Expert: Bruce E. Johnson - 10/10/2009

Question
Other than perimeter and load bearing walls what is the recommended max. spacing for interior grade beams?

Answer
Hi Frank, it depends on the floor system you are going to use.  If you are using a floating concrete slab floor then you don't need anything else, the perimeter and load bearing footings supply all of your support and the slab floats on a stabilized base or you incorporate the support footings into the slab for a monolithic pour.  

If you are going to install an elevated floor system then as a "rule of thumb" figure your grade beam spacing as follows: One foot of span per inch of joist or floor beam thickness..In other words if your floor system is 4x8 beams at 48" spacing then your supporting grade beams would need to be 8 feet apart for the 8" thickness of the beam.  Similarly if you are using 2x8 joists at 24" on center then your grade supports should be 8 feet apart, 2x10? 10' grade beam spacing, and so on.  If you are using a precast concrete slab insert then your grade beam spacing will correspond with the size of the precast panel.

There are steel beam floor systems as well.  I recommend that a professional Engineer be included into any significant structural design.  I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com