Bangalore Metro to switch to U-Girders for ORR, KIA Metro lines

BENGALURU: To expedite much-delayed Silk Board-K R Puram (19.5km) on Outer Ring Road and KR Puram-Kempegowda International Airport (36km) metro corridors, Bangalore Metro Rail Corporation Ltd (BMRCL) has decided to use U-girders for constructing the viaduct.

This is the first time BMRCL is using U-girders for Metro work. “In the ORR-KIA route, U-girders would be used for almost 80% of the corridor’s length. Another 12% of the length with sharp curves will have I-girders. About 8% will be below the road level,” BMRCL managing director Ajay Seth told. “Phase 1 and 2 mostly used ‘box-girders with pre-cast post-tensioned segmental construction’ and I-girders at transition spans next to each station,” he said.

U-girders will involve casting the entire span of a viaduct between two piers in single piece whereas box-girders will have many segments between two piers.One U-girder between two pillars can be erected overnight, against 5-6 days required in case of box girders.

“U-girders are cast in yard like box-girders. But in case of box-girders, it takes longer to join the segments at the site. In U-girders that time can be saved. However, this means we will need larger casting yards, larger cranes and wide roads at construction sites. The ORR-KIA line enables such wider road access,” said Seth. The BMRCL’s earlier plan to use U-girders for the elevated section between Gottigere and Swagath Road Cross in Reach 6 of Phase 2 did not fructify.

“Now there is enough experience available in the country for that technology. Metro corporations in Delhi, Chennai, Kochi and Mumbai have used that. Systra, an engineering firm, will provide the design since they hold the patent. IISc will do the proof-checking,” said Seth.

The BMRCL has set 2023 deadline for ORR-Airport lines. However, it is yet to invite tenders for both corridors. To avoid further delays, BMRCL has already commenced soil testing and issuing notices to the property owners for land acquisition for these two-corridors.

Techies commuting on the ORR as well as passengers and airport staff travelling to and from KIA have been demanding for Metro connectivity for the past several years.

Experts say U-girders will also save time as well as lower construction cost. The U-girders will also have built-in sound barriers, apart from built-in cable support. Compared to other types of girders, the much longer U-girders drastically reduce construction time.

However, due to their length, they require absolute precision during construction. Also, their length makes transporting them from the casting yard to the site a tricky proposition. While the conventional box girders and I-girders are 2.2m to 2.5m long, the length of U-girders varies between 22 and 27 metres. The longer length means a span between two pillars can be constructed overnight compared to 5-6 days in case of conventional girders. Delhi Metro has used these girders in Airport Express Line, the Badarpur to Faridabad section of Violet Line and a section in Noida.