 “Indeed, perseverance is what always propels one to pursue all profitable objects. It makes the actions it inspires in living beings successful.” (Hanuman, Valmiki Ramayana, Sundara Kand, 12.11)anirvedo hi satatam sarva artheṣu pravartakaḥ | karoti saphalam jantoḥ karma yac ca karoti saḥ || The difficulty of a task and the fear of what might result with failure take away rational thought. In every aspect of adult life there is some sort of fear, as this is ingrained into the mature human being. Even the animals have fear, along with desires to eat, sleep and mate. With the human being lacking full knowledge of the spirit soul and its relationship to the Supreme Lord, fearing can reach a new height. The fear easiest to identify is that of dying. Lord Rama
 The comparison Shri Rama makes is to the ripened fruit. The fruit starts off as nothing but a tiny seed within a larger plant. Through the gifts of nature and the influence of time, gradually the seed matures into a full blown fruit. Maturity takes a while, so there must be good fortune for the fruit to reach ripeness. Any of the inhibiting influences of material nature can take effect at any second. The Vedas, the ancient scriptures of India, group miseries into three categories. There are those inflicted by the heavenly forces, acts of God if you will. These miseries include hurricanes, tornadoes, and just about any general unpleasant weather event. Then there are the punishing influences of other living entities, those who infringe upon others’ natural rights. These people may also harm us with their insulting words aimed at breaking our resolve.Last but not least, there are the miseries inflicted by the body and mind. These can include diseases and also fearing. The mature fruit does not have the advanced consciousness to fear, but once it reaches its peak development, its destiny is to fall off the plant and get eaten. It has nothing else to wait for. Similarly, the human being matured through the various stages of life, after acquiring so many objects relating to the senses and forging so many relationships, has nothing left to do but die.
As the body was at one time accepted, a sober person can realize that it must also be renounced. In this sense there is nothing to fear, but the forces of material nature are such that fear nevertheless takes over. From the fear of death come all other types of worries and concerns. For young children, there is a fear of not performing well in school. In our youth particularly, in every year of school we were afraid of being left back, forced to repeat the same grade. We even had a recurring dream where we regularly skipped a particular class, only to have a final exam forced upon us at the end of the year that we weren’t prepared for. This isn’t an uncommon dream, as the fear of failing in the classroom is quite widespread.  As one gets older, the fears turn toward the safety and security of family members and occupation. Since fear of failure is so common, and since frustration is found at every corner of the material existence, young children are given lessons on perseverance. Famous fables relay the importance of sticking with a task and seeing it through to completion. Sayings like, “early to bed, early to rise”, “the early bird gets the worm”, and “put your nose to the grindstone” reinforce the need for regulation and dedication in attaining a beneficial end.In the sports world there are so many instances of athletes overcoming obstacles and defying the odds through perseverance. When a famous athlete fails on the big stage, the impulse reaction for the fans and sportswriters is to label that person a bum. “Oh, they choked. They can’t win the big one. Perhaps they will never win. They will stay a failure forever.” Driving these fatalistic predictions is fear, the thought of what would happen should such and such player or team never achieve their desired end. Those who do overcome obstacles and finally achieve the victory that everyone thought was impossible become almost folklore. Their stories are honored and remembered for many years, as everyone takes comfort in their triumph over inner demons. The more stories there are like this, the more people will have faith in their abilities and the benefit of fighting through adversity. When you throw spirituality into the mix, the benefits of perseverance take on a divine nature. What does this mean exactly? Just as the human being is destined to renounce his body, or die, the rewards accumulated that relate to the enjoyment within that body are also short-lived. Perseverance in running the race can bring victory, a trophy indicating an achievement. Fighting through the difficulties in school can result in a degree, and fortitude at work can make others happy, but all of these things will be given up along with the body at the time of death. The spirit soul, on the other hand, continues its existence. Like a natural acrobat, a space traveller not requiring any rocket ship or spacesuit, the soul jumps from one body type to another through what is known as reincarnation
 How do we know that a specific type of consciousness can stop death? Aside from the fact that Lord KrishnaDepending on consciousness, even defeat and frustration can be handled as favorable conditions. We can take the example of Shri Hanuman to see how this works. During Rama’s time on earth many thousands of years ago, His wife Sita Devi Hanuman didn’t have fears over death. He is an eternally liberated soul, which means that material nature can’t affect him. His consciousness is properly situated, so his mind is always connected to God. Even still, this doesn’t mean that defeat and frustration, and even fear, become totally absent. Rather, they just take on a different nature; their influence is different. The same goes with the invocation of perseverance, as using fortitude in bhakti-yoga, or devotional service  After searching long and hard, Hanuman had yet to find Sita. It just didn’t seem fair. He had braved so many elements to reach Lanka. No one had before done what he did, so there was no comparing his activities to anyone else’s. Nevertheless, the mission still wasn’t complete. After clandestinely searching through Lanka for Sita, Hanuman still didn’t see her. The fear in his mind now related to Rama and the monkeys in Sugriva’s army. Sugriva was the king of the Vanaras in Kishkindha, and it was at his command that the monkeys were dispatched to search for Sita.The monkeys searched valiantly, and at one point it looked like they wouldn’t succeed. Hanuman remembered his friends and what they had been through together. The last thing he wanted to do was return to them a failure. He couldn’t bear to see the look of disappointment and sadness in their faces. He didn’t want to return to Kishkindha and meet Sugriva’s wrath, for nothing makes a ruler angrier than learning that his subjects disobeyed his orders. Sugriva told the monkeys to find Sita or face severe punishment. Hanuman also didn’t want to disappoint Rama, who was showing signs of suffering from separation from His wife. Before moving forward, Hanuman decided to think things over. In the above referenced verse from the Ramayana, he reminds himself of a few adages relating to perseverance. Tireless determination, indefatigability, leads a person to their profitable end. This makes sense, for if you don’t really want something, you won’t be willing to put in the work to see the successful outcome. Hanuman wanted to find Sita more than anything, so motivation was there. He remembered that when there is desire, strong motivation, the chances for success will greatly increase. The famous saying, “God helps those who help themselves”, applies nicely to this situation. It is actually most appropriate with Hanuman because he was involved in a mission seeking to please God. In cases not relating to the Supreme, the saying doesn’t really apply. For instance, if our perseverance is dedicated towards procuring liquor and beer to get intoxicated, why would the Lord help us? If our desire is to perpetrate some horrible deed, something against the law, surely the divine forces will not provide any aid.  In fact, any activity performed for our personal benefit does not catch the Lord’s eye in the least. There is really no such thing as good or bad when it comes to the body that is destined for destruction. There is only favorable and unfavorable with respect to the advancement of consciousness. Hanuman was engaged in the constitutional activity of bhakti; therefore for him there was all sorts of aid from the Supreme Lord. The motivation itself, his strong perseverance, was enough to forever endear him to Sita and Rama.Not surprisingly, Hanuman would end up succeeding in his mission. While taking shelter of a truth of life known to the wise, he ended up giving countless generations a real-life example of those principles put to use. As our mission in life is to become God conscious, the best way to adapt to every circumstance is to regularly chant the Lord’s names, “Hare Krishna Hare Krishna, Krishna Krishna, Hare Hare, Hare Rama Hare Rama, Rama Rama, Hare Hare In Closing: Tendency to fear will always be there, Thought of losing everything causes scare. For mature fruit no fear but to fall, For adult human tolling bell death's call. Perseverance in worker thus praised, Famed is champion with victory's arms raised. Hanuman knew that perseverance guided will, Brings fruit to those who through trouble stand still. Success when from fortitude action springs, Lesson of Hanuman insight brings. |
Thursday, December 8, 2011
If At First You Don’t Succeed
Neuroscientists find greater complexity in how we perceive motion
How we perceive motion is a significantly more complex process than previously thought, researchers at New York University's Center for Neural Science, Stanford University and the University of Washington have found. Their results, which appear in the journal Current Biology, show that the relationship between the brain and visual perception varies, depending on the type of motion we are viewing.
