be necessary to take quick responsibility for their own safety and planned action from the fire fighters.

Fire and Safety Features of High-Rise Buildings and Structures

Construction Concerns in High-Rise Building Construction

When compared with other form of building construction, high rise building construction have a major focus on the building fire and emergency concerns. To understand this, the first major requirement is to study the number of floors of the building under consideration.

The number of floors both above and below the grade have to be evaluated for the same purpose. The firefighting operations is very much dependent on how these levels are identified and labeled in the building.

For example, if there is a floor which is numbered 13; it is found out whether there exist any other levels like the concourse levels or the mezzanines, or it is found that the floor 13 is where the mechanical level is located and have different mechanical levels. Or sometimes the floor has a penthouse level.

An occurrence of a high-rise fire or any other emergency will ask many questions regarding the high-rise building construction features, that are available.

Hence the Incident Commander (IC) have the role to assign different teams to conduct an ongoing reconnaissance. The team consists of group leaders and division supervisors. Most of the fire department consists of a system officer who is positioned inside the fire command center (FCC).

The system Officer is responsible in the monitor of different building systems like the fire alarm panel, the HVAC system, the elevators etc. The system officer is the important source for the incident commander to gather the data and information of the building which are considered very critical.

The quick determination of the age of high rise buildings and the generation which it falls is also identified by the IC. This is very important to know whether the building make use of any lightweight components, for example, any truss assemblies.

This idea will give us an estimate on how long the fire fighters can operate inside the buildings with reasonable safety.

Concerns on the Structural System the Building Possess

The concerns or question that comes to mind during a fire operation is whether:

1. The building is core type or not
2. If not Core type, what structural system do the building possess
3. If it’s a core type, is the core center or some other type
4. Does the building possess a central HVAC system?

This information will help in bringing a quality pre-fire plan. When you consider the use of high rise construction in the world, almost all countries possess high rise building to an appreciable range.

The lack of proper resources and the time constraints will affect in bringing a quality pre-fire plan for the building. But many firms work for developing these pre-fire plan which are very expensive.

It’s a matter of fact that most of the buildings do not possess a plan by themselves nor approach any other company for the same.

Concerns on Static and Dynamic Features of Building

Once the pre-fire plan is considered for a high-rise building, it will stay active until the features of the building remain static. This is hence the main issue because most of the high-rise buildings are dynamic.

With demand, more and more up gradation is brought to the buildings. Any change on features of the building system will affect the plan and the considerations. Hence these plans must be updated accordingly.

Concerns on the Materials used in High Rise Building

In the case of a core type structural building, emphasis is on finding out what material forms the structural components, the core, the structural frame, the floor components; whether concrete or steel or both.

Most of the modern high-rise building construction consist of floors where the concrete is poured over a metal deck. Regarding this, the questions arise on:

1. Whether the load of the floor is taken by the structural frame?
2. Is there any I – section to support the floors?
3. Is there any fire proofing material used to protect the steel components?

Concerns regarding the roof construction of the building: The material type of the roof, the type of equipment on the roof, the load that is carried by these structural components are the concerns regarding the roof construction.

In high rise construction, the question on whether the roof have the capacity to take the load of a helicopter is raised. Other concerns are on the roof obstructions.

The roof with high parapet walls provide additional safety for the firefighters who are operating on the roof also the ones who have be evacuated. Shorter parapet won’t provide no protection in situations where the visibility is lost due to the smoke or during night operations.

Concerns with Fire Detection and The Protection Systems in High- Rise Buildings

It is very essential to determine and identify the fire detection and the protection system that is available in the building to bring the best fire safety plan.

There may be different types of fire detection devices installed in the building. These include the smoke detectors, the heat detectors, manual pull stations, the rate of rise and etc. It is very essential to determine the location of the fire alarm in the building.

If there is any indication from multiple alarms that are in different locations it fixes the probability of actual fire in the building. When multiple locations of alarms are heard it is recommended to check the lowest alarm.

The sprinkler system in the building is identified if installed. The position or the location of the sprinkler system is very essential. It is also essential to determine the type of sprinkler system in the building; whether it is partial sprinkler protection, or full sprinkler protection or no sprinkler protection. A check on the operation of the sprinkler system has to be examined.

Water Supply in High-rise construction

To construct a fire protection system in the high-rise building, it is very essential to have a comprehensive knowledge on the built-in fire protection systems. Water supply is an essential concern with the fire safety measure.

