Comparing shear strain in a solid web and diagonal structural elements that replace a solid web of a bending beam is confusing for most of us.
A solid web of a bending beam supports combined forces of tension, shear and compression, each of which has a different distribution and magnitude. Shear forces created by the pure bending are zero at the spar and maximum at the middle or neutral line and their direction is longitudinal. Shear forces created by lift of the wing run perpendicular to the web and are more or less uniformly distributed.
The elements of a bending truss can only support forces of tension and compression.
As we all know, solid webs and Warren type trusses have been used for the structure of fuselages as much as for wings.
Please check the following links:

http://en.wikipedia.org/wiki/Shear_stress
http://en.wikipedia.org/wiki/Shear_strain
http://en.wikipedia.org/wiki/Truss_b...polar.29_truss

Warren truss
The Warren truss was patented in 1848 by its designers James Warren and Willoughby Theobald Monzani, and consists of longitudinal members joined only by angled cross-members, forming alternately inverted equilateral triangle-shaped spaces along its length, ensuring that no individual strut, beam, or tie is subject to bending or torsional straining forces, but only to tension or compression. Loads on the diagonals alternate between compression and tension (approaching the center), with no vertical elements, while elements near the center must support both tension and compression in response to live loads. This configuration combines strength with economy of materials and can therefore be relatively light. It is an improvement over the Neville truss which uses a spacing configuration of isosceles triangles.

Use in full scale airplanes:
http://www.americanflyers.net/aviati.../chapter_1.htm
http://nptel.iitm.ac.in/courses/Webc...20and%2029.htm