## 1 Introduction
### The History of the Inca Trail and the Bridges in the Territory of Peru
The Incas designed them from an architectural and engineering point of view and then built three types of bridge systems according to the shape of the structure: (i) beam type bridges, where the riverbed was not very wide and where mainly tree trunks were used as beams (Kaspichaka: Kaspi is wooden sticks and Chaka is bridge in the Quechua language), (ii) stone bridges, where on both banks of the river, they had rocks and a flat stone tile as support (Rumichaka: Rumi is stone and Chaka is bridge in the Quechualanguage); the Incas did not use arches, whereas semicircular arches with stones existed in Europe, but in Europe and in Spain at that time, there were no suspension bridges such as there were in South America, (iii) huaro-type bridges consisting of a cable and basket, (iv) floating bridges, consisting of totora rafts, Coya-Ichu ropes and a wooden log board, a bridge currently known as pontoon type, and (v) suspension-type bridges, where the Incas used to build bridges where the riverbeds were very wide, and ropes made from an Andean plant called Coya-Ichu and other plant fibers were used [13]. The Spanish conquistadors admired the variety of bridges along the Qhapaq Nan. Europeans had never seen a suspension bridge and were surprised to find hundreds of them along the Inca Trail system. The Q'eswachaka (Q'eswa is rope and Chaka is bridge in Quechua language), the last Inca grass (Coya-Ichu) suspension bridge, spans a high ravine above the Apurimac River in Peru and has been in continuous use for 500 years [13]. The bridge is 30 m in span length and is suspended 15 m above the river. It is made of grass fibers and other organic materials. Each year, 1000 villagers from four neighboring Quechua communities gather, twist, and hand-braid native grass Coya-Ichu to make 16 km of rope and then work continuously for four days to rebuild the bridge [13]. Asking permission from the Apus (saccred mountains in the Quechua language) and making offerings to [PERSON] ([PERSON] in the Quechua language) are important activities that take place in conjunction with the reconstruction of the bridge. The community also holds a festival to celebrate the new bridge [8]. The Q'eswachaka is a key component of the Qhapaq Nan and a masterpiece of Inca engineering; its main features are presented in Table 1. It also serves as an important example of sustainable engineering, a living expression of knowledge, practices, and traditions that have been passed down through generations of the Quechua people in the Andes [14] (see Figure 2). The Q'eswachaka bridge is built from strong, locally harvested, and fully biodegradable materials, and it is a concept of sustainable architecture, engineering, and construction. The Incas used a variety of fibrous materials such as grass, cotton, and wool from llamas and other camelids. So, it was only natural that the Incas would find an engineering solution using abundant, locally grown grass fiber that could be woven into rope. Individual grass fibers can easily break and tear, but twisting and braiding them together results in a stronger, more flexible material. This is because the strength of the material increases with the more elements there are to share the load, or forces, acting on them [15]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Data** & **Unit** \\ \hline Bridge Typology & \multicolumn{2}{c}{Symmetrical suspension bridge with two pylons of} \\  & 2 m high stone masonry & - \\ Location & Quehue district, Canas province, Cusco region & - \\ River Name & Apurimac & - \\ Span Length & 28 & m \\ Bridge Width & 1.2 & m \\ Number of Lanes & One lane for pedestrian use or passage of high & - \\  & Andean camelids & - \\ Materials & Qeswa or braided rope based on plant fiber from & - \\  & weeds or straw or Coya-Ichu & - \\ Tensile Strength & 30 (dry condition)/35 (wet condition) & MPa \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main characteristics of the Q’eswachaka bridge.