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Reinforced concrete structural systems can be formed into virtually any geometry to meet any requirement. Regardless of the geometry, standardized floor and roof systems are available that provide cost-effective solutions in typical situations. The most common types are classified as one-way systems and two-way systems. Examined later are the structural members that make up these types of systems.

It is common for one type of floor or roof system to be specified on one entire level of building; this is primarily done for cost savings. However, there may be cases that warrant a change in framing system. The feasibility of using more than one type of floor or roof system at any given level needs to be investigated carefully.

A one-way reinforced concrete floor or roof system consists of members that have the main flexural reinforcement running in one direction. In other words, reactions from supported loads are transferred primarily in one direction. Because they are primarily subjected to the effects from bending (and the accompanying shear), members in one-way systems are commonly referred to as flexural members.

FIGURE 1 One-way slab system.

Members in a one-way system are usually horizontal but can be provided at a slope if needed. Sloped members are commonly used at the roof level to accommodate drainage requirements.

Illustrated in Fig. 1 is a one-way slab system. The load that is supported by the slabs is transferred to the beams that span perpendicular to the slabs. The beams, in turn, transfer the loads to the girders, and the girders transfer the loads to the columns.

Individual spread footings may carry the column loads to the soil below. It is evident that load transfer between the members of this system occurs in one direction.

FIGURE 2 Standard one-way joist system.

Main flexural reinforcement for the one-way slabs is placed in the direction parallel to load transfer, which is the short direction. Similarly, the main flexural reinforcement for the beams and girders is placed parallel to the length of these members. Concrete for the slabs, beams, and girders is cast at the same time after the forms have been set and the reinforcement has been placed in the formwork. This concrete is also integrated with columns. In addition, reinforcing bars are extended into adjoining members. Like all cast-in-place systems, this clearly illustrates the monolithic nature of reinforced concrete structural members.

A standard one-way joist system is depicted in Fig. 2. The one-way slab transfers the load to the joists, which transfer the loads to the column-line beams (or, girders). This system utilizes standard forms where the clear spacing between the ribs is 30 in. or less. Because of its relatively heavy weight and associated costs, this system is not used as often as it was in the past.

FIGURE 3 Wide module joist system.

Similar to the standard one-way joist system is the wide-module joist system shown in Fig. 3. The clear spacing of the ribs is typically 53 or 66 in., which, according to the Code, technically makes these members beams instead of joists. Load transfer follows the same path as that of the standard joist system.

Reinforced concrete stairs are needed as a means of egress in buildings regardless of the number of elevators that are provided. Many different types of stairs are available, and the type of stair utilized generally depends on architectural requirements. Stair systems are typically designed as one-way systems.

FIGURE 4 Two-way beam supported slab system.

A two-way beam supported slab system is illustrated in Fig. 4. The slab transfers the load in two orthogonal directions to the column-line beams, which, in turn, transfer the loads to the columns. Like a standard one-way joist system, this system is not utilized as often as it once was because of cost.

The word structure (pronounced as “strək(t)SHər”) is a noun as well as a verb. As a noun, it means “the arrangement of and the relations between parts or elements of something complex,” and as a verb it refers to the process to “construct or arrange according to a plan; give a pattern or organization.” This term is used extensively in literature, and many disciplines signify these properties and processes.

When applied to the physical and built environment, the term “structure” means an assemblage of physical components and elements, each of which could further be a structure in itself, signifying the complexity of the system. The discipline of “Structural Engineering” refers to the verb part of the definition, dealing with the ways to arrange and size a system of components for construction according to a plan and serving the intended purpose. The primary purpose of any structure is to provide a stable, safe, and durable system that supports the desired function within the physical environment, of which the structure is a part of. The role of the structural engineer, therefore, is to “conceive, analyze, and design” the structure to serve its purpose.

There is hardly any aspect of our built environment or human activity that does not rely on physical structures. Buildings that provide useful places to live and work, bridges that provide us means to move across obstacles, factories that are needed to manufacture almost everything we need, dams to store water to generate power and irrigate lands, transmission towers to distribute electricity; all require structures to function, and structural engineers to design such structures. The role and importance of structural engineering and structural engineers is often underestimated and misunderstood. While a well-conceived and well-designed structure is the backbone of our built environment, a poorly conceived, designed, or constructed structure poses a serious hazard to the safety and well being of people and property. Collapse or failure of structures can claim a large number of lives and result in extensive economic loss. The role of structural engineers is, therefore, critical for overall economic development as well as for improving the community resilience to disasters.

