The first time I designed a building with two levels, the boss told me to make the floor-to-floor height as tight as possible because that would keep the exteior wall cost as low as possible. I did it.
My main recollection of the construction of that building was the constant phone calls and trips to the site to explain how to run the ducts, conduits, and place the lights so everything would fit. I came to the conclusion that the boss may have been focused on the wrong issue.
Eventually I came to this Rule Of Thumb that I am going to share with you. But I always gave the floor-to-floor height more thought than on that first project.
- Decide on the typical ceiling height based on the kind of spaces your building will have. This is often close to 9'-0". If you have a couple of spaces that need a higher ceiling, you can often work that out. So go with the typical ceiling height.
- Look at the most likely structural layout. If your spans are in the neighborhood of 30', use 2'-6" to accommodate the depth of the structural system. Longer spans need more depth. An inch per foot of span is often the ratio. At some point your structural engineer will pick the actual sizes of the structural members, but this will give the engineer something to work with. There is a lot of leeway in sizing structural members.
- Allow 2'-0" for duct space. Usually this is enough. Sometimes a rooftop unit will enter the building larger than this. But in a short distance, the main ducts can usually widen out so that the depth can be reduced. This will require attention when you are choosing a location for the rooftop unit.
- Set the floor-to-floor height to an increment of concrete block coursing - a multiple of 8". You will be glad you did. It is really rare to not have some block for exit stairs, elevator shafts, fire walls or demising walls in a building. When you have to work around an odd dimension for the floor-to-floor height you will learn what I mean. Working out door heads and what the exposed coursing looks like will take more time than it should and the results probably will be a compromise.
So here is an example calculation:
- TYPICAL CEILING HEIGHT = 9'-0"
- STRUCTURAL SYSTEM DEPTH = 2'-6"
- DUCT SPACE = 2'-0"
- TOTAL = 13'-6" (round up)
- FLOOR-TO-FLOOR HEIGHT = 14'-0"
So my advice is to start with a 14'-0" floor-to-floor assumption and adjust by 8" increments if it becomes necessary. Share this with the design team and ask them to verify at the first opportunity that it works for them.
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