a very different kind of Fosters building

Ombú, a large industrial building turned office space, in Madrid city centre, is probably the least Fosters-like Fosters building I have seen. Yet, despite the lack of glassy, slick façades stereotypical of the practice’s high-tech new-builds over five decades, the clean, perfect lines are still apparent here – even in the retrofit of a historic structure. It makes for an incredibly satisfying and striking project and, on top of that, it is carbon-neutral in operation.

Described by the practice as ‘an innovative exemplar of building reuse’, Ombú completed in the Spanish capital in June this year. Its client, Acciona, is one of the largest clean energy providers in the world.

The large-scale retrofit project has transformed this historic building and the surrounding area adjoining Méndez Álvaro Station, creating over 10,000m2 of office space for an undisclosed single tenant. The immediate public and private land around the station has been unified with landscaping suited to the Spanish climate.

Méndez Álvaro Station itself opened in 1997 and is Spain’s busiest bus station. With the transformation of Ombú also comes a new entrance to the back of the station, opening almost straight on to the new office complex. To the back of Ombú runs Calle de Pedro Bosch, a busy road which unites the districts of Retiro and Arganzuela. The bridge running over the Metro tracks is in the process of being knocked down – part of the municipal corporation’s intention to recreate an 18th-century-type boulevard to make a more sustainable neighbourhood.

It is hoped that this, along with the Foster + Partners-designed regeneration of Méndez Álvaro, will improve what has for long been a less than desirable part of Madrid’s centre.

Fosters has worked before in the Spanish capital, most notably on the Torre Cepsa in 2009. In 2016, the AJ100-topping practice was selected from a shortlist that included David Chipperfield and OMA for the commission to overhaul Madrid’s iconic Hall of Realms. Consequently, it has a 12-strong team based in the city but shares its European projects with the main London office, where associate partner Emilio Ortiz Zaforas, one of Ombú’s project architects, was previously based.

Originally built in 1905 by the architect Luis de Landecho, who also designed Madrid’s Ritz Hotel and Athenaeum cultural institute, the brick-built warehouse is the only surviving building of the former Cerro de la Plata Industrial Gasification Society, which once supplied energy to the surrounding areas. It fell into disuse before Acciona acquired it in 2017, essentially saving it from demolition.

Historical photo of the Cerro de la Plata Industrial Gasification Society

Source: El Financiero Hispano-Americano

Fosters’ concept for transforming this warehouse was very simple. It has stripped it bare, cleaned it up and inserted what is basically a giant tiered cake of staggered platforms formed of glulam slabs and columns.

One could say that the project team have been particularly lucky with what they had to work with here. The existing building was in such good shape they have really been able to capitalise on the freestanding loadbearing structure that supports the steel trusses of the lightweight timber pitched roof. As such, the historic building envelope has been entirely retained, conserving over 10,000 tonnes of original brick, mitigating its environmental impact.

The main design move is a lightweight timber structure finished in chestnut overlaid on spruce glulam and inserted into the space. It has been made entirely from sustainably sourced timber from forests in northern Spain. Aiming to save more than 1,600 tonnes of CO2, it is recyclable and demountable, allowing for spatial flexibility while also integrating lighting, ventilation and other services.

The solid masonry walls – albeit cleaned up, whitewashed within and looking as good as new – are still raw in contrast with the crisp new timber structures. Together, the whole thing has a quality of construction rarely seen in the UK.

To understand why this is, Ortiz Zaforas mentions how the building’s owner was also contractor, energy provider and structural engineer. This, he says, cut out the middleman and ‘most’ of the arguments typically witnessed between client and contractor.

As you stand at the eastern entrance of the warehouse, its layout is immediately readable, rectangular in shape with an additional existing annexe to the right. The timber insert has been pushed to this side with a new core connecting the annexe to the main space.

Ortiz Zaforas describes the construction of the timber insert as being ‘completely dry’. It doesn’t touch the brick walls, its construction used limited – if no – metal fixings and it is free-standing. Services are invisibly pulled down between the 2x13m timber slabs and fed through joints in the wood to avoid the need for suspended ceilings or additional finishes – practically invisible.

The warehouse’s most striking features in this main space are its historic timber roof and large cast-iron windows. The roof structure has been retained, luckily being in good shape and requiring very little work. An existing crane – once used to carry coal – was conserved, cleaned up and cleverly adapted with new engine and wireless control during the construction to carry the 2m-long pieces of timber. Long thin delicate grey pipes running along the roof’s sloped interior provide a sensitive way of hiding the sprinkler system.

Where once a chimney sat in the centre of the roof, there is now a skylight formed of PV glass, generating electricity while filtering light, reducing the need for artificial lighting within the vast space.

