5. We are the Main Contractor engaged by the Employer to demolish an existing high-rise residential development & to rebuild on the existing site a new high-rise development. We have engaged a Subcontractor who claimed to be experienced in demolition works to proceed with all related demolition works. However, after 1 month since he took possession of the site, he is still unable to submit to us a proper Method of Statement with Professional Engineer’s endorsement, we are worried and we would like to check on his work.
Our Answer to you:
1. You are right to be concerned, as demolition of any high-rise development is definitely not a straight forward and easy task and it would need special Professional Supervision.
It is common in the market that we use special demolition equipment or explosives to assist in the demolition works, but you need to bear in mind that whatever you do; you need to ensure that there is no “unforeseen collapse situation” which could cause damage to nearby Structures / Foundation Systems or the loss of lives or injury to humans. Thus, you need Professional Supervision for this work.
2. As the process of demolition would differs with structural configurations and systems, I would only highlight to you the major & basic check items that you need to look out for and I would leave the rest to your in-house Professional Engineer to advise you.
3. It is common for any Main Contractor who engaged in such massive construction work to engage a part-time / full-time in-house Professional Engineer to perform checks, to approve any submitted works of the Subcontractors, and also to advise the Construction Team, to ensure that the Construction Process is conducted in a safe and efficient environment, in order to minimize or eradicate possible delays. Based on my past experience, if the Project Contract Sum is of the range of $500 million for example, the liquidated damages that would be imposed on the Main Contractor is definitely in the range of at least $300,000.00 per day.
4. I suppose your Subcontractor has already decided on which Professional Engineer he would engage for his work. You need to ask him to submit to you the name and contact of the Professional Engineer so that you be able to co-ordinate or contact him immediately in the event that you see some “strange” demolition practices on site.
5. A good and responsible Civil and Structural Professional Engineer who engages in such demolition works would proceed with the following basic tasks (apart from other special tasks and precautions which differ with different structural configurations, and neighboring environment etc.):
A. Study the As-Built Drawings of the existing structure that is going to be demolished (inclusive of all Structural and Architectural drawings);
B. Deduce from the aforesaid the load paths i.e. how the loads are transferred from the higher floors to the lower floors and ultimately to the foundation or otherwise.
C. Deduce from the aforesaid the sequence of construction when the aforesaid structure was built.
6. Based on the above mentioned, you would understand that the planning of the demolition process is definitely not a simple task, and one would need to plan and take care of all safety precautions to ensure no possible injury to all the workers and also the general public.
7. If the Building is located in the middle of many nearby buildings, then the use of explosives may be limited as shock waves generated could cause detrimental effects to nearby buildings. It is obvious that one would start the demolition of the non-structural elements of the building first. Thus, one would need to take all precautions to ensure that in the demolition of these non-structural elements, no possible damage is done to the structural elements causing premature damage and possible unforeseen collapse situations.
8. In addition, safety measures needs to be implemented to ensure that no debris fall outside the building in the work process. Thus, safety nets and proper barricades need to be installed. It is also common practice for some Demolition Specialist to erect temporary platforms outside the building to catch possible debris falling off in order not to injure any personnel working on site. You must remember that all the different precautions that you are going to adopt are to ensure safety of the workers and not to endanger the general public. There is no hard and fast rule concerning these matters, you can afford to be innovative in your safety measures implementations, to spend the minimum to obtain the maximum protection. Thus your imagination is your only limit. However, you are advised to consult the Ministry of the Manpower if you need help, to ensure that you have taken the minimum safety precautions as required in compliance with Statutory Requirements.
9. There is always been a misconception that the roof followed by the higher storey structural elements to removed first before that of the lower storey. This is only correct if the load path is coming down from the roof to the higher floor then to the lower floors, then to the foundation.
10. However, this is not always the case, as experienced and innovative Design Engineers, there are possibilities that we would design part of the building such as the higher floors are supporting the lowers floors…This means that part of the lower floors need to be removed before the upper floors… to ensure that there is no possible collapse in such cases...
11. You are puzzled… Let me explain to you. There are situations that we Design and Construction Engineers would prefer to have the center core of the building to be erected “bottom up” from the foundation, then we have the external structural elements cantilever either from the Roof and / or the structural elements of each floor to support the perimeter structural elements such as perimeter vertical elements, possibly made with steel construction to achieve a slender design as required by the Architects.
12. Any inexperienced person would think that the vertical elements at the perimeter are columns (load acting in compression). But in actual fact, it was designed and constructed to be used as a tension tie (load acting in tension). Thus, if one removed that part of the building under the “Top-Down” sequence, it would be a confirm collapse case…Believe me, it would be a real tragedy then…
13. From the aforesaid, I want to highlight and reinforce to all that the process taken by the Professional Engineer specified in Paragraph (5) mentioned above is very important. This is not something that any structural engineer could perform. Only one who is an experienced Professional Engineer with many years of experience working in Construction sites and Design Office would able to do it beautifully and ensure no possible mistake causing unforeseen collapse.
14. In summary, I have highlighted to you the basic and most important considerations in any demolition work. You are again reminded that in addition to the aforesaid, there are numerous other considerations that are particularly important to ensure that you are able to proceed with the demolition work smoothly. Since there are so many possible different building configurations, it would be practically impossible for me to discuss with them one by one here, I will leave them to the Professional Engineer that you engaged to advise you further.
15. With the aforesaid, you know that demolition of building is not a straight forward task, it would require a good and experienced Professional Engineer and all involved to cooperate with him / her to ensure that it would be done smoothly without possible premature collapse or injury to anyone. The conclusion is, as long as you have planned and done it properly, you would have peace of mind… Please contact us if you need our help as we have advised many such cases in the past. Finally, we wish you all the best.
Er. Goh Cheow Leng Maria
First drafted on 10 September 2008.