Reinforced Concrete Design CHAPTER COLUMNS

1 1 A. J. Clark School of Engineering Department of Civil and Environmental EngineeringFifth EditionCHAPTER9aReinforced Concrete DesignENCE 355 - Introduction to Structural DesignDepartment of Civil and Environmental EngineeringUniversity of Maryland, College ParkCOLUMNSPart I Concrete Design and AnalysisFALL 2002 ByDr . Ibrahim. AssakkafCHAPTER 9a. COLUMNSS lide No. 1 ENCE 355 AssakkafIntroductionQAxial Compression COLUMNS are defined as members that carry loads in compression. Usually they carry bending moments as well, about one or both axes of the cross section. The bending action may produce tensile forces over a part of the cross section. Despite of the tensile forces or stresses that may be produced, COLUMNS are 2 CHAPTER 9a.

2 COLUMNSS lide No. 2 ENCE 355 AssakkafIntroductionQAxial Compression Generally referred to as :compression members because the compression forces or stresses dominate their behavior. In addition to the most common type of compression members (vertical elements in structures), compression members include: arch ribs Rigid frame members inclined or otherwise Compression elements in trusses shellsCHAPTER 9a. COLUMNSS lide No. 3 ENCE 355 AssakkafIntroduction3 CHAPTER 9a. COLUMNSS lide No. 4 ENCE 355 AssakkafIntroductionReinforced Concrete ColumnsCHAPTER 9a. COLUMNSS lide No. 5 ENCE 355 AssakkafIntroductionPontPont--dudu--Gard Gard. Roman aqueduct built in 19 to carry water . Roman aqueduct built in 19 to carry water across theacross theGardonGardonValley to Valley to NimesNimes.

3 Spans of the first and second. Spans of the first and secondlevel arches are 53level arches are 53--80 feet. (Near80 feet. (NearRemoulinsRemoulins, France), France)4 CHAPTER 9a. COLUMNSS lide No. 6 ENCE 355 AssakkafOhio River Bridge. Typical cantilever and suspended span bridge,Ohio River Bridge. Typical cantilever and suspended span bridge,showing showing the truss geometry in the end span and cantilevered portion of tthe truss geometry in the end span and cantilevered portion of the main he main span. (Madison, Indiana)span. (Madison, Indiana) CHAPTER 9a. COLUMNSS lide No. 7 ENCE 355 AssakkafIntroduction5 CHAPTER 9a. COLUMNSS lide No. 8 ENCE 355 AssakkafIntroductionCHAPTER 9a. COLUMNSS lide No. 9 ENCE 355 AssakkafIntroductionQColumn load transfer from beams and slabs 1) Tributary area method:Half distance to adjacent columnsyxLoad on column = area floor loadFloor load = DL + LLDL = slab thickness conc.

4 Unit : x = ft, y = ft, LL = lb/ft2, slab thickness = load = (150)/12 + = lb/ft2 Load on column = ( )( )( ) = 10,800 kg = kips6 CHAPTER 9a. COLUMNSS lide No. 10 ENCE 355 AssakkafIntroductionQColumn load transfer from beams and slabs 2) Beams reaction method:B1B2RB1RB1RB2RB2 Collect loads from adjacent beam endsC1B1B2B3B4 CHAPTER 9a. COLUMNSS lide No. 11 ENCE 355 AssakkafIntroductionQLoad summation on column section for designDesign sectionDesign sectionDesign sectionROOF2nd FLOOR1st FLOORF ootingGround levelLoad on pier column= load on 1st floor column + 1st floor + Column on 1st floor column= load on 2nd floor column + 2nd floor + Column on 2nd floor column= Roof floor + Column 9a. COLUMNSS lide No.

5 12 ENCE 355 AssakkafIntroductionQTypes of Reinforced Concrete Columns1. Members Reinforced with longitudinal bars and lateral Members Reinforced with longitudinal bars and continuous Composite compression members Reinforced longitudinally with structural steel shapes, pipe, or tubing, with or without additional longitudinal bars, and various types of lateral 9a. COLUMNSS lide No. 13 ENCE 355 AssakkafQTypes of Reinforced Concrete ColumnsIntroductionTieLongitudinalsteelT ied columnSpirals= pitchSpirally Reinforced column8 CHAPTER 9a. COLUMNSS lide No. 14 ENCE 355 AssakkafQTypes of Reinforced Concrete ColumnsIntroductionComposite columnsCHAPTER 9a. COLUMNSS lide No. 15 ENCE 355 AssakkafIntroductionQTypes of COLUMNS in Terms of Their Strengths1.

