Exposing Falsehoods and Revealing Truths
Role of the Structural Engineer in Demolition
Rob Clarke, BEng, CEng, MICE, MIStructE
Keywords: structural engineer, demolition, hazards, temporary works, sequence, collapse, mechanism, pre-weakening, explosive, long-reach, pulverise
Rob Clarke has thirty years’ experience as a civil & structural engineer. After a career with Buro Happold designing new structures, five years ago he joined one of the UK’s most prominent demolition contractors, Controlled Demolition. He is now practicing as a freelance structural engineer for the demolition industry.
This paper is intended to help other structural engineers as well as construction and health & safety (H&S) professionals to appreciate some of the common structural issues encountered during demolition. It also provides an insight into the role of the structural engineer working for a demolition contractor.
“Although HSE statistics show that demolition is a high-risk activity, it would appear that it is either not given much consideration or is included at the end of the planning process and given whatever time is left in the programme. The avoidance of accidents depends on the quality and thoroughness of the Designer’s plan for the project.” CDM Guidance for Designer, Construction Industry Council.
According to the HSE Statistics Unit, reportable injuries in demolition are twice as high as those in construction. For the 12 month period 2007 – 2008 there were 10 reportable injuries for every 1000 demolition workers (of 18,000 total) compared with 5 reportable injuries for every 1000 construction workers (of 500,000 total). Clearly, demolition is still a relatively dangerous industry to work in.
Risks associated with the industry do not only impact on the demolition contractor and his staff, but can also affect the building owner, developer, local authority, government agencies and the wider community.
The content of this paper is equally relevant to refurbishment projects where demolition is a necessary process but not always fully appreciated by a construction industry that focuses on new-build.
The Health & Safety and CDM regulations are an essential aspect of demolition and much has been written from these perspectives. This paper focuses more on the structural detail.
Role of the structural engineer The structural engineer can play a crucial part in reducing the hazards associated with demolition, refurbishment, dismantling and decommissioning. From power stations to tower blocks and from bridges to Victorian institutional piles. From the robust and modern to the crumbling, damaged and ancient. From city centre buildings trapped between other buildings to those located on greenfield sites. The following text deals predominantly with buildings, however its content can be related to many other types of structure.
Presenting the structural risks clearly at the beginning of a project reduces accidents and saves time and money. Good communication helps ensure that the potential hazards are understood by the client team, site managers, supervisors and the workforce.
Demolition may be viewed as the poor cousin of construction, but an experienced demolition contractor will often know more about the ultimate strength of materials and collapse mechanisms than many structural engineers. During demolition the true strength of a structure and its components are exposed. The demolition engineer has physically worked with, manipulated and pulled apart more buildings than probably most structural engineers will ever design in a lifetime. He will understand plastic and elastic behaviour, serviceability and ultimate limit states, even though he cannot recite the relevant formula.
The scenarios are often complex; getting the demolition sequence right can be like a game of 3D chess, thinking many moves ahead. The structural engineer must be able to visualise the part-demolished structure.
Demolition should not be thought of as de-construction. In construction, there is relatively little choice in the methods and sequence available to build a structure. In demolition there are always many alternative methods and sequences that can be employed. Different contractors have different plant and equipment, different skills and specialisms and different learned experiences. These factors will greatly influence the cost and chosen method. With this variety of methods there are varying hazards and degrees of risk to consider. There may be a best solution in terms of programme or hazards but this may not necessarily be the cheapest. Remote demolition, such as long-reaching or explosives, is often safer because the number of man-hours working at height can be considerably reduced.
In order to obtain a better understanding of the demolition process it helps to be able to recognize the potential hazards. These are not always obvious to those who are in a position to influence decisions made on site. If they were obvious then demolition would be a much safer industry.
The principle concern of the structural engineer is uncontrolled collapse.
In demolition only planned collapse is acceptable. An unplanned collapse of elements greater than three tonne should be reported to the HSE as a dangerous occurrence.
