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Page 1 Design of Beams Contents Introduction Beam types Lateral stability of beams Factors affecting lateral stability Behaviour of simple and built - up beams in bending (Without vertical stiffeners) Design strength of laterally supported beams Design strength of laterally unsupported beams Shear strength of beams Maximum deflection Design of Purlins Page 2 Design of Beams Contents Introduction Beam types Lateral stability of beams Factors affecting lateral stability Behaviour of simple and built - up beams in bending (Without vertical stiffeners) Design strength of laterally supported beams Design strength of laterally unsupported beams Shear strength of beams Maximum deflection Design of Purlins Introduction Beams are structural elements subjected to transverse loads in the plane of bending causing BMs and SFs. Symmetrical sections about z-z axis are economical and geometrical properties of such sections are available in SP (6) The compression flange of the beams can be laterally supported (restrained) or laterally unsupported (unrestrained) depending upon whether restraints are provided are not. The beams are designed for maximum BM and checked for maximum SF, local effects such as vertical buckling and crippling of webs and deflection. Beams can be fabricated to form different types of c/s for the specific requirements of spans and loadings. Section 8 shall be followed in the design of such bending members. Types of beam cross sections Beams can be of different cross sections depending on the span and loadings and are shown below - Page 3 Design of Beams Contents Introduction Beam types Lateral stability of beams Factors affecting lateral stability Behaviour of simple and built - up beams in bending (Without vertical stiffeners) Design strength of laterally supported beams Design strength of laterally unsupported beams Shear strength of beams Maximum deflection Design of Purlins Introduction Beams are structural elements subjected to transverse loads in the plane of bending causing BMs and SFs. Symmetrical sections about z-z axis are economical and geometrical properties of such sections are available in SP (6) The compression flange of the beams can be laterally supported (restrained) or laterally unsupported (unrestrained) depending upon whether restraints are provided are not. The beams are designed for maximum BM and checked for maximum SF, local effects such as vertical buckling and crippling of webs and deflection. Beams can be fabricated to form different types of c/s for the specific requirements of spans and loadings. Section 8 shall be followed in the design of such bending members. Types of beam cross sections Beams can be of different cross sections depending on the span and loadings and are shown below - C / S of Plate Girders Page 4 Design of Beams Contents Introduction Beam types Lateral stability of beams Factors affecting lateral stability Behaviour of simple and built - up beams in bending (Without vertical stiffeners) Design strength of laterally supported beams Design strength of laterally unsupported beams Shear strength of beams Maximum deflection Design of Purlins Introduction Beams are structural elements subjected to transverse loads in the plane of bending causing BMs and SFs. Symmetrical sections about z-z axis are economical and geometrical properties of such sections are available in SP (6) The compression flange of the beams can be laterally supported (restrained) or laterally unsupported (unrestrained) depending upon whether restraints are provided are not. The beams are designed for maximum BM and checked for maximum SF, local effects such as vertical buckling and crippling of webs and deflection. Beams can be fabricated to form different types of c/s for the specific requirements of spans and loadings. Section 8 shall be followed in the design of such bending members. Types of beam cross sections Beams can be of different cross sections depending on the span and loadings and are shown below - C / S of Plate Girders Simple I sections are used for normal spans and loadings with all the geometrical properties available in IS 800 : 2007. All the other sections indicated in the figure are built up sections. These sections are used when the normal I sections become inadequate due to large spans and loadings. These sections are also used due to other functional requirements. I section with cover plates are used when the loads are heavy and the spans are large. If the depth of the beam is restricted due to functional reasons, smaller depth I sections with cover plates can be used Additional cover plates increases the lateral load resistance with increase in I YY . The properties of ISMB and ISWB sections with cover plates are available in SP (6). (Z PZ has to be obtained from calculations) Page 5 Design of Beams Contents Introduction Beam types Lateral stability of beams Factors affecting lateral stability Behaviour of simple and built - up beams in bending (Without vertical stiffeners) Design strength of laterally supported beams Design strength of laterally unsupported beams Shear strength of beams Maximum deflection Design of Purlins Introduction Beams are structural elements subjected to transverse loads in the plane of bending causing BMs and SFs. Symmetrical sections about z-z axis are economical and geometrical properties of such sections are available in SP (6) The compression flange of the beams can be laterally supported (restrained) or laterally unsupported (unrestrained) depending upon whether restraints are provided are not. The beams are designed for maximum BM and checked for maximum SF, local effects such as vertical buckling and crippling of webs and deflection. Beams can be fabricated to form different types of c/s for the specific requirements of spans and loadings. Section 8 shall be followed in the design of such bending members. Types of beam cross sections Beams can be of different cross sections depending on the span and loadings and are shown below - C / S of Plate Girders Simple I sections are used for normal spans and loadings with all the geometrical properties available in IS 800 : 2007. All the other sections indicated in the figure are built up sections. These sections are used when the normal I sections become inadequate due to large spans and loadings. These sections are also used due to other functional requirements. I section with cover plates are used when the loads are heavy and the spans are large. If the depth of the beam is restricted due to functional reasons, smaller depth I sections with cover plates can be used Additional cover plates increases the lateral load resistance with increase in I YY . The properties of ISMB and ISWB sections with cover plates are available in SP (6). (Z PZ has to be obtained from calculations) Two I sections with cover plates can be used when very heavy loads and spans act on the beam. The properties of these sections are not available in SP (6) and have to be calculated. Two I sections placed one above the other are used when the loads are light with large spans, where deflection is the main criteria. The properties of these sections are not available in SP (6) and have to be calculated Gantry girders are used in industrial buildings to lift loads and typical sections used are indicated in the figure. The properties of these sections are available in SP (6), (Z PZ has to be obtained from calculations) Plate girders are used where the spans exceed 20m and the loads are heavy. The properties of these sections are available in SP (6), (Z PZ has to be obtained from calculations) Box sections have large torsional rigidity and can be used as single cell, twin cell or multi - cell sections. The openings are advantageously used for service lines. Castellated beams are special sections fabricated from I sections and are used for light loads and large spans. The openings are advantageously used for service lines. In all built up beams, the fabrication cost is higher due to the provision of connections between the elements. Section Classification There are four classes of section namely Plastic, Compact, Semi - Compact and Slender sections as given in IS 800 : 2007. [cl. 3.7.2 pp - 17] For design of beams, only Plastic and Compact sections are used. Lateral Stability of Beams A beam transversely loaded in its own plane can attain its full capacity (Plastic moment) only if local and lateral instabilities are prevented. Local buckling of beams can be due to web crippling and web buckling. They are avoided by proper dimensioning of the bearing plate and through secondary design checks. Flanges shall always satisfy the outstand to thickness ratio as per IS 800 : 2007 so that local failures of flanges are avoided. Plastic and Compact sections are used. Lateral buckling of beams is the out of plane bending and is due to compressive force in the flange and is controlled by providing sufficient lateral restraint to the compressive flange.Read More
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