From On18 Trestle – Pt IV
– On18 Engine Weight –
With those numbers .. we have a place to at least start.
It has been found, as a result of calculation and observations in practice, that the greatest economy of metal is obtained where d is made from one-eighth to one-fifteenth of the span, one-tenth or one-twelfth being a good average value.
The depth of a girder is often limited by considerations of headroom, clearance, etc., but whenever possible the limits given above should not be exceeded. The standard depth should be one-tenth to one-twelfth of the span, and this should not be departed from in a single web plate girder of varying section without due consideration.
If a depth greater than one-eighth span be adopted it will be found that the amount of metal in the flanges will be comparatively small, but that in the web will be very much increased. The reason for this latter is, that although a thinner web, thinner than that required for one-tenth span, would suffice to carry the shearing forces, a thicker one would be required in practice to allow for the greater buckling tendency due to the increased depth. If a depth less than one-fifteenth is taken, difficulty will be experienced in providing for the heavy end shear on the web.1
- The Practical design of plate girder bridges – Harold Hughes Bird, J.B. Lippincott Co., Philadelphia, 1920 [↩]