The physical structure, or structure for brevity, can be assembled in infinite ways using very few basic element types or forms, some of which are shown in Fig. 1.1. As is evident, these are derived from or are consistent with basic geometric primitives such as line, curve, plane, surface, and solid. This compatibility can often be used to blend the form, function, and structure.

The assemblage of these components can be of many types and configurations. Some are made entirely of the skeleton-type members, some from surface-type members, and some from solid-type members, but most structures are a combination of more than one member type. Based on the member types, the structures can be broadly categorized as

• cable structures
• skeletal structural
• spatial structures
• solid structures, and
• a combination of the aforementioned categories.

Cable Structures: Using Cables as the Main Member Type

These structures primarily transfer forces and internal actions through tension in individual cables or a set of cables. The shapes or geometry of cable profile often govern the behavior. Examples of such structures are:

• Cable nets and fabric structures
• Cable stayed structures
• Cable suspended structures

Skeletal Structures: Using Beam-Type Members

These structures are composed of bar members that mainly resist the loads and forces through a combination of tension, compression, bending, shear, torsion, and warping. Such skeletal members are often called ties, struts, beams, columns, and girders. Typical application of skeletal structures can be found in the following:

• Truss structures
• Framed structures
• Grid and grillage structures

Spatial Structures: Using the Membrane/Plate/Shell-Type Members

These are structures created by spatial or surface type of elements and they transfer loads through a combination of bending, compression, torsion, and in-plane and out-of-plane shear of the element surface. The cross-sections of such members are generally rectangular. Examples of such structures include:

• General shell structures
• Dome-type structures
• Slab, wall structures
• Silos, chimneys, and stacks
• Box girder bridges

Sometimes, surface members and structures can be created by using a large number of skeletal members, covered by a skin or cladding, combining the fabric, cable, and bars to create surfaces.

Solid Structures: Using the Solid-Type Members

These structures comprise of solid bodies or members in which the forces or loads are transferred through the member bodies. Such members or structures do not have a cross-section in the conventional sense. Some of the structures are:

• Dams, thick arches, thick tunnels
• Pile caps, thick footings, thick slabs, pier heads, large joints, etc.

Mixed Structures: Using One or More of the Basic Element Types

Most often, the real structures are composed of one or more types of basic elements. For example, a typical building is made of columns and beams (skeletal), slabs and walls (shell), footings (solid) structures, etc.

There are several other types of structures that are either a combination of the basic forms or especially developed for a particular application or usage. These include stressed ribbon bridges, fabric structures, skeleton spiral structures, floating offshore structures, pneumatically inflated structures etc.

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.

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].

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.

The cycle bridge is part of the Noord-Om project which is a new section on the ring road around the village of Gemert in Netherlands. Although the team claims it to be World's 1st 3D printed bridge, there was already a 3D printed pedestrian bridge built this year in Madrid, Spain which in our opinion is the rightful deserver of the title '1st 3D printed bridge'. Another project was to be initiated by a company named MX3D to print a steel bridge with huge robotic arms but the project appears to be still.

Inauguration of the 3D printed bridge in Netherlands (Image courtesy: BAM Infra)

This Dutch bridge is a practical bridge for cyclists which was printed using a custom built cement printer at the Technical Universty of Eindhoven. Individual layers each having a thickness of 1cm are printed by injecting liquid mortar into the printhead through the storage. Steel reinforcing wire is fed down before a layer dries and the process goes on. The printer builds up sections layer by layer which are approximately 1m high.

3D-printed concrete bridge from Royal BAM Group nv

Six of the printed sections were transported to the site to be glued together to form a bridge that is 8 m long, 3.5 m wide and is 0.9 m thick. During the testing phase, the bridge was found to support an imposed load of 5 Tons which is far greater than that of the cyclists. The expected life of the bridge is 30 years. The bridge was built by the company BAM Infra and the team claims to have gained valuable experience from this versatile project which will allow the members to print much larger sections for much bigger structures.

3D printing in construction has endless possibilities and benefits. A very important aspect is the economical use of cement and other materials. Another aspect is not requiring a formwork or minimum if at all needed which also saves costs and material wastage.