The statement windows around the edges of the existing masonry structure are huge. Rather than replacing them completely – the obvious solution – their cast-iron frames have been retained but with the glass panes replaced, reducing draughtiness by inserting new plastic pieces to minimise thermal bridging. As a result, the vast space retains its existing light-filled atmosphere and, upon a late afternoon visit, beautiful shadows are cast across the floor, the thermal mass of the structural masonry keeping the space cool even on a sunny 30°C day in early September.

The concept for designing the working environment, says Ortiz Zaforas, was that employees could ‘work anywhere and everywhere’ within the building and surrounding area – a notion the client was unconvinced by until the pandemic hit, when this became the norm. Each balustrade surrounding the staggered timber floorplates has a little ridge, just large enough to support a laptop. And taking advantage of Madrid’s temperate climate, a new courtyard offers the option to comfortably work outdoors. This is described by Ortiz Zaforas as an ‘English’ courtyard, due to it being enclosed by four walls – a nod to the Oxbridge college quad.

This courtyard – filled with water features to cool the air – connects via steps to a large 12,400m2 space with over 350 freshly planted trees and featuring outdoor working spaces and areas for informal meetings sheltered by their canopies. Local species have been carefully selected to reduce overall water consumption from irrigation, which will also end up coming from local sources.

The aim is that this green, semi-public space will create connections between the building and surrounding community with the adjacent, space owned by the town being re-landscaped as a new park. Ombú’s outdoor space is gated but will be open to the public during work hours. 

Timber-clad pop-ups, hidden among the trees, conceal exhaust chimneys, as services from the basement must be taken away from the building owing to its special historic nature. All of these pass through the timber core inside, emphasising just how hard these work to keep the services in one place without affecting the existing structure.

There are two main entrances to Ombú: one up Calle de Ombú leading into the building at ground level. Another, just below, leads down steps to basement level through an outdoor lobby within the courtyard.

The basement was incomplete as of September but is due to become a gym and cafeteria. Here a post-tension concrete structure enables a column-free zone, adding value to the space for tenants. ‘We tried to convince Acciona to let us [Fosters] have our own office space here,’ laughs Ortiz Zaforas.

Despite the construction cost being confidential, he mentions that, surprisingly, the cost per m2 is akin to that of a typical office block. The team pre-empted material inflation costs by subcontracting out the timber structure well before construction started.

According to Fosters, Ombú has a 1.0 planet ecological footprint, meaning its carbon emissions will be absorbed by the current capacity of the earth. This achieves the balance of sources and sinks required by the Paris Agreement, with its environmental impact compatible with the original +2°C target.

As a sceptic of truly ‘sustainable’ buildings – too often jargon and ‘greenwashing’ is used to fluff issues – I found this project very convincing. Does this herald a new generation of buildings by Fosters, sensitive to our existing and precarious environment? Hopefully. As the largest practice in the UK, it should be setting a precedent; one adapted to the times, approaching retrofitting as a valid alternative to new build.

However, given the mild climate of Spain and very different requirements in terms of planning and building regulations to those of the UK– specifically in terms of fire – I’m not sure whether similar could be achieved here.

Of course, the team have been particularly fortunate with the existing situation they inherited – the building was in good nick, requiring little work other than aesthetic tweaks. There was also ample land within which to strategically nestle the scheme, giving scope to make connections with its surroundings.

But in terms of what’s been achieved spatially in what was a vast empty space, using a limited material palette and (apparently) ‘typical’ budget, there’s lots for the UK industry to learn from here.

Architect’s view

Acciona’s vision for the future aligns with the practice’s commitment to developing bespoke design solutions that are optimised for their operations and the planet. Foster + Partners has addressed sustainability holistically to realise this unique retrofit project and rejuvenate the surrounding area.

The historic building envelope has been retained to conserve over 10,000 tonnes of original brick and mitigate the environmental impact. A lightweight structure inserted inside the space is made from sustainably sourced timber from local forests and allows for spatial flexibility, while also integrating lighting, ventilation and other services. The timber structure will save more than 1,600 tonnes of CO2 and is recyclable and demountable. A central skylight brings natural light to the interior, reducing the need for artificial lighting, while the glazing incorporates photovoltaic technologies that generate electricity.

Using the concept of ecological footprint, the impact of the project was quantified and improved across all aspects of the development; its carbon footprint has been carefully measured and controlled. The design reduces embodied carbon by 25 percent when compared to a new build over the whole life of the project, while making allowances for future refurbishment. The operational energy is calculated to be 35 per cent below normal expectations.
Chris Trott, partner and head of sustainability, Foster + Partners


Engineer’s view

Timber engineering has advanced considerably over the last 50 years. The panoply of tools available for the trade of structural design is extremely wide, be it in terms of new materials, building procedures, formal configurations, or structural safety verification.

