BIM software is the design software in which architects and engineers draw a building as a 3D model, where every wall, pipe and beam is an object with a size, a material and a cost. BIM stands for building information modelling. In older drawing software, a wall is just two lines on a sheet. In BIM software, a wall is an object that knows it is a 230 mm brick wall with plaster on both sides, 3 metres high and 6 metres long.
Because the model is made of objects, the drawings come from the model. The architect draws the building once. The software then produces the floor plans, the sections, which are drawings of the building cut through from top to bottom, and the elevations, which are drawings of each outside face. If the architect moves a window, every drawing that shows that window changes with it. The general idea is explained on the page on BIM.
Different designers work on the same model. The architect draws the rooms and walls. The structural engineer adds the columns, beams and slabs. The engineers for building services add the ducts for air conditioning, the water pipes and the trays that carry electrical cables.
Here is a typical case. A 300-bed hospital is being designed in Nagpur. On the fourth floor, an air-conditioning duct 600 mm deep must pass under a beam 750 mm deep. The space above the false ceiling is only 1,000 mm high, so the two cannot both fit. The software runs clash detection. It checks every object against every other object and reports each place where two of them occupy the same space. On the fourth floor alone it finds 85 clashes, including this duct and beam. The engineers reroute the duct on screen in an afternoon. Found on site, the same clash would mean cutting the duct, or waiting for a new design while the workers stand idle.
The model is also a source of numbers. Every object has a size and a material, so the software can list the volume of concrete, the length of each pipe and the number of doors. Construction estimating software reads these lists to price the job. The contractor's team often reviews the drawings as PDFs and marks changes on them with PDF markup for construction. The latest drawings are then shared with everyone on the project through construction management software. Some Indian cities also accept building plans online, through permits and licensing systems that check the plan against the city's building rules before a permit is issued.
How is BIM software different from computer-aided design (CAD) used for machines? Mechanical CAD models one product, such as a gearbox, in very fine detail. BIM software models a whole building, with the materials, quantities and costs that builders need.