A steel structure is an assembly of structural elements (beams, columns, floor, plates, roof members, side wall covering members etc) It contains steel members of various sizes and shapes and they are all connected to each other by bolting or welding.
The design engineer in his design drawing indicates the sizes of the members, the orientation, the member location in 3D space, and how they are to be connected to each other. He also indicates the exact material specification from among options available. He gives the overall bay / aisle dimensions of the building, the floor levels etc. He also indicates the paint specification.
This design drawing enables the contractor/fabricator/erector to assess the total volume of work, assess the material quantities for each member size, the time to execute the job, the difficulties and the complexities of execution and the resources and time required so that he can quote a fair price to execute it.
Once the contract is awarded, the contractor proceeds to execute the job but the engineer’s drawings are not good enough for him to start cutting the material to exact size, length, shape, and drill the bolt holes in the correct places to enable assembly with other connected members. He needs to “detail the structure”. He must prepare a fresh set of drawings for each structural element, indicating the shape, size, exact cutting length, the exact diameter and location of the connecting bolt holes, the details of the fixtures and locations, the detailed bill of materials listing all the materials required. Such a drawing is called a shop drawing, or a fabrication drawing, or manufacturing drawing. He then assigns a unique identifying tag (called erection mark) to each assembled piece and then ships them to the construction site where these fabricated pieces are unloaded with the identifying marks punch marked or painted on them. A new drawing called GA drawing (general arrangement drawing) also called Marking Plan, or Marking Elevation is prepared that indicates where each fabricated piece goes at the site and to which other fabricated member it is connected to.
These drawings are prepared by the contractor (in his drawing office) or off loaded to specialised agencies. During the past, they were painstakingly prepared by expert draftsmen but now Modelling software produces these drawings.
This entire process is called Structural Steel detailing and it is an essential and important part of the structural steel construction activity. It is time consuming, strenuous and accuracy is very important. A mistake in these drawings will cause difficulties at site in assembling them and will boost up the cost as the members may have to be reefabricated if there was a mistake in the drawing. These drawings are required urgently always and those responsible for producing these drawings are always under tension as everybody will be putting pressure on them to hurry up, as hundreds of skilled workmen at site are waiting for the drawings with expensive equipment which cannot be kept idle.
For those who did not understand, here is an analogy.
If a large company wishes to have uniforms for all its hundreds of employees, a dress designer will present a design for the uniform indicating general parameters like cloth material, colour, type of shirt, type of collar, number of pockets, type of sleeves , buttons, etc.
But this information is not sufficient for the tailor. He must take measurements for each employee and depending on the waist and neck sizes and other parameters make a cloth cutting diagram for each employee if the uniform is to fit the employees correctly.
In structural steel detailing the fabrication drawings correspond to the tailor’s cutting diagram. The tailor’s mistake is not every expensive. The material is only cloth. The fabricator’s mistake is very costly. Steel costs thousands more per tonne than cloth!
Of course, you can have standard uniform sizes like small, medium, large, Extra Large etc and mass produce them but still you need cutting diagrams for each size. The fitting may not be perfect unless you cut the cloth for each person exactly as per his body size.