6 Short ColumnsA column is said to be short when its length is such that lateral buckling need not be considered. Most of Concrete COLUMNS fall into this Slender ColumnsWhen the length of the column is such that buckling need to be considered, the column is referred to as slender column. It is recognized that as the length increases, the usable strength of a given cross section is decreased because of buckling 9a. COLUMNSS lide No. 16 ENCE 355 AssakkafIntroductionQBuckling Buckling is a mode of failure generally resulting from structural instability due to compressiveaction on the structural member or element involved. Examples Overloaded metal building COLUMNS . Compressive members in bridges. Roof trusses. Hull of 9a.

7 COLUMNSS lide No. 17 ENCE 355 AssakkafIntroductionQBucklingFigure 1a10 CHAPTER 9a. COLUMNSS lide No. 18 ENCE 355 AssakkafIntroductionQBucklingFigure 1bCHAPTER 9a. COLUMNSS lide No. 19 ENCE 355 AssakkafIntroductionQThe Nature of BucklingDefinition Buckling can be defined as the sudden large deformation of structure due to a slight increase of an existing load under which the structure had exhibited little, if any, deformation before the load was increased. 11 CHAPTER 9a. COLUMNSS lide No. 20 ENCE 355 AssakkafIntroductionQBuckling Failure of Reinforced Concrete ColumnsFigure 2 CHAPTER 9a. COLUMNSS lide No. 21 ENCE 355 AssakkafIntroductionQCritical Buckling Load, PcrThe critical buckling load (Euler Buckling) for a long column is given bywhereE= modulus of elasticity of the materialI= moment of inertia of the cross sectionL= length of column22 LEIPcr =(1)12 CHAPTER 9a.

8 COLUMNSS lide No. 22 ENCE 355 AssakkafStrength of Reinforced Concrete COLUMNS : Small EccentricityQIf a compression member is loaded parallel to its axis by a load P without eccentricity, the load Ptheoretically induces a uniform compressive stress over the cross-sectional the compressive load is applied a small distance eaway from the longitudinal axis, however, there is a tendency for the column to bend due to the moment M = 9a. COLUMNSS lide No. 23 ENCE 355 AssakkafQEccentric Axial Loading in a Plane of Symmetry When the line of action of the axial load Ppasses through the centriod of the cross section, it can be assumed that the distribution of normal stress is uniform throughout the section. Such a loading is said to be centric, as shown in Fig of Reinforced Concrete COLUMNS : Small Eccentricity13 CHAPTER 9a.

9 COLUMNSS lide No. 24 ENCE 355 AssakkafQEccentric Axial Loading in a Plane of SymmetryFigure 3. Centric Loading PPStrength of Reinforced Concrete COLUMNS : Small EccentricityCHAPTER 9a. COLUMNSS lide No. 25 ENCE 355 AssakkafStrength of Reinforced Concrete COLUMNS : Small EccentricityQEccentric Axial Loading in a Plane of Symmetry When the line of action of the concentrated load Pdose not pass through the centroid of the cross section, the distribution of normal stress is no longer uniform. Such loading is said to eccentric, as shown in Fig 9a. COLUMNSS lide No. 26 ENCE 355 AssakkafStrength of Reinforced Concrete COLUMNS : Small EccentricityQEccentric Axial Loading in a Plane of Symmetry Figure 4. Eccentric Loading PPCHAPTER 9a.

10 COLUMNSS lide No. 27 ENCE 355 AssakkafQEccentric Axial Loading in a Plane of SymmetryThe stress due to eccentric loading on a beam cross section is given byStrength of Reinforced Concrete COLUMNS : Small EccentricityIMyAPfx =(2)15 CHAPTER 9a. COLUMNSS lide No. 28 ENCE 355 AssakkafQColumns Loaded with Small Eccentricities The Concrete column that is loaded with a compressive axial load Pat zero eccentricity is probably nonexistent, and even the axial/small eccentricity combination is relatively rare. Nevertheless, the case of COLUMNS that are loaded with compressive axial loads at small eccentricity eis considered first. In this case we define the situation in which the induced small moments are of little significance.