This article was first published on the Access Engineering blog.
With the increase in land prices especially in urban areas, designers are compelled to plan taller structures with a minimum number of vertical load carrying elements (columns, shear walls etc.), to maximise floor space. 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. Column free spaces give tenants the flexibility to outfit the space in any way the see fit, allowing them to easily install partitions as well as HVAC and electrical systems. As per previous experience and research data, it was observed that in Reinforced Cement Concrete (RCC) structures, which are economical, the maximum column spacing can be up to 7.5m – 8m intervals. Post tensioning can be used to more effectively and economically increase the length of the intervals between these columns.
Post Tensioned (PT) systems provide a means of strengthening concrete by exploiting its compressive strength and by removing the tension placed on on it. The major difference between RCC and PT structures from a designer’s point of view is that the tendons are stressed prior to exerting its dead, weight of the building, basic furnishings, fixtures and service loads; to meet building load load bearing standards. Pre-compression is applied to the slab or the beam so as to counteract the tensile stresses due to external loading.
Many leading companies in Sri Lanka hold office spaces in Access Towers (P2)
The Access Tower II Experience
Access Tower II is a 30 story office building with a lobby, 7 levels of car parking, and its own lifestyle center on its top 3 floors, which include; a gymnasium, banquet hall, members lounge, restaurant and rooftop bar. From the beginning the high potential of implementing a post – tension system for the building was recognised. During the brainstorming sessions to prepare the initial structural plan for building, the main issue faced by the architects and structural designers was how to make a the ramp structure in an effective way. There was a proposal to take the ramp part out of the building to maximize the number of parking areas. However in order to do that, it would have been necessary to drive additional piles under the existing basement which would incur a cost as well as require the demolition of the ramp of the existing tower I. Considering this, an alternative, and the finally selected, proposal included a Post Tension system which would allow construction of the new Tower without any alterations of the existing basement of the 1st Tower. The Post Tension beam depth at the start of the cantilevered area was fully optimized to achieve maximum clear height of the circulating area of ramp. Doing this enabled a huge cost saving together with eliminating any demolition and rework that may have been needed for the basement and ramp of the existing tower 1.
Steel tendons are encased in the concrete slabs and tensioned to improve the load bearing capacity of the floor (P3)
Implementation of a Post Tensioned (PT) System
Considering an office floor arrangement, it was finally decided to use a 10.5m x 11.5m main grid, ie column spacing, as it was the most suitable arrangement in view of the buildings structural and architectural aspects. The result is a massive open floor area, with only a single middle column on one side of the building’s “L shaped” design. Having an initial discussion with all related parties, mainly; the architect, structural engineers and M&E engineers was one of the main key points to achieve such a cost effective, buildable and functional structure.
In addition to increase the clear span of the vertical elements, there are several other advantages of using a post tension system in the Tower II project and they are as follows;
Thin slab and beams.
Lighter structure, reducing the dead load on the buildings foundation which consequently reduced the number of piles.
Rapid construction. The typical construction cycle per floor In Tower II project decreased by 6 days.
Improved water seal.
Low floor to floor height reducing building height and enabling construction of higher number of floors within the same building envelope.
A Low floor to floor height ratio also significantly reduces the cooling load of the building resulting in an environmentally friendly design.
The result is single wide open space in both the car park and office floors (P4)
The Post Tension Concept
Tendons with 15.2 mm high strength strands were used according to design profiles as per the post tensioned designed. RAPT (Reinforced And Post Tensioned) software was used to do the post tension analysis and designs. Tendons with two to five numbers of strands were used in a single duct to the ease distribution and spacing of tendons. Dead ends of strands were monolithically casted to the floor and live ends were 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 the ends were treated with non-shrink grout. The insides of the post tensioned ducts were also infilled so that it become bonded post tension system.
The following table shows basic material usage comparison per typical office floor for the PT and RCC system of Access Tower.