Reinforced Concrete Design :introduction 6e4q11

REINFORCED CONCRETE DESIGN Engr. Randy G. Policarpio

Introduction Concrete is a mixture of sand, gravel, crushed rock, or other aggregates held together in a rocklike mass with a paste of cement and water. • Sometimes one or more ixtures are added to change certain characteristics of the concrete such as its workability, durability, and time of hardening. • As with most rocklike substances, concrete has a high compressive strength and a very low tensile strength. Reinforced concrete is a combination of concrete and steel wherein the steel reinforcement provides the tensile strength lacking in the concrete. • Steel reinforcing is also capable of resisting compression forces and is used in columns.

Concrete Mixing and Proportioning Workability • Workability measured by slump test 12” 1 1. 2. 3. 4.

slump

2 3 4 Layer 1: Fill 1/3 full. 25 stokes Layer 2: Fill 2/3 full. 25 stokes Layer 3: Fill full. 25 stokes Lift cone and measure slump (typically 2-6 in.)

Advantages of Reinforced Concrete as a Structural Material 1.

It has considerable compressive strength per unit cost compared with most other materials. 2. Reinforced concrete has great resistance to the actions of fire and water and, in fact, is the best structural material available for situations where water is present. During fires of average intensity, with a satisfactory cover of concrete over the reinforcing bars suffer only surface damage without failure.  3. Reinforced concrete structures are very rigid. 4. It is a low-maintenance material. 5. As compared with other materials, it has a very long service life. Under proper conditions, reinforced concrete structures can be used indefinitely without reduction of their load- carrying abilities. This can be explained by the fact that the strength of concrete does not decrease with time but actually increases over a very long period, measured in years, because of the lengthy process of the solidification of the cement paste. 6. It is usually the only economical material available for footings, floor slabs, basement walls, piers, and similar applications. 7. A special feature of concrete is its ability to be cast into an extraordinary variety of shapes from simple slabs, beams, and columns to great arches and shells. 8. In most areas, concrete takes advantage of inexpensive local materials (sand, gravel, and water) and requires relatively small amounts of cement and reinforcing steel, which may have to be shipped from other parts of the country. 9. A lower grade of skilled labor is required for erection as compared with other materials such as structural steel.

Disadvantages of Reinforced Concrete as a Structural Material 1. Concrete has a very low tensile strength, requiring the use of tensile reinforcing. 2. Forms are required to hold the concrete in place until it hardens sufficiently. In addition, falsework or shoring may be necessary to keep the forms in place for roofs, walls, floors, and similar structures until the concrete gain sufficient strength to themselves. • Formwork is very expensive. • It should be obvious that when efforts are made to improve the economy of reinforced concrete structures, the major emphasis is on reducing formwork costs. 3. The low strength per unit of weight of concrete leads to heavy . This becomes an increasingly important matter for long-span structures, where concrete’s large dead weight has a great effect on bending moments. Lightweight aggregates can be used to reduce concrete weight, but the cost of the concrete is increased. 4. Similarly, the low strength per unit of volume of concrete means will be relatively large, an important consideration for tall buildings and long-span structures.  5. The properties of concrete vary widely because of variations in its proportioning and mixing. Furthermore, the placing and curing of concrete is not as carefully controlled as is the production of other materials, such as structural steel and laminated wood.  

Design Codes • NATIONAL STRUCTURAL CODE OF THE PHILIPPINES (NS) VOLUME I : Building, Towers and other Vertical Structures.  VOLUME II: Roads, Highways and Bridges.