The mode of water supply for fire protection system must be determined. Not only the source but also the water flow. The water supply equipment as a part of fire protection system will include the gravity tanks, fire pumps, the city water mains and other different components.

In a situation where the primary water supply becomes insufficient or it fails to operate, a backup water supply will be served by the fire department connection (FDC) coming under the area and the locality.

Concerns with the Stairs in High-rise building during a Fire Emergency

Early Identification of number of stairs in the building must be considered. Most of the high-rise buildings will possess two stairs well that runs throughout the height of the building. But most probably one among the two will have the access towards the roof.

In modern high-rise buildings, when the fire alarms of specific zones are activated, these primary stairwells are pressurized. In old buildings, under a fire emergency, one of the stairwell will behave like a smoke tower.

Whatever be the type of stair, scissor type or return type, both are found to be critical under a fire emergency. All information regarding, which stair can be easily accessed for evacuation and which is easily occupied by the fire are very valuable during an emergency plan.

Always a pre-plan during the good performance of the building itself it must be categorized that which stair is used for evacuation and which is the attack stair.

Concerns with the Elevators

All high-rise building will possess elevators. Large buildings will have elevators in more number that are arranged in separate banks that serves separate locations.

Elevators can be considered as an essential parameter for the firefighters. They behave as valuable tools in emergency situations. Operational success is attained by accounting the number of elevators and their type, before undergoing the operation.

The HVAC Systems in High Rise buildings

Most of the modern high rise buildings are equipped with well modernized and sophisticated Heating ventilation and the air conditioning system.

Many buildings are equipped with ventilation system to get rid of smoke and to control the air movement because of fire within the building.

The first and the second generation high-rise building make use of horizontal ventilation. Any chance of smoke found, make them to open the windows. This way blocking of smokes was avoided. This strategy is not possible in the present day modern high rise buildings.

The third-generation buildings where more of sealed ones. These were referred to as windowless buildings. This is not because of the absence of the windows in the building, but the fact that they are designed to be opened or easily broken out.

In a third-generation building, a smoke generated will be collected in a remote area of the building. This problem is well solved by the installation of HVAC system. Sometimes the HVAC system can result in very tragic negative problems also.

The problem of ventilation in high rise buildings is a great issue. Mostly the issue is with the smoke than with the fire problems. The smoke has resulted in countless injuries and deaths during most of the fires in the high-rise building.

The issue with the ventilation in high rise buildings must be considered in the initial planning stage of the buildings, to reduce additional expense.

Concerns with the Utilities in The High-Rise Buildings

The daily utilities in the building like the electricity, water, steam and the natural gas have a very important role in the daily operations of a given building. These operations do have an important role during a fire emergency. A control of these operations is a simple matter and are of less effort which can promote large safety.

Generalization of Fire Damage in High Rise Buildings

The fire damage will be categorized under three ways.

1. The detrimental effect for the occupant’s life safety
2. Structural Damage
3. Damage to the properties-Non-structural damage

Among most of the fire incidents that have been recorded, it is observed that the injuries and the loss of life is less. What everyone wish is to bring a less effect on the property damages also.

The migration of heat and the smoke within the high rise buildings is a great threat for the occupants within the building. Most of the death due to fire accidents are caused within the dwellings.

The fire will result in the formation of toxic gases that is very dangerous to the human health. In such situations, the structural damages have least importance when compared with the life safety. After extinguishing the fire, it is found that the structure is subjected to water damage. The repair and the maintenance is a great economic loss.

The high-rise buildings are stuffed with large equipment which are numerous are costly. Fire will bring lots of property loss due to these reasons. These reasons have resulted in the bankrupt of many companies as their production process was completely stopped and lost the market.

There are many factors that affect the fire and safety concerns of a high-rise building. It is always recommended that how extreme be the fire damage, the measures and operations must bring life safety as the primary concern. This will ask for fire and building codes that will possess both passive and active fire protection systems in order to reduce the fire damages.

Performance Based Design for Structural Safety Against Fire

Performance based design concept is the design the buildings according to the main goals. PBD is hence regarded as one of the best solution for this problem of fire issues in structural point of view.

Fig.1. Different Performance Levels in Performance Based Design

The selection of a level from the levels, Immediate Occupancy (IO), Life Safety (LS), Collapse Prevention (CP) is the basis on which the building design. This is mainly a design concept used in earthquake resistant structures.

