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Central Valley truss bridge

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Central Valley truss bridge
Posted by JohnWPowell on Thursday, July 2, 2020 4:11 PM

I have 2 cv truss bridges together when a train goes over them the bridge flexes so much motors uncouple from each other. What's a good way to fix this?

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Posted by Overmod on Thursday, July 2, 2020 4:19 PM

Easiest way is likely to be to make up rigid reinforcing beams and cement the upper flange as continuously as possible to the underside of the bridge, either on the centerline if double track or under each track center.  You could design slight camber into this as preload against sag if you want to keep the beam section down for visual reasons.  If visibility is a concern you can use Plexiglas or similar optically-clear material for the web.

i'd recommend against trying to make fancy modeled lattice trusses for these beams -- that sort of fabrication at "HO scale") will seldom have the necessary strength and rigidity for long-term effectiveness; this has been proven time and again at 'engineering challenge' competitions.

Ed will be able to provide you with a number of pictures of prototype bridges (e.g. perhaps in the P&LE) where truss bridges were strengthened for heavier service instead of being replaced.  Some of those might work in your situation...

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Posted by GMTRacing on Thursday, July 2, 2020 4:27 PM

Assuming HO and assuming you fit the steel reinforcements you can reinforce the main parts underneath with metal box tube or wood though the box tube would be easier to hide. Both structures deflect? I have one of the 150ft versions and I haven't had that happen yet. Let us know how you progress.   J.R.

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Posted by Wolf359 on Thursday, July 2, 2020 4:48 PM

I'd try putting some beams along the bottom near the edge on each side and disguise them as something like a pipe or some kind of decorative fairing.

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Posted by BigDaddy on Thursday, July 2, 2020 4:56 PM

Is their a pier in the middle?

Henry

COB Potomac & Northern

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Posted by JohnWPowell on Thursday, July 2, 2020 5:01 PM

Yes there a plier between them

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Posted by jjdamnit on Thursday, July 2, 2020 5:02 PM

Hello All,

JohnWPowell
I have 2 cv truss bridges together when a train goes over them the bridge flexes so much motors uncouple from each other. What's a good way to fix this?

  • What scale- -HO or N?
  • Did you tie the bridges together in any way or just but them back-to-back with no structural members above (a pronounced "V" in the center of the span)?
  • What's the span of a single bridge: 150' or 200' (scale)?
  • How big a gap are you spanning (in scale feet or actual measurement)?
  • Is the span supported in the center in any way?
  • What era are you modeling: early steam, transition, early diesel, or modern?
  • Is this a mainline or branch line?

All of these questions need consideration even in model railroading.

If you can answer these questions the great folks on these forums can help you better.

Hope this helps.

 

"Uhh...I didn’t know it was 'impossible' I just made it work...sorry"

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Posted by JohnWPowell on Thursday, July 2, 2020 5:03 PM

Yes they are ho and the steel reinforcements are installed

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Posted by jjdamnit on Thursday, July 2, 2020 5:09 PM

Hello All,

JohnWPowell
Yes there a plier between them

Can you describe what a "plier" is?

Hope this helps.

"Uhh...I didn’t know it was 'impossible' I just made it work...sorry"

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Posted by jjdamnit on Thursday, July 2, 2020 5:10 PM

Hello All,

JohnWPowell
Yes they are ho and the steel reinforcements are installed

What "steel reinforcements" are you referring to?

Hope this helps.

"Uhh...I didn’t know it was 'impossible' I just made it work...sorry"

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Posted by BigDaddy on Thursday, July 2, 2020 5:20 PM

A plier is a a pier that lies at the juction of the two bridges.

My assumption is the flexing is going on at the center of each span since there is a pier.  There are lots of CV truss bridges in the forum and this is the first I've heard of the problem.