• AMERICAN CONCRETE INSTITUTE (ACI Code)  Building Code Requirements for Structural Concrete (ACI 318)  This code is used primarily for the design of buildings.  Design requirements for various types of reinforced concrete are presented in the codes along with a “commentary” on those requirements.  . The commentary provides explanations, suggestions, and

Portland Cement Portland cement is a mixture of calcareous and argillaceous materials which are calcined in a kiln and then pulverized. When mixed with water, cement hardens through a process called hydration. Types of Portland Cement Type I—The common, all-purpose cement used for general construction work. Type II—A modified cement that has a lower heat of hydration than does Type I cement and that can withstand some exposure to sulfate attack. Type III—A high-early-strength cement that will produce in the first 24 hours a concrete with a strength about twice that of Type I cement. This cement does have a much higher heat of hydration. Type IV—A low-heat cement that produces a concrete which generates heat very slowly. It is used for very large concrete structures.   Type V—A cement used for concretes that are to be exposed to high concentrations of sulfate.

ixtures MOST COMMON TYPES OF IXTURES • Air-entraining ixtures, conforming to the requirements of ASTM C260 and C618, are used primarily to increase concrete’s resistance to freezing and thawing and provide better resistance to the deteriorating action of deicing salts. The air-entraining agents cause the mixing water to foam, with the result that billions of closely spaced air bubbles are incorporated into the concrete. When concrete freezes, water moves into the air bubbles, relieving the pressure in the concrete. When the concrete thaws, the water can move out of the bubbles, with the result that there is less cracking than if air entrainment had not been used. • The addition of accelerating ixtures, such as calcium chloride, to concrete will accelerate its early strength development. The results of such additions (particularly useful in cold climates) are reduced times required for curing and protection of the concrete and the earlier removal of forms. (Section 3.6.3 of the ACI Code states that because of corrosion problems, calcium chloride may not be added to concretes with embedded aluminum, concretes cast against stay-in-place galvanized steel forms, or prestressed concrete.) Other accelerating ixtures that may be used include various soluble salts as well as some other organic compounds. • Retarding ixtures are used to slow the setting of the concrete and to retard temperature increases. They consist of various acids or sugars or sugar derivatives. Some concrete truck drivers keep sacks of sugar on hand to throw into the concrete in case they get caught in traffic jams or are otherwise delayed. Retarding ixtures are particularly useful for large pours where significant temperature increases may occur. They also prolong the plasticity of the concrete, enabling better blending or bonding of successive pours. Retarders can also slow the hydration of cement on exposed concrete surfaces or formed surfaces to produce attractive exposed aggregate finishes. • Superplasticizers are ixtures made from organic sulfonates. Their use enables engineers to reduce the water content in concretes substantially while at the same time increasing their slumps. Although superplasticizers can also be used to keep water– cement ratios constant while using less cement, they are more commonly used to produce workable concretes with considerably higher strengths while using the same amount of cement. (See Section 1.13.) A relatively new product, self-consolidating concrete, uses superplasticizers and modifications in mix designs to produce an extremely workable mix that requires no vibration, even for the most congested placement situations. • Waterproofing materials usually are applied to hardened concrete surfaces, but they may be added to concrete mixes. These ixtures generally consist of some type of soap or petroleum products, as perhaps asphalt emulsions. They may help retard the penetration of water into porous concretes but probably don’t help dense, well-cured concretes very much.

Failure Mechanism of Concrete Shrinkage Microcracks are the initial shrinkage cracks due to carbonation shrinkage, hydration shrinkage, and drying shrinkage.

Failure Mechanism of Concrete Bond Microcracks are extensions of shrinkage microcracks, as the compression stress field increases, the shrinkage microcracks widen but do not propagates into the matrix. Occur at 15-20 % ultimate strength of concrete.

Failure Mechanism of Concrete Matrix Microcracks are microcracks that occur in the matrix. The propagate from 20% fc. Occur up to 30-45 % ultimate strength of concrete. Matrix microcracks start bridge one another at 75%. Aggregate microcracks occur just before failure (90%).

