Lift Rod classification of load bearing

Time：2021-04-22 View：

Load is a professional word in human society. It refers to the external resistance overcome by machines or active machines, the requirements for the business capability of a certain system (such as circuit switching stations, postal services, Railways), and the weight carried by objects. Extended to the proportion of resources occupied.

 

Load Classification

 

Power load
Load refers to the power and current passing through wires, cables and electrical equipment (Transformers, circuit breakers, etc.). The load is not a constant value, but a variable value that changes over time. Because electrical equipment does not run at the same time, even if it runs at the same time, it cannot reach the rated capacity at the same time. In addition, the working system of each electrical equipment is also different, which can be divided into long-term, short-term and repeated short-term. In the design, if the capacity of each electrical equipment is simply added up as the basis for selecting the capacity of wire, cable section and electrical equipment, the result is not scientific. Either too large, making the equipment underloaded and uneconomical; Or too small, overload operation occurs, resulting in overheating insulation damage and increased line loss, affecting the safe operation of wires, cables or electrical equipment. In severe cases, it will cause fire accidents. In order to avoid this situation, the design adopts a Assumed load, namely calculated load, to represent the total load response of the system. The selection of conductor, cable section and electrical equipment by calculating load is close to reality, because the thermal effect of calculating load is equal to that of variable load.
Computing load is also called required load or maximum load. Computational load is an imaginary continuous load, and its thermal effect is equal to the maximum thermal effect generated by the actual variable load in a certain period of time. In the design of power distribution, the maximum average of 30 minutes is usually used as the basis for selecting Electrical Industrial conductors according to civil thermal conditions.
The method of calculating load is called load calculation. The methods commonly used in our country to determine the calculation load have the need coefficient method and binomial method. The advantage of coefficient method is that it is simple and suitable for the calculation of substation load in the whole plant and workshop. Binomial method is suitable for the calculation of load in the main line and branch line affected by large-capacity equipment in the machining workshop. However, when determining the calculation load of branch trunk lines with small number of equipment and wide difference in equipment capacity, binomial method is more reasonable than required coefficient method, and the calculation is also simpler.

Fire load
Fire load is a parameter to measure the quantity of combustibles contained in buildings, and is the basic element to study the characteristics of the comprehensive development stage of fire. Simply speaking, it is the total energy that all combustibles in the building volume may release due to combustion. When a fire occurs in a building, the fire load directly determines the duration of the fire and the change of indoor temperature. Therefore, it is necessary to understand the concept of fire load and reasonably determine the fire load value when designing the fire protection of building structures.
Combustibles in buildings can be divided into fixed combustibles and contained combustibles. Fixed combustibles refer to combustibles used in structural materials such as walls, ceilings, floors, and decoration materials, as well as Combustibles Used in doors, windows, fitments, etc. Contained combustibles refer to combustibles composed of furniture, books, clothing, Thai latex, furnishings, etc. The quantity of fixed combustibles can be easily obtained accurately through the design drawings of buildings. It is difficult to accurately calculate the amount of contained combustibles. Generally, it is determined by survey statistics.
Combustibles in buildings
There are many kinds of combustibles in buildings, and their combustion calorific value varies depending on the material properties. In order to facilitate the study, in practice, some materials are often converted into wood with equivalent calorific value according to combustion heat value, and the amount of combustibles is expressed by the weight of equivalent wood, which is called the amount of equivalent combustibles. Generally speaking, large space contains more combustibles than small space, so the amount of equivalent combustibles is related to the size of building area or volume. In order to facilitate the study of fire characteristics, the amount of equivalent combustible Wood per unit floor area within the fire range is generally defined as fire load.
Determination of fire load
The relationship between the fire load and the severity of the fire is obvious. There is no fire without combustibles; The more combustibles there are, the more serious the fire is. Therefore, the calculation of fire load is very important. However, not only the quantity of combustibles is important, but also the type of combustible materials in unit space. Because some materials release more energy per unit than other unit materials when burning. This is why fire loads are often expressed in MJ instead of kg. Sometimes, we use some hot data, such as converting the heat energy of all combustible materials in a space into equivalent Wood quantity to represent the fire load in this range.
Fire load density
That is, Fire Load Density, the Fire Load per unit of building area.

