Hello to my readers..
If you own a business in one of these industries, the choice of a crane is critical for the success and efficiency of your business. So, here are some important factors that you must take into account for buying a crane.
Types of Crane
To begin with, you must know the different types of cranes available in the market. You must decide whether you need a mobile or fixed crane. You must also learn about overhead cranes, gantry cranes, monorail cranes, material lifting cranes, heavy duty cranes and other options to decide which one suits your requirements.
Lifting Capacity
An important consideration while choosing a crane is the maximum weight lifting capacity specified by the manufacturer. Different applications require materials with different weights to be lifted. So, the crane you select must be capable of supporting the maximum weight to be lifted at your workplace. Make sure that you dont oversize the capacity of the crane purchased.
Usage Limit
Another deciding factor to buy one of the industrial cranes is the limit of its usage. You must know how many times the crane would be used per hour or per shift. Besides, you must also know the shifts per day and the days per week for which the crane would be used. This should help you choose the crane that withstands a particular usage limit.
Learn about Components
Cranes with different components are suitable for different working environments. So, you must take into account the components like mechanical components and electrical components. Besides, you must also learn about the motor controls and operator controls to ensure that the right crane is purchased.
Speeds and Distances
Learn about the speed with which the crane lifts a particular quantity of eight. At the same time, you must learn about the maximum height or horizontal distance that the crane can move. All these factors help you choose the suitable industrial cranes that add efficiency to your working style.
Maintenance Required
Finally, you must consider the maintenance required for different parts of the crane, including motors, gearboxes, bearings, wire ropes and so on. Ask for warranty from the seller of industrial cranes.
Taking all these things into consideration should help you invest money in right types of gantry cranes, heavy duty cranes or other cranes.
Ok, thats all for this post..
See u next post..
Saturday, October 23, 2010
Sunday, August 29, 2010
Avoiding Pitfalls In Crane Projects
You have a shiny new building with a shiny new crane and everything looks great. For some reason, though, the crane won’t clear the building columns, even though the contractor and the crane manufacturer are saying everything is to spec and it’s not their problem. Common sense says somebody is wrong and that somebody should have to pay (because it’s going to cost a bundle). Unfortunately in this case, there’s a giant crack in the building specs, and you’ve just fallen through it. This means that after all the arguing and legal costs, you’re still going to have to pay to get it fixed. If you’ve already fallen in this black hole, there’s not much you can do, but if you are about to embark on a new building with an overhead crane, this article will show you where the cracks are and suggest how to bridge them safely.
What Is Required?
The runway alignment specs—written by the Crane Manufacturers Association of America (CMAA) and adopted by the Metal Building Manufacturers Association (MBMA), the American Institute of Steel Construction (AISC), and the Association of Iron and Steel Engineers (AISE)— fill an entire page and take considerable time to interpret. A simplistic summary is that runways must be ±1⁄4 inch in a single bay and no more than ±3⁄8 inch over the full length of the runway.
These tolerances must be maintained in four ways: left/right, up/down, parallel to each other, and level in respect to each other. Figure 1 shows an actual AISC/ CMAA chart.
A second set of crane-related numbers to remember are the crane-to-building tolerances. CMAA and the Occupational Safety and Health Administration (OSHA) require that all moving
objects (the crane and hoist) must clear all stationary objects (the building) horizontally by 2 inches and clear all vertical objects (roof trusses, lights, pipes, etc.) by 3 inches. Although this meets the legal requirements, this author highly recommends the horizontal be increased to
4 inches and the vertical to 6 inches to allow for unforeseen problems.
Where’s the Villain?
