CIBSE Case Study: Ingenious Library
CIBSE Case Study: Ingenious Library
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Article from the November 2013 edition of the CIBSE Journal written by Andy Pearson.
With its shimmering façade, Birmingham's new landmark library is a decadent nod to the city's glittering past. Thankfully, the ingenuity of the engineer means that the building's performance more than matches its head-turning appearance.
Looking for all the world like a gigantic pile of boxes stacked one on top of another, Birmingham's new library towers nine storeys above the city's Centenary Square. These boxed floors are veiled beneath a brash filigree skin of ornamental metal hoops - latticework that serves little purpose other than as an architectural nod to the city's industrial past and its role as a centre for jewellery manufacture.
Given its showy overcoat of architectural embellishment, what is surprising about Europe's biggest lending library is that it is a gleaming example of clever, low energy design. The £189m scheme not only creates a new focus for the city, but answers Birmingham City Council's brief for a library to have a BREEAM Excellent rating.
The design competition was won by engineers Buro Happold, working with Netherlands-based Mecanoo Architecten. From the outset, the team set out to minimise the building's energy demands by maximising passive performance to deliver the brief in the most cost-effective way. "We worked with the architect on developing the massing so that it had a positive impact on the way the building would perform," says Sam Haston, building services engineer at Buro Happold.
Visitors enter the 39,000m2 building, which opened in September, on the ground floor. From here they ascend an escalator through a circular atrium, formed by a series of off-set openings in the floorplates.
On their journey the visitor passes through the open-plan fi rst, second and third floors, which house the bulk of the lending library's books. On the fourth fl oor is a closer space where the city's archivists work. Floors five and six house the archive repositories floor seven the offices, and floor eight is mostly plant space.
The building is crowned by a gold cylinder at level nine containing the Shakespeare memorial room. Below ground, the building has a mezzanine level and giant basement. Form and layout resulted, partly, from the engineer's desire to maximise the potential of natural ventilation as part of the low-energy, mixed-mode ventilation solution. Critical to the design was a series of acoustic studies of the site and its surroundings, combined with a monitoring exercise to test air quality.
"The studies showed it was possible to naturally ventilate the building, with the exception of the north elevation, which is close to a major road," says Haston.
As a result of these studies, the building is arranged with cellular spaces. Rooms requiring mechanical ventilation are located on its noisier north side, with the open-plan spaces on the east, west and south elevations.
The atrium is the building's circular heart; the full-height opening helps drive Buro Happold's natural ventilation solution.
"A clear focus from our perspective was to position the spaces that could be naturally ventilated - such as the library and open-plan areas - near the bottom of the building where we could utilise the biggest stack effect," Haston explains.
Clustering the open-plan spaces on the first four floors of the building has enabled a hybrid, mixed-mode ventilation strategy to be adopted for these floors. "The sealed archive spaces located above the open plan floors help drive the stack for the natural ventilated lower areas," says Haston.
The British Standard was used as the basis for the archive design although there were a number of derogations made, such as moving air conditioning into the archive vaults, for reasons of space, energy and fire.
The strategy allows natural ventilation to condition the floors when external temperatures are between 13° C and 21° C. With the building open from 8am to 8pm throughout the year, Buro Happold estimates that natural ventilation will be possible for approximately 30% of the time.
The mixed mode system has been designed so that when conditions allow, fresh air is introduced to the lower floors through openings above the ceiling. They are fitted with a weather louvre, a mechanical damper, and an attenuator, to help keep external noise out of the library. A louvred ceiling allows fresh air to enter the open-plan spaces. "We needed a good stack ventilation system to be able to cope with the higher pressure losses from having to draw fresh air through the extensive louvre arrangement," says Haston.
Fresh air is pulled across the floors to the atrium. From here it rises up and out of the building through roof-level louvres. "There was a lot of work and computational fluid dynamics (CFD) analysis done in trying to resolve the architect's desire for off-set openings in the atrium, against our need to provide an efficient path to draw the air up easily through the atrium," Haston explains.
In winter, when outside air temperatures are too low to use the natural ventilation system, the roof-level louvres close. Fresh air is supplied to the reading rooms mechanically by a series of handling units, which are housed in the eighth-floor plantroom. Again, the atrium provides a return path for the warmed exhaust air. However, in winter, fans pull the air up through the atrium to the roof level where a heat exchanger recovers heat to pre-heat the supply air, before it is discarded. "It's an efficient solution to use the atrium as an exhaust air path, from both an energy and space perspective," explains Haston.
Extra heat is supplied to the perimeter of the floors from trench heaters set into the raised access floors. Underfloor heating performs a similar role on ground and lower ground levels, which have no raised floors.
In summer, when external temperatures are too high to maintain comfort conditions using natural ventilation, the dampers shut off the façade fresh air intakes. As in winter, a mechanical ventilation solution keeps the spaces supplied with variable quantities of fresh air-based on measured CO2 levels.
Unlike winter, however, mechanical extract is not used in the summer, since there is no need to reclaim heat. Instead, the warmed air will enter the atrium, where it is allowed to rise up and out through the roof-level louvres to minimise fan energy use. A nightcooling strategy helps cool the buildings post-tensioned concrete floor slabs to keep daytime cooling loads to a minimum.
Cooling is provided by chilled beams hidden above the slatted ceilings. They are kept cool by ground water pumped from an aquifer below Centenary Square. This subterranean solution supplies water at 14° C. According to Haston, the aquifer is capable of providing the library with up to 40kW cooling. He says this will be: "enough to take the edge off the load at peak cooling duty and should be sufficient to meet the cooling demand at other times of the year".
