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Assembly and Erection

The fabrication of spatial structural members is generally performed by plane cutting, branch cutting, flattening, plate welding or coned. Nodes of spatial structures are typically fabricated by: flat plates welded into node assemblies; plates pressed into shapes; nodes made from cast steel; nodes forged into solid or hollow spheres; or extruded into the desired shape (aluminum nodes).

Since spatial structures are usually made of a large number of members, very strict tolerances are used in their fabrication (about +1/32 in.). In the design of nodes, it is typical to include an adjustment mechanism to have some level of dimensional control during the assembly and erection process. In the past, it has been common to construct test assemblies to check any problems that may arise during the site assembly and erection of the structure. However, computer sequencing simulations to check the stages of construction has now substituted for this.

There are various methods used for the erection of spatial structures. The method selected for erection depends on the system used, the module size, and site access. A large portion of a total construction budget is sometimes used for the erection of spatial structures. Modular systems are easier and less costly to erect than nodular or piece-small systems using bars and nodes. The high stiffness of spatial structures results in small deflection under loads, which is also important for erection. Large spatial structures are usually assembled at the ground level on site and are lifted hydraulically or by crane. Reactions from the hoisting equipment are generally distributed in all directions without causing overstress in the members.

There are three general methods of assembly and erection of spatial structures:

1. Cantilever Method
In this method, the structure is erected starting from a support point and a section or strip of the structure is erected to span between two adjacent supports with the help of movable scaffoldings. Once part of the structure is stabilized, the rest of the structure is assembled by adding separate pieces or units to already erected structure. This method is usually used for nodular systems made of member and nodes (connectors) assembled through bolting (such as Mero System) or placing the member ends inside the connectors’ slots (such as Triodetic System). This method is normally used when the site condition does not allow the use of heavy cranes or other lifting equipment or when unskilled labor is easily available.

To see an animation of the Cantilever method of assembly and erection. CLICK HERE



2. Lift Slab Method
In this method, the entire spatial structure is assembled on ground and lifted to the final location using cranes or other heavy lifting equipment. This method is usually used for the erection of flat double or triple layer girds. Double layer grids with spans up to 300 ft weighting up to 2,000 kips have been erected using this method.

To see an animation of the Lift Slab method of assembly and erection. CLICK HERE


3. Subassembly Erection Method
This method is a combination of the two methods mentioned above. Part of the spatial structure is first assembled on the ground and lifted, then placed and secured into its final position. Subsequent sections are then assembled, lifted, and connected to the previously in-place parts of the spatial structure (similar to the cantilever method). This method has the basic advantage of the first two methods. First, the labor-intensive part of the process, which involves connecting pieces in-place in the air (as used in the cantilever method) is minimized. Also, since only part of the spatial structure is being lifted, smaller cranes or lifting equipment are needed for the erection. The size of the segments to be assembled on the ground can be adjusted based on the maximum capacity of the available lifting equipment.

To see an animation of the Subassembly Erection method of assembly and erection. CLICK HERE

The selection of the erection and assembly method depends mainly on the site condition. If the site is clear and open, it is usually more desirable to assemble the entire spatial structure and erect it using cranes or jacks (Lift Slab Method). If the construction site is clear but there is not enough space for cranes to maneuver around the building site, the Subassembly Erection Method is used. When spatial structure have large and heavy members, the Cantilever Method is more effective.

When cranes or jacks are used to install part or the entire structure, it is important that the lifting points are selected such that the structural members will not be subjected to overstress.

When the Lift Slab or Subassembly Erection  Methods are used, the roof decking and electrical/mechanical systems can be installed while the structure is on the ground. This has two main advantages: (1) it significantly reduces the construction costs, and (2) weather protection of the building is provided as soon as the structure is in place and facilitates the remaining construction operations.

The labor costs for the assembly and erection is an important factor that should be considered, and it generally depends on where the structure is located. For typical spatial structures the assembly and erection constitute about 30% to 40% of the total construction costs. Using an efficient erection method along with a good design can substantially reduce this cost. It has to be noted that if the structure (or any segments of it), is limited to a length of 60 ft and a width of 15 ft, it can be transported by a truck.


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