 

Physiological load
The load or resistance that muscles encounter before contraction is called preload. It means that the muscle has a certain initial length before contraction. Within a certain range, the tension generated by muscle contraction is proportional to the initial length of the muscle before contraction, and if it exceeds a certain limit, it will show a reverse relationship. That is, in the initial stage, the muscle tension also increases with the increase of the initial length; When the muscle is in the optimum initial length (the optimum preload), the contraction produces the maximum tension. If the initial length is increased, the muscle tension decreases instead. When the muscle is in the optimum initial length, if it starts to contract with equal length, the shortening speed is the fastest, the shortening degree is the largest, and the work efficiency is the highest.
Afterload refers to the load or resistance encountered after muscles begin to contract. When muscles contract under the condition of afterload, tension increases first, and then muscle contraction occurs. Within a certain range, the greater the afterload, the greater the tension will be generated, and the time when muscles begin to shorten will be delayed, and the slower the shortening speed will be. When the afterload increases to a certain value, the tension generated by the muscle reaches its maximum. At this time, the muscle may not Shorten at all, and the initial speed is equal to zero, the tension generated by the muscle has a reverse relationship with its initial speed when contracting. Therefore, only when the muscle is under moderate afterload can the best effect of muscle work be obtained.

Cognitive load
Cognitive load refers to the total amount of psychological resources that people need in the process of information processing. Cognitive load theory was first proposed by John Swiller (John Swdler), a cognitive psychologist at the University of New South Wales, Australia, in 1988. it is based on the early research of Miller and others, its purpose is to minimize the cognitive load that hinders learning in the design of teaching materials and teaching process, optimize and promote the cognitive load of learning, and enable learners to reasonably use limited cognitive resources, achieve maximum learning effect.
Cognitive load theory holds that the level of "cognitive load" caused by certain learning materials is mainly determined by three basic factors: the complexity of learning materials, the organization and presentation of learning materials, knowledge and experience of learners. Thus, three types of cognitive load are formed: external cognitive load, internal cognitive load and related cognitive load.
In 1988, psychologist J.Sweller of the University of New South Wales in Australia formally proposed "cognitive load theory" (CognitiveLoadTheory, referred to as CLT) from the perspective of cognitive resource allocation on the basis of the research results of modern cognitive psychology. Then cognitive load theory began to be widely studied in the field of practical teaching and achieved certain research results. Many studies have proved that cognitive load theory provides a new theoretical framework for studying cognitive processing in the teaching process and has extremely practical guiding significance for teaching practice. As demonstrated in Alexander(1997), cognitive load is regarded as an important factor in the design of teaching activities. Whether teaching activities are effective depends (or partly depends) on whether it can reduce unnecessary cognitive load.

 

I. Basic viewpoints of cognitive load theory
Cognitive load theory is developed based on the limitation of human brain working memory (workingmemory). In human information processing system, short-term memory (short-termMemory) is a kind of working memory, which mainly processes information extracted from sensory memory (sensorymemory) and long-term memory (long-termmemory), it plays a role in controlling the information flow in the information processing system and plays a vital role in the smooth development of cognitive activities. Cognitive load theory pays great attention to the function of working memory in the learning process. Its basic viewpoints can be summarized as follows:(1) the capacity of working memory is extremely limited, and the capacity of long-term memory is essentially infinite, all information must be processed in working memory before entering long-term memory;(2) the learning process requires that working memory be actively used for understanding (and processing) materials and coding the information to be acquired to be stored in long-term memory;(3) if the information capacity to be processed by the learner exceeds the information capacity that the learner can process in the working memory, then learning will become invalid. On this basis, J.Sweller and others believe that all kinds of cognitive processing activities in the process of problem solving and learning need to consume cognitive resources, A certain cognitive load is generated. If the total amount of resources required for all activities exceeds the capacity of working memory, it will cause insufficient resource allocation, thus affecting the efficiency of individual learning or problem solving, this condition is called cognitive overload (cognitiveoverload).

 