Concrete Properties The standard strength test generally uses a cylindrical sample. It is tested after 28 days to test for strength, fc. The concrete will continue to harden with time and for a normal Portland cement will increase with time as follows:

Concrete Properties • Compressive Strength, f’c

• Normally use 28-day strength for f design strength Ec c

• Poisson’s Ratio, 

f’c

• ~ 0.15 to 0.20 0.45f’c • Usually use  0.17

o

u

c

Concrete Properties • Modulus of Elasticity, Ec

• Corresponds to secant modulus at 0.45 f’c • ACI 318-99 (Sec. 8.5.1):1.5

Ec ( psi )  33 w

f 'c ( psi )

where w = unit weight (pcf) 90 pcf < wc <155 pcf For normal weight concrete (wc  145 pcf)

Ec ( psi )  57,000

f 'c ( psi )

Concrete Properties • Concrete strain at max. compressive stress, o

• See Fig. 3.17 for typical  curves in compression • o varies between 0.0015-0.003 • For normal strength concrete, o ~ f 0.002 E c

c

f’c 0.45f’c

o

u

Concrete Properties • Maximum useable strain, u

• ACI Code: u = 0.003 • Used for flexural and axial compression fc

Ec

f’c

0.45f’c

o

u

Concrete Properties Typical Concrete Stress-Strain Curves in Compression

Concrete Properties Types of compression failure There are three modes of failure. [1] Under axial compression concrete fails in shear. [2] the separation of the specimen into columnar pieces by what is known as splitting or columnar fracture. [3] Combination of shear and splitting failure.

Concrete Properties 2. Tensile Strength

• Tensile strength ~ 8% to 15% of f’c • Modulus of Rupture, fr

f r deflection  7.5 calculations, f 'c ( psi • For use:) P

• Test: Mmax = P/2*a

ACI Eq. 9-9

unreinforced concrete beam

fr

fr 

Mc 6 M  2 I bh

Concrete Properties 2. Tensile Strength (cont.) • Splitting Tensile Strength, fct • Split Cylinder Test P Concrete Cylinder Poisson’s Effect

Concrete Properties 2. Tensile Strength (cont.) 2P f ct  ld f ct  (5 to 7) f 'c ( psi )

(Not given in ACI Code)

Reinforcing Steel • The reinforcing used for concrete structures may be in the form of bars or welded wire fabric. • Reinforcing bars are referred to as plain or deformed. • The deformed bars, which have ribbed projections rolled onto their surfaces (patterns differing with different manufacturers) to provide better bonding between the concrete and the steel, are used for almost all applications. • Instead of rolled-on deformations, deformed wire has indentations pressed into it. • Plain bars are not used very often except for wrapping around longitudinal bars, primarily in columns.

Reinforcing Steel 1. General •

Most common types for non-prestressed : • •

hot-rolled deformed bars welded wire fabric

Reinforcing Steel

Reinforcing Steel

Grades of Reinforcing Steel • Reinforcing bars may be rolled from billet steel, axle steel, or rail steel. • There are several types of reinforcing bars, designated by the ASTM. • These steels are available in different grades as Grade 50, Grade 60, and so on, where Grade 50 means the steel has a specified yield point of 50,000 psi, Grade 60 means 60,000 psi, and so on. •

•

•

ASTM A615: Deformed and plain billet steel bars. These bars, which must be marked with the letter S (for type of steel), are the most widely used reinforcing bars. Bars are of four minimum yield strength levels: 40,000 psi (280 MPa); 60,000 psi (420 MPa); 75,000 psi (520 MPa); and 80,000 psi (550 MPa). ASTM A706: Low-alloy deformed and plain bars. These bars, which must be marked with the letter W (for type of steel), are to be used where controlled tensile properties and/or specially controlled chemical composition is required for welding purposes. They are available in two grades: 60,000 psi (420 MPa) and 80,000 psi (550 MPa), designated as Grade 60 (420) and Grade 80 (550), respectively. ASTM A996: Deformed rail steel or axle steel bars. They must be marked with the letter R (for type of steel).

• When deformed bars are produced to meet both the A615 and A706 specifications, they must be marked with both the letters S and W.

