Bringing in reinforcements

Update on Qatar’s IIB Tower from VSL

After being awarded a sub-contract by HBK to carry out the design, supply and installation of the post-tensioned slabs in the International Islamic Bank (IIB) tower, Qatar, post-tensioning specialist VSL has now completed 36 of the building’s 44 floors.
With responsibility of the slabs material supply, equipment provision, design and detailing, VSL expects to finish the project situated in West Bay, Doha, Qatar, by September this year.
Deputy general manager Stephen Burke has high hopes for the structure, which is expected to be used for both commercial and residential purposes.
“The IIB is going to be a very iconic headquarters for the bank as it is located in a very prestigious part of Doha and will certainly hold its own among the other high rises in West Bay,” he says.
The IIB tower’s floor slab edges flow, and are contoured with no corners or sharp edges, which Burke considers to be a real design credit to the architects. The slabs are a two way post-tensioned arrangement with a central core and perimeter columns rising throughout the structure’s storeys.
VSL complies with national and international standards approved in every country where the use of post-tensioning requires official certification. The company’s technology is based on the post-tensioning process during which the prestress is permanently introduced into the structure after the concrete has hardened.
The technique, which is achieved by the stressing of suitably arranged, high-strength pre-stressing tendons generates
favourable stress conditions in the structure, enabling efficient use of building materials while controlling deformations
under service conditions.
Among the benefits of using post-tensioning is the faster floor-to-floor construction cycle time gained in comparison to traditional construction methods. The early stripping of the horizontal formwork is possible once the tendons have been stressed using stateof- the-art hydraulic jacks and pumps.
Once the stressing of the tendons has been completed the ducts can be injected under pressure with a cement grout, which protects the strands from corrosion and creates a monolithic slab.
With the post-tension technique being used to strengthen all the floors in the IIB tower, the project benefits from all the advantages of the construction method.
“The major advantage of employing post-tensioning is the ability to span longer distances than the traditional reinforced concrete technology.In the case of IIB tower this gives a greater rentable floor space and larger columnto- column spacing,” Burke says.
“Another major advantage is that the slab thickness can be optimised thus saving concrete, reinforcing steel and weight, which is especially important when considering the benefits with respect to the reduction of the carbon footprint on the structure,” he adds.
VSL, which uses the company’s own brand of flat slab system has also used the method to perform structural upgrades and strengthen and reinforce existing buildings using carbon fibre technology for clients who want to
change a structure’s use.
Burke confirms clients are increasingly requesting to change the use of buildings from hotels to offices or offices to apartments.
Michael Rosales, VSL deputy project manager, who has been working on the IIB tower says that despite the list of advantages the use of post-tensioning can bring to projects it is used on, working with the method of construction means challenges such as congestion problems on the slab where the plumbing pipes and electrical lines are located. He confirms issues can arise when plumbing works such as urinals, lavatories and drains need to be relocated.
The slab edge locations — where inserts for curtain walls are generally located — also require the post-tension anchorages and the curtain wall brackets to be well co-ordinated in order to avoid potential clashes.
“An important challenge which needs to be mitigated on all post-tension slab projects is the co-ordination of the MEP services. It is essential at the early design stage that the builders’ work drawings are frozen,” he says.
The sketches show the layout of the services which run inside the slab such as electrical conduits and also slab penetrations for pipe risers and floor drain gulleys.
“As post-tensioning slab designers we need to take care of the designated wet areas such as bathrooms and balconies which are usually depressed in the slab and lower than the finished floor level, these challenges are overcome by early co-ordination with other trades on the project,” he adds.
Due to the complexity of post-tensioning works all labourers currently constructing the IIB tower flooring are suitably trained engineers having received VSL training to use the technique.
The training academy was first established in Bangkok in 2007 to serve the Asian region after which it expanded with a second training organisation in Dubai.
Courses the project’s supervisors and labourers have taken include introductions to the VSL post-tensioning system, post-tensioning bridge labour, post-tension building labour and strand handling and install as well as stressing operation.
In addition to ensuring the building’s high quality the training has helped ensure both the workers supervising and performing the techniques are aware of the hazards associated with the operations.
Burke confirms that although the construction markets in Middle Eastern countries have responded to post-tensioning
at various paces, VSL has used the technique on many projects in Qatar.
“The clients we have dealt with in Qatar have now realised the benefits of using posttensioning. For instance in Saudi Arabia they are still a little bit behind in terms of adopting post-tensioning. They are still doing a lot of reinforced concrete traditionally. We are starting to overcome that momentum now in Saudi but Qatar has adopted the technology quite quickly,” he says.
One of the first companies to employ posttensioning technology on projects in Qatar VSL has also used the method for Umm Bab Cement Silos, a cement plant including all plant structures, silos, infrastructure works like roads and cable trenches, Ras Laffan LNG tank, a giant gas field off the coast of Qatar, Doha city Centre, the Sports Tower project and the Salwa and Immigration bridges.
VSLl — which has also established a branch office in Qatar — is currently tendering infrastructure projects in anticipation of the World Cup in 2022. Burke confirms he expects many tenders to be let after Ramadan in Doha.
“We’ve got a branch office in Qatar so we’re basically tendering new projects, not new buildings but infrastructure, and bridges in anticipation of gearing up for the world cup in 2022. There’s a lot of tendering going on at the moment and we expect after Ramadan they’ll start to let some of these tenders,” he says.
Looking to the future Burke reports that VSL is investing heavily in research and development, a direct result of which, is the introduction of the new VSLAB – S– Series flat slab anchorages, to the market. This new range of slab anchors covers all anchorage unit sizes between one and five strands with ultimate capacity between 260 kN and 1,395 kN allowing
a stepless selection of the required tendon force and spacing.
Another advantage of the new slab system is the “slap-on” principle — the installation of the anchorage after casting of the concrete, which offers benefits to contractors by separating the critical paths of concreting and posttensioning
while minimising finishing works.
Other accessories include grout caps, an optional accessory which further enhances the system’s durability by providing maximum protection against aggressive environmental agents.
Burke says the new systems are in preparations to be used on projects by the end of this year and that VSL’s range of construction projects will increase, as a result.
“In terms of the new sub-system we’ve developed, the advantages are that it’s a new compact system and it’s a more efficient system in terms of pre-stressing and cycle times. So it’s designed to comply with the new European approvals.
“It has been fully tested and approved and will be launched here in the last quarter of 2011,” he reports.
“We’re still using the tried and tested conventional system because obviously with the new technology there’s lots of testing and R&D that has to be done before you can put it into the market. That’s all complete now. It’s all been
signed off so we’re extending the range and it will make a far better, productive component or technology as it’s rapped,” he concludes.