Here is a diagram that shows a few truss designs

Types of Supporting Trusses

According to information provided in the World Guide, the 880 surviving covered bridges are supported by 18 distinct truss configurations, and multiple variations thereof.The Burr arch is by far the most popular, with 224 surviving examples. The Howe truss is used in 143 bridges, followed by 135 Town lattice bridges, 101 queenpost truss bridges, and 95 multiple kingpost truss bridges.

Table 2. Truss configurations-numbers and lengths*

Truss Configuration	Number	Title	Single Span Length ("L") Without Variations
			Shortest		Longest		Ranking
			feet	meters	feet	meters
Brown	2		100	30.5	116	35.4
Burr	224	and variations	32	9.8	222	67.7	1st in L
Childs	7		50	15.2	104	31.7
Haupt	2	and variations	85	25.9	134	40.8
Howe	143		20	6.1	200	61.0	2nd in L
Inverted bowstring	2		75	22.9	100	30.5
King	30		22	6.7	70	21.3
Long	27		51	15.5	170	51.8	5th in L
Multiple king	95	and variations	36	11.0	124	37.8
Paddleford	10		35	10.7	120	36.6
Paddleford arch	11		78	23.8	183	55.8	3rd in L
Partridge	6		64	19.5	154	46.9
Post	1		165	50.3		0.0	6th in L
Pratt	8	and variations	36	11.0	96	29.3
Queen	101	and variations	25	7.6	130	39.6
Smith	20		59	18.0	172	52.4	4th in L
Tied arch	4		37	11.3	136	41.5
Town	135		25	7.6	162	49.4	7th in L
Town and supplemental	14		80	24.4	129	39.3
Warren	6	and variations	30	9.1	87	26.5
Double multiple	4	and variations	63	19.2	111	33.8
Double town	4		98	29.9	122	37.2
Double warren	1		116	35.4		0.0
Other / unknown	23
	880

*From the World Guide to Covered Bridges, published by the National Society for the Preservation of Covered Bridges, Inc., 1989 edition.

Reasons for Using Covered Bridges vs. Other Alternatives

As stated earlier, the reason these timber structures were covered was simply to help protect the timber from the ravages associated with periodic wetting. Those involved with early truss structures have stated that timber truss structures without coverings would often fail after 10-20 years of service. Coverings quickly proved their worth by greatly extending the life of the structure-so much so that the use of timber structures without coverings was only for a brief period of time.

The development of the timber truss allowed these bridges to span greater distances than those with beam-only structures. They were also able to surpass the spanning capability of arch structures, whether of stone, masonry, or timber.

The development of processes that produced wrought iron and cast iron in larger capacities during the mid-1800s soon led to truss types made of progressively more metal. Timber trusses quickly lost their popularity in the late-1800s and were used less and less after the early 1900s, except in those areas of plentiful large timber

Patents and Covered Bridges

The United States established its first patent office in 1790. Tragically, for the purposes of historical research, a fire destroyed this office in 1836 with the loss of all patent records to that date. Efforts were made to restore as many of the patents as possible, yet many remain lost forever. Hence, any definitive statements of fact regarding the earliest patents related to the developments of timber trusses and covered bridges are suspect. Not surprisingly, some historians have made heroic efforts to compile as many of the lost pieces as possible. Richard Sanders Allen deserves special recognition for his compendium of "Thirty-Two Lost Bridge Patents." As his title suggests, even just the recovered patent variations alone are too numerous to fully describe in this manual. In an ongoing effort to focus on the surviving authentic examples of North American covered bridges, the following discussion includes only the more prominent developments.

Early North American bridge builders actively pursued patents for their designs in an attempt to gain more bridge construction contracts. A few of the very first patents involved general bridge construction, but by 1797, there were several that involved specific schemes for timber arches. Among others, Timothy Palmer received a patent that year, the details of which remain unknown, but he began construction of his Permanent Bridge only a few years after this, his initial patent.

