IES MASTER GATE MATERIAL
GATE – PSU – IES – GOVT EXAMS – STUDY MATERIAL
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- General Design Requirements
- Design of Tension Members
- Design of Compression Members
- Design of beams
- Design of Girders
- Design of Gantry Girders
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DESIGN OBJECTIVE : The objective of design is the achievement of an acceptable probability that structures will perform satisfactorily for the intended purpose during the design life.With an appropriate degree of safety, they should sustain all the loads and deformations during construction and use and have adequate resistance to accidental loads and fire.
METHODS OF DESIGN : Structure and its elements shall normally be designed by limit state method as per ls 800-200m. Where the limit state method cannot be conveniently adopted working stress method shall be used. LOADS AND FORCES For the purpose of designing any element, member or a structure, the following loads and their effects shall be taken into account, where applicable; with partial safety factors and combinations Dead loads: b. Imposed loads: Liveload, crane load. snow load etc.) (c) Wind loads(d) Earthquake loads(e) Erection loads Accidental loads such as those due to blast g) Secondary effects due to contraction or expansion resulting from temperature changes, differential settlements of the structure as a whole or of its components, eccentric connections.
100% CLASSIFICATION OF CROSS SECTIONS : Basis of Classification The plate elements of a cross-section may buckle locally due to compressive stresses. When plastic analysis is used, the members shall be capable of forming plastic hinges with sufficient rotation capacity(ductility) without local buckling to enable the redistribution of bending moment required before formation of failure mechanism When elastic analysis is tied, the member shall be capable of developing the yield stress under compression without local buckling On the above basis, four classes of sections are defined as follows(a) Plastic Cross-sections, which can develop plastic hinges and have the rotation capacity required for failure of the structure by formation of a plastic mechanism(b) Compact Cross sections, which can develop plastic moment of resistance, but have inadequate plastic hinge rotation capacity for formation of a plastic mechanism. (c) Semi-compact Cross-sections, in which the extreme fibre in compression can reach, yield stress, but cannot develop the plastic moment of resistance, due to local buckling(d) Slender Cross-sections
GENERAL DESIGN REQUREMENTS LIMIT STATE DESIGN OF STEEL STRUCTURES : Basis for Limit State Design In the limit state design method, the structure shall be designed to withstand safely all loads likely to act on it throughout its life. It shall also satisfy the serviceability requirements, such as limitations of deflection and vibrations and shall not collapse under accidental loads such as from explosions or impact or due to consequences of human error to an extent not originally expected to occur The acceptable limit for the safety and serviceability requirements before failure occurs is called a limit state. The objective of design is to achieve a structure that will not become unfit for use with an acceptable target reliability. In other words, the probability of a limit state being reached during its lifetime should be very low. In general, the structure shall be designed on the basis of the most critical limit state and shall be checked for other limit states In limit state design, structures are designed on the basis of safety against failure and are checked for serviceability requirements LIMIT STATE DESIGN limit states are the states beyond which the structure no longer satisfies the performance requirements
LIMIT STATE DESIGN : Limit states are the states beyond which the structure no longer satisfies the performance requirements specified. The limit states are classified as: a) Limit State of Strength b Limit State of Serviceability Limit State of Strength The limit state of strength are those associated with failures(or imminent failure), under the action of probable and most unfavorable combination of loads on the structure. Limit State of Serviceability The limit state of serviceability includes: Deformations and deflections Vibrations Repairable damage due to fatigue Corrosion and durability The major innovation in the limit state method is the introduction of the partial safety factor format which essentially splits the factor of safety into two factors one for the material and one for the load. In accordance with these concepts, the safety forma limit state codes is based on probable maximum load and probable minimum strengths. So that a content level of safety is achieved
Following points must be kept in mind while checking for various limit states. Collapse strength limit states are related to the maximum design loads under extreme conditions. The partial load factors are chosen to reflect the probability of extreme conditions, when loads act alone or in combination. Stability shall be ensured for the structure as a whole and for each of its elements. It includes overall frame stability against overturning and sway, uplift or sliding under factored loads Serviceability limit states are related to the criteria governing normal use. Hence unfactored loads are used to check the adequacy of the structure. Load factor Yy of value equal to unity shall be used for all loads leading to serviceability limit states. Following service criteria comes under limit states of serviceability(a) Deflection The deflection under serviceability loads of a building or a building component should not impair the strength of the structure or components or cause damage to finishing.