Top Down Demolition - Working & Benefits

Demolition of buildings is a very messy process that would result in a humongous amount of waste and dust. This challenge is solved innovatively by a unique method of demolition called top-down demolition.

A lone excavator digger working on the roof of a 12 story building in Taiyuan, Shanxi province, China.

Top-down demolition or deconstruction involves the demolishing of high-rise buildings from the top floor level progressively floor by floor, down to the ground. All the equipment, machinery, and waste-collecting bins are carefully taken to the top floor for safe and systematic removal and collection of demolition waste.

This article comprehends the working and features of the top-down demolition method in construction. The post also encounters various potential hazards that are encountered during the process, for better planning of the demolition work.

Working Features Top-Down Demolition

The top-down demolition method is applied only to certain high-rise or low-rise buildings that have a limited amount of space. Given the stability of the building and safety conditions around the site, it is always safe to demolish a building, in the reverse order of building it.

Top Down Demolition Sequence

When carrying out top-down demolition, the plant and machinery are craned onto the topmost floor of the building. Based on the stability of the building, additional props are employed to support the weight of the plant and the machines.

Propping for Deconstruction

During the process, one story at a time is demolished and then moved to the below floors systematically. As the demolishing progresses, debris formed is dropped down through a lift shaft to the waste collecting bin.

The demolition process is monitored continuously to ascertain that the load transfer is occurring as designed and that the vibration from the work does not have a significant effect on the floors below the working floor.  

The detailed sequence of processes involved in top-down demolition is dependent on the features, i.e., the structural layout, site conditions, age, strength and stability, etc. Hence, each building follows a different top down demolishing strategy.

Top Top-down methods can be performed manually, where the project is small and the labor is cheap. But this method cannot be considered safe compared to new top down demolition technologies available.

New top-down demolition technologies involve modular machines that can adapt the shape of any structure and descend down as the demolishes progress as shown in the figure. These systems are also available in automatic which helps to monitor the movement and the safety.

 Some of the plant and machinery used for deconstruction are cranes, tower cranes, crawler or mobile cranes, excavators, skid-steer loaders, 360-degree excavators, pneumatic breakers, etc. In case any of the plants is electric, it must refueled, which is done by lifting a fuel browser to the appropriate floors and then refueling the plant.

All the machinery loads, including the fuel browsers, are included in the calculation for the weights of floor loadings. The location of the fuel browser should be mentioned in the fire plan and fire risk assessment.

Potential Hazards During Top-Down Demolition

In addition to understanding how effective top-down demolition methods are, we also must realize the potential hazards expected during the process. This will help in effective planning and taking precautionary measures before conducting the demolition. Some of the specific hazards are:

1. Shear Failure

Shear failure during top down demolition is more significant in buildings with flat slabs and hollow pot floors. It is mainly caused by due to overloading during the implied loads of machinery for demolition, which would result in uncontrolled collapse.

2. Disproportionate Collapse

Disproportionate collapse is expected during the demolition of high-rise buildings made out of panel construction. This is why the principal contractor and designer must identify the construction method to implement the right controls on site before demolition.

3. Floor Loading

Overloading of floors is one major cause of building collapse. It would result in shear failure as explained before. Overloading can be caused by the additional machinery craned up, the collection of demolition waste, or point loading caused by certain remote-controlled machines.

Hence it is inevitable to conduct a non-destructive assessment of floors for floor load test, install temporary works for additional supports, and remove demolition debris immediately before the loads build up.

4. Lateral Floor Movement

The machinery during the operation undergoes slewing or turning movement that would result in lateral movement of the building floors. This would result in shifts and the appearance of openings.

This is common in buildings with floors tied together using dowelled contraction joints or starter bars. Hence, it is essential that the principal contractor study the building properly and the correct method of deconstruction be used.

5. Buckling

During the demolition process, the formation of openings, removal of a main load member, etc. would reduce the effective length of certain members. In certain cases, some members have to take loads that they are designed for, some lose axial capacity, while some experience high bending, or are subjected to asymmetric loading. These changes in load path would result in buckling of the structures.

6. Fire

It is vital to keep an up-to-date fire plan for all demolition and structural alteration work. Fire is a constant and immediate risk in demolition works. Its risk is higher for buildings greater than 18 meters in height.

During the demolition works the already set escape routes, fire set points, and means of raising fire alarms vary throughout the demolition process. But, it must be updated, reviewed, and made aware to all present throughout the demolition process.

Benefits of Top-down Demolition

1. Enhanced Safety: Minimizes risks to workers and surrounding structures.
2. Controlled Debris Removal: Efficiently manages debris as it falls.
3. Reduced Environmental Impact: Limits dust and noise pollution.
4. Quicker Project Completion: Accelerates demolition progress.
5. Preservation of Valuable Components: Salvages materials for reuse.
6. Minimal Disruption: Decreases disturbances to neighboring areas.
7. Cost-Efficiency: Optimizes resources and labor.
8. Access to Utilities: Maintains access to essential services during the process.
9. Structural Stability: Maintains stability of lower levels during demolition.
10. Improved Site Cleanup: Simplifies post-demolition cleanup efforts.

Key Takeaways

Top down demolition is one of the most common and only methods designed for simple and cost-effective demolition works. They can be used to reduce the impact on surrounding public services and properties close to the structure. The key to a successful deconstruction is having a correct assessment of the weight of the building components to ensure that it is adequate to take all the equipment and related loads for the demolition process.

Read More: Construction and Demolition Waste Disposal: How to Get Rid of It?