Theodore Burr obtained the first of his many patents in 1804 or 1806, (again, according to the source), which regrettably remains among the unrecovered records. His second patent was issued in 1817. Burr's trademark design dates from this patent. He extended curved lower ribs that had reached only bottom chords, up along the trusses, all the way to the top chord. This superposition of arch and truss forms seems to have been influenced by earlier bridges built in Switzerland. The resulting structure has been described as a combination of conventional trusses (parallel chords with compression diagonals) and supplemental arches. One of Burr's early examples of this bridge form, and probably the basis for his 1817 patent, was his Union Bridge crossing of the Hudson River between Lansingburgh and Waterford, NY, circa 1804. This was a significant structure; 244 m (800 ft) long, with four spans. The structure was rebuilt after being in service for some time, to include a roof and siding. This heavily braced and counterbraced structure exemplified what today is called a Burr arch.

Lewis Wernwag was born in Germany in 1769 and obtained a patent (which is also lost) in 1812. The patent most likely described a structure similar to his crossing of the Schuylkill River at Philadelphia, PA's Upper Ferry. The huge 104-m (340-ft) trussed arch span was quickly termed the "Colossus" and represented a major triumph in bridge construction, with its attractive and apparently efficient use of timber, supplemented with iron rod bracing members. Wernwag owned a metal works company and relied more on early forms of metal connections and components rather than on traditional timber joinery only. He received a second patent in 1829 for improvements in his structure. Regrettably, the bridge was lost to fire in 1838.

Ithiel Town (1784--1844) of New Haven, CT, was a prominent architect known for designing many types of buildings. He also planned many bridges, initially experimenting with various truss arch combinations. However, Town wanted to devise a structure that would require fewer carpentry skills than was required by the intricate joinery details of some of the early bridges. Using only planks joined with round wooden pegs, he began developing a lattice style of truss construction and obtained his first patent in 1820. He was nearly as good a promoter as an inventor, and the lattice truss became very popular, although it has been criticized for its apparent waste of material. This truss layout proved to be very adaptable. It could include heavier members for longer spans, and could even be doubled up to include two layers of web members and three layers of chords for heavy loads, such as those generated by the railroads. A few of his bridges were built with such heavy members that they became identified as a timber lattice, as compared with the more common plank lattice. The most famous of the surviving timber lattices is found in the Windsor, VT-Cornish, NH, covered bridge over the Connecticut River, which remains one of the longest two-span covered bridges in the United States.

Stephen Long (1784--1864) had a varied background and career. He gained his experience as a timber bridge builder while serving in the U.S. Army. Long was commissioned to locate, plan, and build the Baltimore and Ohio Railroad. He chose to use a standardized truss for all his spans, with timber counterbraces in all the panels. With the addition of timber wedges at the bearing joints between the posts and diagonals, he found that he had better control over the trusses' as-built geometry. He obtained his first bridge patent in 1830. Subsequent printed materials pronounced that these wedges allowed the truss builders to induce member forces in the trusses that effectively prestressed the structure, to employ today's terminology.

William Howe (1803--1852) made a major contribution to the evolution of timber covered bridges by being the first to use metal components as primary members within an otherwise timber truss. He used parallel timber chords, with timber diagonals and counters in the panels, but he used round iron rods for the vertical tension members. The threaded rod ends allowed easy adjustment of the structure, to keep it tight both during and after erection. Many modifications were made over the years to Howe's original design to address various desired details, but his truss was quickly adopted to withstand the heavy loads on railroads. The popularity of the Howe truss continues today. It is often selected when constructing new covered bridges. Howe's modification was a major reason for the short life and reduced popularity of Stephen Long's truss-which was essentially the same, but without the iron rod verticals.

Types of Longitudinal Trusses

The truss types described here are presented in the order of their span length, starting with the shortest. The first three truss types (kingpost, queenpost, and multiple kingpost) are ones used in the earliest North American covered bridges. No patents were ever taken on their configurations, and no individual is specifically credited with their development. The other truss types that follow were developed and ultimately named after enterprising early builders/engineers (usually in recognition of a patent obtained for the details of the truss).