Identifying Marks on Reinforcing Bars Markings are described in the following list: 1. The producing company is identified with a letter.  2. The bar size number (3 to 18) is given next. 3. Another letter is shown to identify the type of steel (S for billet, R in addition to a rail sign for rail steel, A for axle, and W for low alloy). 4. Finally, the grade of the bars is shown either with numbers or with continuous lines. A Grade 60 bar has either the number 60 on it or a continuous longitudinal line in addition to its main ribs. A Grade 75 bar will have the number 75 on it or two continuous lines in addition to the main ribs.

Identifying Marks on Reinforcing Bars 1. General •

Standard Reinforcing Bar Markings

Identifying Marks on Reinforcing Bars

Identifying Marks on Reinforcing Bars

Identifying Marks on Reinforcing Bars

Reinforcing Steel 3. Stress versus Strain • Stress-Strain curve for various types of steel reinforcement bar.

Reinforcing Steel GR 60 (less ductile)

• Es = Initial tangent modulus = 29,000 ksi (all grades)

GR 40

Stress E • Note: GR40 has a longer yield plateau

1s 0.20 Strain

Loads • Perhaps the most important and most difficult task faced by the structural designer is the accurate estimation of the loads that may be applied to a structure during its life. • No loads that may reasonably be expected to occur may be overlooked. • After loads are estimated, the next problem is to decide the worst possible combinations of these loads that might occur at one time.

Loads • Dead Loads • Dead loads are loads of constant magnitude that remain in one position. They include the weight of the structure under consideration as well as any fixtures that are permanently attached to it. • For a reinforced concrete building, some dead loads are the frames, walls, floors, ceilings, stairways, roofs, and plumbing. • To design a structure, it is necessary for the weights or dead loads of the various parts to be estimated for use in the analysis. • The exact sizes and weights of the parts are not known until the structural analysis is made and the of the structure are selected. • The weights, as determined from the actual design, must be compared with the estimated weights. If large discrepancies are present, it will be necessary to repeat the analysis and design using better estimated weights.

Loads

Live Loads • Live loads are loads that can change in magnitude and position. They include occupancy loads, warehouse materials, construction loads, overhead service cranes, equipment operating loads, and many others. In general, they are induced by gravity. Among the many other types of live loads are: • Traffic loads for bridges—Bridges are subjected to series of concentrated loads of varying magnitude caused by groups of truck or train wheels. • Impact loads—Impact loads are caused by the vibration of moving or movable loads. It is obvious that a crate dropped on the floor of a warehouse or a truck bouncing on uneven pavement of a bridge causes greater forces than would occur if the loads were applied gently and gradually. Impact loads are equal to the difference between the magnitude of the loads actually caused and the magnitude of the loads had they been dead loads. • Longitudinal loads—Longitudinal loads also need to be considered in deg some structures. Stopping a train on a railroad bridge or a truck on a highway bridge causes longitudinal forces to be applied. It is not difficult to imagine the tremendous longitudinal force developed when the driver of a 40-ton trailer truck traveling at 60 mph suddenly has to apply the brakes while crossing a highway bridge. There are other longitudinal load situations, such as ships running into docks and the movement of traveling cranes that are ed by building frames. • Miscellaneous loads—Among the other types of live loads with which the structural designer will have to contend are soil pressures (such as the exertion of lateral earth pressures on walls or upward pressures on foundations), hydrostatic pressures (such as water pressure on dams, inertia forces of large bodies of water during earthquakes, and uplift pressures on tanks and basement structures), blast loads (caused by explosions, sonic booms, and military weapons), and centrifugal forces (such as those caused on curved bridges by trucks and trains or similar effects on roller coasters).

Live Loads

Live Loads

Environmental Loads • Environmental loads are loads caused by the environment in which the structure is located. • For buildings, they are caused by rain, snow, wind, temperature change, and earthquake. • Strictly speaking, these are also live loads, but they are the result of the environment in which the structure is located. Although they do vary with time, they are not all caused by gravity or operating conditions, as is typical with other live loads.

