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Conc Notes S05

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ARCH 3126 Steel, Timber & Concrete Spring 2005 Monday, March 28th Homework #23 is due Today (Chapter 3 Steel). Homework #24 is due Wednesday, March 30th (Chapter 12 - Steel).The text is written for the ACI 318-95 code. The current edition, ACI 318-02, contains some major revisions and we will cover most of the difference in the two codes that pertain to homework solutions. The newest version of the code 318-05 has not been published yet.Chapter 1 -- Introduction, Materials, and Properties1.1 -- Reinforced Concrete StructuresThe three most common building materials used today from which structures are built are wood, steel, and concrete (including prestressed). The three most common structural systems are better termed timber, structural steel, and reinforced concrete.The primary reason that reinforced concrete is a logical union of plain concrete and steel in regards to there strength is due to the high compressive (but low tensile) strength of concrete and the high tensile strength of steel reinforcing (Draw a beam showing the location of the reinforcement in the tension zone). Also they have similar rates of thermal expansion.1.2 -- Historical BackgroundVery interesting, but this is not a history class.1.3 --ConcretePlain concrete is made by mixing cement, fine aggregate, coarse aggregate, water, and frequently admixtures.1ARCH 3126 Steel, Timber & Concrete Spring 20051.4 -- CementThe cement has adhesive and cohesive properties, which bond mineral fragments. Cements used in reinforced concrete construction are called hydraulic cements, which set and harden in the presence of water (Type I Portland cement). Strength is normally attained in 28 day (varies with strength). Table 1.4.1 pp. 6 shows the types of Portland cement as follows:Type PropertiesI Ordinary constructionII Moderate sulfate resistanceIII High early strengthIV Low heat of hydrationV High sulfate resistanceK ExpansiveIn addition an A indicates that the concrete be air-entrained which provides durability.1.5 -- AggregatesAggregate occupies about 75% of the total volume of concrete (least expensive part). Fine aggregate (sand) is material less than 3/16 in. and coarse aggregate (gravel) large than that. The nominal maximum size of coarse aggregate from ACI - 3.3.2 is governed by the clearance between the sides of a form and the adjacent bars as follows:1. 1/5 the narrowest dimension between sides of forms2. 1/3 the depth of slabs3. 3/4 the minimum clear spacing between reinforcing bars2ARCH 3126 Steel, Timber & Concrete Spring 2005Structural lightweight concrete is usually made from kiln dried aggregates of expanded shale or clay (some are natural). Typical weights are from 70 to 115 pcf (145 for normal weight). All-lightweight concrete contains both lightweight fine and coarse aggregate. Sand-lightweight concrete contains only lightweight coarse aggregate. Sand replacement is a term used to define concrete with all or part of the lightweight fine aggregate replaced with natural sand. See Figure 1.5.1 pp. 7 for some approximate unit weights of lightweight aggregate concrete.1.6 -- Admixtures1. The most widely used admixture in concrete is air-entraining which provides an increase in durability. 2. Accelerating admixtures will decrease the time required for curing (best for cold weather placement).3. Water-reducing and Set-controlling admixtures may be used for higher strength (less water) and durability (also for hot weather placement).4. Admixtures for flowing concrete are used to produce slump rates of 7 inches or greater and increase workability (commonly termed plasticizers).Other admixtures can produce gas, expansion, color, fungus-germ-insect protection or provide dampproofing, reduced permeability and aggregate expansion or inhibit corrosion.1.7 -- Compressive StrengthThe strength of concrete is controlled by proportioning of cement, coarse and fine aggregate, water and admixture. The most important variable in determining strength is the water to cement (w/c) ratio.The slump test is the measure of the workability of concrete where a truncated cone-shaped 12 inch metal mold is filled with fresh concrete and the lifted off. The distance the top of the wet mass is the slump (3 to 4 inched is normally desired).3ARCH 3126 Steel, Timber & Concrete Spring 2005The strength of concrete is denoted by f'c which is the standard 28 day compressive strength in psi of test cylinder 6 in. in diameter and 12 in. high. (200-mm cube test in other parts of the world). The maximum ultimate strain of concrete is 0.003 per ACI - 10.2.3. Strengths of concrete range from 3000 psi up the 18000 psi (3 to 10 normal for slabs to high strength columns).1.8 -- Tensile StrengthThe strength of concrete in tension greatly effects the extent and size of cracking in structures and is measured by the split-cylinder test. Tensile strength in flexure is known as the