This article will show you how to export a solid created in the Sandbox Editor into DCC tools like 3ds Max or Maya.
To export solids, follow these instructions:
Select the solid in the perspective viewport, to select a group of solids hold the CTRL key and select each solid you need.
Open the File menu and click Export Selected Objects:
Save the file to a folder, anywhere on your PC.
Load the File in in your DCC tools. It is important to disable the Rotate model flag in the OBJ Importer from 3ds Max.
Also, please refer to the tutorial relating to exporting objects from 3ds Max and Maya, to help you get the completed object in-game.
Solids are a great way to get ideas into the level, without worrying about creating actual detailed geometry.
Efficiently using them to produce layouts and initial drafts will give you a good idea of how the final product may work, but in a fraction of the time.