Amazing Old Trestle (a Haupt Bridge) - Trains Magazine

The Haupt arch truss was developed as a timber truss with the design addressing the characteristics of wood.  It is a design where the primary load-carrying element is the timber arch, and the latticework maintains the shape of the arch, so the arch can maintain its load carrying ability. 

Each of the two arches is split into halves with the latticework sandwiched between.  This would make the latticework perform like a web to the arch, similar to the function of a steel web with a steel girder bridge. 

A feature of the Haupt arch truss is that it could be set up for travel on either the upper level, the lower level, or both.  This photo of the Bull Run Bridge shows it with the tracks running on the lower level, whereas, the Farmville High Bridge has the trains running on the top deck.

Latter day timber truss bridges were assembled by the use of iron bolts, nuts, and incorporated iron and tension rods, but these Haupt trusses were held together with “tree nails.”  That is a carryover from shipbuilding where a wooden peg is driven into a hole, and then the peg is expanded by splitting its end and driving a wooden wedge into it.

One problem with this intricate timber construction was its many water-trapping joints, and the rot they encouraged.  Even with treated lumber, which was not available then, this type of timber construction exposed to the weather would have been a challenge.  Some wood is naturally rot resistant, but may not have other characteristics that are desirable.  This problem led to the development of the covered bridge, which basically placed the structure in a protective building with a roof.

Indeed, Haupt was also a designer of covered bridges.  But ultimately, timber bridge construction was replaced by iron and steel.  It seems that only the basic pile trestle has survived into the modern age as a practical timber railroad bridge.  Or generally at least, the trend seems to have moved away from the intricacy and numerous joints of the latticework, and toward heavier timber and piling construction with fewer joints.

I have posted this photo before in a discussion of railroad bridges.  This is the first bridge over the Minnesota River at Carver, MN on the Minneapolis & St. Louis RR, built in 1871: 

Here is the Bull Run Bridge with a single Haupt truss for comparison to the M&StL bridge at Carver:

It seems to me that the principles of the Haupt truss are identical to those of the arched truss on the M&StL bridge.  For all I know, Herman Haupt might be responsible for this M&StL bridge design.  There was another M&StL bridge at Excelsior, MN that was identical to this Carver Bridge.  I have seen a couple others on other railroads, so it must have been a somewhat popular design.  It is hard to say where this design originated. 

Historical research may dead end before it gets back to the origin of the Carver truss.  However, the Haupt designs of just a few years earlier may offer a clue.  They seem to portend something like this Carver Bridge—basically seeming to be a refinement of the Haupt arch truss.  The refinement was that part of the original Haupt truss design could be cut away and removed because it was not essential to the overall structure.  

Like the Bull Run Bridge, this Carver Bridge has arches made as plank laminations with their form being reinforced with timber latticework that makes up the sides of the truss.  So, with both bridges, on each side, the latticework acts as a structural web to the arch, which is the primary load-carrying member. 

However, the Carver Bridge omits the latticework structure continuing upward from the top of the arch, whereas the Bull Run Bridge extends the latticework on up to a horizontal top plane, and then placing cross timbers in that plane, and thus developing a full box truss. 

Also, the Carver Bridge has the arches set higher, so their ends terminate perhaps six feet above the bottom chord of the truss, whereas, the Bull Run Bridge has the ends terminating right at the bottom chord.  So the Carver Bridge is made as if you took the Bull Run Bridge, raised its arch up, and chopped off the excess latticework above the arch.

Both bridges have cross-timbers at their tops, although the Carver Bridge has them right on top of the arch, whereas the Bull Run Bridge has them as the top of the box truss.  It is difficult to tell from the photo, but it might be that the cross timbers on top of the arches of the Carver Bridge are set in an “X” pattern similar to side latticework of the smaller box truss in the foreground.

Interestingly, the arches of the Carver Bridge have more plank plies toward their centers.  You can see those arches getting thicker in height as they rise.  This must have looked like one mighty impressive piece of carpentry when it was new.  But its life was short, as it was replaced with a new iron bridge about ten years later.  And that bridge was replaced by today’s bridge in 1919. 

I would guess that before these early timber bridges gave way to newer replacements, they evolved from the use of wooden pegs as fasteners, into the use of bolts, nuts, and washers made of iron and steel.  I am guessing that this Carver Bridge in the photo is being built with a considerable quantity of bolts, nuts, and washers.  Unlike wooden pegs, the use of bolts with nuts and washers would have worked well to draw those plank laminations of the arch together.