Which wood framed structure would you expect to be stronger?

Posted by in Construction & Shortcuts, Structural terms | August 31, 2015

Which wood framed structure would you expect to be stronger?

Number 1 or Number 2?

  1. Wood framed construction built in accordance with prescriptive framing methods. (Prescriptive framing methods are established upon rule of thumb guidelines that have developed from the past performance of similarly framed structures.  For example, the walls of a home are framed with 2×4’s or basement walls that have five feet or less of unbalanced earth backfill are built with 8-inch thick concrete block.)
  2. Wood framed construction built in accordance with science based engineering methods.

But, before you answer, consider these general principles:

Science-based engineering provides a more accurate assessment of the actual strength of the structure and a more accurate assessment of the possible loading conditions.  Knowing the actual strength and the possible loading conditions provides more certainty against failure, and thus, more certainty should allow a reduction in safety factors.  However, a reduction in the safety factor will result in less “extra” capacity in the structure.

Prescriptive framing provides the comfort of knowing that relatively few structures have failed in the past when framed in accordance with commonly accepted rules of thumb.  However, less certainty should require higher safety factors, and thus, more “extra” capacity.

Are you ready to give your answer?

I’ll give you some time to think about it ………

(do you here the “Final Jeopardy” tune in the background) ……

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According to an August 2015 article in the SBC Magazine the International Residential Code (IRC) considers a prescriptive (rule of thumb) shear wall (a wall being loaded [pushed or pulled] along its length by wind or seismic forces) to have more than twice the strength of an engineered shear wall.  That is, given the example of a common wall, eleven feet long, with two window openings, the IRC specification would consider a non-engineered wall to be more than twice as strong as a similar wall that was engineered.  Restated, computations show that the prescriptive shear wall would actually be weaker than the engineered shear wall.

The discrepancy is due to safety factors.  Apparently politics has figured into the development of the code and those who build walls by the prescriptive methods (some associations of home builders) have a stronger lobby or better political ties than those who manufacture engineered walls.  Because of that political power, safety factors are significantly reduced in the code for the prescriptive walls.  However, engineered walls are still required to comply with the safety factors developed for engineering analysis and design.

We have noticed this anomaly with a common concrete block basement wall.  The prescriptive design and construction method allows five feet of well-drained earth backfill against an eight-inch thick block basement wall.  Practically speaking a well-drained earth backfill is a figment of the imagination, however, home builders assume that their backfill is well-drained.

Engineering computations show that an eight-inch thick wall is overstressed by the lateral earth pressures from five feet of earth backfill.  If groundwater pressures are considered that overstress is even greater.  This overstress is one of the reasons that many basement walls bow inward and crack.  Since eight-inch thick block walls usually don’t collapse, they qualify as a prescriptive method.  However, they do bow inward, and if the inward bowing progresses, they have been known to collapse.

So, here is there answer to the question: the IRC says that the wall built in accordance with the prescriptive methods is stronger, but in reality, the answer is the wall built in accordance with science based engineering methods is stronger.  There may be some exceptions to this rule, but generally, that is how it works out.

The moral of the story: no matter what politicians say, even those developing building codes, reality  always wins in the end.

If you are interested in more on this topic I refer you to Engineered vs. Prescriptive Wall Panel Design in the August 2015 SBC Magazine.

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