Neuroscientists have posited that our perception of motion is derived from a relatively simple process—that is, it relies on a single cortical area in the brain. This region, MT, has a well-established role in processing information about moving visual objects. However, less clear is the scope of its influence—does it dictate our ability to detect all types of motion or is its function more limited or, perhaps, more nuanced?
With this in mind, the NYU-led team examined how the visual system processes two types of motion: local motion, which involves tracking signals that fall within a small retinal area, and global motion, in which several movements are integrated over larger areas. Specifically, they monitored activity in MT to determine if its neurons were active in response to both types of motion.
In normal scenes, local and global motion are normally in agreement. Here, the researchers conducted experiments in which macaque monkey and human subjects watched specially crafted patterns in which the local and global motion information were in competition. The researchers made physiological and perceptual measurements of brain activity.
Their results showed notable differences in how the visual system functions in processing local and global motion. They found that neuronal activity in MT was controlled by the local motion in the stimulus and unaffected by global motion. Under the same conditions, though, humans' perceptual responses were dominated by global motion, which means that their responses were determined by a second brain mechanism that encodes global motion, whose nature is currently unknown.
J. Anthony Movshon, director of the Center for Neural Science and the paper's senior author, says that the work opens a window for further study: "While comprehending the specifics of this process requires more work, it's clear that motion perception does not depend on a single cortical brain area, but, rather, reflects the action and interaction of multiple mechanisms. We now have new tools to help us identify and study brain systems that are currently unknown."
Provided by New York University
"Neuroscientists find greater complexity in how we perceive motion." December 5th, 2011. http://medicalxpress.com/news/2011-12-neuroscientists-greater-complexity-motion.html
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Robert Karl Stonjek
Robert Karl Stonjek
Where is the accurate memory? The eyes have it
(Medical Xpress) -- The witness points out the criminal in a police lineup. She swears she’d remember that face forever. Then DNA evidence shows she’s got the wrong guy. It happens so frequently that many courts are looking with extreme skepticism at eyewitness testimony.
Is there a way to get a more accurate reading of memory? A new study says yes. “Eye movements are drawn quickly to remembered objects,” says Deborah Hannula, assistant professor at the University of Wisconsin Milwaukee, who conducted the study with Carol L. Baym and Neal J. Cohen of the University of Illinois, Urbana-Champaign and David E. Warren of the University of Iowa College of Medicine. Tracking where and for how long a person focuses his or her eyes “can distinguish previously seen from novel materials even when behavioral reports fail to do so.” The findings will appear in an upcoming issue of Psychological Science, a journal published by the Association for Psychological Science.
The researchers gave university students 36 faces to study. These target faces were also morphed to produce images closely resembling them; the morphed phases were not seen during the study phase. The students were then shown 36 three-face displays, one at a time. Told that the studied faces wouldn’t always be there, the participants had to press a button indicating which face was the studied one, or simply choose a face if they felt none had been studied. They then reported verbally whether the studied target face was present or not. While they looked at the 3-face display, their eye movements were recorded, tracking where the eyes focused first and what proportion of time was spent looking there. For the analysis, the psychologists divided the faces into three groups: studied targets; morphs mistaken for the “target” face; and morphs chosen and known to be incorrect.
Participants easily identified the target faces most of the time. They also spent more time looking at these faces, and did so soon after the 3-face display had been presented. “The really interesting finding is that before they chose a face and pressed a button, there was disproportionate viewing of the target faces as compared to either type of selected face,” said Hannula. However, “after the response was made, viewing tended to mimic the behavioral endorsement of a face as studied or not, whether that endorsement was correct or incorrect.” In other words, “pre-response viewing seems to reflect actual experience, and post-response viewing seems to reflect the decision making process and whether or not the face will be endorsed as studied.”
Hannula theorizes as to what is happening: “Early disproportionate viewing of the target face may precede and help give rise to awareness that a particular face has been studied. Subsequently, we begin to think about the choice that we’re making”—we look closely, compare and weigh the options—“these cognitive processes permit us to make a decision, but may also lead us down the wrong path. In this case, leading us to endorse a face as studied despite having never seen it before.”
Aside from the potential for practical application, says Hannula, eye movement methods could be used to examine memory in individuals—like some psychiatric patients and children – who may have trouble communicating what it is that they remember. “Eye movements might provide us with more information about what exactly these individuals remember than behavioral reports alone.”
More information: http://www.psychol … ical_science
Provided by Association for Psychological Science
"Where is the accurate memory? The eyes have it." December 5th, 2011.http://medicalxpress.com/news/2011-12-accurate-memory-eyes.html
Posted by
Robert Karl Stonjek
Robert Karl Stonjek
Properties of Concrete and Steel
Objectives and Methods of Analysis and Design, and Properties of Concrete and Steel
Version 2 CE IIT, Kharagpur
Lesson 2
Properties of Concrete and Steel
Version 2 CE IIT, Kharagpur
Instructional Objectives:
At the end of this lesson, the student should be able to:
• know the properties of concrete in respect of strength, deformation and durability
• know the properties of steel used as reinforcement in concrete structures
• understand the importance of quality control, inspection and testing of concrete and steel in several steps from its basic preparation to the removal of formwork after the construction
•
recommend the acceptance of good concrete based on sample test of specimens, core tests, load test and non-destructive tests
1.2.1 Introduction
It is essential that the designer has to acquire a fair knowledge of the materials to be used in the design of reinforced concrete structure. This lesson summarises the characteristic properties of concrete and steel, the two basic materials used for the design. This summary, though not exhaustive, provides the minimum information needed for the design.
1.2.2 Properties of Concrete
Plain concrete is prepared by mixing cement, sand (also known as fine aggregate), gravel (also known as coarse aggregate) and water with specific proportions. Mineral admixtures may also be added to improve certain properties of concrete. Thus, the properties of concrete regarding its strength and deformations depend on the individual properties of cement, sand, gravel, water and admixtures. Clauses 5 and 6 of IS 456:2000 stipulate the standards and requirements of the individual material and concrete, respectively. Plain concrete after preparation and placement needs curing to attain strength. However, plain concrete is very good in compression but weak in tension. That is why steel is used as reinforcing material to make the composite sustainable in tension also. Plain concrete, thus when reinforced with steel bars in appropriate locations is known as reinforced concrete.
The strength and deformation characteristics of concrete thus depend on the grade and type of cement, aggregates, admixtures, environmental conditions and curing. The increase of strength with its age during curing is considered to be marginal after 28 days. Blended cements (like fly ash cement) have slower rate of strength gain than ordinary Portland cement as recognized by code,
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Depending on several factors during its preparation, placement and curing, concrete has a wide range of compressive strength and the material is graded on the basis of its compressive strength on 28th day also known as "characteristic strength" as defined below while discussing various strength and deformation properties.
(a) Characteristic strength property
Characteristic strength is defined as the strength below which not more than five per cent of the test results are expected to fall. Concrete is graded on the basis of its characteristic compressive strength of 150 mm size cube at 28 days and expressed in N/mm2. The grades are designated by one letter M (for mix) and a number from 10 to 80 indicating the characteristic compressive strength (fck) in N/mm2. As per IS 456 (Table 2), concrete has three groups as (i) ordinary concrete (M 10 to M 20), (ii) standard concrete (M 25 to M 55) and (iii) high strength concrete (M 60 to M 80). The size of specimen for determining characteristic strength may be different in different countries.
