The concrete is required to be poured for curing and formation of the desired shape. The general techniques used for formwork are roadform that is a permanent arrangement of shuttering, and timber shuttering that is a customized temporary structure.
Concrete Formwork Basics
Concrete needs to be poured into an enclosed space and remain there until it adequately consolidates to maintain its shape. Concrete that has been poured newly for concrete slabs can be preserved in shape by existing features like walls and edgings. Alternatively, provisional shuttering, which is also called formwork, could be necessary. On vertical structures, formwork construction can be difficult and is therefore usually carried out by professional formwork erectors. However, slab work at ground level is usually less difficult and needs only a simple formwork. In all the cases, whether vertical structures or work at the ground, the formwork should be strong. It must be able to bear the forces produced by the wet concrete, including the weight of the vibration producing equipment. The formwork joints must be fastened adequately and prevent the mix leak during the vibration and curing. The primary formwork types being used for producing ground slabs are steel roadform and customized timber shuttering.
Roadform
Roadform, a permanent type of formwork used for poured concrete for housing and other structures is usually preferred for use since it is strong, capable of bearing different loads, and is a permanent arrangement. It can beconcrete formwork 1 used for construction work at different sites, involves a moderate skill to configure the necessary arrangement, and is inexpensive. It consists of steel sections that are channel-shaped, binding brackets, and an arrangement for the linking of adjacent sections. The units are stacked to create space for the required concrete depth. However, the maximum height is determined by the height of the steel pins that hold the system in its position. The steel pins are located with the bracket and forced into the ground. Each section is aligned accurately and held by tightening the fastening wedge. It is ensured that the roadform is firmly held in its place. Roadform is erected by aligning the sections with a rigid cord and by ensuring that the sections are accurate vertically. The pins should be firm with no gaps between the sections.
Timber Shuttering
Timber shuttering Roadform is only suitable for simple slabs. Timber shuttering, however, may be necessary for complicated work. The enormous benefit of the timber shuttering is that since it is created at the site, it can be customized to any desired configuration. Timber shuttering is the normal option for vertical concrete works. It is also used in combination with the steel elements. The shutter part in contact with concrete is called "form lining," while the supporting timbers are identified as "bracing." The bracings are created from timber that is straight, robust, and capable of bearing the desired loads. The form lining is generally some type of plywood or hardboard. Formwork is a professional trade, especially for the vertical structures. Bracing that is exceptionally intricate is designed by structural engineers.
Saturday, April 17, 2010
Cement Basic - Structural engineering
Concrete and cement are not the same thing. Learn the difference between the two and learn all about the different types of cement that are available for construction projects.
How many times have you heard someone say walk on the cement sidewalk? How about the concrete sidewalk? Which is correct? Actually, the correct phrase is "concrete sidewalk.” Many people use the terms interchangeably, but actually they are different.
Concrete is a mixture of cement, water, sand, and gravel. By volume, 10 to 15% of concrete is cement. As concrete hardens, the mixture becomes stronger. The hardening process can take years. Concrete has the ability to withstand the pressure of heavy loads because it has high compression strength. It can also be molded into any shape, can be made porous or watertight, and is a relatively cheap material for use in construction projects.
Cement is powder and is one of the main ingredients in concrete. Cement and concrete have been used in construction since at least the Roman Empire. Modern cement is made of limestone, silicon, calcium, and often aluminum and iron.
The type of cement used in almost all concrete is Portland cement. Portland cement has been around since 1824. The name Portland does not refer to a brand name, as many might think. The original inventor, Joseph Aspdin, was a British bricklayer and named his new invention “Portland” because its color reminded him of the color of the natural limestone on the Isle of Portland which is a peninsula in the English Channel.
Although Portland cement is the main cement used in concrete, there are other types of cement. The three types that are often mentioned are Portland, blended, and hydraulic. All Portland and blended cements are actually hydraulic cement. What is hydraulic cement, though? Hydraulic cement is actually the generic term in the construction industry. It refers to any cement that will set and harden after it is combined with water. Most modern construction cements are hydraulic. There are six different types of hydraulic cement:
* Type GU: General Use
* Type HE: High Early Strength
* Type MS: Moderate Sulfate Resistance
* Type HS: High Sulfate Resistance
* Type MH: Moderate Heat of Hydration
* Type LH: Low Heat of Hydration
Portland cement is a particular type of hydraulic cement. Portland cement contains hydraulic calcium silicates. There are eight specific types of Portland cement that fall into categories ranging from Type I to Type V. Type I and Type IA are general purpose cements. Type II and Type IIA contain tricalcium aluminate, but no more than 8%. To compare to the hydraulic cement types, some of the Type II cements meeting the standard for the moderate heat of hydration type.
Type III and Type IIIA are similar to Type I cements. However, they have higher early strengths because they are ground finer. Type IV cements are used in special types of structures that require a small amount of heat to be generated from hydration. Type IV cements develop their strength over a longer period of time when compared to other types. Finally, Type V cement has a high sulfate resistance which means it contains no more than 5% tricalcium aluminate.
The third type of cement is blended cement. Blended cement is also hydraulic cement and is made by mixing two or more materials. Usually the primary materials used in blended cement are Portland cement and slag cement. Fly ash, slica fume, calcined clay, pozzolan, and hydrated lime are also used. There are two main types of blended cement:
* Type IS (X): Portland blast furnace slag cement
* Type IP (X): Portrland-pozzolan cement
The X represents the amount of the second material that is in the mixture.
The reason that there are different types of cements is not only required because of the different uses of the cement, but also because of the type of materials available differ by location. Many of the types described above actually cross-over between the different categories. This allows for flexibility in particular construction projects. For example, different pozzolans and slag are available in different regions. As long as the desired properties of the concrete can be achieved usually, there is flexibility in the final choice of cement that is used.
Read more: http://www.brighthub.com/engineering/civil/articles/42795.aspx#ixzz0lM5zjmp3
How many times have you heard someone say walk on the cement sidewalk? How about the concrete sidewalk? Which is correct? Actually, the correct phrase is "concrete sidewalk.” Many people use the terms interchangeably, but actually they are different.
Concrete is a mixture of cement, water, sand, and gravel. By volume, 10 to 15% of concrete is cement. As concrete hardens, the mixture becomes stronger. The hardening process can take years. Concrete has the ability to withstand the pressure of heavy loads because it has high compression strength. It can also be molded into any shape, can be made porous or watertight, and is a relatively cheap material for use in construction projects.
Cement is powder and is one of the main ingredients in concrete. Cement and concrete have been used in construction since at least the Roman Empire. Modern cement is made of limestone, silicon, calcium, and often aluminum and iron.
The type of cement used in almost all concrete is Portland cement. Portland cement has been around since 1824. The name Portland does not refer to a brand name, as many might think. The original inventor, Joseph Aspdin, was a British bricklayer and named his new invention “Portland” because its color reminded him of the color of the natural limestone on the Isle of Portland which is a peninsula in the English Channel.
Although Portland cement is the main cement used in concrete, there are other types of cement. The three types that are often mentioned are Portland, blended, and hydraulic. All Portland and blended cements are actually hydraulic cement. What is hydraulic cement, though? Hydraulic cement is actually the generic term in the construction industry. It refers to any cement that will set and harden after it is combined with water. Most modern construction cements are hydraulic. There are six different types of hydraulic cement:
* Type GU: General Use
* Type HE: High Early Strength
* Type MS: Moderate Sulfate Resistance
* Type HS: High Sulfate Resistance
* Type MH: Moderate Heat of Hydration
* Type LH: Low Heat of Hydration
Portland cement is a particular type of hydraulic cement. Portland cement contains hydraulic calcium silicates. There are eight specific types of Portland cement that fall into categories ranging from Type I to Type V. Type I and Type IA are general purpose cements. Type II and Type IIA contain tricalcium aluminate, but no more than 8%. To compare to the hydraulic cement types, some of the Type II cements meeting the standard for the moderate heat of hydration type.
Type III and Type IIIA are similar to Type I cements. However, they have higher early strengths because they are ground finer. Type IV cements are used in special types of structures that require a small amount of heat to be generated from hydration. Type IV cements develop their strength over a longer period of time when compared to other types. Finally, Type V cement has a high sulfate resistance which means it contains no more than 5% tricalcium aluminate.
The third type of cement is blended cement. Blended cement is also hydraulic cement and is made by mixing two or more materials. Usually the primary materials used in blended cement are Portland cement and slag cement. Fly ash, slica fume, calcined clay, pozzolan, and hydrated lime are also used. There are two main types of blended cement:
* Type IS (X): Portland blast furnace slag cement
* Type IP (X): Portrland-pozzolan cement
The X represents the amount of the second material that is in the mixture.
The reason that there are different types of cements is not only required because of the different uses of the cement, but also because of the type of materials available differ by location. Many of the types described above actually cross-over between the different categories. This allows for flexibility in particular construction projects. For example, different pozzolans and slag are available in different regions. As long as the desired properties of the concrete can be achieved usually, there is flexibility in the final choice of cement that is used.
