BIM: the challenges of modelling an existing building

In an article on RICS, Les Pickford – freelance writer and former editor of the RICS Construction – talks about the challenges of producing a BIM model for an existing building. This is the second in a series of articles following the RICS HQ BIM project.



Read Part 1: modelling RICS headquarters


“The advantage for the lay person,” says Matt McDermott, RICS Sales and Marketing Director and catalyst for the BIM communications programme, “is that they can see how changes to an existing building will impact their environment. This can mean improved staff engagement and happier clients. A model can provide a centrepiece for better communications.”



“A 3D model allows you to collaborate with structural and M&E engineers, If you wanted to change all of the windows, for example, the model allows you to quickly create a windows schedule that can be used immediately by all parties. Without the collaborative element of BIM, this process would probably mean architects and engineers annotating and emailing documents back and forth until a new specification is agreed, and that this could take weeks.”


The RICS HQ survey

King says the process for surveying the Parliament Square building followed a standard route – with a measured building survey, a topographical survey, elevations, sections, etc – but also created a 3D model with the ability to assign information to building elements. “Previously, we may have done a building survey, and maybe took the elevations from a laser scan, but delivered only 5% of the information we captured. With BIM we’re giving much more information to the client.”


King advises that conducting a laser scan of an existing building presents specific challenges, including:


§  a lot of information isn’t accessible, e.g. because services such as plumbing are behind walls


§  the building is occupied and so it’s often difficult to access certain areas, e.g. server rooms. “That’s generally the case due to confidentiality policies,” he says “so it helped that RICS had CAD drawings to fill any gaps and Facilities staff available to help with access and scheduling of room surveys to work around conferences, etc”


§  windows that don’t open can prevent the laser scanning team from clearly seeing external control points used to ensure the accuracy of scans


§  historic buildings tend to have smaller rooms with more corridors and doors to staircases, etc. This makes the scanning process slightly more complex and time consuming, compared to modern buildings with more open floors, less walls and more columns



§  clients often don’t like any marks left in the occupied building. “If we return to a room, we’d like to use the same control points but we can’t really mark the floor or walls as we can with industrial buildings,” says King. “This is why we try to start and finish on known fixed coordinates and complete a room in a day.”


However, while new builds allow models to be populated with manufacturer’s product details, existing buildings (especially historic ones) could have elements that are hundreds of years old or of unknown origin, so model details have to be populated from scratch. “For example, when doing M&E modelling we can scan a pipe but we don’t know what it’s made from, what flows through it, where it comes from or where it goes to,” warns King. “So we will add parameters to drop-down menus so an engineer can click on the pipe and complete the details of its use (e.g. gas, water or electrics) and what it’s made from (e.g. steel, iron or plastic).


“Not many organisations can afford to immediately populate a new model for an existing building,” King adds. “But through ongoing building maintenance, this can be done room by room, window by window, door by door. Soon, RICS could have a very information-rich model.”


Read part 3: getting value and monitoring progress


Full article can be found on