(b) Other strengths of concrete
In addition to its good compressive strength, concrete has flexural and splitting tensile strengths too. The flexural and splitting tensile strengths are obtained as described in IS 516 and IS 5816, respectively. However, the following expression gives an estimation of flexural strength (fcr) of concrete from its characteristic compressive strength (cl. 6.2.2)
70 .
f
kc
in
N/mm
2
=
rc
f (1.1)
(c) Elastic deformation of concrete
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Figure 1.2.1 shows a typical stress-strain curve of concrete in compression, where
Ec = initial tangent modulus at the origin, also known as short term static modulus
Es = secant modulus at A
Et = tangent modulus at A
εe = elastic strain at A
εi = inelastic strain at A
It is seen that the initial tangent modulus is much higher than Et (tangent modulus at A). Near the failure, the actual strain consists of both εe and εi (elastic and inelastic respectively) components of strain. The initial tangent modulus Ec in N/mm2 is estimated from
E
c
=
5000
f
kc
(1.2)
where fck = characteristic compressive strength of concrete at 28 days
The initial tangent modulus Ec is also known as short term static modulus of elasticity of concrete in N/mm2 and is used to calculate the elastic deflections.
(d) Shrinkage of concrete
Shrinkage is the time dependent deformation, generally compressive in nature. The constituents of concrete, size of the member and environmental conditions are the factors on which the total shrinkage of concrete depends. However, the total shrinkage of concrete is most influenced by the total amount of water present in the concrete at the time of mixing for a given humidity and temperature. The cement content, however, influences the total shrinkage of concrete to a lesser extent. The approximate value of the total shrinkage strain for design is taken as 0.0003 in the absence of test data (cl. 6.2.4.1).
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(e) Creep of concrete
Creep is another time dependent deformation of concrete by which it continues to deform, usually under compressive stress. The creep strains recover partly when the stresses are released. Figure 1.2.2 shows the creep recovery in two parts. The elastic recovery is immediate and the creep recovery is slow in nature.
Thus, the long term deflection will be added to the short term deflection to get the total deflection of the structure. Accordingly, the long term modulus Ece or the effective modulus of concrete will be needed to include the effect of creep due to permanent loads. The relationship between Ece and Ec is obtained as follows:
εc (1.3)
=
fc/Ec
where εc = short term strain at the age of loading at a stress value of fc
εcr = ultimate creep strain
θ = creep coefficient =
(1.4)
ε ε
rc
c
(cl. 6.2.5.1 of IS 456)
The values of θ on 7th, 28th and 365th day of loading are 2.2, 1.6 and 1.1 respectively.
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Then the total strain = εc + εcr =
f E
c
ce
(1.5)
where Ece = effective modulus of concrete
From the above Eq. (1.5), we have
Ece
=
ε
(1.6)
f
c
ε
+
c
=
ε
rc
c
c
Eε ε
+
c
rc
=
E
c
1
+
θ
The effective modulus of Ece of concrete is used only in the calculation of creep deflection.
It is seen that the value of creep coefficient θ (Eq. 1.4) is reducing with the age of concrete at loading. It may also be noted that the ultimate creep strain εcr does not include short term strain εc. The creep of concrete is influenced by
• Properties of concrete • Water/cement ratio • Humidity and temperature of curing • Humidity during the period of use • Age of concrete at first loading • Magnitude of stress and its duration • Surface-volume ratio of the member
(f) Thermal expansion of concrete
The knowledge of thermal expansion of concrete is very important as it is prepared and remains in service at a wide range of temperature in different countries having very hot or cold climates. Moreover, concrete will be having its effect of high temperature during fire. The coefficient of thermal expansion depends on the nature of cement, aggregate, cement content, relative humidity and size of the section. IS 456 stipulates (cl. 6.2.6) the values of coefficient of thermal expansion for concrete / oC for different types of aggregate.
1.2.3 Workability and Durability of Concrete
Workability and durability of concrete are important properties to be considered. The relevant issues are discussed in the following:
(a) Concrete mix proportioning
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The selected mix proportions of cement, aggregates (fine and coarse) and water ensure:
• •
the workability of fresh concrete, required strength, durability and surface finish when concrete is hardened.
Recently more than forty per cent of concrete poured world over would contain admixtures.
(b) Workability
It is the property which determines the ease and homogeneity with which concrete can be mixed, placed, compacted and finished. A workable concrete will not have any segregation or bleeding. Segregation causes large voids and hence concrete becomes less durable. Bleeding results in several small pores on the surface due to excess water coming up. Bleeding also makes concrete less durable. The degree of workability of concrete is classified from very low to very high with the corresponding value of slump in mm (cl. 7 of IS 456).
(c) Durability of concrete
A durable concrete performs satisfactorily in the working environment during its anticipated exposure conditions during service. The durable concrete should have low permeability with adequate cement content, sufficient low free water/cement ratio and ensured complete compaction of concrete by adequate curing. For more information, please refer to cl. 8 of IS 456.
(d) Design mix and nominal mix concrete
In design mix, the proportions of cement, aggregates (sand and gravel), water and mineral admixtures, if any, are actually designed, while in nominal mix, the proportions are nominally adopted. The design mix concrete is preferred to the nominal mix as the former results in the grade of concrete having the specified workability and characteristic strength (vide cl. 9 of IS 456).
(e) Batching
Mass and volume are the two types of batching for measuring cement, sand, coarse aggregates, admixtures and water. Coarse aggregates may be gravel, grade stone chips or other man made aggregates. The quantities of cement, sand, coarse aggregates and solid admixtures shall be measured by mass. Liquid admixtures and water are measured either by volume or by mass (cl. 10 of IS 456).
1.2.4 Properties of Steel
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As mentioned earlier in sec. 1.2.2, steel is used as the reinforcing material in concrete to make it good in tension. Steel as such is good in tension as well as in compression. Unlike concrete, steel reinforcement rods are produced in steel plants. Moreover, the reinforcing bars or rods are commercially available in some specific diameters. Normally, steel bars up to 12 mm in diameter are designated as bars which can be coiled for transportation. Bars more than 12 mm in diameter are termed as rods and they are transported in standard lengths.
Like concrete, steel also has several types or grades. The four types of steel used in concrete structures as specified in cl. 5.6 of IS 456 are given below:
(i) Mild steel and medium tensile steel bars conforming to IS 432 (Part 1)
(ii) High yield strength deformed (HYSD) steel bars conforming to IS 1786
(iii) Hard-drawn steel wire fabric conforming to IS 1566
(iv) Structural steel conforming to Grade A of IS 2062.
Mild steel bars had been progressively replaced by HYSD bars and subsequently TMT bars are promoted in our country. The implications of adopting different kinds of blended cement and reinforcing steel should be examined before adopting.
Stress-strain curves for reinforcement
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Figures 1.2.3 and 1.2.4 show the representative stress-strain curves for steel having definite yield point and not having definite yield point, respectively. The characteristic yield strength fy of steel is assumed as the minimum yield stress or 0.2 per cent of proof stress for steel having no definite yield point. The modulus of elasticity of steel is taken to be 200000 N/mm2.
For mild steel (Fig. 1.2.3), the stress is proportional to the strain up to the yield point. Thereafter, post yield strain increases faster while the stress is assumed to remain at constant value of fy.
For cold-worked bars (Fig. 1.2.4), the stress is proportional to the strain up to a stress of 0.8 fy. Thereafter, the inelastic curve is defined as given below:
Stress
0.80 fy 0.85 fy 0.90 fy 0.95 fy 0.975 fy 1.00 fy
Inelastic strain
Nil 0.0001 0.0003 0.0007 0.0010 0.0020
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Linear interpolation is to be done for intermediate values. The two grades of cold- worked bars used as steel reinforcement are Fe 415 and Fe 500 with the values of fy as 415 N/mm2 and 500 N/mm2, respectively.
Considering the material safety factor γm (vide sec. 2.3.2.3 of Lesson 3) of steel as 1.15, the design yield stress (fyd) of both mild steel and cold worked bars is computed from
fyd = fy / γm
(1.7)
Accordingly, the representative stress-strain curve for the design is obtained by substituting fyd for fy in Figs. 1.2.3 and 1.2.4 for the two types of steel with or without the definite yield point, respectively.
