Abutment of dry laid stone in bamboo gabions
Elevation Section A-A
Section B-B
Transvese bracing to the arch barrel
Plan
Wu Zhi Qiao - Mixia China
WZQ-CD-02
Self-supported Inter-locked Bamboo Arch Footbridge The project requires the design of a river crossing footbridge for a rural mountain area in southwest of China, that is robust, safe, and built out of a cheap and sustainable material.
2. Compression strength perpendicular-to-grain is very low as it induces tension perpendicular-to-grain failures. As this failure involves a large transverse deformation of the circular section, this can be improved by using bundled bamboo elements and a tight transverse arrangement. This will restrict individual bamboo elements' transverse deformation and therefore increase overall compression strength in the perpendicular-to-grain direction.
This proposed scheme is a single span inter-locked bamboo bow arch footbridge. The span of the arch is 20m with a rise of 2.3m. The deck is 2m wide. The arch springing will be set 0.5m above the flood level in wet season.
3. Difficulties in connection: Bamboo elements are lashed with bamboo rope or nylon rope, if necessary the grout-filled-internode technique can be employed in critical joints.
The traditional Chinese arch bridge, bow arch with a curved deck, has been selected, with simple two rail parapets, to create a simple, unpolished look of a countryside bridge. The shape of bridge selected can easily fit in a channel with steep banks. This not only improves the loading condition in the bamboo elements, but also provides better connections to the approaching road on the steep cliff river banks at both ends.
4. The weakness of durability of bamboo material can be overcome using preservative treatment in a similar way to weak timber, together with careful detailing in design.
The simple two rail parapet and the deck edge bamboo create a clean, smooth flowing curve which complements the bow arch and creates a dynamic effect that gives the bridge a pleasing contrast to the heavy rocks of the mountain surroundings.
Construction Method
The bridge together with the pleasing natural appearance of bamboo blends harmoniously with its surroundings.
Stage 1. Build abutments on either side of river bank. Woven strips of bamboo are used to hold dry laid stones in place and to provided a strong abutment that will prevent the bridge from sinking and longitudinal movement at the ends.
The following method assumes that the bridge construction will be carried out during a dry season.
Figure 1 Parapet detail and deck walk surface details.
Bamboo is fast growing sustainable material and probably is the cheapest construction material available locally. It has been traditionally used by local people to build houses. This makes it a favourite material for this rural river crossing footbridge.
Details of Structure A self supported bamboo interlock structure system is used to form the main arch rib and triangular bracing within the arch rib is provided to stabilize the structure (see Figure 2). A triangularly arranged bamboo chord along each side of the arch at each end of the bridge is used to provided an additional external transverse bracing to the the arch rib.
Figure 2. Inter-locked self-supported arch bridge.
Stage 2. The arch rib is constructed in two halves. Each half is built from either side of the river bank (see Figure 3 a). On the side of the river bank with a wider dry river bed, one half of arch rib skeleton will be erected with a temporary gantry support, while on the other side, the arch skeleton will be prefabricated in a vertical open arch position with a temporary cable support system. Stage 3. After both halves of arch skeleton are fabricated the half held by cable is lowered in to position to join to the other half. The central part of arch rib and the triangular bracing legs on each side of arch on both ends of bridge (Figure 3 b) are then installed. A complete arch rib skeleton is formed and the elements are interlocked. In order to make the arch have a symmetric deformation the vertical restrictions from the cable and gantry are removed and the arch skeleton stands on its own. Weave through the rest of arch rib elements, starting from the crown towards both ends and complete the arch rib. This will load the arch gradually and reduce the possible large movement and load built-up at the element joints from one-off loading the structure at the end of erection.
(a)
(b)
(c)
Stage 4. Install the triangular bracing members in the arch rib.
If the level of approaching road permits, a steep arch (low span to rise ratio) is desirable as this can improve the loading condition in the arch bamboo elements and make best use of their advanced mechanical properties and avoid their weaknesses. The bamboo along the deck edge not only provides a clear and smooth curved appearance, but also it is used to lock the deck walking surface (see Figure 1) and support the parapet posts.
Stage 5. Build the deck longitudinal members starting from the crown of the arch barrel toward both ends of bridge (Figure 3 c). Stage 6. Lay up the transverse deck walk surface bamboo and lock them with the edge bamboos at each side. Stage 7. Install the parapets and complete the bridge.
It is desired that the bamboo elements are lashed with bamboo rope or nylon rope in the traditional way as it will be easy for local people to maintain the structure. If necessary, the key connection will use the grout-filled-internode technique.
Figure 3 Construction sequence.
Stage 8. Remove all temporary cable and gantry supports and clean the river channel.
It is proposed that the abutment is built out of bamboo with woven-bamboo-strip-retained-dry-laid-stones. However if the steep river banks are formed by solid, stable rock, it is preferable to cut a pocket into the rock to form the abutment for the arch.
Technical Issues and Considerations Bamboo has unique mechanical properties. It has high strength-to-weight ratio, high bending strength, but a low shear capacity. It is strong in compression and tension in the parallel-to grain direction but weak in compression in the perpendicular-to-grain direction. It also has low modulus of elasticity. It needs to be carefully considered in the structural design. Some major technical issues and problems in use of this material are discussed below: 1. Compression and tension strength of bamboo parallel-to-grain are quite high and shear strength is relatively low. To make best use of these properties in the proposed structure, it is suggested that a higher rise of arch is used if possible to improve loading condition in the arch rib elements.
Wu Zhi Qiao - Mixia China
WZQ-CD-01
Wu Zhi Qiao – Mixia China 2010