What is the best way to strengthen a SIP so that it will clear span 24' over a basement.  I saw Chris Kavala's pictures of placing a beam below the floor but in my case I want to keep the ceiling below free of any beams.  I am wondering if a structural steel member can be placed on each side of a SIP as a spline to clear span 24' to carry a 45 pound live load.  I anticipate using a 4' wide by 26' long EPS SIP of adequate depth to hide the steel members and carry the load.  The total live load for this SIP would be 96 SF times 45# = 4,320 pounds.  With this length of span and load, how much deflection would be expected when using steel members as splines in SIPS?

My questions not only deal with being able to hide a steel member in the SIP as a spline but also with which type of steel member would be the best to use.  Would a heavy gauge "C-channel" as a spline suffice?  In other words, I need to span 24' with a SIP to carry a 45 pound live load and end up with a clean ceiling below that does not sag too much.  Since this floor will be over a heated basement I am not concerned much about R-value which would be reduced by the steel splines.

Normally, I would specify 4x2 floor trusses but in this case, the plan is to install large ductwork under the floor above.  That is why there should be no beams under the floor.  If SIPs can not be used as a floor for this application, then what other systems should be considered?

I am asking for suggestions before I approach a structural engineer.

No, can not live with posts because of the way the rooms will be used.  But I believe that I can shorten the span by converting some of the partitions in the basement to load bearing walls.  By running the SIPs a different direction and using some walls as load bearing, then the longest span is 16 feet.  I believe this shorter span will not only work but allow the use of thinner SIPs.

By the way, how much of a bearing do SIPs require?  Would a four inch bearing be enough?

Chris,

Thanks.