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Module 1: Purlins and Gantry Girders

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Video 1
we are going to discuss about Gantry Girders. Gantry Girders are used in mill and heavy industrial buildings such as factories and workshops, where Gantry Girders are supported by columns and carrying cranes. Gantry girders are utilised to transport the goodsand equipment from one place to another place in the workshop. Gantry Girders are typical example for laterally unsupported beam in industrial buildings. Also Gantry Girders undergo bending moment under both the direction. one is vertically and other is laterally. So biaxial bending movement has to be also checked for design of Gantry Girders. Therefore the when we will be going to design Gantry Girders we need to consider two things, one is the Gantry Girders is a laterally unsupported beam, so we have to design the Gantry Girders considering the lateral torsional buckling effect and also we have to consider the biaxial bending that means that interaction formulad d 1dz dyM MM M has to be checked. And also in Gantry Girders, the loads are moving from one place to another place therefore we need to know little about influence line diagram that means, we have to see the position of load in which the maximum bending moment and maximum shear force is going to occur. So as the wheel is moving from one place to another place, wheels are placed in such a way thatmaximum shear force and the maximum bending moment can be achieved. The gantry girder is designed against that maximum shear force and bending moment. In gantry girder certain impact load will come into picture so some additional load has to be added in the load calculation. It has been recommended in codal guidelines to add certain percentage of additional load. Also certain percentage of load will be acting as a lateral load for which lateral bending moment and lateral shear force will be produced and because of that the lateral bending strength and shear strength should also be checked so that it is not going to fail under this lateral load, right.
The overhead travelling crane running system consists few components like crane, then this crane is compromising the crab or trolley, power transmitting device and the cap which houses the control and operators and also the crane rails and their attachment, also the gantry girder and column with brackets supporting gantry girder. Gantry girder is placed on the column either with the support of a bracket or with the step column. Above the step column packing plates are placed, then gantry girders are consisting of I Section is placed above packing plate and at the top planes along with I section we provide another channel section to take the heavy load coming from crane girder. Crane girder is placed on top of the crane rails. A crab trolley is kept in the crane girder, with the help of the trolley loads are being shifted from one place to another. A diaphragm is used to keep the I section the gantry girder in position throughout its length. Gantry girder is supported between two stepped columns in two sides.
The crab position should closest to the gantry girder to consider the maximum effect. Once the crab position is located then depending on the clearance from gantry girder to the crab wheel location loads coming to the girder can be calculated. So before finding the maximum bending moment and shear force we have to see whatever load are going to be acting on the girder.
The following imposed loads should be considered in the design.
1. Vertical loads from the cranes because crane will be carrying certain instruments,certain heavy machines.2. Impact loads from crane because during operation certain impact will come intopicture3. Longitudinal horizontal force along the crane rail.4. Lateral thrust across the crane railIn calculating the above forces crane should be positioned such that it gives maximum designforces in the girder.
Now according to codal provisions, for vertical load when it is an electric overhead crane then 25 percent of the maximum static wheel load will be added as impact loads. whereas for hand operated cranes it will be 10 percent of the maximum static load. For horizontal forces transverse to rail, for electric overhead cranes it will be 10 percent of the weight of crab and the weight of lifted on the crane, and for hand operated cranes it will be 5 percent of theweight of crab and the weight lifted on the crane. The horizontal forces along rail will be 5 percent of the static wheel load that has to be considerd.
As the crane moves with the load, a lateral load (transverse to the rail) is developed due to application of brakes or sudden acceleration of trolley. IS 875 recommends 10% of W for EOT cranes as horizontal loads, where W is the total weight including lifted weight and thetrolley weight. As the crane moves longitudinally, loads parallel to the rails are caused due to the braking (stopping) or acceleration and swing (starting of the crane). This load is called the longitudinal load and is transferred at the rail level. The longitudinal load per wheel = 5% of the wheel load, i.e., Wg = 5W/100
Permissible deflection has been given in table 6 of IS 800:2007.
So, the maximum deflection is checked against permissible deflection from serviceabilitypoint of view.
Video 2
The maximum vertical load on gantry girder is the maximum reaction of crane girder. To get this, crab should be placed as close to gantry girder as possible. If , LC = Span of crane girder L1 = Minimum approach of crane hook (distance between c.g. of gantry girder and trolley).W = weight of trolley plus the weight liftedw = weight of crane girder per unit lengthRA=W Lc2Wt(Lc−L1)Lc
So, once the wheel load is found, maximum bending moment and maximum shear force in gantry girder can be obtained. Maximum bending moment occur when the mid span of the gantry girder intersects the distance between C.G. of the wheels of and one of the wheel load.Then, the maximum bending moment can be achieved at a position of the nearest wheel load from the mid span.
Maximum shear force can be achieved by placing one of the wheel loads on the support.
Gantry girders cause moving loads that cause fatigue. Fatigue effects for light and medium duty cranes need not to be checked, if normal and shear stress ranges,f ≤ 27γmft Or, if actual number of stress cycle,NSC<5× 106(27γ/ mγfmft)3For heavy duty crane, the gantry girder must be checked for fatigue.γmft = partial safety factor for strength (Table 25 of IS 800-2007)f= actual fatigue stress range γm= partial safety factor for material = 1.10For heavy duty crane the gantry girder must be checked for fatigue.
Normal stress rangef f=f f n√3 5 ×106/N sc for Nsc ≤5 ×106and,
f f=f f n√5 5 ×106/N sc for 5 ×106≤ Nsc ≤108Shear stress rangeτf=τfn√5 5× 106/NscWhere, f f , τf=¿design normal and shear fatigue stress range of the details , respectively forlife cycle of NSCf fn,τfn=¿ normal and shear fatigue strength of the details for 5 × 106 cycles for the detailcategory.