Kingpost

The most elementary heavy timber truss configuration is the kingpost (see figure 25). The inclined members of a kingpost truss serve both as the top chord and as the main diagonals, and resist compression forces. The horizontal member, along the bottom of the truss, is the bottom chord and acts in tension. A central vertical member (the kingpost), also acts in tension to support the floor loads and serves as the connecting element between the opposing main diagonals. The kingpost truss configuration has two panels. A panel is that portion of the truss that lies between any two vertical components.

Figure 25. Diagram of kingpost truss.

In addition to resisting the tensile forces generated by the opposing diagonals, the bottom chord almost always supports the floor beams. In most kingpost truss bridges, the floor beams are located only at the ends of the bridge and next to the center kingpost. The floor beam point loading does not coincide with the intersections of the theoretical centerlines of the truss members. This connection eccentricity induces bending stresses in the bottom chord that may be large or negligible, depending on the distance of the floor beams from the joints and the depth of the bottom chord.

The dead and live loads are applied differently to kingpost trusses. Live traffic loads are carried to the truss through the central floor beam, while much of the bridge dead load is carried in the rafter plate, along the eaves of the roof. As a result, almost half of the bridge weight is carried to the end posts of the bridge, which transfer their loads directly to the foundation. The kingpost truss carries the centerline floor beam(s) and the inner ends of the four eave plates. Technically, the end posts and the eave struts are not structural members of the kingpost trusses, and their connections are not intended to transfer axial loads within the truss; they are simply members of the associated framework.

The inclination angle for the kingpost diagonals is restricted. Generally, steeper diagonals are more efficient at resisting shear forces in a truss. There are, however, compromises to consider when laying out the members in any truss. For instance, given a set span for a two-panel kingpost truss, steeper diagonals make taller trusses. Beyond the aesthetic issues of building unusually tall, but short-span structures, there are practical limits to the height of the bridge involving bracing and its connections. Hence, the span limit for this simplest truss is quite short, typically only about 7.6 to 9.1 m (25 to 30 ft).

Longer kingpost trusses have been built by including subdiagonals. These members act as braces, from the bottom of the kingpost up to the midpoint of the main diagonals, thereby producing a minitruss within the larger kingpost truss. Short struts often extend above this junction to support the load from the roof eave plate. Vertical metal rod hangers may also be used from the intersection of these subdiagonals downward to the bottom chord, allowing installation of floor beams at this quarter point of the bridge. These modifications allowed builders to increase kingpost spans out to about 10.7 to 12.2 m (35 to 40 ft).

Figure 26. Diagram of kingpost truss with subdiagonals.

Most kingpost trusses were built with single member components, usually large sawn or hand-hewn timbers. The most critical connection in kingpost trusses is the heel connection of the main diagonals to the bottom chord. These connections are prone to several weaknesses discussed in more detail later.

The kingpost truss is not very common in the extant United States covered bridge population. There are only about 30 kingpost covered bridges¹ remaining in the United States, with spans ranging from 6.7 to 21.3 m (22 to 70 ft).^[1] It is very unusual for a kingpost bridge to span 6.7 m (70 ft)-approximately 15.2 m (50 ft) would be the more common upper limit. The extant kingpost bridges were built between 1870 and 1976. ^[1]

Queenpost

The next range in span lengths commonly includes trusses developed from a simple modification of the kingpost. The queenpost truss is, conceptually, simply a stretched-out version of the kingpost truss, accomplished by adding a central panel with extra horizontal top and bottom chords (see figure 27). Classic examples of queenpost trusses do not have any diagonal web members in the central rectangular panel. Therefore, the most simple queenpost trusses are not true trusses at all,; but rather frames (although this distinction is not relevant to this discussion). The vertical members are termed queenposts. These trusses are considered to have three panels.

Figure 27. Diagram of queenpost truss.

