2426 Methods of Erection

The method chosen for erection of a space frame depends on its behavior of load transmission and constructional details, so that it will meet the overall requirements of quality, safety, speed of construction, and economy. The scale of the structure being built, the method of jointing the individual elements, and the strength and rigidity of the space frame until its form is closed must all be considered. The general methods of erecting double-layer grids are as follows. Most of them can also be applied to the construction of latticed shells.

1. Assembly of space frame elements in the air. Members and joints or prefabricated subassembly elements are assembled directly on their final position. Full scaffoldings are usually required for this type of erection. Sometimes only partial scaffoldings are used if cantilever erection of space frame can be executed. The elements are fabricated at the shop and transported to the construction site, and no heavy lifting equipment is required. It is suitable for all types of space frame with bolted connections.

2. Erection of space frame by strips or blocks. The space frame is divided on its plan into individual strips or blocks. These units are fabricated on the ground level, then hoisted up into its final position and assembled on the temporary supports. With more work being done on the ground, the amount of assembling work at high elevation is reduced. This method is suitable for those double-layer grids where the stiffness and load-resisting behavior will not change considerably after dividing into strips or blocks, such as two-way orthogonal latticed grids, orthogonal square pyramid space grids, and the those with openings. The size of each unit will depend on the hoisting capacity available.

3. Assembly of space frame by sliding element in the air. Separate strips of space frame are assembled on the roof level by sliding along the rails established on each side of the building. The sliding units may slide one after another to the final position and then be assemble together or assembled successively during the process of sliding. Thus, the erection of space frame can be carried out simultaneously with the construction work underneath, which leads to saving of construction time and cost of scaffoldings. The sliding technique is relatively simple, requiring no special lifting equipment. It is suitable for orthogonal grid systems where each sliding unit will remain geometrically nondeferrable.

4. Hoisting of whole space frame by derrick masts or cranes. The whole space frame is assembled at ground level so that most of the assembling work can be done before hoisting. This will result in increased efficiency and better quality. For short and medium spans, the space frame can be hoisted up by several cranes. For long-span space frame, derrick masts are used as the support and electric winches as the lifting power. The whole space frame can be translated or rotated in the air and then seated on its final position. This method can be employed to all types of double-layer grids.

5. Lifting up the whole space frame. This method also has the benefit of assembling space frame at ground level, but the structure cannot move horizontally during lifting. Conventional equipment used is hydraulic jacks or lifting machines for lift-slab construction. An innovative method has been developed by using the center hole hydraulic jacks for slipforming. The space frame is lifted up simultaneously with the slipforms for reinforced concrete columns or walls. This lifting method is suitable for double-layer grids supported along perimeters or on multipoint supports.

6. Jacking-up the whole space frame. Heavy hydraulic jacks are established on the position of columns that are used as supports for jacking-up. Occasionally, roof claddings, ceilings, and mechanical installations are also completed with the space frame at the ground level. They are appropriate for use in space frame with multipoint supports, the number of which is usually limited.

24.3 Latticed Shells 24.3.1 Form and Layer

The main difference between double-layer grids and latticed shells is the form. For double-layer grid, it is simply a flat surface. For latticed shell, the variety of forms is almost unlimited. A common approach to the design of latticed shells is to start with the consideration of the form — a surface curved in space. The geometry of basic surfaces can be identified, according to the method of generation, as surface of translation and surface of rotation. A number of variations of form can be obtained by taking segments of the basic surfaces or by combining or adding them. In general, the geometry of surface has a decisive influence on essentially all characteristics of the structure: the manner in which it transfers loads, its strength and stiffness, the economy of construction, and the esthetic quality of the completed project.

Latticed shells can be divided into three distinct groups forming singly curved, synclastic, and anticlastic surfaces. A barrel vault (cylindrical shell) represents a typical developable surface, having a zero curvature in the direction of generatrices. A spherical or elliptical dome (spheroid or elliptic paraboloid) is a typical example of synclastic shell. A hyperbolic paraboloid is a typical example of anticlastic shell.

Besides the mathematical generation of surface systems, there are other methods for finding shapes of latticed shell. Mathematically, the surface can be defined by a high-degree polynomial with the unknown coefficients determined from the known shape of the boundary and the known position of certain points at the interior required by the functional and architectural properties of the space. Experimentally, the shape can be obtained by loading a net of chain wires or a rubber membrane or a soap membrane in the desired manner. In each case the membrane is supported along a predetermined contour and at predetermined points. The resulting shape will produce a minimal surface, which is characterized by a least surface area for a given boundary and also constant skin stress. Such an experimental model helps to develop a first understanding about the nature of structural forms.

The inherent curvature in latticed shell will give the structure greater stiffness. Hence, latticed shell can be built in single layer, which is a major difference from the double-layer grid. Of course, latticed shell may also be built in double-layer. Although single- and double-layer latticed shells are similar in shape, the structural analysis and connecting detail are quite different. The single-layer latticed shell is a structural system with rigid joints, while the double-layer latticed shell has hinged joints. In practice, single-layer latticed shells of short span with lightweight roofing may also be built with hinged joints. The members and connecting joints in a single-layer shell of large span will resist not only axial forces as in a double-layer shell but also the internal moments and torsions. Since the single-layer latticed shells are easily liable to buckling, the span should not be too large. There is no distinct limit between single and double layers, which will depend on the type of shell, the geometry and size of the framework, and the section of members.

0 0

Post a comment