The height limit of mobile scaffolding needs to be determined comprehensively based on the type of scaffolding, installation method, and usage scenario. It is not a fixed value, but its height is closely related to stability and safety. The following provides a detailed explanation from two aspects: specific restriction standards and risk of exceeding the limit:
1、 Height limit standards for mobile scaffolding
The height limit of mobile scaffolding is mainly based on the "Safety Technical Code for Construction Tool Scaffolding" (JGJ202) and product design parameters. The core principle is "height to width ratio ≤ 3:1" (i.e. the height of the scaffolding body does not exceed 3 times the width), which can be divided into the following categories:
Standard portal mobile scaffolding (width 0.9-1.2m): When erected independently in a single group, the height usually does not exceed 6m (meeting a 3:1 ratio, such as a height ≤ 5.4m when the width is 1.8m); If it is necessary to increase the height, wall connecting components must be installed to rigidly connect with the building (one for every 2-3m increase), and the maximum extension can be up to 12m, but special scheme verification is required.
Lightweight mobile scaffolding (used for indoor decoration, load ≤ 1kN/㎡): The height generally does not exceed 4m, and double-layer operations are prohibited (i.e. only one layer of work platform is allowed to carry people and goods).
Wheeled mobile scaffolding: Due to the small contact area between the wheels and the ground and poor stability, the height must not exceed 5m when used independently, and the load on the working layer needs to be reduced by 20% (such as the original design load of 3kN, but not exceeding 2.4kN in reality).
In addition, the height limit is also affected by ground conditions: when the ground is uneven, soft foundation, or outdoor wind force is ≥ 4, the height needs to be reduced by an additional 10% -20%. For example, a frame that originally allowed 6 meters should not exceed a maximum of 5.4 meters at this time.
2、 The main risks of exceeding the height limit
After the mobile scaffold exceeds the specified height, the anti overturning ability, structural strength, and stability of the scaffold will significantly decrease, which may cause the following risks:
1. Overturning and collapse of the structure
The stability of mobile scaffolding depends on the balance between the center of gravity and the bottom support surface, and an increase in height will cause the center of gravity to move upward. When the height exceeds the limit, slight external forces (such as personnel walking or gusts of wind) can break the balance, especially for wheel frames. The lateral shear resistance of the wheels is weak, and they are prone to overall tipping due to lateral forces. For example, if a mobile scaffold with a height of 6m is increased to 8m, the anti overturning moment will decrease by more than 40%, and the probability of overturning will reach more than 70% when encountering a level 5 wind (wind speed of 10.8-13.8m/s).
2. Structural deformation or fracture
The upright pole is the core component that bears vertical loads. When the height exceeds the limit, the slenderness ratio (length to section radius ratio) of the upright pole increases, which is prone to "unstable bending". For example, a standard pole (diameter 48mm × 3.5mm) with a slenderness ratio of 150 at a height of 6m meets safety standards; When the aspect ratio increases to 9m, it reaches 225, exceeding the specification limit (210). At this time, if subjected to a load of 2kN, the pole may bend laterally, causing the connection between the crossbar and the fastener to loosen.
The fastening nodes of the horizontal and vertical bars may experience stress concentration under ultra-high conditions, which may result in fastener slippage (insufficient friction) or bolt fracture, leading to local structural disintegration.
3. Risk of falling of operators
The shaking amplitude of the elevated structure will increase with height: at a height of 4m, the shaking amplitude is usually ≤ 5cm, and at 8m, it can reach 15-20cm. At this time, the operators find it difficult to maintain balance and are prone to falling from the edge of the platform.
If the frame tilts due to excessive height, the protective railing (height ≥ 1.2m) may fail as the frame tilts, causing the originally vertical railing to become tilted and the protective height to be insufficient, further increasing the risk of falling.
4. Secondary disasters (such as object strikes)
If materials and tools on a super elevated structure fall due to shaking, the increase in falling height will lead to a sharp increase in impact force (according to the free fall formula, the impact force of a 2kg tool falling from an 8m height is about 160N, which is twice that of a 4m height), which may cause fatal injuries to ground personnel or equipment.
3、 Summary
The height limit of mobile scaffolding is based on mandatory standards of structural mechanics and safety testing. In actual use, it is necessary to strictly follow the requirements of "height to width ratio ≤ 3:1" and product manual, and unauthorized height increase is strictly prohibited. If the working height exceeds the limit, more stable equipment such as floor standing scaffolding or high-altitude work vehicles should be used, and a special safety plan should be developed. In any case, the core should be "stable center of gravity and balanced structural stress" to avoid safety accidents caused by excessive height.