The member forces and behavior in queenpost trusses are very similar to those found in kingpost trusses: therefore, the design considerations for these two basic truss styles are equally similar. A number of similarities exist between kingpost and queenpost trusses:

• Truss components are usually of single members.
• The key area of interest is the heel connection.
• Some of the longer spans use subdivided panels, with subdiagonals, hanger rods, and extra floor beams.

The span lengths of queenpost truss bridges range from about 12.2 to 18.3 m (40 to 60 ft), although there are a few examples that are longer. The longer span requires that many of their bottom chords be spliced longitudinally from separate timbers. This tensile connection is another area of weakness in the truss and is discussed in more depth later.

There are approximately 101 bridges supported by queenpost trusses, or slightly more than 10 percent of all the surviving covered bridges in the United States. Their spans range from 7.6 to 39.6 m (25 to 130 ft), and they were built between 1845 and 1985.^[1]

Multiple Kingpost

A straightforward way to stretch the span capability of the queenpost truss is to add panels to the kingpost truss to create what is known as multiple kingpost trusses (see figure 28). Accordingly, the basic kingpost truss is sometimes referred to as a simple kingpost truss. (The image depicted in figure 28 demonstrates verticals that have been cut down to accept the diagonal; —some refer to these as gunstock verticals. The verticals depicted in figure 29 are, perhaps, more commonly notched to accept the diagonal.) Most of these trusses were built with an even number of panels so that all the diagonals are in compression and all the verticals are in tension under normal loading. Very few multiple kingpost trusses have an odd number of panels, with opposing (or crossing) diagonals in the center panel.

Figure 28. Diagram of multiple kingpost trusses.

There is a lack of tensile capacity of the connection of the diagonals to posts. In this instance, the compressive force in the diagonals under the influence of the dead load of the bridge is usually much larger than the tensile force resulting from the passage of vehicles. Hence, under normal circumstances, the diagonals remain in compression under all combinations of loading, and the tensile connection is unnecessary.

The longer spans of the multiple kingpost truss, without increasing truss depth significantly, generate higher member forces, which require more capacity. Multiple kingpost truss chords are often comprised of twin members that sandwich a central plane of single web (vertical and diagonal) members. The longer chord members also usually require splices that typically are staggered along the truss length. This critical detail is meant to ensure that, at any particular cross section along the bridge, there is at least one unspliced bottom chord (tension) member in each longitudinal truss; —more specifically, there should be 1 m (3.28 ft) separation between splices of adjacent members of the bottom chord.

The panels in multiple kingpost trusses are often quite short, which means that the transverse floor beams could be located abutting each vertical member. This minimal eccentricity between load application and truss joint location greatly reduces bending stresses in the bottom chord. In addition, these more closely spaced web members tend to have smaller member forces in the diagonals due to their geometry, so that the connection forces are somewhat smaller than those associated with kingpost or queenpost trusses.

Burr Arch

Theodore Burr obtained the first U.S. patent issued for a specific timber truss configuration in 1806. The Burr arch is, basically, a combination of a typical multiple kingpost truss with a superimposed arch (see figure 32). The arch was added to allow heavier loads on the bridges and to stretch their span capabilities to greater lengths. Surviving examples of Burr arch bridges have spans of up to 67.7 m (222 ft).^[1]

Burr's development was immediately popular with bridge builders and has proven durable. More existing North American covered bridges use the Burr arch than any other type. The classic, or conventional, Burr arch supports the ends of the arch components at the abutment, with no connection between the bottom chord and arch as they pass each other (the chord is supported by the abutment directly separated from the arch end). A variation of the Burr arch (sometimes referred to as a modified Burr arch) terminates (and ties) the arch with a connection directly to the bottom chord, which is supported on the abutments.

Figure 32. Diagram of conventional and modified Burr arch.

