Here’s the twist
The huge concrete structure that forms Birmingham’s new library, with its spectacular internal atrium, challenged the very rules governing frame design. Jan-Carlos Kucharek reports.
It might be a building whose frenzied circular facade has been inspired by the old gasometers of industrial Birmingham, but it’s the huge circular holes cut into its square grid plan of Dutch architect Mecanoo’s £193m city library that have caused the real headaches.
With its massive atrium rising up through libraries, conference spaces, archive rooms and theatres, terminating at the transplanted Victorian rotunda of the city’s Shakespeare Memorial room at the top, it has demanded engineering gymnastics to ensure structural integrity.
As Buro Happold project engineer Jeremy Morris explains, this is due, in no small part, to loadings. “There are a lot of books so there are high loads on every floor, but on the archive levels on the 5th and 6th floors, they are particularly large. Here we’ve designed for live loads of up
to 15kN/m2,’ he says.
The concrete structure has been designed for high loads — up to 15kN/m2 on the fifth and sixth floors — despite the huge atrium holes compromising it running up through the building
The new library replaces the city’s existing Brutalist structure with a 10-storey building of stacked boxes, and one basement level that reaches out under Centenary Square to form a new sunken public amphitheatre. It is being built on an NEC3 contract. “The architects, who appointed Davis Langdon as design manager on the project, were initially keen on a 10m x 10m grid steel structure to reference the city’s industrial heritage, but with these loads there were implications for both steelwork sizes and facade deflections. We value engineered this to a 7.2m x 7.2m grid — a lesser span, but better for controlling deflections,” says Morris.
But while steel was originally proposed, contractor Carillion was clear that it wanted a concrete frame — their concerns mostly being about acoustics and fire protection. “With steel, full-depth trusses would have to be built on site, and the impact on local residents and traffic would have been enormous,” says Simon Dingle, Carillion project manager. “We sat down early with the design team and discussed concrete, because while steel frame might go up quicker, making it fire proof would require materials and time.”
But Dingle says the main challenge with the concrete was the temporary works. “Every floor is different, with different loadings and uses, so we had to work in combination with fabricator Morris Rowe, to detail.” Dingle explains that it was all about optimising the frame construction from the outset. “We used slip-form for the cores, but the temporary works for casting the floors was horrific. But it was needed as the atrium from floor to floor changes constantly, so bespoke works were needed for each slab cast.” Pre-tensioned concrete floors, which are more capable of dealing with larger structural loads, were also specified.
The finished building will feature a spirograph frieze unifying the various functions within
But there were further complications due to a 30m long by 20m wide studio theatre that rises through the building from ground floor and which has to be acoustically isolated from the rest of the structure, and is effectively a frame within a frame, explains Morris. Between the atrium and this, the concrete frame has to work around these “exceptions” to the conventional structure.
“As a result a lot of the middle of the building is hanging from the concrete box and transfer walls that form archive levels five and six,” adds Morris. “These are used to hang steel columns, which perform excellently in tension, supporting levels two, three and four. It’s no coincidence the walls were placed at archive levels — these are not public access and so can be solid, uninterrupted, and fully devoted to taking the floor loadings below.”
Patrick Arends, project architect for Mecanoo, explains that The National Archive would have insisted on four-hour protection for the structure here, which would have meant enormously thick concrete walls. But some lateral thinking from the project team meant the fire rating on this structure could be halved. “Things were solved with a fire engineering approach where we negotiated two-hour fire ratings for
the archive levels through proposing a dedicated hypoxic environment,” says Arends. “This is a sealed nitrogen-rich atmosphere fed by isolated, specialist plant, creating an environment with reduced oxygen content, and therefore a lesser fire risk.” The nitrogen level increase means that library staff will only be able to access the archive for a limited amount of time each day.
Hanging floors also obviated the need for all the columns to go to ground, where the architect wanted clear spans. Transfer structures at first floor level were considered but beam sizes would have been enormous. Thus the columns that do appear in the entrance area taking high loads are composite steel/concrete to reduce their overall size.
The atrium rises through the library, conference spaces, archive halls, book wall and studio theatre, topped by the Shakespeare Memorial room
Carillion’s Simon Dingle adds: “At lower levels the 12m high columns have been cast with a single pour using single 12m steel tubes with a finish so good, the architects don’t want them to be clad.”
The post-tensioned concrete slabs are typically 250mm thick. Morris says the engineers worked with Mecanoo and moved the atrium voids and reduced their diameters to make them fit as much as possible within the grid. This reduced the number of structural columns that would need to be taken out. Where possible, the design team used the architecture of the building to gain additional structural purchase. “We used the steelwork of the ‘book wall’ between levels three and five to tie these slabs together and let them share load,” says Morris. “In this way, up to 20mm deflections at the atrium edges are shared and reduced.”
Arends says perimeter deflections in the order of 15mm do occur, partly as a result of the 2.3m set-back of the columns from the building’s edge, meaning that movement joints had to be considered for the hanging curtain glazing. The 5.4m aluminium box sections of the frieze cladding are connected to this via spigots that set it 900mm off the face of the glass, to allow for facade access cradles
at levels three, seven and nine. The frame is powder coated aluminium. Iron and steel were considered for historical reasons, but loads on the structural frame would have been enormous. As it is, the 5.4m and 1.8m diameters of the box frame sections all finally align with the 7.2m structural grid.
With all the bespoke structural approaches, the new Birmingham Library seems like a strange exercise in setting up structural rules, by way of the concrete frame, only to then break them. Why set up the frame only to knock atrium holes through its grid, use steel columns to hang it, and use steel frames to stiffen it?
Morris says: “The starting point was always the 7.2m grid, and while the resulting structure is reasonably complex, the principles remain simple. In fact, as part of the construction process, the 7.2m grid was used to build the concrete floors — we just knocked away the temporary props that allowed us to build them where the atrium voids occur to let the hanging columns take on the job of supporting them.”
So the complete concrete frame is there in principle, even if marked by its absence, carrying the library’s post-tensioned slabs with their 20-year resilience built in, ready to assume the dense matter of as yet unwritten knowledge.
With the atrium voids moving relative to each other as they go up the building, the logistics of erecting temporary scaffolding to allow floors to be cast was a major issue
The book wall from levels 2-4 employs a steel frame which the concrete atrium perimeter uses for additional stability
Ventilation earns sustainable rating
Due to complete in early 2013, the Birmingham Library will not only be a cutting-edge facility for the city, it will also achieve a BREEAM excellent rating. This is due in part to its ability to switch from mechanical to natural ventilation.
The building is predominantly mechanically ventilated, with a bespoke air conditioning system feeding the 5th and 6th floor archive levels to create its hypoxic environment. But “stack effect” ventilation is also possible via the internal atrium, which acts as a chimney, with external air being drawn in through vents in the building’s facade. These vents have actuators connected to the BMS system, so when the climate allows it, the louvres in the facade deploy and the building switches to natural ventilation. Heat exchangers draw heat from the air before it is expelled.
Natural ventilation is likely to be used for about 35% of its typical yearly use. Patrick Arends explains that air velocities were modelled to ensure there was not too much draught up through the stack — in what he terms the “Marilyn Monroe” effect — but also to check that there was enough overlap of voids to ensure that air volumes passing up were sufficient to ventilate the library’s spaces efficiently.