Berlin crowdsourcing platform Jovoto has launched an open design competition to repurpose a defunct railway bridge spanning the Danube river in Linz.

Jovoto is asking members of the public to submit and vote on ideas to recycle parts of a 393-metre-long steel bridge in "the world's largest upcycling project".

The 120-year-old railway bridge is reaching the end of its technical life and the campaign has been launched to preserve some of the historic structure within a civic project.

Over 30 proposals have been submitted to the online competition so far, which is open to architects, engineers, artists and designers and runs until 31 August 2015.

Among the ideas are a globe-shaped observation tower, a zip-wire line and an ice rink, while one applicant has pledged to convert the bridge into a giant crocodile-shaped play park.

The construction of the winning design will be funded by the bridge owner Linz AG, unlike crowdfunded projects that rely on public donations. Last month, a Dutch studio completed the "world's first crowdfunded public infrastructure project" – a pedestrian bridge in Rotterdam.

Crowdsourcing and crowdfunding are becoming popular ways for organisations to seek public input and funding on proposals. BIG has recently turned to the crowdfunding platform Kickstarter to finance a prototype for its Waste-to-Energy Plant, while the National Museum of Qatar has launched a nationwide competition to design its visual identity.

Jovoto claims that public involvement in the redevelopment of the Linz bridge could help to overcome negative responses to the potential scheme – citing the scrapping of Zaha Hadid's Tokyo Olympics stadium as a key example.

"There's a growing movement towards the use of online crowd-based platforms to publicly weigh-in on civil projects," said a statement from Jovoto. "Crowd-approved results tend to significantly increase the chance of a win-win situation for all stakeholders."

"We only have to look at the recently scrapped Olympic stadium in Tokyo, Japan, for evidence of what happens if the public is not engaged during the design phase."

On 27 September 2015, Linz residents will be asked to vote for their favourite design, or to leave the bridge intact alongside its replacement. A similar process is underway in New Zealand, where the public is being given the choice to select a new flag design from a range of 40 options or keep the country's existing banner.