II. Concept and structure of cognitive load
Since the advent of cognitive load theory in the 1980 s, people have always had different opinions on the concept of cognitive load (cognitiveload). For example, Cooper(1990) thinks that cognitive load refers to the psychological energy level required by processing specific quantity of information; J.Sweller et al. (1998) believed that cognitive load was the load generated when specific work was combined with learners' cognitive system; Xin Ziqiang et al. (2002) it is believed that cognitive load can be regarded as the mental work required by processing a specific amount of information. With the increase of the amount of processed information, cognitive load also increases; Lin Chongde et al. (2005) it is believed that cognitive load refers to the number of intellectual activities imposed on working memory in an example; Lai Risheng et al. (2005) it is believed that cognitive load refers to the total number of intellectual activities applied to working memory in certain occasions; Yang Xinde et al. (2007) it is believed that cognitive load is all the mental energy needed for mental activities in working memory to complete a certain task. Combined with the above different explanations, the author believes that cognitive load is the sum of cognitive resources that people actually devote to working memory in order to successfully complete specific work tasks, including necessary and unnecessary.
Structurally, cognitive load consists of internal cognitive load (IntrinsicCognitiveLoad), external cognitive load (ExtraneousCognitiveLoad) and related cognitive load (GermaneCognitiveLoad):(1) the internal cognitive load is determined by the inherent characteristics of the material itself (such as difficulty and complexity), the original knowledge level of the learner, and the interaction between the two. It is generally believed that the internal cognitive load is relatively fixed, it cannot be changed by teaching design (InstructionDesign), but recently some scholars believe that the internal cognitive load can be changed (Pollocketal,2002;Peteretal,2004);(2) external cognitive load comes from the presentation mode and teaching design of teaching materials, which is generally related to the unreasonable Organization and Design of teaching contents and can be adjusted through the reorganization and design of teaching contents;(3) relevant cognitive load is information related to learners' subjective fields, Refers to the amount of cognitive resources that learners intend to invest in the process of Schema construction and automation, which is related to students' cognitive efforts and improves students' related cognitive load, it can guide students to use the remaining cognitive resources for deep schema construction. Therefore, Bannert(2002) regards it as a tool for schema construction and automation.

 

III. Enlightenment of cognitive load theory to teaching work
In order to avoid that the total cognitive load of middle school students exceeds their working memory capacity, the teaching principle of cognitive load theory is to reduce students' internal cognitive load and external cognitive load as much as possible; on the premise of ensuring the surplus of working memory resources, students are properly guided to invest more psychological efforts (MentalEffort), improve their related cognitive load, and realize the acquisition of schema and the automation of rules. As mentioned earlier, the internal cognitive load of middle school students in teaching activities is related to the characteristics of teaching materials and students' cognitive level, while the external cognitive load is related to the presentation mode of teaching materials and the level of teaching design, related cognitive load is related to students' cognitive effort (MentalEffort). Therefore, we can optimize the cognitive load structure of middle school students in teaching activities from the following aspects to improve the teaching effect.

 

(1) controlling internal cognitive load: fully consider the characteristics of teaching materials, students' cognitive level and their interaction
It is generally believed that the internal cognitive load is determined by the nature of learning materials and cannot be changed through the change of teaching design, but the experiments of Pollocketal(2002) and Peteretal(2004) raised objections. The common point of the two experiments is to reduce the internal cognitive load by using the information sorting principle of materials. Pollocketal first show students the learning materials in batches, and then show them all at once. This sorting method of learning materials, especially for beginners, can better promote a deep understanding of learning materials (quoted from Bannert,2002). However, Peteretal changed the traditional sample teaching method (ExampleInstruction) and decomposed the sample into several components that can be understood separately. The final results of his five experiments show that, the new sample presentation method reduces the internal cognitive load brought by materials to students.
TracyClarke,PaulAyres and JohnSweller also studied a progressive method to change the inherent cognitive load in their book "TheImpactofSequencingandPriorKnowledgeonLearningMathematics:ThroughSpreadsheetApplications. They use Spreadsheet software to form students' math skills. Students are divided into two categories. One class of students is not familiar with Spreadsheet, and the other class of students have a better understanding of Spreadsheet. Each category of students is divided into two parts, receiving two kinds of training first: one kind of training is to use Spreadsheet software first, and then train to use this software to develop mathematical skills; another kind of training is to practice Spreadsheet software skills and math skills at the same time. The results show that for students who are not familiar with Spreadsheet in advance, sequential presentation is better than concurrent presentation, and the test scores are also higher; But for students who have known Spreadsheet in advance, the situation is just the opposite. This shows that for students with low knowledge and skills, adopting the strategy of "learning skills first and then learning concepts in specific content fields" can improve their learning; For them, technical content is likely to have high element interactivity. If you learn technical skills and concepts in specific subject areas at the same time, the internal cognitive load may increase. However, for students with low skill level, the technical content does not have high element interaction, while learning the concepts of technical skills and specific subject areas will not increase the internal cognitive load. When determining the sequence strategy, students' technical skill level and content element interaction level are the key variables (Kalyuga et al., 2003;VanMerrienboer,Kirschner & Kester,2003).