Various kinds of environmental loads. 1. Snow and ice. 2. Rain. 3. Wind 4. Soil pressure 5. Seismic loads. 6. Temperature Differentials

ANALYSIS vs DESIGN •  ANALYSIS Given the cross section, concrete strength, reinforcement size and location, and yield strength, compute the resistance or strength and the compare to the strength required. In analysis there should be one unique answer.

• DESIGN Given a factored design moment, normally designated as , select a suitable cross section, including dimensions, concrete strength, and solve for required reinforcement. In design there are many possible solutions.

LIMIT STATE • When a structure or structural element is no longer acceptable or unfit for its intended use, it is said to have reached the limit state. Major groups for Reinforced Concrete Structural Limit States: 1. Ultimate 2. Serviceability 3. Special

ULTIMATE LIMIT STATE These involve a structural collapse of part or all of the structure ( very low probability of occurrence) and loss of life can occur. The major Ultimate Limit States are as follows: a.

Loss of equilibrium of a part or all of the structure as a rigid body (tipping, sliding of structure).

b.

Rupture of critical components causing partial or complete collapse (flexural failures, shear failure).

c.

Progressive collapse Minor local failure overloads causing adjacent to failure until entire structure collapsed. d. Formation of plastic mechanism- yielding of reinforcement to form plastic hinges at enough sections to make structure unstable. e. Instability cased by deformations of structure causing buckling of . f. Fatigue- can fracture under repeated stress cycles of service loads (may cause collapse).

SERVICEABILITY LIMIT STATES Functional use of structure is disrupted, but collapse is not expected. More often tolerated that an ultimate limit state since less danger of loss of life. The major Serviceability Limit States include the following: a. Excessive deflections for normal service caused by possible effects: 1.

Malfunction of machinery.

2.

Visually unacceptable.

3.

Damage to nonstructural .

4.

Changes in the distribution of forces.

5.

Ponding of roof (collapse of roof).

b. Excessive crack width may be unsightly and may allow. 1. Leakage 2. Corrosion of the reinforcement 3. Gradual deterioration of the concrete

c. Undesirable Vibration Undesirable vibrations may disturb the s includes, 1. Vertical vibrations of floors and bridges. 2. Lateral and torsional vibrations of tall buildings. 3. Vibrations due to change in loading.

SPECIAL LIMIT STATES Damage or failures caused by abnormal conditions or abnormal loadings and includes: a. Damage or collapse in extreme earthquakes, floods. b. Structural effects of fire, explosions, or vehicular collisions. c. Structural effects of corrosions or deterioration. d. Long term physical or chemical instability (normally not a problem with concrete structure).

DESIGN PHILOSOPHY 1. •   WORKING STRESS METHOD Also called the Allowable Stress Design, it is a method of design based on elastic stage of an structural member. Hence, the member is designed on working loads, also referred to as service loads or unfactored loads. 2. STRENGTH DESIGN METHOD Also called the Ultimate Strength Design, it is a method based on ultimate limit state design. In the Strength Method, the service loads are increase sufficiently by factors to obtain the load st which failure is considered to be “imminent”. This load is called the factored load or factored service load. The Strength Design Method requires:

STRENGTH DESIGN METHOD for: •s   a. Probability of understrength of member due to material strength and dimension. b. Inaccuracies in design equation. c. Degree of ductility. d. Importance of the member in the structure.

Factored Load combinations for required strength (u) ACI 318-99 Dead and Live Load U = 1.4 DL U = 1.4 DL + 1.7 LL (ACI 9.2.1) Dead, Live, and Wind Loads U = 0.9 DL ± 1.3 WL U = 0.75 (1.4 DL + 1.7 LL ± 1.7 WL)

(ACI 9.2.2)

Dead, Live, and Earthquake Loads U = 0.9 DL ± 1.3 * 1.1 EL U = 0.75 (1.4 DL + 1.7 LL ± 1.7 * 1.1 EL) (ACI 9.2.3)

Factored Load combinations for required strength (u) ACI 318-02

Factored Load combinations for required strength (u) ACI 318-08

Factored Load combinations for required strength (u) ACI 318-11