1. REPAIRS USING EPOXY-BONDED STEEL PLATES

1.1 Introduction

It appears that the use of steel plates bonded to the tension face of reinforced concrete beams was developed in France and South Africa in the 1960s. In the UK the first recorded use of resin-bonded steel plates to strengthen an existing building was in about 1966; for the strengthening of road bridges the first use was in 1975 for the Quinton Interchange on the M5.

The use of steel plates bonded to the sides of beams to increase shear resistance would appear to be a possibility, but I have not found any record of such use. However, it has been used to stiffen reinforced concrete floor slabs by bonding the steel plates to the top surface of the slabs, and to strengthen the connections between beams and columns.

1.2 Information on the technique

This technique depends on composite action between the steel plates and the concrete, and therefore requires maximum bond between steel epoxy resin-concrete. This in turn requires very careful preparation of the contact surfaces. The steel must also be adequately protected against corrosion on the exposed surfaces. When the work is properly carried out and the bond strength has fully developed, the strength of the epoxy joint should exceed that of the concrete which will fail in horizontal shear.

The strength of epoxy resins deteriorates rather rapidly at temperatures in excess of about 65°C and this constitutes a fire hazard when used in building structures, but is of little significance in bridges.

A considerable amount of work has been carried out on the development of this technique at the Transport and Road Research Laboratory at Crowthorne, and at Sheffield University, and the universities of Dundee and Warwick.

The technique is discussed in detail in the Report by the Standing Committee on Structural Safety, set up by the Institution of Civil Engineers and the Institution of Structural Engineers. The Report recommends caution in the use of this method and emphasizes that it should not be used to strengthen structures in which the concrete is poor quality, or the reinforcement has suffered chloride attack, or the concrete has been damaged by alkali-silica reaction, unless these basic defects are first corrected.

2. THE USE OF FIBER-REINFORCED PLASTICS

According to Research Focus, No. 21, April 1995, the use of fiber reinforced plastic (FRP) as a substitute for steel has been investigated in Switzerland, Germany and the USA, and at Oxford Brookes University under an EPSRC (Engineering and Physical Sciences Research Council) grant.

2.1 Carbon fibre composites

1. Introduction

Deep cracks in massive concrete members and cracks which pass right through a structural member can often be repaired satisfactorily by crack injection using a selected polymer such as an epoxy resin.

When properly carried out, crack injection will significantly improve the structural strength of the member. If corrosion or rebars and spalling of concrete has already occurred, then a satisfactory method of repair may be to remove defective concrete down to the rebars, clean off the rebars, remove all grit and dust etc., inject the crack with a suitable resin and then fill in the cut-out section of concrete with an SBR modified mortar. In appropriate cases, this repair method can be combined with crack injection, e.g. to avoid cutting out on both sides of the member.

2. Essential features of crack injection

The essential crack injection consists of injecting a suitably formulated resin into the cracks. This should bond the concrete together across the crack and should form an effective seal against ingress of water or other liquids, and reduce the ingress of carbon dioxide.

Correct formulation of the resin is of vital importance; present-day polymer resins provide considerable scope for variations in the formulation so as to obtain optimum characteristics for each particular job.

The resins in general use are epoxy, polyurethane and polyester, in that order. Desirable qualities for the formulated resin include:

a. low viscosity, (to facilitate penetration into the crack);

b. formation of good bond to damp concrete;

c. suitability for injection in a wide temperature range;

d. low-curing shrinkage;

e. toughness (low modulus of elasticity combined with high yield point);

f. curing time to suit injection conditions;

g. resistance to aggressive chemicals may be required;

h. durability under service conditions.

3. The injection process

This work is highly specialized and should only be entrusted to firms with a proven record of success. Unsuccessful crack injection by one firm is likely to result in another (more experienced firm) being unable to rectify the situation; I have come across this unfortunate state of affairs on more than one occasion.

The crack injection process is carried out in the following phases:

a. preparation of the cracks;

b. location of injection points and surface sealing;

c. injection of resin;

d. removal of injection nipples (if used) and plugging the holes

e. removal of sealing strip and application of any surface treatment which may be required.

4. Preparation of the cracks prior to injection

This should consist of the removal of any dirt or loose weak material on the surface, followed by cleaning out of the crack if this is considered necessary. It is seldom that cracks less than 0.5 mm wide require cleaning unless they have been fouled by the use to which the structure has been put. Compressed air can be used for this cleaning work; solvents may be required in special cases.