Causes of collapse can include:
When the structural issues are understood, collapses can be prevented as the following examples demonstrate:
Cut lines. It is much better to pulverise and munch up to a cut line rather than prop and diamond saw cut. Providing lateral stability to the propped element can be difficult. Also, do not munch up to the face of an in-situ column, leave a stub of reinforced concrete beam protruding so as not to destroy the reinforcement anchorage.
When working towards the end of a framed building i.e., an un-braced frame with no shear walls, it is sensible to leave a minimum of two bays, which is much more stable than one bay.
Long-reach demolition has now become common place. The track-mounted plant with an extra long boom and dipper arm, sometimes telescopic, can reach 40m easily and there are some giants that can reach 90m high.
Long Reach Sequence – working towards the stiff core to maintain stability.
When long-reaching, it is good practice to follow a sequence that works back towards the stiff core so that the lateral stability of the structure is always maintained. The distance from the long-reach machine to the face of the building should preferably be a minimum of half the height of the building so that the operator is a safe distance from the falling demolition arisings.
Normally, when long-reaching tower blocks, the floor slab will be able to safely carry the demolition arisings from the floor above. If a floor is overloaded with the demolition arisings from a number of higher storeys, a progressive vertical collapse can occur. It is generally safer to long-reach bay by bay full height rather than floor by floor. This prevents excessive debris built up on floors and the potential for a progressive collapse.
The large panel residential tower block system building can be at risk of a Ronan Point type failure (the full height collapse of one structural bay) during demolition. These buildings have pre-cast concrete floors, walls, stairs and stair cores. The floors and walls are relatively slender and the fixity at the slab/wall interface is poor compared to current practice. There is little slab end bearing and limited slab continuity reinforcement connecting adjacent pre-cast slabs across the wall support. Often remedial strengthening work was undertaken in later years but the effectiveness of such work can be variable and as-built records are hard to find. For this type of building the long-reach method should be modified slightly. Extra care should be taken to limit the weight of arisings on the lower slabs. As the building is reduced, a relatively wide building base area should be maintained and the floors tiered back to preserve the lateral stability of the building.
Weak structure & high risk of collapse. Strong structure & low risk of collapse.
Basements. The presence and location of any basements should be identified in the pre-tender health and safety plan. It is good practice is to keep a marked up drawing on site recording the extent of the filled basement as the demolition front progresses.
Explosive Demolition. A ‘stand-up’ can present a serious hazard if, after the charges have detonated, the structure does not fall or collapse and break up. The risks associated with entering a partially collapsed structure are high. Contingency measures for this should be provided where possible, for example, having long-reach plant on stand-by. Pre-weakening is required to help the structure break up under gravity. There is a fine balance between carrying out enough pre-weakening to avoid a ‘stand-up’ and too much pre-weakening, making the building unsafe to work in, especially during high winds. The calculations which justify the extent of pre-weakening must also consider accidental damage and over-break. The structural engineer has to work closely with the demolition contractor to produce the most appropriate pre-weakening arrangement. Special internal strengthening work can be designed to influence the direction of fall in conjunction with the delay sequence for the charges.
“Commercial pressures imposed by the Client to expedite work often impose severe limitations on the time available for a demolition contractor to investigate the design and construction features in detail and then to plan the demolition work carefully. Contracts tend to leave responsibility for investigation of the existing structure to the demolition contractor and then rely on cost penalties to enforce prompt completion. Contractual arrangements framed in this way increase the risks to both the general public and site workers, and limit fair competition.” 8th SCOSS Report 1989. This was written 20 years ago and is still pertinent today. The preparation time is so important that client pressures to reduce this period should be resisted firmly.
Commercial pressures imposed by the demolition contractor can often complicate the required demolition solutions. The competitive tender price can necessitate imaginative solutions to avoid expensive methods. However, the demolition contractor must be able to clearly demonstrate that all the potential hazards have been considered and that the risk has been reduced to an acceptable level. Compliance with the CDM regulations should prevent risk taking or inexperienced contractors slipping through the net.