The actual arches of most Burr arches are in pairs; these sandwich a single multiple kingpost truss between them. The most common connection uses a single bolt to join the arches through each of the vertical members of the truss. This means that the load sharing between the truss and the arch components is largely dependent on the relative stiffnesses of those bolts. The floor beams carry the live loads to the truss bottom chords, and the roof loads bear on their top chords. For these vertical loads to be distributed into the arch, the bolts must resist significant vertical shear forces. The initial, traditional Burr arches used arch components sawn from large, single timbers that were lap-spliced to each other at the verticals. Later, use of continuous but laminated (multiple-layer) timber arches became popular with some builders.

In addition to the critical areas of interest cited above for the multiple kingpost truss that comprises the central portion of the Burr arch structure, special attention should be paid to the ends of the arches and the interconnections of the arch to the truss. Figure 33 shows the connection of timber arch with post using only a single bolt. This Burr arch happens to have a dual timber arch, —one above the other.

Figure 33. Connection of arch to post—Wehr Bridge, Lehigh County, PA

There are about 224 remaining bridges supported by the Burr arches and its multiple variations (about 25 percent of all covered bridges).^[1] The Burr arch has individual spans that range from 10.0 to 67.7 m (33 to 222 ft); this longest span is 10 percent longer than the next rival configuration of truss (the Howe). The extant Burr arches were built between the early 1800s and 1988.^[1]

Town Lattice

Ithiel Town, an architect by education, obtained his first patent for a unique type of timber truss in 1820 (see figure 34). All the other trusses mentioned above, and those that follow this subsection, principally rely on large and heavy timbers that require skilled artisans to properly craft the rather elaborate joinery between the various components. Town sought a means of constructing bridges that would rely on an easily adapted design and would require less skilled labor. His patented truss developed a configuration that could be extended to a wide range of span lengths with relatively little modification of the configuration. In the opinion of many informed bridge aficionados, his patented truss represents arguably the most important development in the history of covered bridges, and one that remains a popular and enduring style. Later portions of this manual will examine the merits of this truss configuration.

Town's lattice configuration relies on assembling relatively short and light planks that were available and easy to handle. He connected the overlapping intersection of members with round timber dowels or pegs, termed treenails—pronounced trunnels (and so spelled hereafter in this manual). The plank intersections in the web may have from one to three trunnels. Where chord members intersect with web or lattice members, the overlapping zone may contain as many as four trunnels. The dowels are often 38 to 51 millimeters (mm) (1.5 to 2 inches) in diameter. The parallel and closely spaced web members are joined to chords along both the top and bottom of the trusses. Two levels of chords commonly are used as the bottom chords. The top chords may have one or two levels of members. The lowest bottom chord provides the seat for the transverse floor beams.

Figure 34. Diagram of Town lattice truss.

Town, or lattice, trusses are most commonly comprised of thin members with pairs of chords on each side of the lattice webs. In this case, the truss is sometimes termed a plank lattice. The chord members generally are not spliced to abutting pieces at their ends, but the terminations are staggered so that any panel of chord has at least one unspliced member. A few Town lattice trusses were fabricated of heavier components using single chord members on each side of the lattice. In this case, the truss is termed a timber lattice. The chord members require splices at their ends.

There remain about 135 bridges supported by Town lattice trusses.^[1] Town lattice trusses support varying span lengths, from relatively short (only 7.6 m (25 ft)), up to some of the longest covered bridge spans in the world. Individual Town lattice trusses span up to 49.4 m (162 ft).^[1] The oldest surviving Town lattice bridge (the Halpin Bridge in Middlebury, VT) was purportedly built about 1824.^[1] New examples of Town lattice covered bridges are still being built.

Long Truss

Colonel Stephen H. Long first patented a truss configuration in 1830. His focus was on a parallel chord truss made with heavy timbers and with crossed diagonals in each panel (see figure 35). A special feature of his bridge included the use of timber wedges at the intersections of the chords, posts, and diagonals. The wedges allowed builders and maintainers to adjust the shape of the panels, and provided the opportunity to adjust the initial camber.

Figure 35. Diagram of Long truss.