5. Location of injection points and surface sealing

The distance apart of the injection points will depend largely on the depth and width of the crack. The object is to have as few injection points as possible consistent with maximum resin penetration with low operating pressure. They are either holes drilled on the line of the crack, or nipples screwed into the concrete; the use of nipples is usually reserved for high pressure work (See Figure 1.).

6. Injection of the resin

As stated above, crack injection work should only be entrusted to specialist firms with a good ‘track record’ and preferably to firms which formulate their own resins. The correct formulation of the resin to suit the particular requirements of each job is of primary importance. If the formulator and the applicator are the same firm, any question of divided responsibility is avoided. For larger jobs, some adjustments in formulation may be required during the execution of the work to meet unforeseen conditions and this requires close co-operation between the staff on site and the laboratory.

Firms specializing in crack injection develop their own techniques and equipment. Some firms favor simple means of injection by gravity feed or pressure guns, the resin being premixed in batches. Others use sophisticated equipment for continuous feed of freshly mixed resin and hardener through separate feed pipes, bringing the two materials together at a specially designed nozzle. One firm supplements pressure-feed by the use of a vacuum mat to assist penetration.

Figure 2 Diagram of crack injection and coating of thermal contraction crack in concrete wall.

In appropriate cases, the degree of penetration can be checked by coring or by a UPV survey or impulse radar. The aim of all injection processes is to obtain uniform penetration of the resin and complete filling of the cracks. It has been found in practice that a deliberate fluctuation in the injection pressure can be more effective than an increase in continuous pressure. The volume of resin used in filling cracks is very small.

Where cracks are inclined or vertical, it is usual to commence injection at the lowest injection point and work upwards. For horizontal cracks there is no fixed order of work; the injection can start at one end and work along the crack to the other end, or start in the middle and work first left and then right, or alternately left and right.

7.  Final work following injection

It is usual to remove the injection nipples (if they are used) and seal the holes when the resin has set. Crack injection, except where the cracks are very wide on the surface, is likely to be much less conspicuous than cutting out and repairing with mortar. Nevertheless, the injected crack will be visible. Where appearance is important, surface grinding and some ‘cosmetic’ treatment will help to mask the crack.

Figure 2 shows a repair using crack injection.

INVESTIGATIONS FOR STRUCTURAL DEFECTS

1. What is a failure?

A failure can be considered as occurring in a component when that component can no longer be relied upon to fulfill its principal functions. Limited deflection in a floor which caused a certain amount of cracking/distortion in partitions could reasonably be considered as a defect but not a failure. While excessive deflection resulting in serious damage to partitions, ceiling and floor finishes could be classed as a failure.

2. Introduction

This section considers the situation if the initial inspection/investigation detailed in Chapter 4 indicated that some parts of the structure may require strengthening. This can arise for three basic reasons:

a. serious deterioration of some of the structural members;

b. serious overloading of members.

c. proposed change of use involving substantial increase in floor loading.

3. Indications of structural defects

What are the likely signs of structural distress? No precise answer can be given to this question, but the following brief notes are relevant:

a. Diagonal cracks in beams and walls usually denote high shear stress and should be investigated.

b. Excessive deflexion in beams and floor slabs indicates that the members are over-loaded. This is also likely to show as cracking in the soffit at right angles to the main reinforcement (flexural cracking).

c. Bowing in columns and load-bearing walls is likely to cause cracking parallel to the main reinforcement.

4. Bowing in wall panels may be due to differential shrinkage/thermal effects between one face and the other.

5. Errors in the location, design and/or execution of movement joints, isolation joints, stress relief joints and sliding joints can result in cracking, spalling and distortion. This type of defect can be very difficult to rectify.

4. Investigation procedure

It will be seen from the previous section that visible cracking plays an important part in indicating that the structure or parts of the structure are suffering from structural distress. In other words, the members affected were unable to carry the loads imposed on them with an adequate factor of safety.

Such a state of affairs may be brought about by:

a. error(s) in design;

b. errors in construction (workmanship and/or materials);

c. actual loading significantly in excess of the design load;

d. physical damage, impact, explosion fire etc.

e. serious corrosion of reinforcement, which may be the result of many factors.

