Here is a non-technical example.

Take a 30 foot by 50 foot building, no interior walls. The wind blows against the 50 foot walls and blows it in.

Now, build a shear wall perpendicular to, and against, the center of that 50 foot wall, effectively creating two 25 foot walls with a 'T' wall in the center. The same wind blows against that 50 foot wall and the shear wall in the center holds the wall up.

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Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )