Could you confidently gallivant under huge mushroom-like structures knowing that they had been glued – not bolted – together? The architects and engineers of the “Parasols” in Seville, Spain, certainly hope so because the design features components that are stuck to each other in such a way. Understandably, they say the biggest problem was finding a glue that could withstand 60°C (140°F) and therefore wouldn’t melt in Seville’s summer heat. This is a fairly important criterion for the free-standing parasols that cover an area of 150m x 70m - one of the largest architectural timber structures ever built. Researchers from the Fraunhofer Institute for Wood Research WKI in Germany have adhered to the challenge and stuck with a formula they believe will do the job.

Rising up like gigantic metallic mushrooms (as it has been described) the Parasols began in 2004 as an international ideas competition conducted by the Sevilla Urban Planning Agency. It is hoped to be completed this year. The site began as a large covered market square known as the Plaza de la Encarnacion in 1842 before the stalls were knocked down in 1973 and replaced with a car park. It wasn’t until an archaeological dig in the 1990s – during planned construction of an underground carpark – that the site was given a change of direction.

From Roman times to futuristic construction

The square is now playing host to some pioneering construction techniques (along with its eye-catching design), because conventional mechanical joining methods were ruled out for structural reasons. They have been replaced with somewhat adventurous glued in threaded rods to connect even the load-bearing structural components that consist of finely-wrought laminated veneer lumber beams.

As a typical Seville summer is bathed in relentless sunshine which keeps the mercury constantly extremely high, finding a glue that wouldn’t melt in the heat – and hence lose its ability to keep the structure together – was a job given to researchers from the Fraunhofer Institute who have been working on behalf of the building inspection authorities to determine how close the thermal load is likely to come to this limit.

“We ascertained the temperatures that might occur at the site and used simulations to determine the temperature this would trigger within the construction materials,” explains head of department at WKI, Dirk Kruse. “Our results revealed that the temperatures in the adhesive could potentially reach almost 60°C, which is obviously too close to the limit for comfort.”

If the researchers couldn’t create a suitable glue, the building inspection authorities would have been forced to bring building work to a halt. The group began “tempering” the structural components, which means that once the components had been glued in place, they were heated up again.

“This causes post-curing reactions to occur,” said Kruse. The result is that the glue can now withstand up to 70°C and delivers an acceptable safety margin over and above the thermal stress that is actually expected to occur, which means that the building work can now be continued as planned and Seville will soon be featuring a brand new landmark.

As the structure nears completion, what once looked like a random collection of six interlinked mushrooms is actually a huge, four-level public space, comprising a farmers’ market, retail spaces, a metro station, many bars and restaurants and an archeological space. The museum connected with the Roman excavation is in the basement below street level with an entrance in the thickest parasol column, and a large stairway leading up to ground zero. The air-conditioned, covered market is on the ground floor at street level, and above this is an elevated plaza with bars and restaurants, designed for the local people and tourists to gather to watch a concert, theater or a sporting event.

The panorama deck occupies the second floor, level with the eaves of the surrounding buildings, and also hosts the Sky Cafe. A walkway offers views of Seville's old city, 22m above the ground. Visitors can also catch glimpses of the other platforms down to the museum through cutouts in the platforms, and look out over the restaurants from smaller terraces.

The architects chose Kerto-Q light timber beams with a polyurethane coating, which is said to be cheaper than metal but still durable. The polyurethane coating protects the wood, which is sustainably-grown, and allows it to breathe.

The architects say the Parasols are self-cleaning and only need repainting every 20-25 years.