BIM News

BIM Use in Retrofits: Ask the Right Questions First to Make the Job Easier


November 26, 2014  │By: Kate Morrical  │Posted on:

A structural engineer’s scope in a renovation is often smaller than that of the architects or mechanical engineers. It may not be an efficient use of time and resources to model areas of a building that do not require structural or mechanical modification. On the other hand, the owner or client may decide that the current project is an excellent opportunity to update the documentation for the entire building, in which cases the scope is adjusted accordingly.

Below are questions you should be asking first to help make BIM use in retrofits projects easier.

What Should You Model? The absolute minimum model requirements for an existing building project are the new elements being added. The rest of the existing conditions can be shown in the background, either in the form of 2D CAD files from a prior project or as underlays of architectural information from the current model.

The next level up involves modeling new elements and any existing structure they connect to or affect. For example, if you are adding a new beam to support a new opening in a floor, you would model the new beam and the elements (beams, columns, walls) that support it. You would also model the existing floor, so that you can more easily show the new opening.

If the extent of the renovation justifies it, or if the contract requires it, you may end up modeling the entire existing structure. This simplifies the question of “should we model that?”, but it adds complexity to the question “what exactly are we modeling—and how much do we really know about it?”

How True Is Your Model? Implementing BIM for an existing building always comes with some level of uncertainty. How much uncertainty depends on what kind of existing documentation you have, and your level of confidence in it.

Usually you have some kind of construction documents for a building. But do they represent the current state of the building? If there have been previous renovations, do you have the plans for those, too? If they’re old, are they still legible? Have you done a field survey to supplement and confirm the documentation? Were you able to survey the entire building or just some of it? If you can put the answers to these questions in the model itself, it can help you down the road when you’re trying to figure out if that beam really is where it’s modeled, or if it just represents the best guess at the time.

One possible approach to this challenge is a three-level classification scheme for documenting the confidence in the accuracy of the information contained in the model. At the top of the scale are elements that have been verified in the field. Someone has gone out with a tape measure, laser scanner, or other survey equipment and confirmed the dimensions, plan location, and elevation of the object.

Read full article on

Thanks for reading!

Please enjoy a limited number of articles over the next 30 days.

For total access log in to your The BIM Hub account. Or register now, it's free.

Register Sign in