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The role of short members in Baltimore truss bridges

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The role of short members in Baltimore truss bridges
Posted by BN7150 on Monday, January 9, 2017 11:03 AM

Many bridges of this style are used in railroads, for example the BNSF line over the UP's at Cheyenne, Wyoming.
I have a question about this truss design.
I understand that tensions act on the inner diagonals and lower chords in the Pratt truss, but in the Wikipedia description says that the short members of the Baltimore truss prevents buckling. The short members connect to the inner diagonals and lower chords only. Of course, buckling occurs in compression members. The Baltimore truss is certainly a subclass of the Pratt truss. Is the Wikipedia correct?
Is here a well informed civil engineer?

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Posted by BigDaddy on Monday, January 9, 2017 1:42 PM

Way above my paygrade, but for people who don't know a Baltimore truss from a Warren truss a picture is handy

Henry

COB Potomac & Northern

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Posted by 7j43k on Monday, January 9, 2017 7:10 PM

I think Wikipedia is correct, except they left something out.

The Pratt truss uses the intersection of the verticals and the lower horizontal tension members to anchor the supports for the short-span girders under the tracks (among other things).  With the Baltimore truss, there are almost twice as many points for this to happen because the short verticals will also be used to anchor the supports.  Thus the short-span girders can be made lighter because their span is shorter.

When the short-span girders are loaded, they will place the short (and long) verticals in tension.  Note that the short verticals are anchored mid-span of the diagonal compressive beams.  To lessen the deflection of these beams (this is where the Wikipedia comment comes in) by the short verticals in tension, additional compressive beams are added--the short diagonals.

If you didn't have the short diagonals, the loaded short verticals would add a nasty bend into the diagonal compressive beams.

If you didn't have the short verticals, there would be very little purpose for the short diagonals.  Possibly the compressive beams could be made a little lighter duty.

Note also the Waddell "A" truss, above.  And compare it to a kingpost truss, of which there is not a drawing.  But a kingpost truss looks just like the Waddell "A" truss if you remove the short verticals and diagonals.  Which do exactly the same thing that they do in the Baltimore and Pennsylvania trusses.  So, just as a kingpost truss is a simplified version of a Pratt truss; so it the Waddell "A" truss a simplifed version of the Baltimore/Pennsylvania.

 

 

Ed

 

PS:  notice in the above drawings of bridges that the Baltimore and Pennsylvania are really the same bridge.  It would be interesting to do an analysis of each and see the numbers.  Perhaps the Pennsylvania is slightly more efficient.

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Posted by BN7150 on Monday, January 9, 2017 11:16 PM

Thank you for your comments. But I can not understand....

I think that the force applied to the members of the truss is as shown in the attached drawing.

Isnot it that only the tensions act on the long diagonals and lower chords where the short members support the middle?

Force to join the Baltimore Truss

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Posted by 7j43k on Monday, January 9, 2017 11:27 PM

The drawing looks strange to me.  

For example:  I see that all the short verticals are in tension.  If the short diagonals at the ends are also in tension, as drawn, they will deflect the long end diagonals.  It is my impression that those short diagonals would instead be in compression.  And would thus block the deflection.

 

Ed

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Posted by BN7150 on Tuesday, January 10, 2017 2:05 AM

Ichinoto-river bridges

The attached photo is the Ichinoto-river bridges. I cited from the following address.

https://ja.wikipedia.org/wiki/%E4%B8%80%E3%83%8E%E6%88%B8%E5%B7%9D%E6%A9%8B%E6%A2%81

In this case it can be clearly judged whether the stress applied to each member is tension or compression. Unfortunately it is a deck truss.

This bridge was built by the American Bridge Company in 1908, and is still in use today.

Tags: bridge , Japan , JR-East
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Posted by BN7150 on Tuesday, January 10, 2017 3:52 AM

As you said, the previous diagram was wrong. Next is correct, I think.

Baltimore truss diagram

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Posted by richhotrain on Tuesday, January 10, 2017 5:21 AM

I am no engineer but maybe this presentation will help.

https://prezi.com/h-kihghauqfu/baltimore-truss-bridge/

The Baltimore truss is a modified version of the Pratt truss.
The difference between the Pratt truss bridge and the Baltimore truss bridge is the extra diagonal beams located on the lower half of the truss to help support against buckling from compression and to help control deflection.

Rich

Alton Junction

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Posted by richhotrain on Tuesday, January 10, 2017 5:24 AM

The discussion in this link may be helpful as well.

https://mysite.du.edu/~jcalvert/tech/machines/bridges.htm

Rich

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Posted by 7j43k on Tuesday, January 10, 2017 11:44 PM

BN7150

Ichinoto-river bridges

 

That is a beautiful bridge (though perhaps I could do without the catwalk).  Yes, you can certainly see which are the tension members.  And it is fun to see the short verticals turn from tension in the through version to compression in the deck version.  As you note, it would be nice to see a through version for comparison.

