Openings can be stiffened by applied side pilasters (174:4) and central pilasters (174:5), but the top and bottom members must be stiff also and the connecting joints as strong as possible. Other methods include pilaster attached to a stiff apron (174:6)" L frame (174:7)TU frame(174:8)rinverted U(174:9); T frame"(174:10); I frame(174:ll) and H frame (174:12). Methods of increasing the stiffness of flat panels include increasing the thickness (174:13); sandwich panel (174:14); closed box (174:15); cross-ribs reinforcement (174:16); double cross-rib (174:17) and pyramid bracing (174:18). Carcasses can also be stiffened by arbitrary fixed cross-rails or shelves or divisions, which in effect either adopt an 'H' or T formation and break up the carcass into smaller boxes, with increased wood thickness relative to the opening. This method is often adopted in open bookcases with loose shelves, where one or more shelves are housed and glued into the sides to increase the stiffness, but such carcasses will still distort fractionally if stood on an uneven surface, no matter how sturdy the wood sections are, for increasing the wood dimensions also increases the overall weight, therefore some form of pilaster, stiff apron rail or box plinth is preferable. The principles of deflection and the appropriate methods of combating them are examined in detail in the Design Manual for Cabinet Furniture published by the Furniture Development Council.
In all carcass work it should be remembered that while a well-fitted carcass back, grooved, screwed or glued in position will help to achieve overall stiffness and squareness, it will not automatically ensure such rigidity.
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