Read more: http://www.brighthub.com/engineering/civil/articles/42795.aspx#ixzz0lM5zjmp3
Friday, April 2, 2010
Collapse Modelling of Soft-Storey Buildings
Studies undertaken by the authors in recent years have indicated that the existing building stock at most risk of damage and collapse from earthquake excitation in lower seismicity regions such as Australia are unreinforced masonry buildings and soft-storey structures. Soft-storey buildings possess storeys that are significantly weaker or more flexible than adjacent storeys, and where deformations and damage tend to be concentrated. Soft-storeys commonly occur at the ground floor, where the functional requirements dictate a higher ceiling level or a more open configuration, such as for car parking or retail space, resulting in an inherently weaker and more fl exible level, as shown in figure 1. In high seismic regions, soft-storey structures and unreinforced masonry are banned, yet in regions of lower seismicity such building types and configurations are common, and are often occupied by organisations with a post-disaster function or house a significant number of people. This paper will address the performance of soft-storey buildings under earthquake excitations specifically. Research findings presented in this paper are directly relevant to low-moderate seismic regions worldwide such as Thailand, Vietnam, Hong Kong, China and Singapore, where similar soft-storey structures of limited ductility are commonly constructed.
Soft-storey buildings are considered to be particularly vulnerable because the rigid block at the upper levels has limited energy absorption and displacement capacity, thus leaving the columns in the soft-storey to defl ect and absorb the seismic energy. Collapse of the building is imminent when the energy absorption capacity or displacement capacity of the soft-storey columns is exceeded by the energy demand or the displacement demand. This concept is best illustrated using the Capacity Spectrum Method shown in figure 2, where the seismic demand is represented in the form of an acceleration-displacement response spectrum (ADRS diagram) and the structural capacity is estimated from a non-linear push-over analysis expressed in an ADR (as illustrated in Wilson & Lam, 2006).
The structure is considered to survive the design earthquake if the capacity curve intersects the demand curve, and collapse if the curves do not intersect. In regions of high seismicity, the maximum displacement demand could exceed 200-300 mm, which translates to a drift in the order of 5-10% in a soft-storey structure. Such drift demands are significantly greater than the drift capacity of soft-storey structures even if the columns have been detailed for ductility. This is the reason soft-storey structures have behaved poorly and collapsed in larger earthquake events around the world. In high seismic regions, buildings are configured and detailed so that in an extreme event a rational yielding mechanism develops to dissipate the energy throughout the structure and increase the displacement capacity of the building. Ductile detailing in reinforced concrete columns includes closely-spaced closed stirrups to confine the concrete, prevent longitudinal steel buckling and to increase the shear capacity of columns (Mander, 1988; Park, 1997; Paulay & Priestley, 1991; Watson et al, 1994; Priestley & Park, 1987; Bae et al, 2005, Priestley, 1994; Bayrak & Sheikh, 2001; Berry & Eberhard, 2005; Pujol et al, 2000; Saatcioglu & Ozcebe, 1992). The emphasis is on the prevention of brittle failure modes and the encouragement of ductile mechanisms through the formation of plastic hinges that can rotate without strength degradation to create the rational yielding mechanism.
Current detailing practice in the regions of lower seismicity typically allow widely spaced stirrups (typical stirrup spacing in the order of the minimum column dimension) resulting in concrete that is not effectively confined to prevent crushing and spalling, longitudinal steel that is not prevented from buckling, and columns that are weaker in shear. Design guidelines that have been developed in regions of high seismicity (ATC40, FEMA273) recommend a very low drift capacity for columns that have such a low level of detailing. The application of such standards in the context of low-moderate seismicity regions results in most soft-storey structures being deemed to fail when subject to the earthquake event consistent with a return period in the order of 500-1500 years.
Soft-storey buildings are considered to be particularly vulnerable because the rigid block at the upper levels has limited energy absorption and displacement capacity, thus leaving the columns in the soft-storey to defl ect and absorb the seismic energy. Collapse of the building is imminent when the energy absorption capacity or displacement capacity of the soft-storey columns is exceeded by the energy demand or the displacement demand. This concept is best illustrated using the Capacity Spectrum Method shown in figure 2, where the seismic demand is represented in the form of an acceleration-displacement response spectrum (ADRS diagram) and the structural capacity is estimated from a non-linear push-over analysis expressed in an ADR (as illustrated in Wilson & Lam, 2006).
