PRECAST BRIDGE FOR KYALAMI

February 2, 2016  3293  3293   

Manufactured by CMA member, Silverton Precast in Pretoria, the 13m precast T-beams reduced construction time by at least six weeks on site and allowed a far simpler and more cost-effective system to be developed for the world-class race track.

After initially weighing up the options, lead engineers WSP Group Africa (Pty) Ltd designed the structure to be carried on top of reinforced earth embankments spanned with 40 precast beams. These would act as both the backbone of the bridge and as a permanent formwork for an in situ concrete deck, as well as road surface layers.  

 

Technical detail

In order to ensure the smooth progress of the project, Silverton Precast owner, George Ellinas, invested a considerable amount of time and infrastructure to ensure the 40 oversize beams were delivered on time and to the tight specifications of the engineer.

He explains: “We were awarded the tender to produce 40 TY5 pre-stressed beams weighing 10 tons each and measuring approximately 13m by 700mm wide and 600mm high. This required us to prepare casting beds with the capacity to produce two beams per bed at a time. We also had to install tensioning equipment to ensure that pre-stressed cables were correctly tensioned on each beam.

“By putting in two sets of shutters and increasing the strength of our concrete mix, we were then able to remove the shutters within just five days to ensure the strand could be removed in time. While the sides were removed, the soffits were designed to be left in place to speed up the process.”

 

Stringent regulations

With final preparations in place, the first pour started early on 21 May this year and the entire project was completed just six weeks later on 2 July. This was a remarkable achievement considering that the team at Silverton Precast had to cure the concrete to at least 40MPa to ensure the shutters for the concrete could be safely removed.

George explains that the pre-stressing process required up to 500 tons of pressure to conform to National Highway Regulations, which is necessary to allow the heavy weight of the road structure as well as the heavy construction equipment to move over the bridge with no effect on the overall integrity of the structure.

Bridge designer, Hugo Louw of WSP Group Africa, says the bridge project forms part of the massive upgrade of the Kyalami Race Track, which entails lengthening and upgrading the surface of the track, as well as providing improved access to the inside perimeter via the underpass bridge.

 

Need for speed

“We had to design a bridge that could be constructed quickly to keep up with the rest of the construction work. Precasting was the obvious choice as this meant that there were few or no disruptions on site (as would have been the case with in situ construction). It also gave us the option to be innovative and opt for reinforced earth walls which not only speeded up the project considerably, but also significantly reduced the overall cost of the project.

“The project relied on sub-contractor, Reinforced Earth, to prepare the walls with steel strap mesh and specially designed panels made from 140mm of reinforced concrete. Once completed a concrete strip was cast in situ at the abutment to form a solid footing for the beams. Elastomeric bearings were then installed with a spacing of 765mm between beams. The upside-down T- shape beams were then lowered into place and spaces were grouted to form permanent formwork that was then filled in with a deck of in situ concrete.

“This provided a strong base for construction work to proceed and carry the weight of the heavy road surfacing machines, as well as several road base layers and the special bitumen running course. As an added benefit, the reinforced earth panels as well as the exposed beams at the bottom of the bridge provide a more attractive and smooth face,” explains Louw.

He adds that the end result is satisfying as it met all the criteria for speed, cost and aesthetics. “We are proud of the rapid deployment of the bridge as well as the overall look, which fits in with the sophisticated nature of the track. This is a job we can truly be proud of and all sub-contractors involved in the job need to be commended.”

 

Specifications

•    Manufactured in six weeks

•    Weight per beam 10 tons

•    Measuring approximately 13m

•    Dimensions 700mm wide x 600mm high

•    Overall bridge span 9m

•    Bearing-to-bearing span 11m

•    Overall span 13m

•    Beam is tensioned and prestressed

•    Readymix concrete used due to time constraints

•    Readymix ensured consistent strength and workability

•    Mix designed by Frank Cutugno of Pronto Readymix

•    Truck Conveyor Mounted pouring of concrete which allowed concrete to enter 300mm space

•    End strength of beams after 28 days approximately 70MPa

 

About Silverton Precast

The company was started in 1998 by George’s father, Costa. Together the father-and-son team established a concrete yard to manufacture lintels for the Pretoria building market. From humble beginnings manufacturing approximately 300m of lintels per day, the company has expanded to become one of the largest lintel manufacturers in the capital, producing 4 000m per day and seven days per week.

In 15 years the company has manufactured enough lintels to go end-to-end from Cape Town to London. It has also expanded into the fields of civil and infrastructure work manufacturing a range of products that range from light mast bases to road barriers, stormwater channels, grids and a variety of road-related infrastructure.

“We know concrete, we know steel and we know shutters. If you give us a plan and a steel bending schedule, we will manage everything else from there. By adhering to all quality management specifications, the end result will be a product that conforms to all specifications and criteria and that is exactly what we aim to achieve,” concludes George.