Detailed explanation of large-span truss construction

Because of its light weight, high strength, high stiffness and good seismic performance, large span truss is widely used in airport terminal building, gymnasium, exhibition hall and many other building types. For example, airport terminal building adopts large span truss structure to provide spacious interior space to meet the needs of travelers' movement and waiting for flights. Large sports stadiums, swimming pools, ice rinks, etc., often use large-span truss structures to support large roof areas and provide column-free viewing spaces. These building types cover a wide range of areas from large public facilities to special purpose buildings, reflecting the importance of long-span truss structures in modern architecture.

Due to the constraints of site conditions, the area available for truss assembly and lifting is very compact in some projects. In order to improve efficiency and save costs, it is necessary to formulate a reasonable construction process that can meet the needs of its own construction without affecting the operation of other processes.

1、Programmatic choices

The height and width of the completed concrete structure at the site of a large-span project are usually large, and the steel joist installation location is usually in the middle of the roof, so out-of-span lifting is not possible. At the same time, the construction program also needs to consider the terrain and lifting equipment. In addition, due to the presence of a basement, complex reinforcement measures would be required if a large crane was selected for the overall lifting. Therefore, the program selection also needs to consider the construction progress and economic efficiency comparison.

According to the actual situation of the construction site, it is usually determined that the main and secondary trusses can be assembled as a whole on the ground, the main trusses can be lifted in the whole joist or in sections within the collapse, and the secondary trusses can be lifted as a whole. Crane can be used for both assembling and hoisting. According to the performance of the crane, part of the main joist is divided into 2 or 3 sections according to the actual need. Segmentation point can not be chosen outside the concrete structure, otherwise more safety measures are needed to guarantee the construction of butt joints, so the segmentation point is chosen inside the concrete structure, and the floor can be utilized to build the operation platform. The bracing frame is placed at the lower chord node near the segmentation point of the main truss, and the bracing frame is placed on the top of the concrete beam or column on the roof.

2、Truss construction details

2.1 Joist assembly

In order to avoid the accumulation of error, the main and secondary trusses are assembled by the method of whole bulk assembly, and the iron bench is made of 16-gauge channel steel as the assembling platform. In order to ensure the straightness precision of the truss, the chords should be copied strictly by level meter, and at the same time, fine steel wires are tightened at the outer ends of the upper and lower chords for the chords to be straightened.

The positioning edge line of the web is measured and placed at the inner node position of the stringers, and the web is installed according to the position of the edge line. Immediately after the adjustment of chord rods, some web rods are installed at the end, middle and joint position, so that the truss shape can be fixed to avoid the deformation when other web rods are installed.

2.2 assembling position and support car position selection

In order to improve the construction efficiency and avoid the situation of secondary inverted transportation and blocking the traveling route of the crane, the trusses are assembled near the projection position of the installation, and the assembling table is arranged on both sides of the channel parallel to the direction of the channel.

In addition, the number of crane shifts should be minimized when hoisting, so it is necessary to determine the crane support position in advance. The principle is that the crane can lift two adjacent main trusses at the same time in the same position. When the trusses are lifted from the assembling position, the slewing radius of the hook position should be larger than the slewing radius of the hook when it is put into position as far as possible, so that the crane's action in the process of lifting is to lift the hook, rotate the arm, and lift the arm, and the slewing radius is getting smaller and smaller, and the safety coefficient is getting larger and larger, so that the safety of the aerial lifting is guaranteed to the greatest extent.

2.3 Main truss lifting

(1) Construction sequence

Due to the constraints of site conditions, the truss installation adopts the construction method from one side to the other side. The construction sequence should meet the requirements of construction organization design and be managed in strict compliance with construction delivery.

(2) Joist lifting

The plane position and elevation of the support should be adjusted accurately before truss lifting, and welded firmly according to the requirements of drawings after adjustment. Measure and place the truss positioning axis on the surface of the support.

When lifting the whole joist, two-point lifting is adopted. In order to avoid lateral instability of single joist, cables are set up at 1/3 position from the end of joist on both sides of the joist during lifting, and the joist is fixed with cables after it is put into position.

When the truss is lifted in two sections, two-point lifting is also adopted, the shorter section is lifted first, the overhanging end is put on the top of the supporting frame and the elevation is adjusted with a level meter, after that, the longer section is lifted, and the upper and lower chord butt joints should be welded firmly before the hook is taken off by the crane, and then the webs between the butt joints are welded.