1.2.5 Other Important Factors
The following are some of the important factors to be followed properly as per the stipulations in IS 456 even for the design mix concrete with materials free from impurities in order to achieve the desired strength and quality of concrete. The relevant clause numbers of IS 456 are also mentioned as ready references for each of the factors.
(a) Mixing (cl. 10.3)
Concrete is mixed in a mechanical mixer at least for two minutes so as to have uniform distribution of the materials having uniform colour and consistency.
(b) Formwork (cl. 11)
Properly designed formwork shall be used to maintain its rigidity during placing and compaction of concrete. It should prevent the loss of slurry from the concrete. The stripping time of formwork should be such that the concrete attains strength of at least twice the stress that the concrete may be subjected at the time of removing the formwork. As a ready reference IS 456 specifies the minimum period before striking formwork.
There is a scope for good design of formwork system so that stripping off is efficient without undue shock to concrete and facilitating reuse of formwork.
(c) Assembly of reinforcement (cl. 12)
The required reinforcement bars for the bending moment, shear force and axial thrust are to be accommodated together and proper bar bending schedules shall be prepared. The reinforcement bars should be placed over blocks, spacers, supporting bars etc. to maintain their positions so that they have the
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required covers. High strength deformed steel bars should not be re-bent. The reinforcement bars should be assembled to have proper flow of concrete without obstruction or segregation during placing, compacting and vibrating.
(d) Transporting, placing, compaction and curing (cl. 13)
Concrete should be transported to the formwork immediately after mixing to avoid segregation, loss of any of the ingredients, mixing of any foreign matter or loss of workability. Proper protections should be taken to prevent evaporation loss of water in hot weather and loss of heat in cold weather.
To avoid rehandling, concrete should be deposited very near to the final position of its placing. The compaction should start before the initial setting time and should not be disturbed once the initial setting has started. While placing concrete, reinforcement bars should not be displaced and the formwork should not be moved.
The compaction of concrete using only mechanical vibrators is very important, particularly around the reinforcement, embedded fixtures and the corners of the formwork to prevent honeycomb type of concreting. Excessive vibration leads to segregation.
Proper curing prevents loss of moisture from the concrete and maintains a satisfactory temperature regime. In moist curing, the exposed concrete surface is kept in a damp or wet condition by ponding or covering with a layer of sacking, canvas, hessian etc. and kept constantly wet for a period of 7-14 days depending on the type of cement and weather conditions. Blended cement needs extended curing. In some situations, polyethylene sheets or similar impermeable membranes may be used to cover the concrete surface closely to prevent evaporation.
(e) Sampling and strength of designed concrete mix (cl. 15)
Random samples of concrete cubes shall be cast from fresh concrete, cured and tested at 28 days as laid down in IS 516. Additional tests on beams for modulus of rupture at 3 or 7 days, or compressive strength tests at 7 days shall also be conducted. The number of samples would depend on the total quantity of concrete as given in cl. 15.2.2 and there should be three test specimens in each sample for testing at 28 days, and additional tests at 3 or 7 days.
(f) Acceptance criteria (cl. 16)
Concrete should be considered satisfactory when both the mean strength of any group of four consecutive test results and any individual test result of compressive strength and flexural strength comply the limits prescribed in IS 456.
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(g) Inspection and testing of structures (cl. 17)
Inspection of the construction is very important to ensure that it complies the design. Such inspection should follow a systematic procedure covering materials, records, workmanship and construction.
All the materials of concrete and reinforcement are to be tested following the relevant standards. It is important to see that the design and detailing are capable of execution maintaining a standard with due allowance for the dimensional tolerances. The quality of the individual parts of the structure should be verified. If needed, suitable quality assurance schemes should be used. The concrete should be inspected immediately after the removal of formwork to remove any defective work before concrete has hardened.
Standard core tests (IS 516) are to be conducted at three or more points to represent the whole concrete work in case of any doubt regarding the grade of concrete during inspection either due to poor workmanship or unsatisfactory results on cube strength obtained following the standard procedure. If the average equivalent cube strength of cores is equal to at least 85 per cent of the cube strength of that grade of concrete at that age and each of the individual cores has strength of at least 75 per cent, then only the concrete represented by the core test is considered acceptable. For unsatisfactory core test results, load tests should be conducted for the flexural members and proper analytical investigations should be made for non-flexural members.
Such load tests should be done as soon as possible after expiry of 28 days from the date of casting of the flexural members subjected to full dead load and 1.25 times the imposed load for 24 hours and then the imposed load shall be removed. The maximum deflection of the member during 24 hours under imposed load in mm should be less than 40 l2/D, where l is the effective span in m and D is the overall depth of the member in mm. For members showing more deflection, the recovery of the deflection within 24 hours of removal of the imposed load has to be noted. If the recovery is less than 75 per cent of the deflection under imposed load, the test should be repeated after a lapse of 72 hours. The structure is considered unacceptable if the recovery is less than 80 per cent.
There are further provisions of conducting non-destructive tests like ultrasonic pulse velocity (UPV), rebound hammer, probe penetration, pull out and maturity, as options to core tests or to supplement the data obtained from a limited number of cores. However, it is important that the acceptance criteria shall be agreed upon prior to these non-destructive testing. There are reports that UPV tests conducted three days after casting after removal of side formwork give very dependable insight about the quality of concrete.
1.2.6 Concluding Remarks
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The reinforced concrete consisting of plain concrete and steel reinforcement opened a new vista fulfilling the imaginations of architect with a unified approach of the architect and structural engineer. This has been made possible due to mouldability and monolithicity of concrete in addition to its strength in both tension and compression when reinforced with steel. However, concrete is produced by mixing cement, sand, gravel, water and mineral admixtures, if needed. Therefore, the final strength of concrete depends not only on the individual properties of its constituent materials, but also on the proportions of the material and the manner in which it is prepared, transported, placed, compacted and cured. Moreover, durability of the concrete is also largely influenced by all the steps of its preparation.
Steel reinforcement though produced in steel plants and made available in form of bars and rods of specific diameter also influences the final strength of reinforced concrete by its quality and durability due to environmental effects.
Concrete cover provides the protective environment to embedded steel from rusting that would need presence of both oxygen and moisture. Not only the extent of cover but the quality of cover is important for this reason.
Accordingly, inspection of concrete work, sample testing of specimens, core tests, load tests and non-destructive tests are very important to maintain the quality, strength and durability of reinforced concrete structures. Moreover, it is equally important to remove small defects or make good of it after removing the formwork before it has thoroughly hardened.
Thus, starting from the selection of each constitutive material to the satisfactory construction of the structure, the designer's responsibility will only produce the desired concrete structure which will satisfy the functional requirements as well as will have its aesthetic values exploiting all the good properties of this highly potential material.
1.2.7 Practice Questions and Problems with Answers
Q.1: What are the constituent materials of plain concrete?
A.1: The constituent materials of plain concrete are cement, sand (fine aggregate), gravel (coarse aggregate), water and mineral admixtures in some special cases.
Q.2: Define characteristic strength fck of concrete.
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A.2: Characteristic strength of concrete is defined as the compressive strength of 150 mm size cube at 28 days and expressed in N/mm2 below which not more than five per cent of the test results are expected to fall.
Q.3: How and when the characteristic compressive strength fck is determined?
A.3: Characteristic compressive strength is determined by conducting compressive strength tests on specified number of 150 mm concrete cubes at 28 days after casting. It is expressed in N/mm2.
Q.4: What do the symbols M and 20 mean for grade M 20 concrete?
A.4: The symbol M refers to mix and the number 20 indicates that the characteristic strength fck of grade M 20 is 20 N/mm2.
Q.5: Express the relation between flexural strength (fcr) and characteristic compression strength fck of concrete.