The structure is considered to survive the design earthquake if the capacity curve intersects the demand curve, and collapse if the curves do not intersect. In regions of high seismicity, the maximum displacement demand could exceed 200-300 mm, which translates to a drift in the order of 5-10% in a soft-storey structure. Such drift demands are significantly greater than the drift capacity of soft-storey structures even if the columns have been detailed for ductility. This is the reason soft-storey structures have behaved poorly and collapsed in larger earthquake events around the world. In high seismic regions, buildings are configured and detailed so that in an extreme event a rational yielding mechanism develops to dissipate the energy throughout the structure and increase the displacement capacity of the building. Ductile detailing in reinforced concrete columns includes closely-spaced closed stirrups to confine the concrete, prevent longitudinal steel buckling and to increase the shear capacity of columns (Mander, 1988; Park, 1997; Paulay & Priestley, 1991; Watson et al, 1994; Priestley & Park, 1987; Bae et al, 2005, Priestley, 1994; Bayrak & Sheikh, 2001; Berry & Eberhard, 2005; Pujol et al, 2000; Saatcioglu & Ozcebe, 1992). The emphasis is on the prevention of brittle failure modes and the encouragement of ductile mechanisms through the formation of plastic hinges that can rotate without strength degradation to create the rational yielding mechanism.
Current detailing practice in the regions of lower seismicity typically allow widely spaced stirrups (typical stirrup spacing in the order of the minimum column dimension) resulting in concrete that is not effectively confined to prevent crushing and spalling, longitudinal steel that is not prevented from buckling, and columns that are weaker in shear. Design guidelines that have been developed in regions of high seismicity (ATC40, FEMA273) recommend a very low drift capacity for columns that have such a low level of detailing. The application of such standards in the context of low-moderate seismicity regions results in most soft-storey structures being deemed to fail when subject to the earthquake event consistent with a return period in the order of 500-1500 years.
Structural Testing - Engineering
Computer models are useful in structural analysis, but are no substitute for laboratory testing. An important facet of structural testing and design, field and laboratory evaluation of engineering materials provides invaluable design and forensic information for structural engineers.
The Need For Structural Testing
In this age of computer modeling and analysis one might begin to think that structural testing of engineering designs and materials is soon to become a thing of the past. Reams of tabulated data on material properties, limits of failure, safety factors, and other pertinent information already exist, so why bother with the trouble and expense of performing actual tests? Aren’t field testing laboratories for construction and civil engineering getting anachronistic? No. Raw materials are never consistent, construction and manufacturing processes are never perfect, and structural requirements constantly change with new applications, designs, and desired functionality. For these reasons, field and laboratory structural analysis, engineering design and testing, and materials engineering and testing will continue for as long as there is a need for structural engineering
Structural Analysis With Field and Laboratory Testing
Structural testing is performed as a means to investigate the performance of materials, assemblies, and designs utilized in civil engineering and mechanical engineering projects. The engineering laboratory testing of materials can evaluate structural components for various physical properties such as response to stress, strain, and loads under a variety of conditions such as varying temperature and accelerated environmental aging. Fatigue cracking, shear flows, compressive and tensile load failures, strain response, elastic and viscous behaviors, sound and vibration dampening, combustion resistance, impact characteristics, creep, and ductility are some of the typical analyses performed. A structural testing laboratory can also analyze assemblies of engineering materials for expected and maximum response under static and cyclic loading, identify unexpected load distribution and/or response, determine the strength of connections (such as pull out resistance of fasteners, welding joints, etc.), gauge wind resistance and airflow characteristics, seismic response, racking characteristics of panels and assemblies, beam and truss load response, airframe resilience, forensic analysis, and so on. These analyses help pinpoint structural weaknesses and verify design safety factors that can help acquire the structural engineer’s stamp of approval, or help the forensic structural engineer determine cause of failure.
And testing is not limited to laboratory facilities. Field testing laboratories for construction and civil engineering perform special inspections for foundations, concrete cure, anchoring adhesives and connections, welding joints, presence of specified structural elements such as high strength fasteners, structural steel assemblies, and other critical design elements required for the safe performance of civil engineering structures. Field testing is not limited to civil applications but can also be conducted for mechanical structural testing as well. By installing load cells, strain gauges, and various other transducers and sensors the response of automobile and aircraft frames, for example, can be ascertained under actual usage conditions. Impact analysis of these structures is an especially active field as part of an ongoing effort to improve the protection of the occupants during accidents, crashes, and explosive events.
About The Author:
John Moehring is a practicing Engineering Technologist in civil, geological, biological, and electrical engineering fields. And one of these days he may actually get it right.
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