 

Ed

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Posted by BN7150 on Sunday, January 15, 2017 10:08 AM

Re-correction of the diagram

The attached figure of this time is the calculation result by a friend. No force is added to the center vertical menber. Since each connection point is a pin joint, it is inevitable to convince this. Of course, measures against buckling are not applied to the compression members.

Baltimore truss diagram

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Posted by fisherdm on Wednesday, January 18, 2017 7:19 PM

7j43k's response is essentially correct. The main function of the short verticals and diagonals is to reduce the distance between panel points along the bottom chord. (A panel point is where the diagonals intersect the top or bottom members.) This reduces the size of the stringer beams that are directly below the rails, which are supported by cross beams located at the panel points. There is a secondary benefit that the sub-truss reduces the unbraced length of the diagonals for in-plane buckling.

The tension/compression diagram above is correct for a uniformly loaded truss. However when the locomotive first enters the bridge, the load is only at one end and some of the "tension" diagonals are in compression. (The same thing occurs as the train leaves the bridge.)

Remember that as truss bridges got longer, they also got taller (to control deflection). This increased the distance between panel points and made the diagonals longer. The Baltimore truss was one way to address the problems created by the taller trusses required for longer bridges.

Finally - yes, the Pennsylvania truss is more efficient - but only in terms of material. The detailing and fabrication costs are higher because the top chord isn't a continuous member. Each section of the top chord has to be fabricated separately and riveted together at the panel points. The angle of each diagonal is also different, requiring more engineering time to calculate the forces along the length of the truss. It's a trade off.

Dan

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Posted by NWP SWP on Wednesday, January 18, 2017 10:18 PM

Fisherdm, I am trying to build a a bridge based off the old Vicksburg bridge and the Thebes bridge could you by chance identify what types of trusses are on those bridges?

Steve

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Posted by BN7150 on Thursday, January 19, 2017 1:50 AM

Thank you Dan. Scales fell from my eyes. I will ask my friend to re-calculate at once.

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Posted by fisherdm on Thursday, January 19, 2017 6:25 PM

cascadenorthernrr

Fisherdm, I am trying to build a a bridge based off the old Vicksburg bridge and the Thebes bridge could you by chance identify what types of trusses are on those bridges?

 

Cascadenorthern,

I looked at photos of both the Vicksburg bridge and Thebes bridge on bridgehunter.com (linked in another thread). Both of these are cantilevered trusses, and therefore fall outside the usual Pratt/Warren/Howe-style designations. Cantilevered truss bridges have to be designed for a very different set of loads, because the forces in the members change as the bridge is constructed. On bridgehunter.com there are some good side views of each bridge, which allows you to see which way the diagonals run at each location.

The Vicksburg bridge has a pair of single cantilevered spans with a suspended span between them. On one side is a series of approach spans. The suspended span and approach spans are all Pennsylvania-type trusses.

The Thebes bridge has two double-span trusses with three suspended spans; one between the cantilevers and one between each shore and the cantilevers. Again, the suspended spans all appear to be Pennsylvania-type trusses.

The bridgehunter site has some good photos of the Thebes bridge under construction. In the first one, the cantilever on the left side is complete and the right side is still being built. Once both cantilevers are complete, the suspended span truss that fits between them will be barged out into the river, lifted up, and connected to both cantilevers.

One thing to consider - cantilevered bridges were used where really long spans were needed. Simple Pratt-style trusses were good up to about 250 feet; Baltimore-style trusses were good up to about 400 feet. It's kind of hard to justify a cantilevered truss on most model railroads. At the Thebes bridge, the center span is 671 feet, with the suspended span being probably about 400 feet long.

Dan

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Posted by NWP SWP on Thursday, January 19, 2017 6:38 PM

Could I compress it slightly? Also what would you suggest I use to construct it?

Steve

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Posted by fisherdm on Thursday, January 19, 2017 7:06 PM

cascadenorthernrr

Could I compress it slightly? Also what would you suggest I use to construct it?

 

You could. But even at a 2:1 compression, the main span is still over 3 feet long. Using your sketch as a guide, I would go for something more like the Vicksburg bridge - two single cantilevers with one long span in the middle. I would eliminate piers 1, 3, 4, and 6. The backup spans for the cantilevers would extend from shore across one pier each.

As for materials - you'll make Central Valley very happy. Big Smile

Dan

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Posted by NWP SWP on Thursday, January 19, 2017 7:15 PM

Ok thank you. And do you have any tips for scratchbuilding it?

Steve

If everything seems under control, you're not going fast enough!

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