modulus of rupture and is governed by ACI - 11.2 as follows:f f normalct c 6 7 . ' ( )f f sand lightweightct c 5 7 . ' ( )f f all lightweightct c 5 ' ( )1.9 -- Modulus of ElasticityThe modulus of elasticity of concrete varies primarily with strength, but also with weight, age and size. The modulus of elasticity per ACI - 8.5.1 is as follows (also in Table 1.9.1 pp. 16 for normal weight):E w f allc c c 331 5 .' ( )E f normalc c 57000 ' ( )1.10 -- Creep and ShrinkageCreep is the property of concrete by which it continues to deform with time under sustained loads at unit stresses within the elastic range. Shrinkage is the property of concrete by which it continues to change in volume with time that is unrelated to load application (both rates will decrease with time).4ARCH 3126 Steel, Timber & Concrete Spring 20051.11 -- Concrete Quality ControlThe specified compressive strength is deemed adequate when both of the following are occur (ACI Average of all sets of three consecutive strength test equal or exceed f'c2. No individual strength test falls below f'c by more than 500 psi when f'c 5000 psi.If these are not met in-place testing may indicate the concrete is adequate. If not re-analysis may indicate acceptance. If not load testing may be used for acceptance. If not then it must be strengthened or removed.1.12 -- Steel ReinforcementSteel reinforcement may consist of bars, welded wire fabric, or wires (usually deformed bars). Sizes are given in Table 1.12.1 pp. 21 (also in back of your ACI code or ACI 340 SP-7(97) REINFORCEMENT 1, (make a big copy of this) and types and strengths of steel in Table 1.12.4 pp. 23. Billet steel (ASTM A615/A615M) is newly made and is sufficiently ductile. Grade 60 is the primary material used (75 is become more popular), but grade 40 is used for smaller bars to be bent. The modulus of elasticity of steel is 29,000,000 psi (ACI 8.5.2) and prestress steel is lower (27,000,000 psi) and more variable, thus it should be obtained from the manufacturer or by test (ACI 8.5.3).1.13 -- SI UnitsYou are the torch bearers for this, Federal government now and others later. We will do very little in terms of SI units.5ARCH 3126 Steel, Timber & Concrete Spring 2005 Tuesday, March 29th Chapter 2 -- Design Methods and Requirements2.1 -- ACI Building CodeThe ACI Building Code Requirements for Reinforced Concrete is based partly on empirical and mostly rational data.2.2 -- Strength Design and Working Stress Design MethodsThe working stress method focuses on conditions at service load and the strength design method focuses on loads when failure may be imminent. 2.3 -- Working Stress MethodThe working stress method (now referred to as alternate design method, but not in ACI 318-02) has set limits on the stresses allowed under service loads (working loads).allowablef f Alternate Design Method ACI App A of old code, not in ACI 318-02. Some of the obstacles to the working stress method are as follows:1. No account for different types of loads.2. Creep and shrinkage are not easily accounted for in elastic stresses.3. Stress is not proportional to strain at concrete crushing therefore the inherent factor of safety is unknown.2.4 -- Strength Design MethodThe strength design method (formerly called ultimate strength method) has the service loads increased by sufficient factors to obtain the load at which failure is considered to be imminent. The strength provided must be greater than the required strength to carry these factored loads.6ARCH 3126 Steel, Timber & Concrete Spring 20052.5 -- Comments on Design MethodsAlthough strength design is currently the philosophy employed most widely, serviceability must be maintained. Working stress is stilled required to calculate deflections and cracking of structure in service load conditions.2.6 -- Safety Provisions--GeneralThe two primary factors used to provide safety in the ultimate strength method of design are U the overload factors (load factors) in ACI 9.2 (new factors) or ACI C.2 (old factors) and the understrength factors (strength reduction factors) in ACI 9.3 (new factors) or ACI C.3 (old factors).u nM M Design Strength 0.005) = 0.75 Compression members, spirally reinforcedCompression controlled sections (t < 0.002 i.e. balance) = 0.70 Compression members, othersCompression controlled sections (t < 0.002 i.e. balance) = 0.85 Shear and torsion = 0.70 Bearing on concrete = 0.65 Bending in plain concrete2.8 -- Safety ProvisionsACI Appendix C Load and Strength Reduction FactorsThe structure must be designed for the most severe of any load combination. These are the factors in the ACI 318-02 code. The load factors for some basic combinations are as follows per ACI 9.2 (the text pp. 40 is old appendix (typo 1.5 vs. 15)):(9-1) U = 1.4(D+F)Dead D & Fluid - F(9-2) U = 1.2(D+F+T) + 1.6(L+H) + 0.5(Lr or S or R)Live - LTemperature TEarth pressure HRoof Live - LrSnow SRain - R(9-3) U = 1.2D + 1.6(Lr or S or R) + (1.0L or 0.8W)Wind

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