BENDING TO SHEAR RATIO APPROACH FOR BEAM DESIGN

Current design criteria for timber beams are often governed by calcula ting shear strength rather than bending strength. However, bending is often observed as the controlling factor in real structures and experi mental studies. This report describes a design procedure based on pred icting whether bending or shear will control the design and designing the member for that consideration. The prediction is based on comparin g the ratio of maximum bending and shear stresses to experimentally de termined bending and shear strengths. Stresses are based on load and s upport conditions and span/depth ratios. A lower limit bending-to-shea r stress ratio of 10 is recommended for design on the basis of a lower bound fit to experimental strength values. For this stress ratio, typ ical designs for most load and support conditions will result in bendi ng-controlled designs that do not require shear calculations. Shear wi ll control the design for a simply supported span with a concentrated load near one support or for a span with a short overhang having a con centrated or uniform load. This approach is limited to unsplit and unn otched beams. Design considerations for notched beams and shear at con nections are outside the scope of this report.