When assessing the safety of an existing building, a significantly different approach is required compared to when planning a new structure. This article explores how to approach the assessment of an existing building, explaining some of the important questions that must be addressed.
Until recently, the appraisal of existing buildings was treated as requiring the expertise of a specialist structural engineer and not necessarily an essential part of every structural engineer’s training. That has since changed especially due to new safety legislation in some part of the world recognizing the need to assess and reassess the safety of existing buildings bearing in mind the overlap between structural and fire safety1.
On a simplistic level, there are at least two reasons why an existing building or structure may require some assessments or appraisal. First, where the existing building is showing signs of distress and there is consequently, the need to verify its structural integrity. Or where an existing building is being modified or considered for reuse. The latter is currently at the centre of a global concern – climate change. The embodied carbon of new constructions is responsible, directly and indirectly for 50% of global emissions. Hence, reusing existing buildings does not only reduce material wastage but also minimizes embodied carbon.
Existing buildings in the most basic form can be described as any structure which have being fully or partly constructed and is already being put to service. In order words, the assumption is that an existing building must have a functional design, which forms the basis of its construction, and a considerable amount of implementation must have been carried out.
It is generally assumed that many existing buildings, particularly those built in the 20th century are safe because many of them have been built from traditional materials, designed to the older, more conservative codes of practices that were more rigid in terms of load and material factor of safeties. Also, it is believed that an existing building must have proven itself if it has stood the test of time. However, what is often overlooked is the fact that many of these codes of practices were written at a time where many aspects of the design process were still evolving. For instance, there was a time where the façade of buildings wasn’t necessarily checked for wind loading. In fact, the current requirement for fire resistance in steel and concrete frames is the most stringent it has ever been, but it was only developed in the 1980s1.
The appraisal of existing buildings can be straightforward, but it can also be relatively complex. Where the appraisal of an existing building is merely to determine its structural integrity. This is fairly straightforward. As long as the engineer is able to demonstrate that applied loads have not exceeded calculated resistances, the structure is considered structurally functional under the assumption that it had performed satisfactorily over an extended period of time. If the reverse is the case, the structural integrity of the structure is compromised, and then deconstruction or some remedial action may be advised.
However, where the subject of the appraisal is modification or reuse, as would often be the case, a proposal to increase loads or reduce structural resistances, then the appraisal becomes very complex.
This article’s focus is mainly on the latter. There are five important questions the structural engineer must answer:
- How much of risk is involved compared to a new build?
- Does past performance matter?
- How safe is the building?
- What level of intrusive investigation and testing is required?
- To what extent is calculation Required?
How much Risk is Involved Compared to a New Building?
One of the very first question that the engineer must answer and perhaps continue to review as the assessment of an existing building for reuse proceed is how much risk is actually involved relative to a new build.
For new buildings, the risk associated with predictability is almost unity. This would seem to be a good incentive for anyone to discountenance the reuse of existing buildings and just consider a new build. However, if we are to encourage greater reuse of existing structures, we must be able to assist clients in navigating the risk and uncertainty involved with work on old buildings, which may be just as crucial as handling the technical components.
There are several causes for these uncertainty in existing buildings, many of which would come to light in the next sections.
Does Past Performance Matter?
The next question the structural engineer must answer is does past performance really matter? If it does, can the engineer rely on the structure’s past performance?
The primary aim of any appraisal is to ensure that a building has an appropriate margin of safety against structural and fire risks. Past performance might justify that a structure can continue to carry the same loads it has actually experienced in the past assuming there are no modifications and alterations. It, however, doesn’t promise that an existing building would have the same factor of safeties that would be required of a modern building.
While on the surface, it would appear that factor of safeties has generally become less stringent due to better understanding of analysis, design and the materials we work with. However, there are certain parts that have even become more stringent. In many appraisals of existing buildings, some part of a structure, when checked to modern codes are found to be very close to failure. Notwithstanding, the fact that many historic forms of construction have now been found to be very defective.
So, does performance matter? It does, but only to the extent that a structure continues to carry the loads it is actually experiencing. The structural engineer realistically, can’t rely on past performance in the case of alteration/modifications. It is in fact, a blunder to assume that an existing building is able to carry the loads for which it was designed for, at the date of construction. An experienced structural engineer knows too well that there is a sharp distinction between theoretical design and actual performance. To assume that a building is able to support the loads it was designed for is to assume a perfect design and a perfect workmanship, which is hardly ever the case.
How Safe is the Building?
One area where the factor of safeties of existing building have become more stringent than in modern buildings is fire resistance and disproportionate collapse1. Today, there is the risk that a structure can be subjected to severe fire such that a disproportionate collapse will be triggered. The next question is therefore, how then do you justify the safety of an existing building? Should you carry out checks, what factors of safeties would you use?
For many cases, it would be appropriate to apply a lower factor of safety. However, there are cases that require more caution, and the modern codes of practices should apply. Examples would include where the structure is in distress, or a proposed alteration or modification is poised to significantly increase loads and change existing load paths. Also, in brittle structures that can fail without any warning1.