The Code of Practice for Demolition (BS 6187) is an extremely good reference document providing an overview of the demolition process. However, the BS cannot be too prescriptive because every structure is different. That said, there are some regularly occurring situations where the following advice may be useful:
Small plant are used wherever possible to avoid hand and arm vibration (HAV) caused when using hand held tools. When considering the safe size of small demolition plant to work on suspended floors, a starting guide is:
In the right situations these weights can be increased using back-propping with calculations to prove the strength of the floor.
Getting the debris away efficiently to avoid a back-log of material which can overload floors is often the key to safe and successful demolition. This is particularly important on constrained city centre sites when internal drop zones and openings through structural walls are often necessary to create a route to get the demolition debris material out of the building. Strengthening masonry walls to turn them into deep beams can allow new openings to be created underneath.
A fully enclosed scaffold with reinforced plastic sheeting is used to surround a building during demolition when it is necessary to protect the public from any debris escape. A demolition scaffold is typically fully boarded with no gaps, sheeted or debris netted and carries 2kN/m2 at the top level. It is important for the structural engineer to consider the scaffold tie back fixings to the existing structure, which may be weak. The vertical cantilever must not be allowed to become too high as the demolition progresses.
When working with pre-stressed concrete, the stressed tendons should never be cut. Generally there is no need. These elements can be demolished quite safely by destroying the concrete around the stressed tendons. Immediately the concrete fails and crushes the tendons relax and become de-stressed. This is true for grouted and un-grouted ducts.
Deep RC beams
Deep RC beams can be very difficult to demolish, particularly at height. One practical solution is to gradually reduce the beam’s cross-section along its entire length with a hammer attachment. This can be much less hazardous than cutting and lifting.
Demolition against a party wall is one of the most complex operations. Sometimes pre-weakening the structure to allow hinges to develop whilst long-reaching can be used to fold the frame away from the retained building.
It should be noted that every situation is different and should be individually checked and assessed.
4. Survey and Inspection
Surveys and inspections form an essential part of a preventative safety system, particularly at the planning stage for demolition operations.
Structural surveys are a form of detective work; the structural engineer has to read the clues to determine how the structure was actually constructed. He has to look for tell tale signs and evidence about connection strength and the general quality of workmanship and attention to detail.
The importance of the Structural Survey cannot be underestimated. Serious hazards are created if, for example, a movement joint or a narrow bearing detail is missed.
A detailed structural assessment should identify any weak elements, supports, connections or structural features which are stability sensitive. Overall stability throughout the demolition process to avoid the danger of an unplanned collapse should be considered. Load paths must be identified and it is important to identify the potential for hazards associated with hangers, ties, cable stayed roofs, barrels, arches and domes, space frames, retained facades, pre-stressed components, structures retained by ground anchors and structures that have been modified.
There are three main opportunities for the demolition contractors to carry out a structural survey:
Pre-demolition survey. A good time to carry out the first survey is during the Type 3 asbestos survey work when intrusive work is acceptable. Usually after the building is vacated and before the soft-strip starts.
The Pre-demolition building survey should include:-
1. Structural form and movement joint locations.
2. A description of how lateral stability is maintained.
3. Any pre-stressed or post-tensioned elements.
4. Novel forms of construction.
5. Gross structural defects, cracks and signs of movement.
6. Hangers and ties.
7. A search for any existing drawings and calculations, noting that actual details may vary.
8. The building age and typical details to expect.
9. Recommendations for any local exploratory work.
A good pre-tender H&S plan will have a pre-demolition structural survey.
During soft-strip. Often the structure is not properly exposed until the soft-strip operation is completed. The original demolition method may then need to be revised. Much of the floor and ceiling finishes will have been removed, exposing the structure to provide a fuller understanding of the building. Early investigation work can now be confirmed. Signs of poor workmanship may be exposed. More intrusive work can be carried out to expose connections and joints and locally check slab reinforcement.
It should be noted that it is quite often difficult for the demolition contractor to supply a set of complete method statements before the structure is fully exposed, although this is often requested. The contractor needs to be given the flexibility to modify his methods in the light of new found information.