As in a detective story, the first move is to round up the suspects. The problem can be found
in one of four areas:
1. Mill steel tolerances
2. Building steel fabrication tolerances
3. Building erection tolerances
4. Overhead crane runway tolerances (measuring and verification methods)
One big problem is that runways usually are built with building steel (wide flanges), fabricated
by building steel fabricators, and installed by building steel erectors, but runway steel is not building steel. In fact, building steel and runway steel are incompatible in the first three ways listed previously. Following is an illustration of just the first point—mill steel tolerances—but the other two items exhibit similar shortcomings. The mill tolerance for structural wide-flange
beams basically is 1⁄8 inch per 10 feet of length, although this oversimplifies the American
National Standards Institute (ANSI)/AISC specification somewhat (see Figure 2).
Therefore, in a common 30-foot bay, the wide-flange beam can have a sweep (horizontal bow) of 3⁄8 inch, which means that putting up this first piece of steel exceeds the acceptable CMAA/MBMA/AISC
runway tolerance already. To compound the problem, the opposing runway can have an equal (but opposite) sweep, doubling the problem.
Solutions
How should this seemingly simple problem be addressed? Three potential solutions exist:
1. Adjust the rail laterally in relation to the girder. Although this solution is the most commonly used, it is bad engineering practice and actually is prohibited by the AISC specifications.
The runway beam/girder is the wide-flange structural shape that supports the runway, while the rail (commonly American Society of Civil Engineers (ASCE) rail, similar to railroad rail) is the track upon which the end truck wheels traverse (see Figure 3). It is a common misconception that the runway beams have no particular installation tolerance and that only the rail is at issue. Further, this assumption seems to be confirmed by the lateral adjustment of the rail fasteners (for example, Jbolts/ hook bolts or patented clips). Actually, the tolerance of the beam installation is governed by the tolerance of the rail installation. This is because, according to AISC Design Guide 7, paragraph 19a, the centerline of the rail should be within ±3⁄4 inch of the girder web thickness. This prevents top flange rollover and subsequent fillet cracking and possibly girder failure.
This conventional wisdom is so commonly accepted that it has evolved into generally accepted practice. Unfortunately, like so much conventional wisdom, it’s wrong, it’s bad for the equipment, it will result in significantly shorter service life, and it can be dangerous.
2. Augment the specs. Just because the generally accepted specs have left the crane runways as an orphan does not mean that you as a prospective new building owner should not include a stop-gap page of specs to cover yourself. If you buy the steel from the same vendor, fabricate
with the same fabricator, and install with the same installation crew, you very likely will end up with the same problem. It defies reason that any efficient contractor can buy, fabricate, and install 20+ pieces of apparently identical red primed steel to a tolerance two to four times tighter than the other several thousand pieces of red steel in that same building. This is not meant to slight building contractors.
Successful contractors have set up a well-disciplined system to produce and install building steel, but runway steel, although similar-looking, is a significantly different animal. While it is unlikely the contractor would adopt this more stringent standard temporarily, it is not impossible.
The silver lining for you, the buyer, in using the augmented specs as part of the contract is that the corrections no longer are your problem or expense.
3. Redefine the scope of building contractor and crane supplier responsibilities. This technically
correct, practically viable solution is the least used of the three, simply because of lack of knowledge and higher up-front costs. The common scope of the crane builder’s contract is to supply and install the crane, runway rail, and conductor bar. This leaves a critical gap in which the buyer is exposed to the previously mentioned problems. The scope should be changed to move responsibility for the runway girders from the building contractor to the
crane builder. Chances are, the crane builder will insist on very tight tolerances from the steel supplier and will take precautions to account for reasonable floor and column tolerances. Also, having the crane builder’s employees install the runways can help to improve installation accuracy because this job is their specialty. If the plant is a union plant, however, the runway conductor bar installation should be awarded to a local electrical contractor, while the crane builder remains responsible for the bar.
Get It Right the First Time
What Is Required?
The runway alignment specs—written by the Crane Manufacturers Association of America (CMAA) and adopted by the Metal Building Manufacturers Association (MBMA), the American Institute of Steel Construction (AISC), and the Association of Iron and Steel Engineers (AISE)— fill an entire page and take considerable time to interpret. A simplistic summary is that runways must be ±1⁄4 inch in a single bay and no more than ±3⁄8 inch over the full length of the runway.
