Bridge Construction in the 19th Century
Research in Maya construction techniques indicates that technological knowledge generated during the Classic Period became a part of the cultural patrimony that has extended to the 21st century. In his landmark volumes Incidents of Travel in Central American, Chiapas, and Yucatán, published in 1841, John Lloyd Stephens describes his travels in the highlands of Guatemala and his encounters with a Maya suspension bridge spanning over a river:
Riding along the river, we reached a suspension bridge of most primitive appearance and construction called by the natives “la hammaca,” which had existed there from time immemorial. It was made of osiers twisted into cords, about three feet apart and stretching across the river with a hanging network of vines, the ends fastened to trunks of two opposite trees it hung about 25 feet above the river, which was here some eighty feet wide and was supported in different places by vines tied to the branches. The access was by a crude ladder to a platform in the crotch of the tree. In the bottom of the hammaca were 2 or 3 poles to walk on. It waved in the wind and was an unsteady and rather insecure means of transportation. From the center of the Vista of the river both ways under the arches of the trees was beautiful and in every direction the hammaca was a most picturesque-looking object.
The detailed observations by Stephens’s keen eye provided a clear overview of the structural mechanisms of a Maya suspension bridge. This type of lightweight structure was used for foot traffic over the river. He described the material and composition of the suspension rope cables and their anchorage attached to immense trees on opposite sides of the river. He describes the bridge walkway platform of laterally oriented poles for supporting foot traffic. He states that suspension ropes were made of osiers, which are willow branches, twisted into cords. Considering the height of the bridge above the ground, it might be difficult to identify the composition of the ropes. His referencing long willow branches braided into length is interesting. The willow native to Guatemala is the bonpland willow (salix bonplandiana). This variety of willow does not have the “weeping willow” or drooping willow branches that were used by the Inca in making rope. The ropes in the bridge described by Stephens were obviously made of henequen to develop a sufficient length for the 80-foot-long suspension cables. The geometry and composition of this basic suspension bridge follows the technical formula for suspension bridge structures developed in the Classic Period and used throughout the Yucatán for centuries, until the 20th century when steel and concrete bridges were substituted for the locally constructed suspension structures. Stephens describes the construction of the bridge as “primitive in appearance.” The bridge may have been primitive in geometry, but not in technology. The bridge owes its irregular geometry to the random geometry of the branches in the anchorage trees. Stephens’s use of the term suspension bridge is interesting; the first suspension bridge built by those of European descent was the Brooklyn Bridge, completed in 1883. This was 42 years after the volumes were published. When he observed the Maya rope bridge, the term suspension bridge was not yet in general use.
Visual evidence of Maya suspension bridge construction to cross rivers in the latter part of the 19th century is shown in Figure 10-1. The 1875 photo is by the famous British photographer Eadweard Muybridge. This is the same area of Guatemala where Stephens encountered the rope bridge described in his book. A close review of the photograph indicates that the bridge is apparently under construction or is undergoing retro-fitting for maintenance purposes. Note the coils of henequen rope laying on the river bank, at the bottom right-hand corner of the photo, in readiness for installation. The braceros are still posed in position during their repair operations in order for the ancient camera to capture that magic moment in time. The suspension rope cables are being attached to substantial trees that will serve as anchorage for the bridge suspension ropes. The tree anchors will resist lateral and vertical forces in the suspension rope system. It appears the upper set of cables on the nearside have been secured to the anchorage, while the far side cables have not been placed with a similar parabolic “sag” in the catenary of the near side cables, nor has it yet been anchored to the large tree to the right of the photo. The bridge is strong and technically correct, but is geometrically asymmetrical due to the irregularity of the randomly placed anchorage trees.
The diameter of the individual suspension rope appears to be approximately 1 inch, and the assembled cables are 2 inches in diameter, compared with the suspension cables on Inca spans that are the “size of a man’s body,” as described by de la Vega. The difference is that henequen rope used by the Maya bridge contractors has a tensile strength of 18,000 psi, whereas willow twigs are substantially weaker and would require a greater area of resistant material and increased diameter of cables. The suspension bridge described by John Stephens in 1841 and the 1875 photographs are obviously structures constructed by small political units for their own use and local access over the river. The bridges do not serve as roadways or support traffic from beasts of burden. The term osier described as the bridge cable material was probably a misnomer; the cables were surely fabricated of henequen. This high-strength rope was used for millennia by Maya for construction. The finished product may not be attractive, but its strength properties and geometry serve the purpose of crossing a natural obstacle and verify the levels of Maya bridge-building technology.
Figure 10-1: Photo of Maya hemp rope suspension bridge (1875). Photo courtesy of Carnegie Institution for Science.
One can be sure that numerous examples of these suspension bridges were constructed in the Yucatán during the Classic Period, and their use continued through the colonial period and into the current era. The design of the bridge includes efficient catenary geometry for foot traffic and can be made with local products by braceros (laborers). The use of henequen fibers was a logical application of high-strength rope invented by the Maya.