Each level has different damage states. In fire-resistant design, the life safety level is used, where we expected to have damages to the building with no harm to the life. It can be sometimes chosen between IO and LS also.

The PBD design will provide more on the provision of hinges in more critical point of the structure making the building more ductile. This ability to provide the performance hinges will describe the ability to have fire dynamics in the spaces of the tall buildings.

How severe be the fire, the structure should stand still letting all the life to be safe, this must be the output of a PBD design.

Understanding and studying the fire accidents happened in a modern building will help to assess the critical components that are involved in the fire safety strategy. As this truly reflects the nature of the tall buildings.

[ Ref: Cowlard[2013], Fire Safety design for tall buildings, Science Direct].

Hence it is very essential during the concreting in cold weather conditions to ensure that the concrete will not undergo freezing in its plastic state.

There are two methods for carrying out cold weather concreting:

1. Provision of normal ambient temperatures for the concrete. This can be done through the heating of the concrete ingredients or bley providing heating enclosures.
2. The addition of chemical admixtures.

Conventional Chemical Admixtures in Cold Weather Concrete

 

Conventional concrete used calcium chloride as accelerating admixtures to offset the retarding effects of slow hydration of concrete in low temperatures. This admixture is not effective below the freezing temperatures.

This is found to be a drawback in the conventional form of admixtures. Hence, for arctic weather conditions, special admixtures are necessary. One such is antifreeze admixtures.
Antifreeze Admixtures for Concrete

The antifreeze admixtures affect the physical condition of the mix water used in the concreting. These can depress the freezing point of the water to a large extent and can be used in temperatures lesser than -30 degrees Celsius. This can enable the extension of the period of the construction activity.

Chemical Composition and Action of Antifreeze Admixtures

There are two groups of antifreeze admixtures that provide the characteristics of antifreeze and the accelerated setting and hardening properties.

They are:

1. First Group

This includes chemicals, weak electrolytes, sodium nitrite, sodium chloride and non-electrolytic organic compounds which lower the freezing point of the water used in the concrete. But these group acts as weak accelerators to promote the setting and hardening.

2. Second Group

These include binary as well as ternary admixtures which contains potash and additives based on calcium chloride, sodium nitrite, calcium chloride with sodium nitrite, calcium nitrite -nitrate-urea and other chemicals.

These have effective antifreeze properties and accelerating property to promote the setting and hardening. These are used in larger dosages compared to that of conventional admixtures.

One such example is the use of 8% of sodium nitrite to keep the liquid at a temperature of -15-degree Celsius.

These admixtures function by lowering the liquid phase freezing point and by accelerating the cement hydration at the freezing temperatures.

Based on the dosage in the mixture, the non-chloride admixture enables the mix (concrete or the mortar) to be placed at sub-freezing temperatures. This hence reduces the need of protective measures required during the cold weathering works.

The method improves the quality of the concrete and as it facilitates early setting, early stripping of formworks can also be carried out. This helps in the reuse of the form within a small duration and hence speed up the construction.

The table-1 shows the significant difference is strength gain at 3, 7 and 28 days for plain concrete and antifreeze admixture used concrete.

Table.1: Concrete Compressive strength with and without antifreeze admixture

 

(As per Ratinov and Rosenburg)

Property	Plain Concrete	Freeze-protection Admixture
Set time (-4 degree Celsius)	–	–
Compressive strength (MPa)
-4 degree Celsius (3 days)	3.4	9.24
-10 degree Celsius (7 days)	8.3	39.3
-10 degree Celsius (28 days)	18.1	49.9

It is possible for the incorporation of other admixtures that contains superplasticizers to be incorporated with the antifreeze admixtures. The main advantage of such combination is that in totality there will be a reduction of water.

The water reduction will reduce the freezable free water content in the mix. This freezable water content is the one that serves as the heat sink for the heat liberated by the initial hydration reactions. This will hence reduce the number of antifreeze admixtures.

Selection of Antifreeze admixtures

The factors based on which the selection of antifreeze admixtures is carried out are:

1. The type of structure
2. The operating Conditions
3. Protecting methods employed in winter concreting
4. Cement brand and aggregate types

A laboratory test must be carried out with the operating materials and the dosage of antifreeze admixtures that are intended to be used in the field.