These tolerances must be maintained in four ways: left/right, up/down, parallel to each other, and level in respect to each other. Figure 1 shows an actual AISC/ CMAA chart.
A second set of crane-related numbers to remember are the crane-to-building tolerances. CMAA and the Occupational Safety and Health Administration (OSHA) require that all moving
objects (the crane and hoist) must clear all stationary objects (the building) horizontally by 2 inches and clear all vertical objects (roof trusses, lights, pipes, etc.) by 3 inches. Although this meets the legal requirements, this author highly recommends the horizontal be increased to
4 inches and the vertical to 6 inches to allow for unforeseen problems.
Where’s the Villain?
As in a detective story, the first move is to round up the suspects. The problem can be found
in one of four areas:
1. Mill steel tolerances
2. Building steel fabrication tolerances
3. Building erection tolerances
4. Overhead crane runway tolerances (measuring and verification methods)
One big problem is that runways usually are built with building steel (wide flanges), fabricated
by building steel fabricators, and installed by building steel erectors, but runway steel is not building steel. In fact, building steel and runway steel are incompatible in the first three ways listed previously. Following is an illustration of just the first point—mill steel tolerances—but the other two items exhibit similar shortcomings. The mill tolerance for structural wide-flange
beams basically is 1⁄8 inch per 10 feet of length, although this oversimplifies the American
National Standards Institute (ANSI)/AISC specification somewhat (see Figure 2).
Therefore, in a common 30-foot bay, the wide-flange beam can have a sweep (horizontal bow) of 3⁄8 inch, which means that putting up this first piece of steel exceeds the acceptable CMAA/MBMA/AISC
runway tolerance already. To compound the problem, the opposing runway can have an equal (but opposite) sweep, doubling the problem.
Solutions
How should this seemingly simple problem be addressed? Three potential solutions exist:
1. Adjust the rail laterally in relation to the girder. Although this solution is the most commonly used, it is bad engineering practice and actually is prohibited by the AISC specifications.
The runway beam/girder is the wide-flange structural shape that supports the runway, while the rail (commonly American Society of Civil Engineers (ASCE) rail, similar to railroad rail) is the track upon which the end truck wheels traverse (see Figure 3). It is a common misconception that the runway beams have no particular installation tolerance and that only the rail is at issue. Further, this assumption seems to be confirmed by the lateral adjustment of the rail fasteners (for example, Jbolts/ hook bolts or patented clips). Actually, the tolerance of the beam installation is governed by the tolerance of the rail installation. This is because, according to AISC Design Guide 7, paragraph 19a, the centerline of the rail should be within ±3⁄4 inch of the girder web thickness. This prevents top flange rollover and subsequent fillet cracking and possibly girder failure.
This conventional wisdom is so commonly accepted that it has evolved into generally accepted practice. Unfortunately, like so much conventional wisdom, it’s wrong, it’s bad for the equipment, it will result in significantly shorter service life, and it can be dangerous.
2. Augment the specs. Just because the generally accepted specs have left the crane runways as an orphan does not mean that you as a prospective new building owner should not include a stop-gap page of specs to cover yourself. If you buy the steel from the same vendor, fabricate
with the same fabricator, and install with the same installation crew, you very likely will end up with the same problem. It defies reason that any efficient contractor can buy, fabricate, and install 20+ pieces of apparently identical red primed steel to a tolerance two to four times tighter than the other several thousand pieces of red steel in that same building. This is not meant to slight building contractors.
Successful contractors have set up a well-disciplined system to produce and install building steel, but runway steel, although similar-looking, is a significantly different animal. While it is unlikely the contractor would adopt this more stringent standard temporarily, it is not impossible.
The silver lining for you, the buyer, in using the augmented specs as part of the contract is that the corrections no longer are your problem or expense.