Bridge Technology During the Maya Classic Period
The golden age of construction peaked at the height of the Classic Period. Maya technical achievement grew with the expansion of cities, the construction of monumental buildings, the installation of infrastructure, and the extension of the sacbe system to connect the flow of communications and commerce between city-states. Maya engineers linked the whole system of intra-city and intercity communications with a technologically based road and bridge network. Bridges were constructed in a variety of short, medium, and long spans using technological, advanced structural systems, and materials and geometry that included cast-in-place concrete, strong tropical hardwood, and high-strength cable-rope suspension structures. Maya bridges completed the missing link in a diverse variety of intra-site and inter-site transportation systems, including bridges in the internal street systems of cities, bridges over flowing rivers, bridges crossing agricultural canals, bridges for sacbeob, and long-span bridges over broad rivers. The representative examples of Maya bridge construction are identified by the standard engineering designations based on the length of their spans. A short-span bridge is less than 3 meters long; a medium-span bridge is 3 meters to 24 meters, and a long-span bridge is one that is more than 24 meters.
Maya Short-Span Bridges
Explorations of Maya cities have uncovered a large number of short-span bridge structures that constituted an integral part of the internal pedestrian flow and entrance routes to the cities. These short-span structures, many of them parts of a monumental structure, are frequently overlooked by archaeologists. They are structural bridges even though they are considered to be part of the architecture. Bridge structures in numerous Maya cities have been investigated by the author. The structures in the city and the entrance ways to the city offered a diverse variety of bridge structures.
Stairway Bridge Spanning Walkway at Hormiguero
The ancient city of Hormiguero is located in the present-day Mexican state of Campeche. The city flourished during its peak period of AD 650–850. Only a minor part of the 84 buildings at the
site have been consolidated. Structure 5 is part of the central group, which is a complex of large temples of the Rio Bec style of architecture. Structure 5 is a towering pyramid with an artistically dramatic facade at its top-level temple. A wide stairway dominates its main facade. Maya engineers enhanced pedestrian pathways of the cities by spanning the broad staircase over a ground-level walkway. They developed a passageway, under the broad staircase, using a Maya arch to span the gap in the supports.
Palace Stairway Bridges at Edzná
The ancient city of Edzná is located in the east of the state of Campeche. This dramatic city with monumental architecture has an extensive network of engineered canals and raised agricultural fields. The acropolis looms high over the site. The tallest and most dramatic structure on the acropolis is the multi-level palace (Figure C-8). The structure rises to a height of five levels, and each level features a network of vaulted rooms. The front of the main facade has a wide stairway that ascends from ground level to the fifth level. The stair has access landings for each of the five levels. The broad monumental stair would bisect the activity on each level without a technological solution for bridging over that level. The second and third levels feature Maya bridging structures for the stairs. The second level uses a flat slab concrete structure to span across the open area way (Figure C-10), and the third level uses a Maya vault to create the bridge spanning mechanism for the stair (Figure C-9).
The Bridge Over the Otulum River at Palenque
Palenque is one of the star attractions in the galaxy of Maya architecture. The city has a surfeit of natural springs, and features aqueduct tunnels built by Maya engineers and formed by the Maya arch. The tunnels form conduits for the springs that flow into the Otulum River to the south of the city center. The sacbe crossing over the river has a Maya bridge structure that spans over the river. The bridge is constructed of cast-in-place concrete and cut-stone masonry, and applies the structural geometry of the Maya arch. This bridge has been in place since the Classic Period.
Medium-Span Bridges
The medium-span Maya bridge structures were constructed of a variety of engineered materials, including cast-in-place concrete and high-strength timber. The medium span structures spanned small rivers and canals.
Bridge Over Defensive Moat at Becán
The city of Becán is a unique Maya city. The city is surrounded by a water-filled moat. The moat has five entrance portals spanning over the waterway and entering the city walls. At each portal a causeway/bridge structure crosses the moat (Figure 10-2). The bridge does not have openings in its superstructure, because it also serves as a check dam to control water management in a sector in the moat. The moat also serves as a dry-season water reservoir for the city. The causeways control access into the city for defense while controlling the water quantities in each sector of the moat. This dual-purpose structure has been in place since the Classic Period.
Figure 10-2: Bridge and causeway over moat around Becán. Author’s image.