Hence, while for new buildings, the engineer is able to apply the prescriptive factor of safeties build into the current codes and standards, for existing buildings he must weigh up the risk against the cost of disruption and strengthening that may be required, in order to make the risks as low as reasonably possible. So, how safe an existing building needs to be is at the discretion of the structural engineer.
What Level of Intrusive Investigation & Testing is Required?
Most existing buildings at the time of appraisal will already be in service. However, for the assessment of an existing building to be worthwhile, there is the need to open up some part of the building in order to identify the vertical and lateral stability system of the building as well as the load paths. Undertaking these investigations helps the structural engineer to identify any potential risks and other issues that may arise during modification.
However, investigations are generally disruptive and expensive hence the structural engineer must aim to minimize them. In fact, no amount of investigation can fully capture what has been built. Because even if we can, it would be impracticable to test every structural member in an existing building. At best, we could group typical structural elements and test some of them, which doesn’t eliminate the risk of those untested from being inadequate. The question therefore is, what level of intrusive investigation and testing is actually required?
Intrusive investigation should aim at finding common flaws in the construction methodology or the codes in place at the time when the building was constructed. This would assist in concentrating the investigation and reveal potential hazards. For instance, being aware of the shortcomings in the evaluation of concrete shear strength in codes before to around 1970 may prompt one to search closely for tiny cracks close to beam supports that could otherwise go unnoticed2.
Consider what you will do with the results, whether positive or negative, before requesting testing. The tests might not be required if the results stay the same. Here are some key points to note when considering intrusive investigation:
- Avoid requesting for too much test at the early stages2.
- Visually inspect the existing building for compliance to typical span-depth ratios, this might start to reveal how hard the structural elements are having to work2.
- Investigate only regions such as those where construction form knowledge indicates potential concealed defects2.
- Ensure that enough tests are conducted to enable the results to be statistically meaningful2.
To What Extent is Calculation Required?
As mentioned in the introduction, there are two reasons why an existing building might be subjected to appraisal. In the first case, when the structure is showing signs of distress. Thus, calculation is required to check the affected sections of the existing building. The second scenario plays out where a proposed change is causing an increase or decrease in load or changes in load path.
Calculation is often required in order to identify the elements that are adequate or inadequate and those with spare capacity. The question however is, how much of these calculations is actually required to assess the existing building?
To answer the question at a basic level, for buildings showing signs distress, all affected parts related to the structural distress need to be checked. While for structures being altered or modified, calculations are to be carried out to the extent of the modification.
Existing buildings have been built, and likely occupied, this means that investigations needed to undertake reliable calculations are going to be expensive, in fact, in some instances the structure needs to be opened up. It therefore makes sense that the need to conduct calculations is targeted towards the identified potential issues only.
Summarily, to identify potential issues one needs to identify how hard the existing building is working. Sadly, this is not a straightforward task. In many instances, there are usually few or no records of the construction available, working drawings not available and even when they are available, they might not represent what was actually built. Worse, some form of modification might have occurred which was undocumented.
Therefore, the assessment of any existing building will always be iterative:
- Review available information1
- Visually inspect structure1
- Postulate the likely load paths and undertake rough calculations to understand if the load path seems credible1
- Undertake investigations to find key missing information and also to check any areas of concern (members are often sized for serviceability, but investigations should be targeted to areas which are critical for strength and hence safety)1.
- If the load path does not prove credible, then investigate alternative paths. This means that it will typically be necessary to undertake two rounds of investigations, with time in between for calculations and assessment1.
For the structural engineer, approaching the design of a new building typically involves selecting a structural form and then applying the required codified rules for design. This is also true for existing buildings, except there is more. For existing buildings that require some form of assessment, particularly where the subject of the assessment is reuse of the existing building, there are additional challenges which the structural engineer must consider. The following is apt.
- The engineer at each level of his assessment must determine the risks associated with reusing the existing building relative to the risk associated with considering a new building and then advise his client appropriately2.
- There will be a need for evaluations investigations and calculations, but the precise number required will depend on the circumstances. In order to focus investigations and assessments, the structural engineer has to be aware of typical features and problems with the relevant kind of construction. But one may never be certain of the structure and condition2.
- Finally, engineer must determine if the structure is sufficiently safe, which involves determining an appropriate factor of safety for each component. This will depend on several considerations, such as if anything is changing, whether there are any highly stressed fragile parts, the likelihood of collapse, the expense and disruption of any strengthening work, and so on2.
Sources & Citations
- Institution of Structural Engineers (2010) Appraisal of existing structures (3rd ed.). London: IStructE Ltd| Gowler P. et al. (2023) Circular economy and reuse: guidance for designers, London: IStructE Ltd
- Institution of Structural Engineers (2023) How to approach the assessment of an existing building London: IStructE Ltd| Lawrence A. et al. (2023 Technical Guidance Notes, The Structural Engineer London: IStructE Ltd