Bulk Active Structure Systems PDF

Bulk Active Structure Systems Structural System and Design - IV  Contents Bulk Active Structure System Types of Bulk Active Structure System Bulk Active Structural Systems Bulk Active Structural Systems Support load by BENDING and their Mo...

Bulk Active Structure Systems Structural System and Design - IV  Contents Bulk Active Structure System Types of Bulk Active Structure System Bulk Active Structural Systems Bulk Active Structural Systems Support load by BENDING and their Moment of Resistance is proportional to the size (BULK) of their cross section Systems of rigid, solid, linear elements having bending rigidity. Most common are Beams, Columns & Slabs Bulk-Active Structural Systems  Bulk-Active Structural Systems Structural elements which support load by BENDING and their MOMENT OF RESISTANCE (MR) is proportional to the size (BULK) of their cross section are BULK ACTIVE STRUCTURES. These structural elements act mainly through material bulk and continuity. External loads produce bending in these elements so these are also known as “Structure Systems in Bending” This system consists of linear and 2D elements, having bending rigidity: most common elements are Beams, Column, Slabs. Beams  Beams Beam is a structural element with cross sectional dimension much smaller than its length supports loads transverse to its longitudinal axis by bending. Generally carry vertical gravitational forces. Characterized by their profile (the shape of their cross section), length, and material. Beams Types of Supports  Types of Beams Beams can be classified according to the manner in which they supported  Types of Beams Floor beams – Major beams supporting the secondary beams or joists Girder – Floor beam in buildings Lintel – Beams used to carry wall load over openings, i.e. doors, windows etc. Purlin – Roof beam supported by roof trusses Rafter – Roof beam supported by roof trusses Rafter – Roof beam supported by purlins Spandrel beam – Beam at outer most wall of buildings, which carry part of floor load and exterior walls Stringer beam – Longitudinal beam used in bridge floors and supported by floor beams Beams Beams Beams Beams Beams  Columns A vertical structural elements that transfers the weight of the structure above (compression) to the elements below. Can be compounded of parts or can be one piece. All columns are subjected to some moment which may be due to accidental eccentricity or due to end restraint imposed by monolithically placed beams or slabs. The strength of a column depends on the strength of the materials, shape and size of the cross-section, length and the degree of the positional and directional restraint at its end. A Column or Strut is defined as a compression member whose effective length exceeds three times the least lateral dimension. A structural element that is predominantly subjected to axial compressive forces is termed a compression member. When a compression member is vertical, it is called a Column, and when it is horizontal or inclined, it is called a Strut.  Columns A column may be defined as an element used primarily to support axial compressive loads and with a height of at least three times its least lateral dimension. All columns are subjected to some moment which may be due to accidental eccentricity or due to end restraint imposed by monolithically placed beams or slabs. The strength of a column depends on the strength of the materials, shape and size of the cross-section, length and the degree of the positional and directional restraint at its end. A column may be classified as short or long column depending on its effective slenderness ratio.  Columns Compression members are structural elements primarily subjected to axial compressive forces and hence, their design is guided by considerations of strength and buckling. Figures in next slide show their examples: pedestal, column, wall and strut. While pedestal, column and wall carry the loads along its length l in vertical direction, the strut in truss carries loads in any direction. The letters l, b and D represent the unsupported vertical length, horizontal least lateral dimension, width and the horizontal longer lateral dimension, depth. These compression members may be made of bricks or reinforced concrete.  Columns Pedestal Column Columns Wall  Columns (a) Effective length: The vertical distance between the points of inflection of the compression member in the buckled configuration in a plane is termed as effective length le of that compression member in that plane. The effective length is different from the unsupported length l of the member, though it depends on the unsupported length and the type of end restraints. The relation between the effective and unsupported lengths of any compression member is le = kl, where k is the ratio of effective length to the unsupported lengths. Clause 25.2 of IS 456 stipulates the effective lengths of compression members (vide Annex E of IS 456). This parameter is needed in classifying and designing the compression members. (b) Pedestal: Pedestal is a vertical compression member whose effective length le does not exceed three times of its least horizontal dimension b (cl. 26.5.3.1h, Note). The other horizontal dimension D shall not exceed four times of b.  Columns (c) Column: Column is a vertical compression member whose unsupported length l shall not exceed sixty times of b (least lateral dimension), if restrained at the two ends. Further, its unsupported length of a cantilever column shall not exceed 100b2 /D, where D is the larger lateral dimension which is also restricted up to four times of b (vide cl. 25.3 of IS 456). (d) Wall: Wall is a vertical compression member whose effective height Hwe to thickness t (least lateral dimension) shall not exceed 30 (cl. 32.2.3 of IS 456). The larger horizontal dimension i.e., the length of the wall L is more than 4t.  Columns The ratio of effective column length to least lateral dimension is referred to as effective slenderness ratio. A short column has a maximum slenderness ratio of 12. Its design is based on the strength of the materials and the applied loads. A long column has a slenderness ratio greater than 12. Its design is based on the strength of the materials and the applied loads. A long column is designed to resist the applied loads plus additional bending moments induced due to its tendency to buckle.  Assumptions The following assumptions are made for the limit state of collapse in compression. 