Henry

COB Potomac & Northern

By the Chesapeake Bay

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Posted by doctorwayne on Thursday, July 2, 2020 5:44 PM

Yeah, with the steel reinforcing plates installed in the bottom members of the span, there shouldn't be any noticeable deflection. I've run my three modified Athearn U-boats (33oz. each) over my Central Valley bridge without any noticeable deflection of the span.

Wayne

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Posted by wp8thsub on Thursday, July 2, 2020 6:29 PM

The CV truss bridges have metal reinforcements, plus connections between the crossmenbers below the track and the actual truss assembly.  Both work together to keep the structures straight when there's weight on them.  If they're deflecting sufficiently that locos come uncoupled, it seems likely to me they weren't built according to the directions, or there's weakness in the assembly from parts that aren't secure.

Rob Spangler

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Posted by SeeYou190 on Thursday, July 2, 2020 8:01 PM

I have built the HO scale Central Valley Pratt Truss Bridge, and never had a problem. The model is very rigid.

How heavy are these locomotives?

-Kevin

Wink Happily modeling my STRATTON & GILLETTE RAILROAD. A Class A line located in a personal fantasy world of semi-plausible nonsense on Tuesday, August 3rd, 1954.

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Posted by peahrens on Friday, July 3, 2020 10:48 AM

I am eyeballing a P2K SD50/SD60 pair crossing my CV truss bridge.  I see, against the background, some very small flex.  I'd say about 1/16", not 1/8".  The locos weigh about 26 oz. each.  How much does yours flex downward with 2 locos atop?

Paul

Modeling HO with a transition era UP bent

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Posted by doctorwayne on Friday, July 3, 2020 11:40 AM

While I no longer own the three 33oz. U-boats, here's the Central Valley truss bridge with two machinist's steel blocks, not quite on the centre of the same bridge....

Total weight (and concentrated, rather than distributed like the just-over 6lbs of the three U-boats) is 7 pounds.

The measured deflection of the bridge's bottom-member was 3"(HO).

Wayne

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Posted by JohnWPowell on Friday, July 3, 2020 7:59 PM

Wow well i wonder what I did or didn't do

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Posted by JohnWPowell on Friday, July 3, 2020 8:02 PM

I thought I built them just like he did in the YouTube videos. Im not sure what is going on. Thank you

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Posted by doctorwayne on Friday, July 3, 2020 9:34 PM

I should have also mentioned that the truss portion of the bridge likely adds little in the way of strength, as it's not cemented to the bridge's deck, but merely snapped in place, as an aid for track cleaning (which is seldom required).

The entire five-span bridge (four deck girder bridges and the truss) are also removeable from their piers and abutments as a single unit...good for those times when the "water" needs to be cleaned.

Wayne

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Posted by wp8thsub on Friday, July 3, 2020 9:38 PM

JohnWPowell
Wow well i wonder what I did or didn't do

Let's try to diagnose this.  You said earlier the steel reinforcements were installed.  There are other places to check for structural issues:

  • What are you using for track?  Is it the Central Valley ties supplied with the kit?  If so, are the rails securely installed?  Is the tie strip secured to the main bridge girders (the ones under the rails) so it doesn't deflect as a train crosses?  If it's some other kind of track, is it installed flush with the top of the girders (which would likely involve either shimming between crossmembers or removing ties that would interfere with them)?
  • Did you retain the pins that attach the bridge deck crossmembers with the truss?  Do they fit tightly into holes in the back side of the lower truss members?
  • Is the truss glued onto the girder assembly, or did you leave it removable?

Rob Spangler

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Posted by hon30critter on Friday, July 3, 2020 11:31 PM

I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!

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Posted by mikeGTW on Thursday, July 9, 2020 4:21 PM
John I've built several of these and no problems I do them like the doctor I do not attach the superstructer to the base I have even built a shorter version from extra parts You state it deflects where at the plier or in the middle of the span I cannot imagine one of these bridges doing that If it is just one certain engine maybe the coupler is hitting something

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