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Module 1: Purlins and Gantry Girders

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Design of Gantry Girder

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Video 1
we are going to discuss about the design steps of a Gantry Girder. First, we will go through the design steps which can be followed and then we will go through one example following those steps. First, we need to calculate the maximum wheel load. Maximum wheel load will come when one wheel is close to the gantry girder. The wheel can move along crane girder, maximum effect will occur when it will be closest to gantry girder. In second step, maximum moment and shear force on gantry girder can be calculated after suitable proportioning of crane. Contribution of impact load should be taken care of. Though the maximum moment due to wheel load is slightly away from the centre of the girder (under the wheel), it is just added to maximum moment due to UDL on girder for simplification and design moment is found. Then in 3rd step, we can find out the maximum shear force due to this vertical load and shear force will be obtained maximum when one of the wheel is placed at the support of the gantry girder. So similar way we can find out maximum bending moment and shear force due to lateral load with similar positions. In next step, we have to find out section modulus. Generally, an I-section with channel section is chosen, though an I-section with a plate at the top flange may be used for light cranes. When the gantry is not laterally supported, the following formula may be used to select a trial section: Z p (trial)=k Zp (k = 1.30 – 1.60)Generally, the economic depth of a gantry girder is about (1/12)th of the span. The width of the flange is chosen to be between (1/40)th and (1/30)th of the span to prevent excessive lateral deflection. Next step, a suitable section is chosen and the properties IZZ , IYY and Zez , Z ey, Zpy ,Zpz are found. Then the section is classified according to b/tf and d/tw ratios. When lateral support is provided at the compression flange, the chosen section should be checked for the moment capacity of the whole section: Mdz=βbZpz f y/γm0<1.2Ze z f y/γm0 However, for laterally unsupported compression flange, the buckling resistance is to be checked with design bending compressive stress fbd .Bending strength about yy axis is calculated because of lateral loading: Md y=βb Zp yf y/γm0<1.2 Ze z f y/γm0Then, Combined local capacity of the flange is checked using the interaction equation Then section is to be checked against shear and local buckling will be checked under wheel load. The girder needs to be checked for bearing. Bearing stiffness will be provided if necessary. The maximum deflection under working load must be checked and the girder is checked for fatigue strength.
Video 2
Next, we will go through one example following these steps.Example: Design a simply supported gantry girder to carry electric overhead travelling crane, given: Span of gantry girder = 6 mSpan of crane girder = 15 m Crane capacity = 200 kN Self weight of trolley, hook, electric motor etc. = 40 kN Self weight of crane girder excluding trolley = 200 kN Minimum hook approach = 1.0 mDistance between wheels = 3 m Self weight of rails = 0.2 kN/m In next lecture we will discuss this, thank you.