There are two structural characteristics of SIPs that are generally not accounted for in creating the usual “stick translations” that predominate in the residential dwelling market.  They are the fact that SIPs are exceptionally high performers when it comes to handling in-plane loads, and that SIP buildings actually behave as thin shell structures, dispersing point loads throughout the entire surface area. These properties may be exploited more consciously in doing the structural analysis of the project and eliminated members that one wouldn’t think of omitting in a conventional stick-frame structure.

1. The first “trick” is to look at dealing with the resolution of ridge or purlin bearing points usually at a gable end wall.  These point loads are usually in the 2,000 lb. – 8.000 lb. Range.  For an extreme case, consider the ridge beam hitting at the peak of a gable end wall.  My SIP load/span charts tell me that for a 6 ½”  thick SIP the per-linear-foot capacity runs from 2000 lbs (12 foot high-30psf wind load) to 8,000 lbs (8 foot high-30psf wind load).  If the design calls for an 8000 lbs  reaction set 20 feet off the floor then it looks like we’re in trouble, as the chart only allows for 5,898 lbs at that height ( all numbers will presume 30 psf wind loading for the rest of this paper).  If the ridge beam is 5” wide, the sill blocking in the bottom of the pocket will be 8” long (1 ½” each side ) which is 2/3 of a foot and therefore capable of only handling 5989 x 2/3 or about 3932 lbs – way under our requirement of 8,000 lbs. 

If we could distribute the load for 2 feet instead of 8 inches we could actually handle 5,898 x 2 or 11,796 lbs, more than adequate with a good safety factor built in. How can we do this?  .....

This question is often asked implying that SIP friendly designs can be built at a very competitive cost compared to stick-built, and that SIP unfriendly designs can be built, but at a premium compared to standard stick-built construction.

I have to say that at least for me this has proven itself to be half true.  We have built some extremely SIP friendly designs that have blown away stick framing schedules by orders of magnitude, but we have also completed some projects where the erection time was been bogged down considerably by some complex geometry.  Let’s review friendly ones first….

The simple questions are always the hardest to fully answer.

Why SIPs – indeed?

Don’t we know the answer?

Well, I know the answers commonly given.  A quick search of the manufacturer’s literature gives the standard list of reasons:

• To provide unmatched comfort in our buildings.
• To make our buildings two to three times stronger than stick-built structures.
• To cut construction time – which is worth money.
• To shift field labor to factory labor which cuts costs.
• To shift construction to a controlled environment which enables higher quality.
• To aid in the tighter scheduling of the whole job.
• To provide a structure that discourages the growth of mold and mildew.
• To provide a building with superior acoustic isolation from it’s environment.
• To build with materials that have a smaller environmental footprint.
• To build with materials that may be recycled.
• To lower our heating and cooling bills.
  
  

I have to say that while all these are true and pretty good reasons for building with SIPs, the most important reason for me is to minimize the greenhouse gas contribution of our building to the atmosphere.

Last year’s devastating hurricanes and flooding in the Gulf Region created an overwhelming need for housing for all those whose homes were destroyed. Eyewitnesses still report that it appears as if the damage was done only just yesterday. The clean-up and re-building job is so huge that all the work done to date seems to be almost insignificant. Recently, in order to help, I have been working on some designs for all types of residential units in all parts of a vast region.  

SIPs have a great deal to offer in this resurrection effort.  ............