To explain how the stiffness of the braces (and the rest of the guitar) works, I need to introduce some engineering measurements that are used to define the stiffness of structures.  These measurements are section properties and material properties.  Section properties are inherent to the shape of the structure.  Material properties are inherent to the material it is made of.  They act together to define strength and stiffness.  Because engineers understand section properties and material properties (and properly apply them), buildings don’t fall down and airplane wings don’t fall off.

Most people have a tactile, “horse-sense” understanding of section properties:  If a wood plank is taller or wider, it will be stiffer (all other things being equal – this caveat applies throughout; I’ll not repeat it.  In real life, things are never equal.)  A thick Sitka spruce trunk is stiffer and stronger than a thin one.  A ¼-inch thick guitar top is stiffer and stronger than a ³/₃₂-inch thick top.  These are all differences in section properties.

Section properties are simply the exact way that engineers use to measure a “stiffer than” or “stronger than” structural shape.  Section properties are determined exclusively by the shape and size of the structure.  Section properties are not affected in any way by the material – this is the key point.  A two-inch by four-inch plank will have the same section properties whether it is made of steel or plastic or butter.  Section properties are essentially perfectly understood by engineers and have been tested and confirmed millions of times.  They are simple to compute and formulas may be found in the suggestions for further reading (SFR) at the end of this article. Please note that a number of topics that are introduced in this article are discussed in much more detail in some of these suggested readings.

Separate from section properties the other important factor is material properties.  A two-inch diameter steel pipe is much stiffer than a two-inch diameter plastic pipe (both pipes have identical inside diameter (ID) and outside diameter (OD)).  This is the material effect.  Because the ID and OD are identical these two pipes have exactly the same section properties; they are different in stiffness (and strength) because they are made of different materials that have different inherent material properties. These inherent material properties are also precisely measured by engineers.  The primary material property of interest here is the modulus of elasticity, abbreviated as E, and generally referred to as “Young’s Modulus.”  The details on Young’s Modulus and how it is determined and used can be found in the SFR.