When lifting with two machines, the end two sections should be lifted first. The length of the middle section of the truss is longer than the clear distance between the concrete. In order to ensure that the truss will not interfere with the concrete structure during the lifting process, the horizontal position of the truss should be inclined before formal lifting. In the lifting project, if the action of two cranes is lifting arm and turning arm, and the radius of rotation is getting smaller and smaller, the safety coefficient is getting bigger and bigger. In addition, since the heights of the two ends of the truss are different, try to make the load of the two cranes consistent. Lifting should be from the direction of rear end, and each crane adopts one point lifting. Weld the butt joints at both ends firmly immediately after seating, and weld the web between the butt joints afterwards.

2.4 sub-truss lifting

Before hoisting the main joist, the control edges of the upper and lower chords of the secondary joist were measured and placed at the corresponding node position of the secondary joist, and the cradle was hung to facilitate the operation of the personnel. After the completion of the hoisting of two adjacent main trusses, the secondary trusses between them were hoisted immediately, so that the main and secondary trusses formed a stable unit to ensure the safety of the structure. After analyzing, the crane boom could only be between the two main trusses when lifting the secondary trusses, otherwise it would cause collision between the boom and the main trusses due to the insufficient boom length.

(On-site optimization of truss segmentation and detailed analysis of crane station location through reasonable placement of assembly site of components, maximizing the performance of crane to reduce the number of lifts and at the same time reducing the number of times of crane shifting, have achieved very good results. Besides, what other problems should be paid attention to in the construction of large-span truss?)

3、Truss Welding Construction

(1) Preparation

Before welding, the interface should be cleaned up in the range of 10-15mm to remove rust and surface stains on the steel. Before formal welding, the starting point and closing arc of position welding should be ground into a gentle slope to make sure that there are no defects such as unfused and shrinkage holes. The ends of steel joist should be reserved for welding shrinkage, and must be corrected before welding due to possible errors in processing and production and possible deformation in transportation.

(2) Quality control

  • (1) Preheat the steel joist and remove moisture before welding;
  • (2)Control the welding speed, and heat input can be increased appropriately;
  • (3) Control the fusion ratio, reduce the proportion of harmful substances in the base material and molten metal in the weld metal;
  • (4) The root weld metal should try to choose a low sulfur and phosphorus content, high manganese content of the welding consumables in order to increase toughness and improve resistance to thermal cracking performance.

(3) Precautions

The first layer of welding precautions before welding to remove the first layer of the raised part, check whether the bevel edge is not fused and depression, if so, it must be removed. Avoid touching the edge of the bevel when grinding the welded joints and other parts. Use large diameter electrodes and moderate current for vertical welding, and use higher current for flat welding. Surface welding precautions welding surface should be selected smaller current, in the bevel edge parts should be extended fusion time, replacement of the electrode should try to shorten the time in order to prevent welding interruption.

4、Emergency plan for truss construction

(1) the establishment of safety warning area in the lifting process if the operation is improper will cause safety accidents, affect the project. Therefore, should set up a warning area, the warning area range is lifting work range, set up a special person to guard the warning area, clear and unified construction of 24h duty system, in the process of lifting prohibit people from walking on the scene.

(2) lifting the process of arranging a person to detect the jack, the use of communication equipment to implement the report of the operating conditions of the jack, to prevent the jack from slipping and other failures.

(3) arrange jack detection at the same time but also arrange special person to detect the situation of the oil pump, if there is overheating, oil leakage and pressure output instability should also be reported in a timely manner, through the commander-in-chief agreed to stop the operation of the whole field for inspection and maintenance, is strictly prohibited unilateral orders.

(4) In the process of lifting, ropes should be set at both ends of truss to ensure the stability of truss and prevent it from shaking.

(5) Carry out welding work after hoisting to the designated position, prevent arc burns during welding, and insulate the stranded wire and anchors.

(6) in order to ensure the smooth progress of the lifting project, in line with the principle of safety first, prevention first, before lifting should be prepared for emergency precautions, develop corresponding precautionary measures.

(7) the lifting personnel into the lifting site must wear a good helmet, if the high-altitude work must be fastened with a safety belt. Professionals to wear a professional sign, pay attention to the signaler's instructions to prevent danger, signaler to carry flags, whistles and talking devices.

(8) crane operations need to know the weight of the work object to avoid overloading operations, in the component lifting is in a reasonable position to tie the slip rope, the first lifting half a meter height to check its ties to confirm that it is firm before lifting. In the component lifting pay attention to the slow rise slow fall, in the component is strictly prohibited to stand people or place the rest of the components, in order to prevent safety accidents.

(9) Signal instructor's language and signal must be consistent with the driver, the commanding officer spits out the words clearly, to avoid misunderstanding, the tower crane driver to listen to the signalman's command to ensure that all parties to coordinate the operation to avoid mistakes.

(10) steel components falling speed slow down, construction personnel in the components of the outer hand-held components, is strictly prohibited to put their hands to the bottom of the components or components of the joints.

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