The incorporation of other admixtures like retarders, superplasticizers with antifreeze admixtures is not restricted in cold weather concreting. The dosage of all the admixture that are used must be established experimentally.

Application and Advantages of Antifreeze admixtures

The antifreeze admixtures are technologically simple and beneficial for cold weather concreting. The admixture helps in improving the cohesiveness, cold joint minimizations, sand streaking, and plasticity. These are estimated to provide large cost saving than other methods of steam curing or concrete enclosures.

The combination of antifreeze agents with water reducing agents or air-entraining agents will help in increasing the resistance of concrete towards the frost action and corrosion.

Materials Used in Bridge Construction

Stones, Timber, Concrete and Steel are the traditional materials that are used to carry out bridge construction. During the initial period, timber and stones were used in the construction, as they are directly obtained from nature and easily available.

Brick was used as a subgroup construction material along with stone construction. Stones as construction materials were very popular because of its durable properties. Many historic bridges made from stones are still present as a symbol of past architectural culture.

But some of the timber bridge have been washed away or are in the stage of degradation due to their exposure to the environmental conditions.

As time passed, the bridge construction has undergone more development in terms of materials used for construction than based on the bridge technology.

The concrete and steel are manmade refined materials. The bridge construction with these artificial materials can be called the second period of the bridge engineering. This hence was the start of modern bridge engineering technology.

Modern bridges make use of concrete or steel or in combination. Different other innovative materials are being developed so that they can well suit with the bridge terminologies.

Incorporation of fibers which comes in the category of high strength gaining materials is now incorporated for the construction of bridges. These materials are also used in order to strengthen the existing bridges.

Stones for Bridge Construction

For a long time in the history, the stone has been used in and as a single form. They are mainly used in the form of arches. This is because they possess higher compressive strength.

The use of stones gave the engineers ease of constructing bridges that are aesthetically top and high in durability.

When considering the history of bridge construction with stones, the Romans were the greatest builders of bridges with stones. They had a clear idea and understanding of the load over bridge, the geometry as well as the material properties. This made them construct very larger span bridges when compared with any other bridge construction during that period.

The period was also competitive for Chinese. China had also developed large bridge called the famous Zhuzhou Bridge. The Zhuzhou bridge is the world’s known oldest open-spandrel, stone and segmental arch bridge. Nihonbashi is the most famous stone bridge in Japan. This is called as the Japan Bridge.

Fig.1: The Zhuzhou bridge, China

With time, the stone bridges have proved most efficient and economical due to the durability and low maintenance guaranty it provides throughout its life period.

Timber or Wood for Bridge Construction

The wood material was used highly in the construction of bridges, unlike today, where it is used for the construction of building works and related. Nowadays, steel and concrete grant a higher range of work flexibility, that the use of wood and timber for mega works diminished.

But, there are innovations related to the preservation of wood, which has helped to increase the demand of wood in structures.

Wood as an engineering material has the advantage of high toughness and renewable in nature. They are obtained directly from nature and hence are environmentally friendly.

The low density of wood makes it gain high specific strength. They have an appreciable strength value with a lower value of density. This property makes them be transported easily.

Some of the disadvantages related to wood as a construction material are that it is:

• Highly Anisotropic in Nature
• Susceptible to termites, infestations, and woodworm
• Highly combustible
• Susceptible to rot and disease
• Cannot be used for High temperature

There are a variety of timber bridges around the world. Figure-2 shows the Mathematical Bridge located in Cambridge. Another bridge is the Togetsu-Kyo Bridge over the Katsura River in Kyoto.

Fig.2: The Mathematical Bridge, Cambridge

Fig.3. The Togetsu-Kyo Bridge, Japan

Steel for Bridge Construction

 

Steel gain high strength when compared with any other material. This makes its suitable for the construction of bridges with longer span. We know that steel is a combination of alloys of iron and other elements, mainly carbon.

Based on the amount and variation of the elements, the properties of the same is altered accordingly. The properties of tensile strength, ductility and hardness are influenced by the change in its constitution.

The steel used for normal construction have several hundred Mega Pascal strength. This strength is almost 10 times greater than the compressive and the tensile strength obtained from a normal concrete mix.

The major inbuilt property of steel is the ductility property. This is the deformation capability before the final breakage tends to happen. This property of steel is an important criterion in the design of structures.