A.5: The generally accepted relation is: fcr = 0.7 N/mm2.
ckf
where fcr and fck are in
Q.6: Draw stress-strain curve of concrete and show the following:
(a) Initial tangent modulus Ec, (b) Secant modulus Es at any point A on the stress-strain curve, (c) Tangent modulus Et at A and (d) elastic and inelastic strain components of the total strain at A.
A.6: Please refer to Fig. 1.2.1
Q.7: Express the short term static modulus Ec in terms of the characteristic compressive strength fck of concrete.
A.7: The suggested expression is : Ec = 5000 N/mm2.
ckf
where Ec and fck are in
Q.8: State the approximate value of total shrinkage strain of concrete to be taken for the design purpose and mention the relevant clause no. of IS code.
A.8: As per cl. 6.2.4.1 of IS 456:2000, the approximate value of total shrinkage strain of concrete is to be taken as 0.0003.
Q.9: Define creep coefficient θ of concrete and express the relation between the effective modulus Ece, short term static modulus Ec and creep coefficient θ of concrete.
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A.9: Creep coefficient θ is the ratio of ultimate creep strain εcr and short term strain at the age of loading (θ = εcr/εc).
The required relation is Ece =
sec. 1.2.2 part (e)}.
cE +1 θ
Q.10: Define workability of concrete.
. {The derivation of Eq. 1.6 is given in
A.10: Workability of concrete is the property which determines the ease and homogeneity with which concrete can be mixed, placed, compacted and finished.
Q.11: Differentiate between design mix and nominal mix concrete.
A.11: In design mix, the proportions of cement, aggregates (sand and gravel), water and mineral admixtures, if any are actually determined by actual design to have a desired strength. In nominal mix, however, these proportions are nominally adopted.
Q.12: What are the different types of batching in mixing the constituent materials of concrete and name the type of batching to be adopted for different materials?
A.12: Mass and volume are the two types of batching. The quantities of cement, aggregates (sand and gravel) and solid admixtures shall be measured by mass batching. Liquid admixtures and water are measured either by mass or volume batching.
Q.13: Differentiate between steel bars and rods.
A.13: Bars are steel bars of diameter up to 12 mm which are coiled during transportation. Rods are steel bars of diameter greater than 12 mm and cannot be coiled. They are transported in standard lengths.
Q.14: Name the types of steel and their relevant IS standards to be used as reinforcement in concrete.
A.14: Please refer to sec. 1.2.4(i), (ii), (iii) and (iv).
Q.15: Draw stress-strain curve of steel bars with or without definite yield point and indicate the yield stress fy of them.
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A.15: Please refer to Figs. 1.2.3 and 1.2.4. For steel bars of Fig. 1.2.3, fy = 250 N/mm2 and for steel bars of Fig. 1.2.4, fy = 415 and 500 N/mm2 for the two different grades.
Q.16: What are the criteria of properly mixed concrete and how to achieve them?
A.16: Properly mixed concrete will have uniform distribution of materials having uniform colour and consistency.
These are achieved by mixing the constituent materials in a mechanical mixer at least for two minutes or such time till those qualities are achieved.
Q.17: What should be the expected strength of concrete structure at the time of removal of formwork?
A.17: The concrete at the time of removing the formwork should have strength of at least twice the stress that it may be subjected to at the time of removal of formwork.
Q.18: Name the sample tests to be performed for checking the strength of concrete.
A.18: The main test to be performed is 150 mm cube strength at 28 days made of fresh concrete and cured. Additional tests should also be conducted on 150 mm cubes at 7 days and beam tests to determine modulus of rupture at 3 or 7 days. There should be at least 3 or more samples of such specimens to represent the entire concrete work. Each sample should have at least three specimens for conducting each of the above-mentioned tests.
Q.19: Mention the specific acceptance criteria of the sample tests of cubes and beams.
A.19: Concrete should be considered satisfactory when both the mean strength determined from any group of four consecutive test results and any individual test result of compressive and flexural strength tests comply the prescribed limits of cl. 16 of IS 456.
Q.20: When is it essential to conduct standard core test?
A.20: Standard core tests are needed if the inspection of concrete work raises doubt regarding the grade of concrete either due to poor workmanship or unsatisfactory cube strength results performed following standard procedure.
Q.21: When do you consider core test results as satisfactory?
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A.21: The core test results are considered satisfactory if: (i) the average equivalent cube strength of the cores is at least 85 per cent of the cube strength of the grade of concrete at that age, and (ii) each of the individual cores has strength of at least 75 per cent of the concrete cube strength at that age.
Q.22: What are to be done for unsatisfactory core test results?
A.22: Load tests are to be conducted for the flexural members and analytical investigations are to be performed for non-flexural members.
Q.23: Prescribe the loading conditions and age of structure for conducting load tests.
A.23: Load tests are to be conducted as soon as possible after expiry of 28 days from the date of casting. The flexural member is subjected to full dead load and 1.25 times the imposed load for 24 hours and then the imposed load has to be removed.
Q.24: When do you conclude the load tests as satisfactory?
A.24: Load tests are considered satisfactory if the maximum deflection in mm of the member during 24 hours under load is less than 40 l2/D, where l = effective span in m and D = overall depth of the member in mm.
For members showing more deflection, the recovery of the deflection within 24 hours of removal of the imposed load has to be noted. If the recovery is less than 75 per cent of the deflection under imposed load, the tests should be repeated after a lapse of 72 hours. The structure is considered unacceptable if the recovery is less than 80 per cent.
Q.25: Name the acceptable non-destructive tests to be performed on structures.
A.25: The acceptable non-destructive tests are ultrasonic pulse velocity, rebound hammer, probe penetration, pull out and maturity.
1.2.8 References
Reinforced Concrete Limit State Design, 6th Edition, by Ashok K. Jain, Nem Chand & Bros, Roorkee, 2002.
Limit State Design of Reinforced Concrete, 2nd Edition, by P.C.Varghese,
Prentice-Hall of India Pvt. Ltd., New Delhi, 2002. 3. Advanced Reinforced Concrete Design, by P.C.Varghese, Prentice-Hall of
India Pvt. Ltd., New Delhi, 2001.
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Reinforced Concrete Design, 2nd Edition, by S.Unnikrishna Pillai and Devdas Menon, Tata McGraw-Hill Publishing Company Limited, New Delhi, 2003.
Limit State Design of Reinforced Concrete Structures, by P.Dayaratnam,
Oxford & I.B.H. Publishing Company Pvt. Ltd., New Delhi, 2004. 6. Reinforced Concrete Design, 1st Revised Edition, by S.N.Sinha, Tata
McGraw-Hill Publishing Company. New Delhi, 1990. 7. Reinforced Concrete, 6th Edition, by S.K.Mallick and A.P.Gupta, Oxford &
IBH Publishing Co. Pvt. Ltd. New Delhi, 1996. 8. Behaviour, Analysis & Design of Reinforced Concrete Structural Elements,
by I.C.Syal and R.K.Ummat, A.H.Wheeler & Co. Ltd., Allahabad, 1989. 9. Reinforced Concrete Structures, 3rd Edition, by I.C.Syal and A.K.Goel,
A.H.Wheeler & Co. Ltd., Allahabad, 1992. 10. Textbook of R.C.C, by G.S.Birdie and J.S.Birdie, Wiley Eastern Limited,
New Delhi, 1993. 11. Design of Concrete Structures, 13th Edition, by Arthur H. Nilson, David
Darwin and Charles W. Dolan, Tata McGraw-Hill Publishing Company Limited, New Delhi, 2004. 12. Concrete Technology, by A.M.Neville and J.J.Brooks, ELBS with
Longman, 1994. 13. Properties of Concrete, 4th Edition, 1st Indian reprint, by A.M.Neville,
Longman, 2000. 14. Reinforced Concrete Designer’s Handbook, 10th Edition, by C.E.Reynolds
and J.C.Steedman, E & FN SPON, London, 1997. 15. Indian Standard Plain and Reinforced Concrete – Code of Practice (4th
Revision), IS 456: 2000, BIS, New Delhi. 16. Design Aids for Reinforced Concrete to IS: 456 – 1978, BIS, New Delhi.