Timber Bridge Structure Over Pusilha River
The Maya city of Pusilha is located in Belize on the river of the same name. The location of the city places the city-state on either side along the river. The river bifurcates the city as it flows eastward between the acropolis and the main part of the city. The acropolis is sited on a steep hill to the south of the river and rises to a height of 79 meters above the river. The north portion of the city is located on level plane. The width of the river at the bridge was reduced to 10 meters by large cast-in-place concrete abutments requiring a span of approximately 10 meters in length. If the concrete bridge abutments had not been built in a manner that reduced the span, the bridge would have required a span longer than 24 meters. The greater span would have placed the beam in the long-span category. A much longer and deeper timber beam would be required. Maya engineering creativity was able to design a shorter-span bridge, which greatly reduced the construction issues, material acquisition, and logistics.
Investigation of the site indicated that the bridge abutements (Figure 10-3) were constructed of composite, cast-in-place concrete and cut stone in the same configuration as large Maya structures. The forensic engineering analysis of the bridge indicated that configuration of the bridge superstructure and buttresses were shown as on the section. A high-strength timber beam of 1 meter in depth and 10 meters in length was required to span the river. The walking surface of the bridge was composed of small-diameter timber members spanning in a traverse direction over the main beams. The surface of the deck was paved with concrete over the timber structure. The advantage of this structural system is that the cast-in-place concrete and high-strength timber beams form a composite structure, increasing the strength. The structural system provides the required strength and the concrete surface waterproofs the timber and resists degradation. The bridge was necessary to connect the bifurcated sections of the city. The spans were elevated some 7 meters above the low water line. This height maintained the spans above the high water level during flood season.
Stairs were used to access the bridge deck level. Other methods of gaining access up to the span elevation would impede the flow of water during the rainy season when the river rose to flood levels. South of the bridge, the terrain rose 79 meters to the level of the acropolis. However, the north buttress was on a more level terrain, and the water could overflow the flat terrain and continue downstream. However if a 10-percent ramp was constructed from the top of the south buttress to a level point on the flat terrain, the length of the ramp would be approximately 61 meters. This length of ramp would result in a virtual dam that would impound the flood water and force the water into the north city-scape. Therefore, the only logical solution was the use of stairways at each end of the bridge that allowed access to the deck and minimized the profile of the bridge, thus reducing the chance of floods while permitting river crossings during the flood season.
Figure 10-3: Drawing of section of timber span bridge at Pusilha. Author’s image.
Maya Long-Span Bridges
Maya engineers used standard technology and the necessary high-strength materials to construct short- and medium-span bridges. However, bridges more than 24 meters in length are considered long spans and require a higher degree of technology. Medium-span bridges used heavy structural members that support loads in bending, but long-span bridges require structures of lightweight tension members. Long-span structures were required to utilize higher strength materials and a structural system applying parabolic geometry acting in pure tension. It is certain that 19th-century Maya knowledge of the methods of construction of suspension bridges was in the folk memory and extrapolated the techniques of building suspension structures developed during the Classic Period. No traces remain of these 19th-century, cable suspension bridges spanning small rivers. These structures have completed degraded. In a similar manner, the degradable rope and timber materials of Classic Period long-span suspension bridges have totally disintegrated, while their tall concrete and stone support towers had been reduced to ruins. The ingenious Maya engineers of the Classic Period knew the secrets of constructing multi-span rope cable suspension bridges across rivers. These bridges were supported on concrete and stone towers high above the river flood waters. The most significant of the great long-span suspension bridges was constructed across the mighty Usumacinta River at the ancient city of Yaxchilan. This bridge was the lifeline for the city and assured a year-round method of traversing the broad river. The accounts of the construction of the bridge and procedures used in the computer-based virtual reconstruction of this marvel of Maya technology are a prime example of the creativity of archaeo-engineering.
Maya Engineers Construct a Lifeline for a Grand City
The ancient Maya city of Yaxchilan is situated within an omega-shaped oxbow formed by the powerful Usumacinta River. The bend in the river is so severe that only a narrow strip of land stands between the two banks of the wide river at the narrow neck (Figure 10-4). For six months a year, the rive
r is in a wildly surging flood stage, creating a 200-meter-wide turbulent barrier around the city. The broad, swirling river embraces the oxbow and converts this magnificent city into an isolated “island” with its perimeter almost entirely bounded by water. Archaeologists have studied this unique city since 1882. However, during that period of study, no one had posed or answered the obvious question: How did this prosperous city survive the six-month cycle of flooding and isolation without the ability to cross the broad river while maintaining their political power and acquiring the necessary sustenance for survival?
The answer was not in the use of cayucos (dugout canoes), for crossing the swirling water of the broad and swiftly flowing river would seize the small craft as it attempted to traverse the river, carry it downstream, and sweep it around the bend. A great distance would be traveled with the cayuco trapped in the swift centrifugal flow of the outside curve of the oxbow. After furious paddling during the crossing, landing the cayuco on the opposite bank would be as perilous as traversing the river—that is, if the cayuco managed to avoid a collision with the giant trees and other debris being swept along the curve. The answer to a safe crossing of the river had to be a Maya engineering solution.
The Lost Secrets of Maya Technology Page 24
