Forms of bamboo scaffolding.

Bamboo scaffolding is used in its various forms according to the utility. Double-layered bamboo scaffolds are most commonly used in new construction and also in major renovation. External works such as rendering, painting, wall tiles and plumbing are carried out from a continuous working platform laid between the inner and outer scaffolds. The inner layer is erected at about 200 mm from the building edge, and the outer layer at about 600 mm from the inner layer. There are many variations to form, articulation, types of bamboos and spacing based on specific site requirements and the craftsmen’s training and preference.

For the outer layer, large-diameter (75 mm) mao jue poles are used as the main vertical posts spaced at about 1 — 3 m centres. They rest on firm ground at ground level and on steel brackets securely fixed to the structural members of the building at higher levels. Buildings Department guidelines require these steel brackets to be provided at 15 m vertical intervals or every fifth floor. Each post should rest on one steel bracket, and the horizontal spacing between the brackets should not exceed 1-3 m. Smaller 40 mm diameter kao jue poles are fixed as standards (uprights) between each pair of mao jue posts.

The posts and standards are connected by mao jue ledgers (horizontals), with a vertical spacing of about 600—750 mm. The scaffolders sit and put their leg over the ledgers and fix the tier above it. The upright standards and posts are also lapped by them sitting in this position. The 600-750 mm gap spacing is just convenient working distance which is the height between the scaffolder’s hip and shoulder. This way, the joints and members are load-tested during the erection stage itself by the scaffolder’s own weight. Cross-bracing is provided by using kao jue poles inclined at an angle of 45—60°. For the inner layer, kao jue poles are used for main posts and ledgers; the intermediate standards are generally not necessary. The main posts of both layers are supported by the same set of triangular steel brackets.

Scaffolds are tied to buildings using 6 mm diameter horizontal mild-steel wires at vertical and horizontal spacings not exceeding 6.6 m and 3 m respectively. These are referred to as “putlogs” though, unlike putlogs in metal scaffolding systems, which transmit vertical platform loads into the building wall, putlogs in bamboo scaffolding provide lateral restraint and wind resistance. They are provided at closer spacing higher up, where wind forces can be significant.

Working platforms for carrying out external works are normally vertically spaced at 2 m. The maximum imposed load on working platforms varies between 0-75 kPa (very light duty) to 3 kPa (very heavy duty) in the Labour Department’s code. The code requires that no more than two working platforms in the former case and no more than one working platform in the latter case should be in use in any bay, that is the space between two adjacent standards along the face of the scaffold. In the double-layered system, kao jue transom, poles spanning between the ledgers are used to support the working platforms, and are therefore provided at every platform level and with a maximum horizontal spacing of about 1.5—2.4 m.

Single-layered bamboo scaffolding is generally used to provide protective cover to renovation works on existing buildings where working platforms are not necessary. It is also used for some new construction, for example where external curtain glass walls are designed to be installed as large panels. Such scaffolds are also used in demolition, where all demolition works are carried out from inside the building including removal of external features. It is mandatory to fit tarpaulin sheets to contain demolition dust and debris, but these need to be removed during typhoons. Fixed external rather than internal ledgers facilitate the removal of tarpaulins.

However, single-layered scaffolds are less safe than doublelayered scaffolds and usage is discouraged. They are not included in either the Labour Department’s code or the Buildings Department guidelines. For minor repair works, including external plumbing, air-conditioning unit replacements and concrete or rendering repair, truss-out bamboo scaffolds are commonly used. This is light- duty, short-duration work and small-diameter kao jue poles are used. The standards, ledgers and raking poles are all supported by triangular steel brackets, which in turn are securely fixed to the structural elements of the building. Putlog ties restrain the top of the standards.

Bamboo scaffolding is also used to erect many of the large cantilevered advertising signs frequently seen over Chinese streets. In Hong Kong, the maximum allowed length to height ratio is 4:3 and the scaffold should be erected in a manner not to obstruct the traffic flow below. Such scaffolds are usually supported by steel wires or hang-poles securely fixed to the structural elements of the building. The lower level is usually decked out with wooden planks to act as a working platform.

Lashing connections.

Joints in bamboo construction have always been tricky. The poles are neither perfectly straight nor round, they have nodes at irregular spacings and their thickness varies. Despite such limitations, practitioners have developed many connection details over the years. Lashing poles with soaked bamboo strips was the preferred method for a longtime, but the strips decayed and needed constant attention and replacement.

By the late 1970s, nylon strips had replaced bamboo strips. The new strips made erection faster and they also lasted longer, but connections still tended to weaken over time due to weathering. This led to Hong Kong researchers specifying engineered connections through experimental investigation. The recommended value for basic characteristic resistance for each lashing is 1.1 kN with a partial safety factor of 1.1—1.25.

The nylon strips are well-specified, with a minimum ultimate strength of 0.5 kN, a width of 5.5 — 6 mm and a thickness of 0.85 — 1 mm. The overlap of two lashed bamboo poles should be 1.5— 2 m, and the distance between the lashings should not be greater than 300 mm.

Other materials such as putlogs, anchor bolts and steel brackets are specified in the Buildings Department guidelines. The established specifications for bamboo poles, however, were given in the Labour Department’s earlier codes. The poles should be 3-5 years old and air-dried in vertical positions under indoor conditions for at least 3 months before use; they should be free from cracks, irregular knots and wormeaten spots.

Despite bamboo and lashing materials having been specified, the effort of codifying the design data is still ongoing.