The engineer should make every effort to obtain copies of the structural calculations and as-built drawings. Unfortunately, this important information is often not available, in which case a ‘structural appraisal’ would be needed and this is time-consuming and expensive.

Assuming that adequate background information is available the general procedure is basically the same but with the emphasis on obtaining information for a practical diagnosis of the structural shortcomings. Additional methods of investigation may include:

a. Ultrasonic Pulse Velocity (UPV) survey

b. an impulse radar survey

c. core testing for strength

d. load tests (seldom used)

It should be noted that the above methods are supplementary to normal investigation techniques, and often used in combination.

4.2 Impulse radar survey

The author is indebted to GB Geotechnics for the information which follows. A transducer containing the transmit and receive antennae is drawn over the surface under investigation at a constant speed. Pulses of energy are transmitted into the material and are reflected from internal surfaces and objects, e.g. changes in density, voids, reinforcing steel. The data is recorded graphically or digitally thus enabling a preliminary assessment on site, followed, if considered necessary, by detailed processing in the laboratory.

Radar responds to changes; it can identify boundaries between layers, measure thicknesses and assess voids and relative moisture content. The radar profile is effectively continuous, radio pulses are transmitted at around 50 000 pulses per second.

Transducers can be hand-held or mounted below survey vehicles, and can be operated up to 200 m from the recording station.

4.3 Core testing for strength

The location of the cores should be carefully selected to provide the information required and for checking the results of UPV and radar surveys.

The cores should be cut, prepared and tested in accordance with the appropriate National Standard; in the UK this is BS 1881: Part 120.

Reference should also be made to BS 6089: Assessment of Concrete Strength in existing Structures, and to Concrete Society Technical Report No. 11.

Misunderstandings sometimes arise over the interpretation of the test results. The actual test on the core will give the compressive strength of the concrete in the core. BS 6089 refers to the ‘estimated in situ cube strength’ which is defined as ‘The strength of concrete at a location in a structural member estimated from indirect means and expressed in terms of specimens of cubic shape’. The Concrete Society Report refers to two types of strength; firstly: ‘Estimated Potential Strength’ which is defined as:

The strength of concrete sampled from an element and tested in accordance with this procedure, such that the result is an estimate of the strength of the concrete provided for manufacture of the element, expressed as the 28 day BS.1881 cube strength, allowance being made for differences in curing, history, age, and degree of compaction between core and BS.1881 cube.

The report also provides for a correction for the influence of included steel. When all these corrections have been made, the result is intended to give the 28-day cube strength of the concrete if cubes had been made and tested in accordance with BS 1881, at the time the member was cast. The intention is to provide an acceptable answer to the questions which arise in new construction when cubes fail. Many experienced engineers feel that with so many corrections only limited reliance can be placed on the results.

The second ‘type of strength’ referred to in the CS Report is ‘The Estimated Actual Strength’; this is defined as:

The strength of concrete sampled from an element and tested in accordance with this procedure, such that the result, expressed as an equivalent cube strength, is an estimate of the concrete strength as it exists at the sampling location, without correction for the effect of curing, history, age or degree of compaction.

The majority of investigations involving existing buildings are concerned with a reasonable assessment of actual strength as defined above, of the concrete in the load-bearing members.

4.4 Load tests

The testing described above should provide information on the general quality of the concrete and condition of the reinforcement. For the engineer to be able to predict with reasonable accuracy the load-carrying capacity of the various structural elements—beams, columns, floor slabs etc.—the following information would also be required:

a. original or, preferably, the as-built drawings of the structure;

b. similar information on any alterations made subsequently;

c. assessment of existing dead and live loads based on the present use;

d. assessment of dead and live loads which will arise from any proposed alterations.

When there is serious doubt about the value of the information available, consideration may have to be given to a load test on selected structural elements. It is accepted that design assumptions do not exactly match the as-built conditions; this is due mainly to the effects of composite action and load sharing. A load test on a beam or floor slab, if correctly carried out, will show how the element under test will react to the applied load under working conditions. During the test it is necessary to record deflections, recovery on removal of load, and details of any crack development.

Load tests must be carried out with great care by an experienced firm with an experienced engineer on site during the test. Provision must be made to deal with any unexpected collapse. All necessary safety precautions must be observed.

Load tests are time-consuming and expensive and should only be carried out after careful consideration of practical value of the results.