When the water temperature returning from the beams rises above 17° C, additional cooling is provided by the library's absorption chiller or from conventional chillers linked to roof-mounted cooling towers.
In contrast to the natural ventilation used for the lower floors, a more sophisticated close-control environmental control is used to protect the valuable material housed in the archive spaces on the building's upper floors.
"The archivists were adamant that they didn't want storage below ground, because they felt the precious material would be safer at the top of the building," says Haston.
The archives are designed to have an air leakage rate of less than 1m3/hr/m2 at 50Pa, which is considerably lower than
the 5m3/hr/m2@50Pa of the main façade. Temperature and humidity are controlled in these sealed, highly insulated repositories using a mechanical ventilation system and mechanical cooling.
There are many archive spaces, each with a slightly different environmental strategy dependent on the contents. The large vaults, for example, have to be maintained at about 16° C, 50% RH +/- 5%C. By contrast, there are smaller spaces that have to be maintained at conditions as low as 5° C, 40%RH.
Initially, Buro Happold's solution was to maintain the precise environmental conditions for each space, using a ducted solution fed from air handling units located two floors above, on the eighth floor.
This scheme was abandoned in favour of individual close-control units located in each archive space, consuming less energy than the ducted solution and helping keep the service riser dimensions to a minimum.
Another significant benefit of changing to the in-room units for the archives was that it allowed the use of an oxy-reduction fire prevention system. This works by reducing the amount of oxygen in the rooms to a level too low for a fire to ignite. This is a much more effective solution for the precious archives than the alternatives of a sprinkler or gas suppression system. Both are triggered to operate only after a fire starts. Once triggered, the sprinkler system would seriously damage the archived materials. The oxy-reduction scheme also required much less space than the gas suppression solution, which would have needed an enormous floor area to store the bottles of inert gas.
Power for the in-room units is supplied by a combined heat and power (CHP) engine, which is housed in the library's basement energy centre. This gas-fired unit generates 725kW of electrical energy and 1MW of heat.
The CHP is coupled to a 400kW absorption chiller, which will make full use of any excess heat. Supplementary heat is provided by two 2.25MW high-efficiency gas boilers.
The library is connected to Birmingham's district heating network. This will not only provide an additional source of heating to the building, but will also enable excess heat generated by the library's CHP to be fed into the district heating system for use by other city centre buildings.
An illuminated read
While the façade makes a bold architectural statement, its impact on the environmental engineering was minimal. Computerised daylight simulations were undertaken by Buro Happold to enable room daylight factors to be calculated in response to the façade's intricate covering. Lighting is delivered to the levels recommended in the SSL Code for lighting using metal halide downlights. These are set out in a grid arrangement to provide background light levels to the floors.
The downlights grid is overlaid with smaller fittings, set closer together, to increase light levels on key areas like bookshelves and reading spaces. Break-out spaces and meeting rooms are lit with a combination of downlights and decorative circular rings of light that appear to echo the motifs of the building's façade. The downlight arrangement is continued out through the façade and on to the external entrance canopy.
To help visitors with navigation and orientation, the walls of circulation routes, stairs and lifts are all illuminated, as is the
central escalator, which glows an electric blue. Feature lighting around the atrium is provided by the rotunda of circular book shelves, which are illuminated by LED lighting integrated into the units.
A central PC controls the lights based on the time of day, daylight levels, and occupancy, to minimise energy usage. To prevent the bling of the façade disappearing into darkness, the building is given a nighttime presence using light fittings mounted at the back of the façade. These illuminate the circle patterns and wash the building in colour. The colours can be programmed by the DMX control system to form a backdrop for events in Centenary Square, ensuring that, even at night, the building can still maintain a brash presence. CJ
Engineering the façade
The library is wrapped in a filigree of overlapping aluminium circles from the first to the eighth floor. The pattern consists of an inner layer of 1,800 mm diameter circles and an outer layer of 5,400 mm diameter circles.
The inner circles are assembled from aluminium extruded, curved, rolled, hollow sections (RHS),100 mm deep, 50 mm high, and 4 mm thick. These interlocking circles are assembled from a series of quarter-circumference extrusions. Two quarter-circumference arches are attached to each other to form a leaf-shaped object and welded at each end. Four of these are connected through a bolted connection plate to form a four-leaf clover shape. The clover leaves are then connected to form a series of interlocking circles. The outer circles, which are also extruded aluminium RHS (200mm deep x 100mm high x 5mm thick), are bolted through the smaller circles to the connection plates.
The frieze was pre-assembled into 5.4m high, 1.8m wide units, before being delivered to site. Gaskets and weatherproofing were provided at bolted connections to prevent water entering the RHS.
The bespoke façade beneath the metal gauze reflects the building
s ventilation strategy. A thermally broken, unitised aluminium-framed flush curtain walling system is glazed with 4,000mm high x 1,800mm wide rectangular glass panes to give visitors glimpses out of the library over the city's skyline.
At the top of the building, where the archives are located, the glazing is replaced by opaque aluminium infill panels, while at the base, where it encloses the plantroom, the façade is louvred.
The life of the building is expected to be 50 - 60 years and the façade has been designed to achieve a similar lifespan.
Performance as designed
Predicted regulated energy consumption 160kWh/m2/a
Regulated carbon emissions 19 kg CO2/m2/a
EPC rating: B