Birmingham’s new library building was originally going to be built using steel, but the decision was taken to convert the design to a concrete frame, offering benefits to both the programme and cost.
- The benefits of changing to concrete
- A complicated building design
- Overcoming technical challenges using concrete
The new Library of Birmingham is one of the flagship projects in the Big City Plan, the ambitious programme for the redevelopment of Birmingham that was launched in 2008.
The city has previously been served by the Birmingham Central Library, the largest non-national library in Europe that is instantly recognisable as one of the UK’s most striking examples of Brutalist design.
A similarly eye-catching building was conceived to replace it. The initial concept for the structure was for a steel frame with significant trusses at various levels to transfer the loads to the load-bearing columns and core structure.
The benefits of changing to concrete
A J Morrisroe & Sons tendered for the concrete works on the project. During this process, Morrisroe saw an opportunity to develop an alternative design using post-tensioned concrete floor slabs and post-tensioned concrete transfer walls and arches. The proposal was accepted by Birmingham City Council.
“After reviewing the scheme, we felt that the design of the frame could quickly be converted to concrete,” says Morrisroe technical director Patrick Sharkey.
“A post-tensioned concrete frame allowed a cost-neutral solution that delivered the required volume while maintaining the planned height and construction programme”
Patrick Sharkey, Morrisroe
The switch in materials offered both programme and commercial savings. Mr Sharkey says the use of concrete to build the frame rather than steel was absolutely crucial in delivering the building to the required specifications and within the required timeframe.
“The original steel frame solution could not deliver the required archive volume without increasing the building height,” he says.
“A post-tensioned concrete frame allowed a cost-neutral solution that delivered this volume while maintaining the original planning height and construction programme.”
A complicated building design
While the change offered some benefits, the project remained very complex. “The building is officially 10 storeys tall, but it is actually equivalent in height to a ‘normal’ 20-storey building,” Mr Sharkey explains. “Not all of the floors were the same size, and many were double-storied.”
The varied size of the floors, along with the increased loading on each, was also a factor in choosing concrete.
“The floor loading in this building is extremely high – much higher than a normal structure,” he says. “When coupled with the large spans, this presented quite a significant technical challenge that the use of post-tensioned concrete slabs was able to solve.”
“The floor loading in this building is extremely high – much higher than a normal structure”
Patrick Sharkey, Morrisroe
The spans on each floor were larger than normal due to the relatively low number of columns incorporated into the design.
“This meant some of these columns were taking very heavy loads,” Mr Sharkey says. “We used slender composite columns using high-strength reinforced concrete combined with heavy structural steel sections.
“Many of the columns did not descend all the way to the foundations, which created a challenge when working out how to support the load.”
Overcoming technical challenges using concrete
The team created transfer structures to support the weight, and concrete was also useful for this. “Concrete transfer walls and arches were used instead of beams, with some of the columns hanging down from these transfer structures,” Mr Sharkey says.
Another challenge was the construction of internal overlapping rotundas up to 21 m in diameter, which form a shifting central atrium through the full height of the building.
“This took a different shape on each floor, which introduced all sorts of cantilevered arrangements,” Mr Sharkey recalls. “Post-tensioned concrete dealt with this.”
“A great deal of temporary works were designed specifically for the canopy, and we put a lot of effort into this”
Patrick Sharkey, Morrisroe
The slabs used in this part of the structure cantilevered up to 7 m and used the three-storey-high bookwall on the second, third and fourth floors as an integral part of the structure.
Cantilevers were also used extensively in one of the building’s most striking elements: the three-storey canopy that extends over Centenary Square.
The initial steel design proposed this part of the structure as a two-storey-deep braced truss. With concrete, single-storey post-tensioned concrete arches were used with a 10 m cantilever span and a 14.4 m backspan, allowing it to be three stories tall.
Several of these arches were connected to the core structures, with each of these connections requiring as many as 36 40 mm-diameter coupled steel dowels to be accurately cast in to the core walls during the slip-forming process.
“A great deal of temporary works were designed specifically for this aspect of the project, and we put a lot of effort into this,” Mr Sharkey says.
The building is set to open later this year, with the design of the concrete frame an important part of it opening on schedule.