City Bridge – Pt XI

Quick Fit Check
Test_Fitting_BridgeTest fitting the bridge where it will go. In fact .. I was wanting to see how much .. if any .. of the upper side of the bridge can be seen when it is in place. (meaning .. do I REALLY have to put in the upper diagonals?) .. seriously though .. most of that will be hidden by the ties I think.
Stumble ..
DiagonalChairI started off earlier planning to simply slice up some styrene to create the bracket that bolts/rivets to where the diagonals cross forming an X-brace. I was suddenly attacked by OCD .. and forced to create a cast-iron shoe for that. Next – plan was to use this as a master for a RTV mold – besides .. got to play with my new micro lens!
CastingsFirstOne of the problems I run into when modeling is much the same thing that I have run into at 12 inches to the foot. When I worked as a machinist I could regularly keep tolerances to under .001 in. When you are making a shoulder for a bearing you want that O.D. to actually be a few ten-thousands larger then the I.D. of the roller bearing. That wasn’t the problem. Every so often I would finish .. smiling happily .. and find that I was off by an order or so .. like .. instead of 2.125 in. .. it would be 3.125 in. I’m willing to bet there is a term for that .. where you are so focused on making it ‘just so’ .. that you miss by a mile.

So. I am going to re-work the Cross-Brace bracket I made. It is too big. It came out pretty good .. in the ‘just so’ area .. but I was so focused on that I missed the size thingie. Oh well. Lesson’s learned and all that ..

In the meanwhile .. here are the brackets. They can go into the scrap box and be re-purposed at some time as scrap.

Once again into the breach ..
Bracket_1So .. let’s talk about where I am going here. The reason for the diagonals is simple. The run from corner to corner in a ‘box’ to keep that ‘box’ from distorting under stress. The work against that stress under tension. In the diagram below the angle I have labeled “Full length Diagonal” runs .. well .. full length . Now .. the diagonal running in the other direction has to be split and a bracket .. shown in blue to tie that “Split diagonal” together.
Bracket_2Here’s a better view of what I am talking about. This is by the way .. a good example of what I label .. “Emagineering”. The exact size of the rivets, spacing and number in “real life” would be calculated by the Bridge Engineer. It boils down (very simplified) in that the cross section of the combined rivets has to equal the cross section of the structure they are riveting .. needed to be structurally equal. But .. I don’t worry about that at the modeling level .. I use what I have on hand and what looks reasonable. That is actually a pretty good way since you can get a sense of “that looks too weak” when looking at something.

So. The brackets sandwich the diagonals into an X-member. There is a minimum of rivets through the Full length diagonal .. and many more on the spit angles. In this case .. I “Emagineered” the size and number.

Bracket_3Finally .. here’s a look at the top and bottom of the assembly. It is really pretty simple .. an angle on top and a plate on the bottom. This by the way isn’t me making this up but taken directly from a book on construction of bridges.

So. I will abandon trying to make ‘cast’ brackets and revert back to the simpler version here. That’s not to say they didn’t also have such things cast up .. but I will leave that to another time or modeler!

Bracket_4Ok. “New and improved” brackets going in. Once all the bits and pieces are glued in I will slap on some primer. I will be using the MicroMart water slide resin rivets and like any decal they work better on a primed/painted surface.
.. and Rivets
rivets3My head almost exploded from applying the Micro-Mark rivets. I suppose that I’m ‘lucky’ in that I am out of the ones I need so this will have to do. Next .. 1) Remove cat hair .. and .. 2) primer
np12


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