1.2.9 Test 2 with Solutions
Maximum Marks = 50, Maximum Time = 30 minutes
Answer all questions. Each question carries five marks.
TQ.1: Define characteristic strength (fck) of concrete
A.TQ.1: See Ans. 2 of sec. 1.2.7.
TQ.2:
How and when determined?
the characteristic compressive strength (fck)
is
A.TQ.2: See Ans. 3 of sec. 1.2.7.
TQ.3: What do the symbols M and 20 mean for grade M 20 concrete?
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A.TQ.3: See Ans. 4 of sec. 1.2.7.
TQ.4: Express the short term static modulus (Ec) in terms of the characteristic compressive strength (fck) of concrete.
A.TQ.4: See Ans. 7 of sec. 1.2.7.
TQ.5: Define creep coefficient θ of concrete and express the relation between the effective modulus (Ece), short term static modulus (Ec) and creep coefficient (θ) of concrete.
A.TQ.5: See Ans. 9 of sec. 1.2.7.
TQ.6: Differentiate between design mix and nominal mix concrete.
A.TQ.6: See Ans. 11 of sec. 1.2.7.
TQ.7: Draw stress-strain curve of steel bars with or without definite yield point and indicate the yield stress fy of them.
A.TQ.7: See Ans. 15 of sec. 1.2.7.
TQ.8: Mention the specific acceptance criteria of the sample tests of cubes and beams.
A.TQ.8: See Ans. 19 of sec. 1.2.7.
TQ.9: When do you consider core test results as satisfactory?
A.TQ.9: See Ans. 21 of sec. 1.2.7.
TQ.10: Prescribe the loading conditions and age of structure for conducting load tests.
A.TQ.10: See Ans. 23 of sec. 1.2.7.
1.2.10 Summary of this Lesson
Properties of concrete and steel are essential to be thoroughly known and understood by the designer. These properties in respect of strength, deformation and durability are summarised in this lesson. The importance of quality control, inspection and testing are emphasised starting from the basic preparation of concrete to the removal of formwork after the construction. The recommendations of Indian Standards are discussed regarding the acceptance of
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good concrete based on sample tests of specimens, core tests and other non- destructive tests.
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Objectives and Methods of Analysis and Design, and Properties of Concrete and Steel
Objectives and Methods of Analysis and Design, and Properties of Concrete and Steel
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Lesson 1
Objectives and Methods of Analysis and Design
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Instructional Objectives:
At the end of this lesson, the student should be able to:
• state the four objectives of the design of reinforced concrete structures, • name the three methods of design of concrete structure and identify the best
method of design, • state the basis of the analysis of the structure, • express the design loads in terms of characteristic loads in limit state and
working stress methods, • define the characteristic load, • name the different loads, forces and effects to be considered in the design, • state the basis of determining the combination of different loads acting on the
structure
1.1.1 Introduction
Reinforced concrete, as a composite material, has occupied a special place in the modern construction of different types of structures due to its several advantages. Italian architect Ponti once remarked that concrete liberated us from the rectangle. Due to its flexibility in form and superiority in performance, it has replaced, to a large extent, the earlier materials like stone, timber and steel. Further, architect's scope and imaginations have widened to a great extent due to its mouldability and monolithicity. Thus, it has helped the architects and engineers to build several attractive shell forms and other curved structures. However, its role in several straight line structural forms like multistoried frames, bridges, foundations etc. is enormous.
The design of these modern reinforced concrete structures may appear to be highly complex. However, most of these structures are the assembly of several basic structural elements such as beams, columns, slabs, walls and foundations (Anim. 1.1.1). Accordingly, the designer has to learn the design of these basic reinforced concrete elements. The joints and connections are then carefully developed.
Anim. 1.1.1
Design of reinforced concrete structures started in the beginning of last century following purely empirical approach. Thereafter came the so called rigorous elastic theory where the levels of stresses in concrete and steel are limited so that stress-deformations are taken to be linear. However, the limit state method, though semi-empirical approach, has been found to be the best for the design of reinforced concrete structures (see sec. 1.1.3.1 also). The constraints and applicabilities of both the methods will be discussed later.
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1.1.2 Objectives of the Design of Reinforced Concrete Structures
Every structure has got its form, function and aesthetics. Normally, we consider that the architects will take care of them and the structural engineers will be solely responsible for the strength and safety of the structure. However, the roles of architects and structural engineers are very much interactive and a unified approach of both will only result in an "Integrated" structure, where every material of the total structure takes part effectively for form, function, aesthetics, strength as well as safety and durability. This is possible when architects have some basic understanding of structural design and the structural engineers also have the basic knowledge of architectural requirements.
Both the engineer and the architect should realize that the skeletal structure without architecture is barren and mere architecture without the structural strength and safety is disastrous. Safety, here, includes consideration of reserve strength, limited deformation and durability. However, some basic knowledge of architectural and structural requirements would facilitate to appreciate the possibilities and limitations of exploiting the reinforced concrete material for the design of innovative structures.
Before proceeding to the design, one should know the objectives of the design of concrete structures. The objectives of the design are as follows:
1.1.2.1 The structures so designed should have an acceptable probability of performing satisfactorily during their intended life.
This objective does not include a guarantee that every structure must perform satisfactorily during its intended life. There are uncertainties in the design process both in the estimation of the loads likely to be applied on the structure and in the strength of the material. Moreover, full guarantee would only involve more cost. Thus, there is an acceptable probability of performance of structures as given in standard codes of practices of different countries.
1.1.2.2 The designed structure should sustain all loads and deform within limits for construction and use.
Anim. 1.1.2 Anim. 1.1.3 Anim. 1.1.4
Adequate strengths and limited deformations are the two requirements of the designed structure. The structure should have sufficient strength and the deformations must be within prescribed limits due to all loads during construction and use as seen in Anim. 1.1.2. Animation 1.1.3 shows the structure having insufficient strength of concrete which fails in bending compression with the increase of load, though the deformation of the structure is not alarming. On the
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other hand, Anim. 1.1.4 shows another situation where the structure, having sufficient strength, deforms excessively. Both are undesirable during normal construction and use.
However, sometimes structures are heavily loaded beyond control. The structural engineer is not responsible to ensure the strength and deformation within limit under such situation. The staircases in residential buildings during festival like marriage etc., roof of the structures during flood in the adjoining area or for buildings near some stadium during cricket or football matches are some of the examples when structures get overloaded. Though, the structural designer is not responsible for the strength and deformations under these situations, he, however, has to ensure that the failure of the structures should give sufficient time for the occupants to vacate. The structures, thus, should give sufficient warning to the occupants and must not fail suddenly.
1.1.2.3 The designed structures should be durable.
The materials of reinforced concrete structures get affected by the environmental conditions. Thus, structures having sufficient strength and permissible deformations may have lower strength and exhibit excessive deformations in the long run. The designed structures, therefore, must be checked for durability. Separate checks for durability are needed for the steel reinforcement and concrete. This will avoid problems of frequent repairing of the structure.
1.1.2.4 The designed structures should adequately resist to the effects of misuse and fire.
Structures may be misused to prepare fire works, store fire works, gas and other highly inflammable and/or explosive chemicals. Fire may also take place as accidents or as secondary effects during earthquake by overturning kerosene stoves or lantern, electrical short circuiting etc. Properly designed structures should allow sufficient time and safe route for the persons inside to vacate the structures before they actually collapse.
How to fulfill the objectives?
