the ribs are located so that the anchor- ages of the prestressing tendons lie between the mullions.
To simplify the analysis and design of the prestressed members and in order to avoid large friction losses due to double curvatures, it was decided to place the prestressing tendons in each rib in the form of simple parabola. This means that the tendons in each rib provide a balancing load or trans- verse component depending on the prestressing force and on the eccentri- city of the tendon at mid span.
Stress-relieved 0.7 in. diameter strands with a maximum initial stress of 169,000 p.s.i. and CCL anchorages were used to make up the tendons. The number of strands varies from two to seven per tendon, giving a prestress- ing force of 116 to 407 kips in dif- ferent ribs.
The required concrete strength was 6,000 p.s.i. at 28 days, but the mix was designed to meet this strength at about seven to ten days to allow the slab to be stressed five days after pouring.
One set of specially developed light- weight fibre-glass pans was used for the formwork and was removed every ten days after the slab had been stressed to be re-used on the floor above.
Air conditioning
The location, orientation and struc- tural concept of the AIA Building set some interesting design considerations for air conditioning. Some of the fac tors which affected this design were:
1. The location of the building at the foot of a hill with no shade from adjacent properties;
2. There are never less than three sunlit faces on any given day of the year;
3. The building is fully glass-cur- tained with tinted glass and has no venetian blinds;
4. There are no interior columns on a typical floor;
5. Interior partitioning is flexible; 6. The sun exposures require a flexible system to cope with the rapid ambient changes of sun, no sun, winter and summer, etc.;
7. The building consists of one floor of garage, three floors of car park, one floor of computer centre, 19 typical floors of offices and the penthouse used for staff and executive dining
rooms.
Due to the extreme sun exposure it was necessary to divide the typical floor area into not less than seven zones. Twenty-one perimeter fan coils were installed on each typical floor
ing from one air handling unit on each floor, which also supplies the fresh air to the perimeter fan coils. A four-pipe fan coil system is used, with pneumatic sequencing valves to mix the required proportion of hot and cold water as called for by the room thermostat.
The computer centre is served by a multi-zone air-handling unit, with cool- ing and heating coils and mixing dam- pers to provide cooling and heating the whole year round.
The penthouse is divided into the staff dining room, which is served by a single zone air-handling unit, and the executive dining rooms and cocktail bars, which are served by fan coils with fresh air supplied from a separate air- handling unit located on the 21st. floor. Heating for this floor is achieved by electric blast heaters installed in the air-handling unit and the fan coils, as a separate 50 h.p. reciprocating system is used to take care of this floor for the purpose of running the air con- ditioning during after hours when the main centrifugal units are shut off.
Mechanical ventilation is used for the garage and the car parks with 30 air changes per hour as a criterion for the garage.
In order to incorporate the special ceiling design which is a feature of the main ground floor of the building, aluminium linear diffusers are used with identical dummy 'diffusers' inter- crossed to form the square ceiling pattern.
dicated that the reverse cycle system was possible.
On these bases the following re- frigeration equipment was selected: Two centrifugal units of 320 tons each, operating on 3.3kv and with the piping so arranged that they operate on reverse cycle for winter heating; one cooling
The results of computer analysis on the air conditioning loading showed that the maximum instantaneous load was 752 tons and the design load, with 15 per cent diversity, was 640 tons. The results of well water analysis in- Slipformed core structure
with one interior zone served by duct- Fibre-glass pans used for floor formwork
Far East BUILDER, February 1969
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