With the increase of land cost especially in urban areas, building designers are planning to develop tall structures with minimum number of vertical load carrying elements (columns, shear walls etc.). This concept is frequently used by the architects for modern building designs especially in car parks up to podium levels or throughout the buildings in office and mix developments. Columns free space for offices are quite pleasing environment as well as partitions, HVAC, electrical systems can be comfortably installed. As per previous experience and research data, it was observed that the RCC structures are economical where the maximum column spacing up to 7.5m – 8m interval. Post tension system can economically and effectively be used for the spans more than the above range.
Post tensioned (PT) system provides a mean of strengthening concrete by exploiting its compressive strength and overcoming the weakness by removing the tension on it. The major difference between RCC and PT structure from designer point of view is the tendons are stressed prior to its dead and service load are exerted. Pre-compression is applied to the slab or the beam so that to counteract the tensile stresses due to external loading.
2. Access Tower II Experience
Access Tower II is mainly an office building including carparks up to level 8. Offices are up to 27th and rest of floors are used as a restaurant, gym, and roof top as open terrace for recreational area. From the beginning it was observed that there was a high potential for the suitability of post – tension system for the building. There were several interactive sessions between the architect and the structural designers of the project at initial stage. During the brain storming session to prepare the initial structural plan for building, the main issue we had to face was how to do the ramp structure effective way. There was a proposal to take ramp part out of the building to maximize the number of parking areas. However in order to do that, it was suggested to drive additional piles under the existing basement which would cost huge amount together with the demolishing of ramp of the existing tower I. Considering all negative consequences due to this, the alternative proposal was done with the help of post tension system to have 6 m cantilevered ramp without touching existing basement. PT beam depth at the start of the cantilevered area was fully optimized to achieve maximum clear height of the circulating area of ramp. By doing this huge cost saving was possible together with saving basement and ramp of the existing tower I.
Considering office floor arrangement, it was finally decided to use 10.5m x 11.5m main grid as it is the most suitable arrangement in view of structural and architectural aspects. Having initial discussion with all related parties, mainly architect, structural engineers and M&E engineers is the main key point to achieve cost effectiveness, buildability and functionality of structure.
In addition to increase the clear span of the vertical elements, there are several other advantages of using post tension system in the Tower II project and they are as follows,
- Thin slab and beams.
- Lighter structure have lesser dead load on foundation consequently reducing the number of piles.
- Rapid construction. In Tower II project the typical cycle per floor decrease up to 6 days.
- Better water tightness.
- Low floor to floor height reducing building height and enabling construction of higher number of floors with the same building envelope.
Low floor to floor height is leading for low cooling load for AC. This is effective during the total life span of the project and hence environmental friendly sustainable design.
3. Post Tension Concept
Tendons with 15.2 mm high strength strands were used according to design profiles as per the post tensioned designed. RAPT software was used to do the post tension analysis and designs. Tendons with two to five numbers of strands were used in single duct to easy distribution and spacing of the tendons. Dead end of strands monolithically casted to the floor and live end was used to stress the strand. Anchor plates and wedges were used retain the strand force applied through hydraulic jack to the tendons. After observing the remaining length of the strands for 3 days, the extended part was cut off and treated the end with non-shrink grout also infill the inside of the post tensioned ducts so that it become bonded post tension system.
4. Material and Cost Saving
Following table shows basic material usage comparison per typical office floor for the PT and RCC system of Access Tower
|ITEM||POST TENSION CONSTRUCTION||CONVENTIONAL RC CONSTRUCTION|
|Concrete quantity per floor||244 m^3||355 m^3|
|Steel weight per floor||17.3 ton||55 ton|
5. Future Development and Conclusion
Post tension system has many advantages if it IS useD at proper locations with proper structural forms since it is eliminating the weakness of concrete tensile carrying capacity. Not only for buildings, the post tension technology is greatly using in bridge structure as well. Innovative designs can be done to develop the concept for other structures such as circular tanks, shell structures etc. Most impotent aspect to be considered for using post tension system effectively and efficiently is the preliminary planning which could facilitate to use it in an efficient manner.