All the above objectives can be fulfilled by understanding the strength and deformation characteristics of the materials used in the design as also their deterioration under hostile exposure. Out of the two basic materials concrete and steel, the steel is produced in industries. Further, it is available in form of standard bars and rods of specific diameters. However, sample testing and checking are important to ensure the quality of these steel bars or rods. The concrete, on the other hand, is prepared from several materials (cement, sand,
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coarse aggregate, water and admixtures, if any). Therefore, it is important to know the characteristic properties of each of the materials used to prepare concrete. These materials and the concrete after its preparation are also to be tested and checked to ensure the quality. The necessary information regarding the properties and characteristic strength of these materials are available in the standard codes of practices of different countries. It is necessary to follow these clearly defined standards for materials, production, workmanship and maintenance, and the performance of structures in service.
1.1.3 Method of Design
Three methods of design are accepted in cl. 18.2 of IS 456:2000 (Indian Standard Plain and Reinforced Concrete - Code of Practice, published by the Bureau of Indian Standards, New Delhi). They are as follows:
1.1.3.1 Limit state method
The term “Limit states” is of continental origin where there are three limit states - serviceability / crack opening / collapse. For reasons not very clear, in English literature limit state of collapse is termed as limit state.
As mentioned in sec. 1.1.1, the semi-empirical limit state method of design has been found to be the best for the design of reinforced concrete members. More details of this method are explained in Module 3 (Lesson 4). However, because of its superiority to other two methods (see sections 2.3.2 and 2.3.3 of Lesson 3), IS 456:2000 has been thoroughly updated in its fourth revision in 2000 taking into consideration the rapid development in the field of concrete technology and incorporating important aspects like durability etc. This standard has put greater emphasis to limit state method of design by presenting it in a full section (section 5), while the working stress method has been given in Annex B of the same standard. Accordingly, structures or structural elements shall normally be designed by limit state method.
1.1.3.2 Working stress method
This method of design, considered as the method of earlier times, has several limitations. However, in situations where limit state method cannot be conveniently applied, working stress method can be employed as an alternative. It is expected that in the near future the working stress method will be completely replaced by the limit state method. Presently, this method is put in Annex B of IS 456:2000.
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1.1.3.3 Method based on experimental approach
The designer may perform experimental investigations on models or full size structures or elements and accordingly design the structures or elements. However, the four objectives of the structural design (sec. 1.1.2) must be satisfied when designed by employing this approach. Moreover, the engineer-in- charge has to approve the experimental details and the analysis connected therewith.
Though the choice of the method of design is still left to the designer as per cl. 18.2 of IS 456:2000, the superiority of the limit state method is evident from the emphasis given to this method by presenting it in a full section (Section 5), while accommodating the working stress method in Annex B of IS 456:2000, from its earlier place of section 6 in IS 456:1978. It is expected that a gradual change over to the limit state method of design will take place in the near future after overcoming the inconveniences of adopting this method in some situations.
1.1.4 Analysis of Structures
Structures when subjected to external loads (actions) have internal reactions in the form of bending moment, shear force, axial thrust and torsion in individual members. As a result, the structures develop internal stresses and undergo deformations. Essentially, we analyse a structure elastically replacing each member by a line (with EI values) and then design the section using concepts of limit state of collapse. Figure 1.1.1 explains the internal and external reactions of a simply supported beam under external loads. The external loads to be applied on the structures are the design loads and the analyses of structures are based on linear elastic theory (vide cl. 22 of IS 456:2000).
1.1.5 Design Loads
The design loads are determined separately for the two methods of design as mentioned below after determining the combination of different loads.
1.1.5.1 In the limit state method, the design load is the characteristic load with appropriate partial safety factor (vide sec. 2.3.2.3 for partial safety factors).
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1.1.5.2 In the working stress method, the design load is the characteristic load only.
What is meant by characteristic load?
Characteristic load (cl. 36.2 of IS 456:2000) is that load which has a ninety-five per cent probability of not being exceeded during the life of the structure.
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The various loads acting on structures consist of dead loads, live loads, wind or earthquake loads etc. These are discussed in sec. 1.1.6. However, the researches made so far fail to estimate the actual loads on the structure. Accordingly, the loads are predicted based on statistical approach, where it is assumed that the variation of the loads acting on structures follows the normal distribution (Fig. 1.1.2). Characteristic load should be more than the average/mean load. Accordingly,
Characteristic load = Average/mean load + K (standard deviation for load)
The value of K is assumed such that the actual load does not exceed the characteristic load during the life of the structure in 95 per cent of the cases.
1.1.6 Loads and Forces
The following are the different types of loads and forces acting on the structure. As mentioned in sec. 1.1.5, their values have been assumed based on earlier data and experiences. It is worth mentioning that their assumed values as stipulated in IS 875 have been used successfully.
1.1.6.1 Dead loads
Anims. 1.1.5 a and
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b
These are the self weight of the structure to be designed (see Anim. 1.1.5a). Needless to mention that the dimensions of the cross section are to be assumed initially which enable to estimate the dead loads from the known unit weights of the materials of the structure. The accuracy of the estimation thus depends on the assumed values of the initial dimensions of the cross section. The values of unit weights of the materials are specified in Part 1 of IS 875.
1.1.6.2 Imposed loads
They are also known as live loads (Anim. 1.1.5a) and consist of all loads other than the dead loads of the structure. The values of the imposed loads depend on the functional requirement of the structure. Residential buildings will have comparatively lower values of the imposed loads than those of school or office buildings. The standard values are stipulated in Part 2 of IS 875.
1.1.6.3 Wind loads
These loads (Anim. 1.1.5a) depend on the velocity of the wind at the location of the structure, permeability of the structure, height of the structure etc. They may be horizontal or inclined forces depending on the angle of inclination of the roof for pitched roof structures. They can even be suction type of forces depending on the angle of inclination of the roof or geometry of the buildings (Anim. 1.1.5b). Wind loads are specified in Part 3 of IS 875.
1.1.6.4 Snow loads
These are important loads for structures located in areas having snow fall, which gets accumulated in different parts of the structure depending on projections, height, slope etc. of the structure (Anim. 1.1.6). The standard values of snow loads are specified in Part 4 of IS 875.
1.1.6.5 Earthquake forces
Anim. 1.1.6
Anim. 1.1.7
Earthquake generates waves which move from the origin of its location (epicenter) with velocities depending on the intensity and magnitude of the earthquake. The impact of earthquake on structures depends on the stiffness of the structure, stiffness of the soil media, height and location of the structure etc. (Anim. 1.1.7). Accordingly, the country has been divided into several zones depending on the magnitude of the earthquake. The earthquake forces are
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prescribed in IS 1893. Designers have adopted equivalent static load approach or spectral method.
1.1.6.6 Shrinkage, creep and temperature effects
Anim. 1.1.8, 9 and 10
Shrinkage, creep and temperature (high or low) may produce stresses and cause deformations like other loads and forces (Anim. 1.1.8, 9 and 10). Hence, these are also considered as loads which are time dependent. The safety and serviceability of structures are to be checked following the stipulations of cls. 6.2.4, 5 and 6 of IS 456:2000 and Part 5 of IS 875.
1.1.6.7 Other forces and effects
It is difficult to prepare an exhaustive list of loads, forces and effects coming onto the structures and affecting the safety and serviceability of them. However, IS 456:2000 stipulates the following forces and effects to be taken into account in case they are liable to affect materially the safety and serviceability of the structures. The relevant codes as mentioned therein are also indicated below:
• Foundation movement (IS 1904) (Fig. 1.1.3) • Elastic axial shortening
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• Soil and fluid pressures (vide IS 875 - Part 5) • Vibration • Fatigue • Impact (vide IS 875 - Part 5) • Erection loads (Please refer to IS 875 - Part 2) (Fig. 1.1.4) • Stress concentration effect due to point of application of load and the
like.