3. Redefine the scope of building contractor and crane supplier responsibilities. This technically
correct, practically viable solution is the least used of the three, simply because of lack of knowledge and higher up-front costs. The common scope of the crane builder’s contract is to supply and install the crane, runway rail, and conductor bar. This leaves a critical gap in which the buyer is exposed to the previously mentioned problems. The scope should be changed to move responsibility for the runway girders from the building contractor to the
crane builder. Chances are, the crane builder will insist on very tight tolerances from the steel supplier and will take precautions to account for reasonable floor and column tolerances. Also, having the crane builder’s employees install the runways can help to improve installation accuracy because this job is their specialty. If the plant is a union plant, however, the runway conductor bar installation should be awarded to a local electrical contractor, while the crane builder remains responsible for the bar.
Get It Right the First Time
In summary, runway steel is not building steel. Poor runways will result in premature wheel failure, motor and or gearbox failure, and premature runway replacement. With a typical wheel replacement costing $8,000 and new runways costing $50,000 or more, not to mention downtime, getting it right the first time can be a real bargain. Using augmented overhead crane runway specs in conjunction with the information provided here can help you to stay out of court and maintain good relations with valuable vendors.
Sunday, August 22, 2010
Overhead Crane - General
There are many types of overhead cranes, all of which can pose hazards if they are not used properly and with adequate safety precautions. Strict guidelines and safety regulations are published by OSHA (US Department of Labor's Occupational Safety & Health Administration Department). By adhering to these standards and rules, you protect your company from some of the injuries and lawsuits that can be served against companies requiring employees to operate heavy machinery.
Some of OSHA's laws on crane design and manufacture govern the following things.
1-Uniform crane control design and manufacture
2-Trolley and bridge bumper design
3-Stairway and ladder design
4-Trolley and bridge brakes
5-Design and manufacture of electrical components
6-Hoisting equipment design and manufacture
There are several basic types of overhead cranes. OSHA's laws and regulations apply to the following cranes.
1-Cantilever gantry
2-Gantry
3-Semi-gantry
4-Storage bridge
5-Wall
All of these require using the appropriate overhead crane parts for all repairs, as substitutions may result in accidents or defective operation of the machinery.
If you or your employees will be operating cranes and other heavy machinery, here are some safety tips to remember.
1-Cranes should only be used and operated by designated persons.
2-If a crane has been installed since 1971, it must meet all OSHA specifications.
3-An equipment manufacturer or engineer must inspect any crane that has been altered to change its load capacity.
4-You must have a cleared walkway on the side of the crane and above it.
5-You must clearly mark the load capacity of a crane so that everyone who might operate it can see it.
6-If a crane has two or more hoists, you have to clearly mark the load capacity of each hoist so that everyone who might operate it can see it.
Some of OSHA's laws on crane design and manufacture govern the following things.
1-Uniform crane control design and manufacture
2-Trolley and bridge bumper design
3-Stairway and ladder design
4-Trolley and bridge brakes
5-Design and manufacture of electrical components
6-Hoisting equipment design and manufacture
There are several basic types of overhead cranes. OSHA's laws and regulations apply to the following cranes.
1-Cantilever gantry
2-Gantry
3-Semi-gantry
4-Storage bridge
5-Wall
All of these require using the appropriate overhead crane parts for all repairs, as substitutions may result in accidents or defective operation of the machinery.
If you or your employees will be operating cranes and other heavy machinery, here are some safety tips to remember.
1-Cranes should only be used and operated by designated persons.
2-If a crane has been installed since 1971, it must meet all OSHA specifications.
3-An equipment manufacturer or engineer must inspect any crane that has been altered to change its load capacity.
4-You must have a cleared walkway on the side of the crane and above it.
5-You must clearly mark the load capacity of a crane so that everyone who might operate it can see it.
6-If a crane has two or more hoists, you have to clearly mark the load capacity of each hoist so that everyone who might operate it can see it.
Saturday, April 17, 2010
Concrete Formwork - Structural Engineer
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.
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.
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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