1. Plane sections normal to the axis remain plane after bending. 2. The relationship between stress-strain distribution in concrete is assumed to be parabolic. The maximum compressive stress is equal to 0.67 fck/ 1.5 or 0.446 fck. 3. The tensile strength of concrete is ignored. 4. The stresses in reinforcement are derived from the representative stress-strain curve for the type of steel used. 5. The maximum compressive strain in concrete in axial compression is taken as 0.002.  Assumptions 6. The maximum compression strain at the highly compressed extreme fibre in concrete subjected to axial compression and bending, but when there is no tension on the section, is taken as 0.0035 minus 0.75 times the strain at the least compressed extreme fibre. 7. The maximum compressive strain at the highly compressed extreme fibre in concrete subjected to axial compression and bending, when part of the section is in tension, is taken as 0.0035. In the limiting case when the neutral axis lies along one edge of the section, the strain varies from 0.0035 at the highly compressed edge to zero at the opposite edge.  Classification of Columns Classification of Columns based on Cross-section Rectangular Columns Square Columns Circular Columns Hexagonal Columns T, L, or + shaped Columns  Classification of Columns based on types of reinforcement Based on the types of reinforcement, the reinforced concrete columns are classified into three groups: (i) Tied columns: The main longitudinal reinforcement bars are enclosed within closely spaced lateral ties. (ii) Columns with helical reinforcement: The main longitudinal reinforcement bars are enclosed within closely spaced and continuously wound spiral reinforcement. Circular and octagonal columns are mostly of this type. (iii) Composite columns: The main longitudinal reinforcement of the composite columns consists of structural steel sections or pipes with or without longitudinal bars. Out of the three types of columns, the tied columns are mostly common with different shapes of the cross-sections viz. square, rectangular, T-, L-, cross etc. Helically bound columns are also used for circular or octagonal shapes of cross-sections. Architects prefer circular columns in some specific situations for the functional requirement.  Braced and unbraced columns The columns in a building are classified as braced or unbraced according to the method applied to provide the lateral stability of the building. 1) Braced column : In braced frames, the lateral loads like wind , earthquake etc. are resisted by some special arrangements like shear walls , bracings or special supports. In other words, the sidesway or joint translation is not possible in such columns. Sidesway or joint translation means that one or both the ends of a column can move laterally with respect to each other. The columns occurring in braced buildings are called braced columns. Braced and unbraced columns  Braced and unbraced columns 2) Unbraced columns : A unbraced frames no special bracing systems are provided to resist horizontal forces. In other words the sidesway or joint translation do occur in such columns. The columns shall have to be designed to resist the lateral loads. The column those occur in the buildings where the resistance to lateral loads is provided by the bending in the columns and beams in that plane are called unbraced columns.  Slabs 2D horizontal supports Just as a continuous bearing wall is more than just a bunch of columns, a slab is more than just a bunch of beams – The load is distributed over a wider area How the slab behaves (how it deflects under load) depends on how it is supported A slab is a flat two dimensional planar structural element having thickness small compared to its other two dimensions. It provides a working flat surface or a covering shelter in buildings. It primarily transfer the load by bending in one or two directions.  Classification of Slabs Slabs are classified based on many aspects Based on shape: Square, Rectangular, Circular and Polygonal in shape Based on type of support: Slab supported on walls, slab supported on beams, slab supported on columns (Flat slabs) Based on support or boundary condition: Simply supported, Cantilever slab, Overhanging slab, Fixed or Continuous slab. Based on use: Roof slab, Floor slab, Foundation slab, Water tank slab Basis of cross section or sectional configuration: Ribbed slab/ Grid slab, Solid slab, Filler slab, Folded plate Basis of spanning directions: – One-way slab – Spanning in one direction – Two-way slab – Spanning in two direction  Slabs When the slab is supported on two opposite sides only, the structural action of the slab is essentially one way. There are beams on all four sides with a intermediate beam. Now if length to width ratio is 2 or greater, slab is one way even though supports are provided on all sides. RCC Slabs  One-way and Two-way action of slab When slabs are supported on two When supports are provided on opposite sides only loads being all sides most of the load is carried by the slab in the carried in the short direction to direction perpendicular to the the supporting beams and one supporting beams. way action is obtained. Horizontal Slab One span Horizontal Slab Continuous in one direction  Horizontal Slab Continuous in both directions Horizontal and Inclined Slab  Types of Slabs Slabs, used in floors and roofs of buildings mostly integrated with the supporting beams, carry the distributed loads primarily by bending. A part of the integrated slab is considered as flange of T- or L- beams because of monolithic construction. However, the remaining part of the slab needs design considerations. These slabs are either single span or continuous having different support conditions like fixed, hinged or free along the edges. Though normally these slabs are horizontal, inclined slabs are also used in ramps, stair cases and inclined roofs.  Types of Slabs While square or rectangular plan forms are normally used, triangular, circular and other plan forms are also needed for different functional requirements. The other types of slabs depending on the nature and magnitude of loadings in respective cases. (a) horizontal or inclined bridge and fly over deck slabs carrying heavy concentrated loads, (b) horizontal slabs of different plan forms like triangular, polygonal or circular (c) flat slabs having no beams and supported by columns only (d) inverted slabs in footings with or without beams (e) slabs with large voids or openings (f) grid floor and ribbed slabs. RCC Slabs