1.1.6.8 Combination of loads
Design of structures would have become highly expensive in order to maintain their serviceability and safety if all types of forces would have acted on all structures at all times. Accordingly, the concept of characteristic loads has been accepted to ensure that in at least 95 per cent of the cases, the characteristic loads considered will be higher than the actual loads on the structure. However, the characteristic loads are to be calculated on the basis of average/mean load of some logical combinations of all the loads mentioned in sec. 1.1.6.1 to 7. These logical combinations are based on (i) the natural phenomena like wind and earthquake do not occur simultaneously, (ii) live loads on roof should not be present when wind loads are considered; to name a few. IS 875 Part 5 stipulates the combination of loads to be considered in the design of structures.
1.1.7 Practice Questions and Problems with Answers
Q.1: Show two reasons why concrete is superior to stone, timber and steel?
A.1:
(i) Stone, timber and steel cannot be fitted to any mould, but concrete during its green stage can fit to any mould.
(ii) Structures made of stone, timber and steel have several joints, but different elements of concrete structures can be cast monolithically.
Q.2: Define integrated structure.
A.2:
Integrated structure is one where each part of the structure satisfies both the structural requirements of strength and serviceability and architectural requirements of aesthetics and functionality.
Q.3: State four objectives of the design of reinforced concrete structure.
A.3: Properly designed reinforced concrete structures should:
(i)
have acceptable probability of performing satisfactorily during their intended life,
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(ii)
(iii)
(iv)
sustain all loads with limited deformations during construction and use,
be durable,
adequately resist the effects of misuse and fire.
Q.4: How to fulfil the four objectives of the design of reinforced concrete structures?
A.4: The four objectives can be fulfilled by:
(i)
(ii)
understanding the strength and deformation characteristics of concrete and steel,
following the clearly defined standards for materials, production, workmanship and maintenance, and use of structures in service,
(iii)
adopting measures needed for durability.
Q.5: What are the three methods of design of reinforced concrete structural elements?
A.5: The three methods are:
(i) limit state method, (ii) working stress method, (iii) method based on experimental approach
Q.6: Which of the three methods is the best?
A.6: Limit state method is the best of the three methods when clearly applicable.
Q.7: What is the basis of the analysis of structures to be designed?
A.7: The basis of the analysis is the employment of linear elastic theory.
Q.8: How to estimate the design loads in (i) limit state method, and (ii) working stress method?
A.8:
(i) In limit state method,
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Design loads = Characteristic loads multiplied by the partial safety factor for loads
(ii) In working stress method,
Design loads = Characteristic loads
Q.9: Define characteristic load.
A.9: Characteristic load is that load which has a ninety-five per cent probability of not being exceeded during the life of the structure.
Q.10: What are the main (i) loads, (ii) forces and (iii) effects to be considered while designing the structures?
A.10: (i) The main loads are:
(a) Dead loads (b) Imposed loads or live loads (c) Wind loads (d) Snow loads (e) Erection loads
(ii) The main force is:
(a) Earthquake force
(iii) The main effects are:
(a) Shrinkage, creep and temperature effects (b) Foundation movements (c) Elastic axial shortening (d) Soil and fluid pressures (e) Vibration (f) Fatigue (g) Impact (h) Stress concentration effects due to application of point loads
Q.11: What are the basis of combining different loads for the design?
A.11: Natural phenomenon and common sense are the basis of selecting the combination different loads acting on the structure while designing.
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1.1.8 References
Reinforced Concrete Limit State Design, 6th Edition, by Ashok K. Jain, Nem Chand & Bros, Roorkee, 2002.
Limit State Design of Reinforced Concrete, 2nd Edition, by P.C.Varghese,
Prentice-Hall of India Pvt. Ltd., New Delhi, 2002. 3. Advanced Reinforced Concrete Design, by P.C.Varghese, Prentice-Hall of
India Pvt. Ltd., New Delhi, 2001. 4. Reinforced Concrete Design, 2nd Edition, by S.Unnikrishna Pillai and
Devdas Menon, Tata McGraw-Hill Publishing Company Limited, New Delhi, 2003. 5. Limit State Design of Reinforced Concrete Structures, by P.Dayaratnam,
Oxford & I.B.H. Publishing Company Pvt. Ltd., New Delhi, 2004. 6. Reinforced Concrete Design, 1st Revised Edition, by S.N.Sinha, Tata
McGraw-Hill Publishing Company. New Delhi, 1990. 7. Reinforced Concrete, 6th Edition, by S.K.Mallick and A.P.Gupta, Oxford &
IBH Publishing Co. Pvt. Ltd. New Delhi, 1996. 8. Behaviour, Analysis & Design of Reinforced Concrete Structural Elements,
by I.C.Syal and R.K.Ummat, A.H.Wheeler & Co. Ltd., Allahabad, 1989. 9. Reinforced Concrete Structures, 3rd Edition, by I.C.Syal and A.K.Goel,
A.H.Wheeler & Co. Ltd., Allahabad, 1992. 10. Textbook of R.C.C, by G.S.Birdie and J.S.Birdie, Wiley Eastern Limited,
New Delhi, 1993. 11. Design of Concrete Structures, 13th Edition, by Arthur H. Nilson, David
Darwin and Charles W. Dolan, Tata McGraw-Hill Publishing Company Limited, New Delhi, 2004. 12. Concrete Technology, by A.M.Neville and J.J.Brooks, ELBS with
Longman, 1994. 13. Properties of Concrete, 4th Edition, 1st Indian reprint, by A.M.Neville,
Longman, 2000. 14. Reinforced Concrete Designer’s Handbook, 10th Edition, by C.E.Reynolds
and J.C.Steedman, E & FN SPON, London, 1997. 15. Indian Standard Plain and Reinforced Concrete – Code of Practice (4th
Revision), IS 456: 2000, BIS, New Delhi. 16. Design Aids for Reinforced Concrete to IS: 456 – 1978, BIS, New Delhi.
1.1.9 Test 1 with Solutions
Maximum Marks = 50, Maximum Time = 30 minutes
Answer all questions.
TQ.1: Tick the correct answer: (4 x 7 = 28 marks)
(i) Properly designed concrete structures should
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(a) sustain all loads likely to come during next 50 years
(b) sustain all loads and deformations without collapse or any damage
(c) sustain all loads with limited deformations during construction and use
(d) sustain characteristic loads and deformations during the next 50 years
A.TQ.1: (i): (c)
(ii) In the limit state method, the design load is
(a) the characteristic load
(b) the ultimate load
(c) the characteristic load divided by the partial safety factor for loads
(d) the characteristic load multiplied by the partial safety for loads
A.TQ.1: (ii): (d)
(iii) In the limit state method, the basis of the analysis of structure is
(a) linear elastic theory
(b) non-linear theory
(c) plastic method of analysis
(d) involving fracture mechanics
A.TQ.1: (iii): (a)
(iv) The characteristic load is
(a) the load at first crack
(b) that load which has a probability of ninety-five per cent of not being exceeded during the life of the structure
(c) the ultimate collapse load
(d) the ultimate collapse load multiplied by the partial safety factor for loads
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A.TQ.1: (iv): (b)
TQ.2: Name the main (i) loads, (ii) forces and (iii) effects to be considered while designing the structures.
(5 + 1 + 8 = 14 marks) (5 marks for five loads, 1 mark for one force and 8 marks for eight effects)
A.TQ.2: See A.10 of sec. 1.1.7.
TQ.3: State the basis of determining the combination of different loads acting on the structure.
(8 marks)
A.TQ.3: See A.11 of sec. 1.1.7.
1.1.10 Summary of this Lesson
This lesson explains the four objectives of the design of reinforced concrete structures. It also mentions the three methods of design and states about the superiority of the limit state method. The basis of the analysis of structure is discussed. The terminologies like design loads, characteristic loads etc. are defined and explained. The different loads, forces and effects to be considered have been listed and discussed. Finally, the basis of the combination of different loads to be considered is also explained.
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