During demolition. Site inspections by the structural engineer during demolition are a sensible precaution. This is an important opportunity, for example, to spot hidden ties which hold a mansard roof together or to inspect a wall which is being subjected to high lateral loads from a rubble pile. Often the temporary works structural engineer is only commissioned to design the temporary works and not inspect progress. A temporary works design needs to be very clear so that it cannot be misinterpreted. Any potential hazards should be brought to the attention of the site manager immediately.
The condition of brickwork and timber is particularly important in refurbishment work. The mortar can vary from rock-hard and fully bonded to a loose sand that flows out of the joints with the slightest disturbance. It can also be patchy, depending on work carried out at different times or work by different gangs. Removing the plaster during refurbishment of old buildings from all faces is recommended to help determine the masonry condition.
During the appraisal of an existing building there are a few basic questions the structural engineer should ask:
Strength and robustness It is a good idea to ask oneself how one would have designed the original structure, considering the site restrictions, the use of the structure and any change of use. Historical plans and a knowledge of historical codes of practice can be of help. Then whilst surveying the building any deviations from your expected design can be further investigated to check for potential difficulties later. When considering the robustness of a structure, the strength and rotational stiffness of connections is key to how the building will behave.
Collapse mechanisms. The Structural Engineer needs to think carefully about how the structure will behave during planned collapses as well as any unplanned collapses. What will be the exact mode of failure: bending, shear, or excessive deflection? Will the failure be slow or sudden? Will there be any warning? Imagine the collapse second by second, in slow motion. How much of the structure will be affected? There are many instances where a load-bearing wall can be removed with no effect because of redundancy in the structure and alternative load paths. One must also consider the possibility of accidental impact by moving plant. What are the key elements which, if damaged, could cause a progressive collapse or catastrophic failure?
Building design details are often simplified to assist construction. For example, reinforcement or bolted connection details may be standardised. This can provide extra load carrying capacity which the Structural Engineer can take advantage of, even if this only increases factors of safety.
Strength of elements When assessing the strength of elements such as reinforced concrete slabs and beams, shear is often the most important failure mechanism to consider because shear failures are generally sudden and without warning. Member stiffness and deflection is less important. Justifying the safe bending capacity can be difficult because the reinforced concrete code limits crack width for durability which is not important during demolition. In the right circumstances the demolition contractor will sometimes safely overload a slab or beam until it fails and the ultimate load can be many times greater than the predicted factor of safety would suggest.
Adjacent buildings An understanding of the sensitivity of any adjacent buildings, their structural frame and stability provision are important.
A structural engineer with the appropriate experience will provide the following services for a demolition contractor:-
Small plant demolishing a multi-storey office block. Remotely controlled breaker.
pre-weakening for explosive demolition (steel). Each column has 5 cuts.
pre-weakening for explosive demolition (concrete)
To give an example of temporary works design for demolition, a long-span trussed roof in a city centre location requires a 500 ton crane to lift down each truss to ground level and all the associated working at height cutting and slinging operations. The alternative was to prop the trusses so that each truss could be safely long-reached in-situ.
Temporary steel props enable long-reach cutting operation
Collapses are often the result of failure to use information that is available somewhere.
Unless you are used to demolition it can be difficult to visualise what a structure will look like during the demolition process. Providing structural engineering advice is a waste of time if it cannot be understood. A simple sketch can replace sides of A4 text, which in reality, few people take the time to read. The sketch can also be easily copied into method statements.
The 3D sketch also helps design development. 3D Collapse simulation movie in real time.
Preferably nothing should be so complicated that it cannot be explained to non-experts. If it is that complicated, one should try to make it simpler. There is much more risk of an incident occurring because, for example, a machine operator or scaffolder has not understood. Also approvals will be easier to obtain. Explaining the reasons behind methods can make them easier to understand and easier for others to remember. This also allows methods to be improved upon and developed by others in the chain.
