An Engineer entering the field of welded design, usually has the background of mechanical or materials engineering, and has very little understanding of the factors that contribute to efficient welded design as welding technology and weld design are not regular subjects in engineering colleges. A successful welded structure design will:
1. perform its intended functions.
2. have adequate safety and reliability.
3. be capable of being fabricated, inspected, transported and placed in service at a minimum cost.
4. cost includes cost of design, materials, fabrication, erection, inspection operation repair and maintenance.
Efficient and economical designs are possible because of:
1. mechanised flame-cutting equipment (smooth cut edges).
2. press brakes are available to make use of formed plates.
3. a wide range of welding processes and consumables.
4. welding positioners are available that permit low cost welds to be deposited in down hand welding position.
One should avoid over designing or higher safety factors and still safe and reliable design.
In developing a design the following factors are of help:
1. Specify steels that do not require pre or post heat treatment.
2. Use standard rolled sections where possible.
3. Use minimum number of joints and ensure minimum scrap.
4. Use stiffeners properly to provide rigidity at minimum weight of material, use bends or corrugated sheets for extra stiffness.
5. Use closed tubular section or diagonal bracing for torsional resistance.
6. Ensure that the tolerance you are specifying are attainable in practice.
7. Use procedures to minimise welding distortion.
8. To eliminate design problems and reduce manufacturing cost consider the use of steel casting or forging in a complicated weldment.
9. Consider cost-saving ideas.
10. Consider the use of hard facing at the point of wear rather than using expensive bulk material.
11. Save unnecessary weld metal use intermittent welds where necessary. Stiffeners and diaphragms may not need full welding.
12. Divide structure into subassemblies to enable more men to work simultaneously.
13. Use mathematical formulae in design don’t use guess work or rule-of-thumb methods.
14. Define the problem clearly and analyse it carefully in regard to the type of loading (steady, impact, repeated-cyclic, tension, compression, shear, fatigue), modulus of elasticity to be considered (tension or shear).
15. Properties of steel sections to consider include, area, length, moment of inertia (stiffness factor in bending), section modulus (strength factor in bending), torsional resistance (stiffness factor in twisting and radius of gyration. Stress is expressed as tensile compressive or shear, strain is expressed as resultant deformation, elongation or contraction, vertical deflection or angular twist.
In the present context we are not discussing the design formulae as it is beyond the escope. For this purpose references on design of welds could be consulted.