Several common construction methods of large span steel structure

1、 High altitude in situ unit installation method

High altitude in situ unit installation method

High altitude in-situ installation method includes high-altitude in-situ bulk method and high-altitude in-situ unit installation method.

The so-called “bulk method” generally refers to a method that members are assembled directly at the design position by cantilever method or full support method.

In this method, full support should be set up to provide high-altitude shelving and workers’ operation platform.

Advantages: due to the light weight of single piece, it can effectively reduce the lifting requirements of lifting equipment.

Disadvantages: it takes a long time for the support to be erected, there are many high-altitude operations, a large span of construction period, and a large number of supporting materials are required, occupying a large number of building sites.

Scope of application: it is widely used in large-span structures such as space grids and latticed shells with small span and short construction period.

This method is fully used in the fabrication and installation of the main arch of Nanjing International Exhibition Center.

The “unit installation method” is to reasonably divide the structure into blocks, and then hoist these block units to the design position for installation.

In order to ensure the smooth splicing of field units, several units should be pre assembled in the factory.

The key point of this method is the reasonable division of hoisting units

1.For example, for long-span steel truss structure, the block position should not be in the middle of truss span;

2.For the beam column structure, it is generally recommended to set the section position at the bending point;

3.For grid structure and shell structure, the scheme of block or strip can be adopted;

4.The element must be self-contained with sufficient stability, stiffness and strength.

Guangzhou Opera House, Shenzhen Universiade Main Gymnasium and Shenzhen Bay Sports Center all adopt “unit installation method”.

2、 Integral lifting installation method

Sliding installation method can be divided into two types: structure sliding method and support sliding method.The construction characteristics of the two methods are introduced below.

Structure slip method

The basic idea is to assemble the whole structure (or part) in the site with assembly conditions, and then use the sliding system to move to the design position.

With this installation technology, the assembly site and mechanical equipment for assembly can be concentrated in a relatively fixed site. Compared with the in-situ installation method, the amount of temporary support and operation platform can be reduced, and the cost of site treatment and management can be saved.

The key consideration is that it is difficult to construct the structure directly at the design position. For example, the construction site around the site is limited, and the normal walking of hoisting equipment cannot be satisfied within the span.The basic element of this construction method is to replace the ground assembly, reaction support and overall lifting into “horizontal movement” in the “overall lifting method”, so it is essentially the same as the lifting method.

At least a few points should be paid attention to in this method

1.The longer the sliding route is, the higher the efficiency is;

2.The sliding element should be geometrically invariant system with enough stiffness and stability in the process of sliding to reduce the resistance of sliding as much as possible.

3.When multi-point traction is used to achieve sliding, the synchronization of traction must be controlled in order to avoid torsion of the structure in the process of sliding. If it is difficult to ensure, the influence of asynchronous traction on the sliding unit should be fully calculated and evaluated. If necessary, temporary reinforcement can be carried out for the sliding unit.

4.Before the sliding element is finally fixed, there is a tendency of “easy sliding” in the moving direction and its orthogonal direction. Because different from the design support conditions, it is necessary to take measures to prevent “sliding” in order to prevent the deformation outside the design (i.e. out of the sliding plane), such as setting self balancing rigid pull rods or flexible cables near the supports on both sides.

In addition, the structural sliding method also includes two different processes: one by one sliding and cumulative sliding.

Sliding one by one: one or several sliding units are connected at high altitude from one end to the design position to form a whole structure directly.

Cumulative slip: that is to say, the sliding unit only slides one section on the slide rail (not moving to the design position for the moment), and then slide a certain distance after connecting the next unit, so as to accumulate one by one until each unit is pushed to the design position.
Generally speaking, in the installation method of large-span and space steel structure, when temporary support cannot be set up or the condition of using installation crane is not good, that is, there is a problem in the construction directly at the building location, the structural sliding method can be used as one of the solutions.

Support slip method

The support slip method is to set up the support frame in the structure design group to provide support and operation platform for the structure in-situ installation. After the installation of this part of structure is completed, the support sliding will be separated from the installed structure.

In this way, it creates conditions for the in-situ installation of adjacent structures, and thus circulates until the overall installation of the structure is completed.

Therefore, different from the structural sliding method, the support sliding method can be summarized as “the structure is not sliding but the supporting mechanism is sliding”, while the structural sliding rule is “the structure is sliding but the supporting mechanism is not sliding”

When the support slip method is adopted, the design of the supporting frame should not only meet the general global and local stability, but also consider the horizontal dynamic load (horizontal inertia force caused by starting and braking), and if necessary, large diagonal brace can be added to improve its lateral stiffness.

In conclusion, because this method needs to occupy the space within the span of the structure, when the surrounding environment is difficult to provide the structure splicing site, the support sliding method can be used as one of the solutions.

It is worth noting that both the structural sliding method and the supporting sliding method, besides the conventional straight-line translation, the curve sliding is also common, which is frequently used in engineering practice.

3、 Integral lifting installation method

Integral lifting installation method

The integral lifting installation method is to assemble the structure to be installed on the ground or appropriate floor projection position, and then use the “lifting system” to lift the whole formed structure up to the design elevation.When the height of large-span steel structure is high, which is not conducive to the erection of support jig frame and the shape of lifting steel structure is regular, the construction method of integral lifting can be used as one of the options.

4、 Unsupported installation method of Large Span Cantilever Steel Structure

Unsupported installation method of Large Span Cantilever Steel Structure

The so-called non support installation method of large-span cantilever steel structure is that under the condition of no supporting mechanism, relying on the stiffness of the cantilever steel structure body, the hoisting machinery is used for the installation of high-altitude bulk parts, and the construction is carried out in the way of gradual extension and stage installation.

Compared with the high-altitude in-situ installation method, although the construction method is also implemented near the high-altitude in-situ installation, because there is no temporary support mechanism, the construction measures are few and the construction technology is simple (no unloading).

It is suitable for cantilever steel structure with high stiffness, good stability and high height.

With this construction method, the cantilever section will deflect under the influence of its own weight. The trouble is that the installation coordinates of the components are unknown. When the installation configuration is required or there is closure situation, the configuration control of the structure in the construction process is one of the biggest technical problems, which is directly related to whether the structure configuration can meet the requirements at the completion and whether the cantilever section can be closed smoothly.

For long-span steel structure buildings, high-altitude in-situ unit installation method, sliding installation method, integral lifting method and large-span cantilever unsupported installation method are several typical and common construction methods at present, but their applicable conditions and key technical points must be grasped and reasonably selected in engineering practice.

Especially in recent years, most of the projects have unique architectural modeling, which brings many difficulties to the construction, especially to the installation work.

It is the highest level of construction technology to deeply understand the structural composition of these projects, understand the load transfer path, be familiar with the surrounding environmental conditions and specific requirements of the project, as well as the available construction resources and proprietary technology mastered by the enterprise itself, and design the most appropriate and new installation scheme according to local conditions.