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Introduction to Design of Purlins
Purlin is basically a flexural member in which a transverse load acts. They are designed for the biaxial moment. The purlins are placed on the rafters and are generally channel or angle sections.
The design procedure for an I/Channel section purlin are:
Find the span length of the purlinCalculate the gravity load and wind load on the purlin in accordance with the codal provisionsDetermine the maximum bending moment and shear force, using the factored loadsFind the approximate plastic section modulus requiredAssume section size and classify the section as per the codal provisionsCheck for shear capacity of the section for both z and y axisCompute the design capacity of the section in both the axisCheck the local capacity using the interaction formula, and also whether deflection in under permissible limits
The basic steps in designing angle section purlins are:
Determine the vertical and wind loadsCompute the maximum bending momentCalculate the required section modulusAssume the depth and width of the angle sectionSelect a suitable section for the calculated value of leg lengths of angle section.
Introduction to Gantry Girders
Gantry girders are utilized to transport the goods and equipment from one place to another place in a workshop. They are a typical example of a laterally unsupported beam in industrial buildings.
In the design of gantry girder for the overhead traveling crane running system, the following load should be considered: (1) Vertical loads from cranes (2) Impact load from crane (3) Longitudinal horizontal force along the crane rail (4) Lateral thrust across the crane rail.
For heavy-duty cranes, the gantry girder must be checked for fatigue. But, a fatigue effect doesn’t need to be checked for light and medium-duty cranes.
The steps in designing a gantry girder for an overhead travelling crane system are:
Calculate the maximum wheel load on the gantry girderWith a suitable position of crane, determine the maximum moment and shear force on the gantry girderCalculate the maximum shear force due to vertical loadCalculate the lateral forces on the girderChoose a suitable section and find properties of the sectionClassify the section to be plastic, semi-compact or compactIf lateral support is provided at the compression flange, check the chosen section for moment capacity of the whole sectionCalculate the bending strength about yy axisCheck the combined load capacity of the flange using the interaction equationCheck the section against shearCheck the section for local buckling under the wheel loadCheck girder for bearingCheck the maximum deflection under working loadCheck the girder for fatigue strength