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Fit for retrofit: Three crucial questions architects should ask their engineer


AKT II director Paul Hutter outlines three questions architects should ask structural engineers before tackling a retrofit

RetroFirst Logos 2019 7

The climate emergency is now indisputable and the built environment industry must rise to tackle this challenge on many levels. Exploiting the embodied energy within the fabric of our current building stock is fundamental to making a meaningful change and achieving Net Zero by 2050.

It is now imperative that the default preliminary position in any city-centre development should be to look for the opportunity to reuse as much of the building as possible before any consideration of demolition and rebuild.

It is important to emphasise that although the urgency of climate change may be forcing this issue higher up the agenda, many of our most successful retrofit projects, such as AHMM’s Angel Building and KPF’s South Bank Tower, were motivated by being the approach that simply made the most commercial sense.

Based on our experience over the last 25 years, we would recommend that any architect should ask their structural engineer these three questions.

Angel orginal

Angel orginal

Source: AKT II

BEFORE: The original Angel Building in Islington

1. How well do you know the existing building?

We spend hundreds, if not thousands, of hours trawling through and falling in love with all the existing drawings on reuse projects. No detail is left behind and, in the case of Hopkins’ 100 Liverpool Street, we analysed over 6,000 steel beams and steel-to-steel connections. Knowing what is there before you start a retrofit project is something the engineer must do in order to unlock the potential within the existing frame and allow the architect to shape the direction of the project.

We spend hundreds, if not thousands, of hours trawling through and falling in love with all the existing drawings on reuse projects.

2. Where is the spare capacity?

We have found that in almost every structural type of building there will be some spare capacity. If this can be identified early on enough, the scheme can be developed around it. One may be able to find the value required to enable the retrofit solution to be viable. The recent remodelling of 100 Liverpool Street in central London, currently on site, involved adding three additional floors to a 1980s steel frame, re-coring the building and increasing the net internal area by 43 per cent. This work was completed without strengthening foundations and working around sitting tenants. It was only possible because the scheme was developed around the spare capacity. 

AFTER: AHMM's retrofit of the Angel Building in Islington

AFTER: AHMM’s retrofit of the Angel Building in Islington

AFTER: AHMM’s retrofit of the Angel Building in Islington

3. How can modern technology help?

Innovation and technology are the main weapons in a structural engineer’s arsenal when trying to unlock the potential in an existing frame. But these need to be paired with an understanding of the architectural aspirations and the construction process. On Horden Cherry Lee’s Finsbury Tower project in central London, we increased the lifespan of the existing building by adding 13 floors on top of the existing 1960s concrete frame. This increased the total floor area of the tower by 125 per cent. In order to do this, we needed to model the entire history of the concrete frame to understand the distribution of load both as materials aged and additional floors were added. To do this we constructed finite element models, with over 200 construction stages, spanning from the 1960s through to 2050. 

The common theme on all our successful retrofit projects is early engagement and a collaborative open working relationship with the client and design team. This allows for detailed upfront early-stage work and it identifies opportunities and what it will take to deliver it. So, while ‘How much does your building weigh, Mr Foster?’ is a good question, a much better question might be: ‘How much can it weigh?’


Readers' comments (5)

  • I wonder if the 'default preliminary position.....' as described above should be embodied in legislation - and not just for city-centre development?

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  • Industry Professional

    An excellent article.
    A key problem is often that old records of buildings no longer exist.
    If one is prepared to put in the time (and thus the money) early on to establish what the construction of a building is, through research and intrusive investigation, then dividends can be reaped.
    I recall decades ago that a refurbishment project was generally said to cost nearly as much as a new build but still had, to at least some extent, the limitations imposed by the previous layout.
    There are also potential problems over the original quality of the construction and unknown "nasties" that only reveal themselves late on.
    Jeffrey (Engineer)

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  • Great pic of the then Angel Centre when brand new... AHMM's work there was exemplary. Perhaps - re Industry Pro's comment - there should be an assumption that all involved in what we might call 'assumed retention' (or at least that phase) do indeed do their due diligence and hoover up everything available, from the planning file to the contemporary write ups, as they would not hesitate to do for e.g. a heritage/restoration property?

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  • What also must not be overlooked is the capacity of foundations to accommodate more load once the building has settled under existing load. To a large extent the pressure on existing foundations is not relevant to whether more load can be applied - it is the incremental increase in load that is significant (subject to not completely overloading the foundations). So the concept of "spare capacity" is not so readily assessed for the foundations and a geotechnical assessment will be required.

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  • Industry Professional

    Jeffrey again. There is a CIRIA document C653 about the reuse of foundations. I recall Arup visiting our old offices in Lewes to view microfiche records of the existing piles of a building so that they could come up with a way of adapting their new design to suit the existing piles. While this was not saving the old building, it still took time to assess. It was fortunate that the records still existed in this case. Too often they have been discarded after 40+ years.
    At the risk of stating the obvious, all such engineering work needs to be done early on in the life of a project and with the Architect so that a reasonable compromise can be found for all before a design is too far advanced. Jeffrey (Engineer)

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