In today's jargon, the wedges allowed builders to induce forced loads in the diagonals in a way that is described as pretensioning. It is extremely difficult to predict the amount of the induced prestressing force. Long's patent applications included images of wedges between the vertical and the chord (as shown in figure 36) and between the counter and the chords (as indicated in figure 35).

However, the wedges do increase the strength of the connection between the horizontal component of the load in the diagonal and the chord. The transfer of load without wedges flows from the end bearing on the diagonal to the cross grain bearing in the post, then from the cross grain bearing at the shoulder of the post back to the end grain bearing at the shoulder of the chord. Introducing the wedge distributes the bearing load from the chord over a much larger area of the post through the wedge in direct cross grain bearing.

Figures 36 and 37 clarify how Long wedges work. The image in figure 36 is from the outside of the bridge (siding and outside chord stick removed) looking back toward the inside of the bridge. The wedge on the right side normally is hidden from view by the floor beam. As the wedge is driven downward, the post is moved with respect to the chord along the shoulders cut in the chord stick. An important engineering aspect of the wedge is to distribute large edge stresses along the vertical face of the shoulder across a wider face of the post at the interface with the wedge.

Howe Truss

William Howe (1803--52) of Massachusetts was granted his first truss patent in 1840 and a second one later in the same year. His second patent used metal rods as the vertical members of what was otherwise a simple timber parallel-chord, cross-braced truss. This was the first truss patent granted with some major structural components made with metal. The configuration used easy-to-erect and readily prefabricated components that could be assembled on site and adjusted via threaded connections at the rod ends. Little skilled labor was involved in assembling and erecting this truss type, and it became an immediate success (see figure 38).

Figure 38. Diagram of Howe truss.

Another factor in the success of Howe's truss type was his inclusion of a detailed structural analysis with the patent application. Up to this time, the selection of member sizes, materials, and overall geometry, was generally left to the judgment of the individual bridge builder. The fledgling structural engineering profession was developing rules and relationships to govern such matters, but no consensus had been attained at the time of Howe's patent.

The initial Howe truss bridges had wooden blocks cut to fit at the connections at the ends of the diagonal members against the chords. Later versions converted to the use of cast iron angle blocks. These blocks were simple to construct and install, and they were a major factor in the popularity of this configuration.

The Howe truss is second only to the Burr arch in popularity of extant covered bridges in the United States. There are about 143 bridges supported by the Howe truss, or about 15 percent of all covered bridges.^[1] The Howe truss has individual spans that range from an unusually short 6.1 m (20 ft) up to an impressive 61.0 m (200 ft), the longest being only 10 percent shorter than the longest Burr arch.^[1] The oldest extant Howe truss was built in 1854, and the configuration remains popular with new authentic examples built today.^[1]

Other

The preceding seven truss configurations support the vast majority of covered bridges. There are many other truss configurations, however, that were patented with a few representative examples still standing, including those identified as Smith, Paddleford, Pratt, Childs, and Partridge trusses. Each of these trusses contains some technical nuance to differentiate it from others, but the basics of their behavior follows those described above.

The Pratt truss deserves special note because it was the precursor of the very popular metal truss of this configuration. In the initial form, Pratt used metal rods for the diagonal tensile elements and timber in the compression posts, taking advantage of the respective strengths of those materials. Very few Pratt timber truss bridges remain, in large part due to the difficult connection of the diagonals to posts, but a very large number of Pratt metal trusses survive, in which the connections with metal were simplified.

While very few exist, the Paddleford trusses (see figure 39) are remarkable in that the assembly of interconnected timbers requires exceptional skill for a proper fit. These structures behave more like frames than trusses, involving shoulder bearing at the frame connections with much of the resistance due to shear and bending stresses in the elements, in addition to the axial forces. The analysis of these structures is especially complex and challenging.

Figure 39. Diagram of Paddleford truss.

There are also a number of covered bridges supported by tied arches (technically not trusses at all). The tied arches are labeled as such due to a horizontal tension element that connects the ends of the arches. The roof and siding are supported by rafter plates and columns above the arches. Rods suspend the floor from the arches.