Flange or Cover Plates form the top of the Main Girder assembly, tying everything together and a major part of the strength of the girder as a cross-section of the girder shows that the cover plates along with the angles and web make up the girde cross-section.
This is one of the areas where I quickly turn to what I an call – Imagineering Simplification – or IS. The math is just .. too much for my poor brain and after all I am not going to actually build build a REAL bridge .. but a model that appears to satisfy the requirements.
For girders with constant depth back to back of flange angles it is sufficiently accurate to assume the effective depth h constant. The required flange area varies then in direct proportion with the bending movement.1
What they are saying is this (simplified for my little brain). Suppose you lay a yardstick across a gap and then press with your finger on the end (where it is supported) and in the center where it isn’t. Obviously .. it bends in the center. What they are doing then is add additional additional thickness toward the center of the girder – in this case – different thicknesses of flange plate.
Note: Some of the engineers liked the idea of adding the thickness to the side of the bridge girders since this allowed the top of the girder to be the same end to end which would enable laying the same tie but *cost* meant that the cheaper ‘vary the Flange Plate’ meant the track crew had to dap the ties differently depending on location.
To calculate the lengths of the cover plates a movement curve is drawn which is then divided proportionately to the various parts making up the flange. When the theoretical length of a cover plate is found they add 6″ for light to 12″ for heaver cover plates. Since I am making this up anyway .. I just used the example in the book.
The diagram shows half the bridge length. I brought this into Sketchup and then I put in the 6.825″ (1/2 of 13.625″). Once I scaled the drawing to that length (now Sketchup knows the proportions) I easily marked off the three dimensions for the three plates. I added 6″ (.125″ – this is O scale) and the numbers in yellow show 1/2 the length needed for each cover plate.
Plate girders should have rived to each end a base plate of a thickness not less than 3/4 in.for railroad (…) bridges. There should be a bedplate between the base plate and masonry, which may be a rolled steel plate of a thickness not less than of the base plate for railroad spans up to 75 ft. (…); it should not project more than 3-1/2 times its thickness beyond the bottom flange angles. Instead of bedplates, cast steel pedestals are now more frequently used (1915).2
– a is the length of a bearing, b the distance from the edge of the bearing to the face of the masonry (Fig 1).
Therefore for my bridge: – which in O scale is .458 in.
Generally, a sole plate 3/4 in. or more in thickness is countersunk-riveted to the girder for a bearing piece, and has a length equal to that of the masonry bridge seat, and a width equal to or a little greater than that of the flange. (…)
|From a table – American Bridge Company’s Single Track Deck Plate Girder Spans – E-40 34,900 lbs. and E-50 38,800 lbs.5|
- Design of Steel Bridges: Theory and Practice for the Use of Civil …, Volume 1, F. C. Kunz, 1915 – Pg.155 [↩]
- Design of Steel Bridges: Theory and Practice for the Use of Civil …, Volume 1, F. C. Kunz, 1915 – Pg.166 [↩]
- Design of Steel Bridges: Theory and Practice for the Use of Civil …, Volume 1, F. C. Kunz, 1915 – Pg.127 [↩]
- Types and Details of Bridge Construction: Plate girders, Frank Woodward Skinner, 1906 – Pg.124-125 [↩]
- Design of Steel Bridges: Theory and Practice for the Use of Civil …, Volume 1, F. C. Kunz, 1915 – Pg.221 [↩]