 

(2) reduce external cognitive load: optimize the presentation mode of teaching materials and improve the level of teaching design
Reducing external cognitive load is the focus of "research on cognitive load theory of teaching. Many researchers have put forward their own views on ways to reduce the external cognitive load of teaching students from different angles. On the basis of referring to many documents, the author thinks that it can be summarized as one sentence, that is, optimizing the presentation mode of teaching materials and improving the level of teaching design. Here are a few examples to illustrate.
1. Present teaching materials according to the principle of proximity, reduce attention dispersion and characterization preservation. That is, the presentation of teaching materials in teaching should be as close as possible in time and space. All information involving multiple sources should be physically integrated to reduce the pressure on limited working memory and release cognitive capacity for other information processing. Take the presentation of images and explanatory text as an example. In space, explanatory text should be integrated into images as much as possible; In time, it should be presented simultaneously rather than continuously to form a whole. In this way, learners can find the corresponding visual and word information with the shortest visual search, which is easy to establish connections between them and promote information integration. Otherwise, the image and explanatory text are presented separately, "learners tend to look at the text part first and then the corresponding chart" (Hegart & Judt,1989). After reading the text, on the one hand, some cognitive resources are needed to preserve text representation and form representation preservation; On the other hand, some cognitive resources are needed to search for corresponding image information and form attention dispersion, thus bringing additional cognitive load. Therefore, "the complete presentation of words and images is more conducive to learners' learning than the separate presentation of words and images (Moreno & Mayer,1999;Mayer,2001)".
2. Remove redundant information to avoid redundant effects (RedundancyEffects). Redundant information is not necessary for the achievement of teaching objectives, also known as redundant information. PaulChandler and JohnSwe11er prove by experiments that redundant information is not neutral and will have negative effects on learning and understanding. The existence of redundant information often forces students to pay attention to some information that they could not pay attention to, thus wasting valuable working memory resources. Therefore, it will bring extra burden to working memory, thus increasing students' external cognitive load. Moreno and Mayer also believe that "when the text is presented separately in narrative mode, the learner's understanding of the presentation material is better than that of presenting the material in narrative mode and screen text at the same time", the text information presented on the screen is redundant and should be cleared.
3. Use the strategy of advance organizer (AdvanceOrganizer) to change students' cognitive readiness. People's understanding of new knowledge is always based on their original cognitive structure (CognitiveConstruction). When the original cognitive structure supports new knowledge, it is relatively easy for people to understand new knowledge; On the contrary, if there is no content supporting new knowledge in the original cognitive structure or relevant knowledge in the original cognitive structure is not activated, people's understanding of new knowledge will become relatively difficult. Teachers should present students with a higher level of abstract and inclusive guiding materials than the current learning task before the start of the new learning task through the strategy of leading organizers, so as to activate the knowledge content related to new knowledge in its cognitive structure and prepare for learning new knowledge, thus avoiding learning new knowledge at the same time, we also need to search for the cognitive burden of relevant knowledge in long-term memory to reduce the external cognitive load.
In addition, by separating audio-visual information, the working memory capacity is expanded; Information is organized to reduce students' cognitive search; Abstract information visualization reduces students' cognitive difficulty; Complex information simplification (such as using charts), simplify students' cognitive processing; Control the presentation speed of teaching materials and leave students enough cognitive processing time; Teach students cognitive strategies (cognitivestrategies) to improve students' cognitive efficiency; Differentiate complex tasks, prevent students from overloaded cognitive load; Adopting the example (worked-example) teaching and simplifying the process of acquiring cognitive skills can reduce students' external cognitive load from different angles.

 

(3) improving relevant cognitive load: stimulating students' learning motivation and guiding them plus-sized their cognitive efforts
Related cognitive load refers to the quantity of cognitive resources that learners intend to invest in the process of schema construction and automation. It is conducive to the acquisition of schema and the automation of rules, and realizes the optimization of students' cognitive structure, combining a large number of complex and disordered information groups into a simple and orderly knowledge system effectively reduces the cognitive load of working memory, thus saving limited working memory resources; the automation of rules also allows us to process information with minimal working memory capacity and reduce the burden of working memory. Moreover, the increase of related cognitive load is also conducive to the construction of meaning and helps students better understand and master what they have learned. Thus, under the condition of ensuring that the total cognitive load is not overloaded, it is beneficial for teaching to appropriately increase the relevant cognitive load of students.
To improve students' related cognitive load, the key is to stimulate students' learning motivation (especially internal motivation) so that students plus-sized their own cognitive efforts. Therefore, educators should transform the learning requirements put forward by society and educators to students into the internal learning needs of learners through various ways and means. For example, cultivate and stimulate students' interest in learning; Carry out education for learning purposes; Adopt heuristic teaching; Establish a reasonable reward mechanism, etc.