RISAFloor uses the Area Load Definitions spreadsheet to define area loads available for use on a floor.
Area loads applied on a floor default to the last area load applied to a floor unless you use the additive feature. Thus, you can define an Office area load over the entire floor using the Area Load Default entry in the Floors spreadsheet.
If you then apply a Storage floor load in an area of the floor, this Office area load will not be used in this area. The same thing would occur if an area load had been explicitly defined on the floor before the storage load.
However, the Additive checkbox allows you to apply multiple area loads over the same area on the floor. This is ideal for situations where piping may be hanging below the floor and the office loading is happening above the floor.
To use this, simply select the Additive checkbox for your piping/other load that you want added to the regular floor loads and apply those loads. Note that if you apply multiple additive loads on top of each other the load will add to itself as well.
Here we have a 5 psf Roof load and a 20 psf Add Piping load. We can see in the image on the right how these loads add to each other.