Fig.4. The Hachimanbashi Bridge

The first iron bridge, Danjobashi Bridge which was built in 1878 in Japan. The figure-4 below shows the Danjobashi Bridge. Danjobashi Bridge was relocated to the present location and was named as Hachimanbashi Bridge in 1929.

It has great historical and technical value as a modern bridge. The bridge was honored by the American Society of Civil Engineers in the year 1989.

The chemical composition and the method of manufacture determines the properties of structural steel. The main properties that are to be specified by the bridge designers when it is required to specify the products are:

• Strength
• Toughness
• Ductility
• Durability
• Weldability

When we mention the steel strength, it implies both the yield and the tensile strength. As the structures are more designed in the elastic stage, it is very essential to know the value of yield strength.

Yield strength is mostly used as it is more specified in the design codes. In Japan, the code recommended is designed for ultimate strength. For example, SS400 designated by the ultimate strength of 400MPa. This is an exception.

The property of ductility is very much relied on by designers and engineers for the design aspects related to the bolt group designs and the distribution of stress at the ultimate limit state conditions. Another important property is the corrosion resistance by the use of weathering steel.

Concrete for Bridge Construction

Most of the modern bridge construction make use of concrete as the primary material. The concrete is good in compression and weak in tensile strength. The reinforced concrete structures are the remedy put forward for this problem.

The concrete tends to have a constant value of modulus of elasticity at lower stress levels. But this value decreases at a higher stress condition. This will welcome the formation of cracks and later their propagation.

Other factors to which concrete is susceptible are the thermal expansion and shrinkage effects. Creep is formed in concrete due to long time stress on it.

The mechanical properties of concrete are determined by the compressive strength of concrete.

The reinforced or the prestressed concrete is used for the construction of bridges. The reinforcement in R.C.C provides the ductility property to the structure. Nowadays, ductility reinforcement is provided as an additional requirement mainly in the earthquake resistant construction.

RCC is nowadays made from steel, polymer or other combination of composite materials. Much sustainable materials is available that can take the role of cement. This is a new innovation in sustainable bridge construction.

When compared with RCC bridge construction, prestressed concrete is the most preferred and employed. A pre-compressive force is induced in the concrete with the help of high strength steel tendons before the actual service load.

Hence this compressive stress will resist the tensile stress that is coming during the actual load conditions. The prestress is induced in concrete either by means of post tensioning or by means of pretensioning the steel reinforcement.

Many disadvantages of normal reinforced concrete like strength limitations, heavy structures, building difficulty is solved using prestressed concrete.

Also Read: Dutch inaugurate the 3D Printed Reinforced Concrete Bridge Designed by Technical University of Eindhoven 

Composite Materials in Bridge Construction

 

Composite materials are developed and used for both the construction of new bridges as well as for the rehabilitation purposes.

Fiber reinforced plastic is one such material which is a polymer matrix. This is reinforced with fibers which can be either glass or carbon. These materials are light in weight, durable, high strength giving and ductile in nature.

New solution and materials are encouraged due to the problems of deterioration the steel and concrete bridges are facing.

Another material is the reactive powder concrete (RPC ) that was developed in Korea. This material is a form of high performance concrete that is reinforced with steel fibers. This mix will help to make slender columns for bridges of a longer span. This also guarantees durability extensively.

Composite materials are used in the repair of bridge columns and any other supporting elements to improve the ductility and the resistance against the seismic force.

Epoxy impregnated fiberglass are used to cover the column (columns that are non-ductile in nature). This is an alternative for the steel jacket technique.

‘Least squares’ is a powerful statistical technique that may be used for ‘adjusting’ or estimating the coordinates in survey control networks. The term adjustment is one in popular usage but it does not have any proper statistical meaning. A better term is ‘least squares estimation’ since nothing, especially observations, are actually adjusted. Rather, coordinates are estimated from the evidence provided by the observations.

The great advantage of least squares over all the methods of estimation, such as traverse adjustments, is that least squares is mathematically and statistically justifiable and, as such, is a fully rigorous method.

It can be applied to any over determined network, but has the further advantage that it can be used on one-, two- and three-dimensional networks.A by-product of the least squares solution is a set of statistical statements about the quality of the solution. These statistical statements may take the form of standard errors of the computed coordinates, error ellipses or ellipsoids describing the uncertainty of a position in two or three dimensions, standard errors of observations derived from the computed coordinates and other meaningful statistics described later.