There are three levels at which communication is important:-
The client’s team need to communicate their views on any assumed demolition sequence to the contractor. They should highlight known hazards and any assumptions made about the existing building that will need to be confirmed by the contractor.
The contractor has to communicate his demolition sequence proposals to the client’s team for approval.
The contractor has to communicate the approved method and sequence to the workforce.
3D drafting software such as Revit and Tekla are being used increasingly to clearly demonstrate complicated demolition sequences, for which they are invaluable tools, particularly where there are a number of different sub-contractors working in the same area at different stages.
3D collapse simulation software such as Extreme Loading from ASI is an impressive tool that is going to become more prevalent, particularly for ‘pulling’ structures (this is where a structure is weakened and pulled to the ground) and for ‘blowdowns’ (demolition by the controlled use of explosives). ASI’s Applied Element Method (AEM) based technology is the only analysis method for accurately analysing the behaviour of a structure during a critical failure and the resultant progressive collapse. The software will analyse and predict the linear, nonlinear, and failure modes of structures in a 4D environment. It will analyse and play out different ‘key element’ removal scenarios. Collapse simulation is an excellent way to demonstrate to clients exactly what to expect and it can be used during the demolition design process to check and refine collapse mechanisms.
The position of structural engineer can be immensely satisfying when, through good communication, one is able to combine the knowledge of experienced construction and demolition professionals to produce innovative, simple and safe solutions to complex and difficult problems.
7. Sustainability Considerations
Designing attractive high quality new buildings that are built to last and can easily be adapted for change of use will produce the most sustainable buildings because they will not need to be demolished.
Demolition contractors have always been efficient at recycling because it makes commercial sense to sell and re-use as much material as possible. Typically 90% of the demolished building is recycled. The 10% that goes to landfill is generally carpets, curtains, blinds, false ceilings and roofing felt.
Water and air pollution is controlled by good site practice, the contract specification and legislative requirements.
When considering the reduction of carbon emissions during demolition operations, diesel fuel is by far the largest single contributor to the quantity of carbon dioxide emissions that are produced. Every 100 litres of diesel fuel consumed results in 268kg of CO2 being produced. It is in everybody’s interest to reduce this as much as possible and there are various methods by which this can be achieved:
8. Design for future demolition and refurbishment
The more as-built information there is available the closer demolition tender sums will be and there will be less opportunity for claims through unforeseen difficulties. CDM health and safety files and operation manuals for new buildings should improve this aspect and help reduce hazards for future generations.
The following design considerations may not be at the top of most designer’s or client’s to-do list, however, they will make future demolition safer and the provisions do assist the creation of robust and adaptable buildings:-
The structural engineer combines technical knowledge with management and communication skills.
With a knowledge of the demolition industry and its practices, the structural engineer can contribute and add value to demolition works from all perspectives – whether the client is a contractor, building owner, developer, local authority or government agency.
Demolition, like most other branches of engineering, is becoming increasingly complex due to factors such as commercial pressures, health & safety, sustainability, community interest/concern, new technologies, new materials and new construction processes. The structural engineer can help clients address these issues and
help the demolition contractor improve productivity and health & safety outcomes through engineering analysis and design.
Engineering analysis and visualisation with CAD provide an opportunity for demolition contractors to improve record keeping and knowledge management, so that practical experience can be passed on to junior staff.
The structural engineer can assist with communication with stakeholders and regulatory authoritories.
Cost efficient, safe and sustainable demolition depends on:
Employ expert advice every time. Early input will reduce costs, financial and H & S risks, contract duration and improve sustainability.
10. Useful links and references
Institution of Civil Engineers: www.ice.org.uk (Demolition Protocol 2008.pdf)
National Federation of Demolition Contractors: www.demolition-nfdc.com
Health & Safety Executive: www.hse.gov.uk
Institute of demolition engineers: www.demolitionengineers.net
Confidential Reporting on Structural Safety (CROSS) scheme: www.scoss.org.uk
Corporation of London: www.cityoflondon.gov.uk (Demolition-control_and_advice)
BS 6187-2000 – Code of Practice for Demolition