For this project we took advantage of some of these tools. Two new timber-based materials were selected: thick CLT of Alpine spruce is ribbed with Spanish chestnut glulam beams. CLT has gained popularity over the last century while hardwood glulam, as a standard industrial factory controlled structural component, is a material from the late 80s.

The high compressive strength of chestnut is used to create relatively slender columns supporting a 60-minute fire rate. The slenderness of the column and beams contributes to the joints’ extremely high degree of rotational restraint. Beams behave as continuous elements and columns have hinged-fixed bars. This is achieved without joints in the span, through a complex node. A high-strength concrete core is connected with fully threaded screws, which are axially inserted into glued steel plates in the beams.

High-strength fully threaded screws, which range in length from 0.2m to 3m, have become an everyday resource. For this project, they allowed us to reach a composite action between beam and slab, and to generate complex composed transversal sections to solve cantilevered areas. Lateral stability is achieved by combining the stiffness of the joint with the vertical nucleus in CLT.
Miguel Nevado, structural timber engineer, Enmadera


Client’s view

As part of Acciona Group, Acciona Real Estate acts in order to respond to the evident climate emergency. We have increased our capacity to invest in the sustainable transformation of the economy, maintaining our authentic and long-term commitment, and accelerating our positive impact.

With these key goals in mind, we began to develop the Ombú project with a holistic approach to sustainability and wellbeing, aligning the project with the original +2°C target of the Paris Agreement.

Its carbon footprint has been carefully measured and controlled, reducing embodied carbon by 25 per cent when compared to a new build over the whole life of the project. In addition, we are continuing to manage the building after construction, to enhance its operational energy performance (calculated to be 35 percent below normal expectations).

It is important that we have a positive impact, both environmentally and socially. Ombú restores the area in which it is located, by refurbishing an obsolete building and increasing the social value of the external areas.
Alejandro Miguel Vicente, technical management and sustainability manager, Acciona Real Estate


Working detail

For the new timber structure, chestnut timber columns follow an optimised diagonal grid, with a typical 8m span. The column shape is driven by the beams above. The columns and beams are connected by a steel plate and a rigid element between them. All timber for the beams and columns is glulam chestnut, which is up to 600mm deep and 13m long. Lamination takes place in Spain and the wood is sourced from local forests to reduce carbon.

The beams are connected to the CLT slab by a series of screws, to reduce the amount of timber required. The 280mm-thick CLT slab is made from spruce. Pieces are typically 2m by 13m. The underside of the slab is exposed and functions as the building’s ceiling.

Services are integrated under the raised floor, in a 500mm cavity that feeds upwards to the level above and downwards to the level below, through a series of machine-milled perforations along the joints between the CLT panels. Terminal elements such as air diffusers, light fixtures and sprinklers are integrated into a series of aluminium extrusions, placed under the CLT joints. The timber structure is FSC certified and has been installed on site using the original bridge crane from 1905, to reduce time and cost.
Taba Rasti and Pablo Urango Lillo, senior partners, Foster + Partners


Project data

Start on site:  2020
Completion date:  June 2022
Gross external floor area:  19,500m2
Construction cost: Undisclosed
Architect:  Foster + Partners
Collaborating architect:  Ortiz León Arquitectos
Client:  Acciona
Structural engineer:  Acciona
M&E consultant: JG Ingenieros
Principal designer: Foster + Partners
Landscape consultant: K8 Paisajismo
Lighting engineer: Artec 3
Timber structure: Enmadera (Miguel Nevado)
Façade consultant: Enar (Envolventes Arquitectónicas)
Planning consultant: Addient
Main contractor:  Acciona
CAD software used:  Revit, Autocad, Rhino, Grasshopper and MicroStation
Annual CO2 emissions: Zero

Performance data

Percentage of floor area with daylight factor >2%: 54%
Percentage of floor area with daylight factor >5%: 38%
On-site energy generation: 0.315%
Heating and hot water load: 0.0049 kWh/m²/year
Carbon emissions (all): Zero
Annual water consumption: 3.11 m³/full-time employee/year
Airtightness value at 50Pa: <9 m³/hr/m²
Overall area-weighted U-value: 1.45 W/m²K
Embodied / whole-life carbon: 1,172 kgCO2e/m²
The number is calculated considering A1-A5 LCA stages and is process based. It includes carbon associated to: structure (40% of total carbon), façade (9% of total carbon), interiors (5% of total carbon) and services (45% of total carbon). Sequestered carbon is not included.
Predicted design life: 60 years