The major practical drawback with least squares is that unless the network has only a small number of unknown points, or has very few redundant observations, the amount of arithmetic manipulation makes the method impractical without the aid of a computer and appropriate software.

The examples and exercises in this material use very small networks in order to minimize the computational effort for the reader, while demonstrating the principles. Real survey networks are usually very much larger.

A‘residual’ may be thought of as the difference between a computed and an observed value. For example, if in the observation and estimation of a network, a particular angle was observed to be 30°0' 0'' and after adjustment of the network the same angle computed from the adjusted coordinates was 30° 0' 20'', then the residual associated with that observation would be 20''. In other words:

computed value − observed value = residual

Any estimation of an over determined network is going to involve some change to the observations to make them fit the adjusted coordinates of the control points. The best estimation technique is the one where the observations are in best agreement with the coordinates computed from them. In least squares, at its simplest, the best agreement is achieved by minimizing the sum of the squares of the weighted residuals of all the observations.

In practical survey networks, it is usual to observe more than the strict minimum number of observations required to solve for the coordinates of the unknown points. The extra observations are ‘redundant’ and can be used to provide an ‘independent check’ but all the observations can be incorporated into the solution of the network if the solution is by least squares.

All observations have errors so any practical set of observations will not perfectly fit any chosen set of coordinates for the unknown points.

Some observations will be of a better quality than others. For example, an angle observed with a 1'' theodolite should be more precise than one observed with a 20'' instrument. The weight applied to an observation, and hence to its residual, is a function of the previously assessed quality of the observation.

In the above example the angle observed with a 1" theodolite would have a much greater weight than one observed with a 20" theodolite. How weights are calculated and used will be described later.

If all the observations are to be used, then they will have to be ‘adjusted’ so that they fit with the computed network. The principle of least squares applied to surveying is that the sum of the squares of the weighted residuals must be a minimum.

A locus line is the line that a point may lie on and may be defined by a single observation. Figure1(a), (b) and (c) show the locus lines associated with an angle observed at a known point to an unknown point, a distance measured between a known point and an unknown point and an angle observed at an unknown point between two known points respectively. In each case the locus line is the dotted line. In each case all that can be concluded from the individual observation is that the unknown point lies somewhere on the dotted line, but not where it lies.

In the following, the coordinates of new point P are to be determined from horizontal angles observed at known points A, B, C and D as in Figure 2(a). Each observation may be thought of as defining a locus line. For example, if only the horizontal angle at A had been observed then all that could be said about P would be that it lies somewhere on the locus line from A towards P and there could be no solution for the coordinates of P. With the horizontal angles at A and B there are two locus lines, from A towards P and from B towards P. The two lines cross at a unique point and if the observations had been perfect then the unique point would be exactly at P. But since observations are never perfect when the horizontal angles observed at C and D are added to the solution the four locus lines do not all cross at the same point and the mismatch gives a measure of the overall quality of the observations. Figure 2(b) shows the detail at point P where the four lines intersect at six different points. The cross is at the unique point where the sum of the squares of the residuals is a minimum.

By far the easiest way to handle the enormous amounts of data associated with least squares estimation is to use matrix algebra. In least squares it is necessary to create a system of equations with one equation for each observation and each ‘observation equation’ contains terms for each of the coordinates of each of the unknown points connected by the observation. So, for example, in a two-dimensional network of 10 points where there are a total of 50 observations there would be a set of 50 simultaneous equations in 20 unknowns. Although this represents only a small network, the mathematical problem it presents would be too difficult to solve by simple algebraic or arithmetic methods.

The Hansen Summer Institute on Leadership and International Cooperation is an exciting international program funded by a generous grant from the Fred J. Hansen Foundation. Six Hansen Summer Institutes took place in San Diego, California between 2007 and 2015. The Institute provides a unique University-based leadership experience and program in international cooperation. For three weeks, young American men and women joined students from a variety of developing countries and regions of social strife. Between then and now, Hansen Alumni have remained in contact via a variety of social networking sites and projects funded by the Hansen Foundation. We are happy to announce the Hansen Summer Institute will host its next group of young leaders in July at the University of San Diego School of Leadership and Education Sciences (SOLES).

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Hansen Summer Institute training aims to provide participants a "leadership toolbox" for resolving conflict and building a better future for their respective countries.

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