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Unusual suspended floor design for Johannesburg tower block
Unique problems were posed for the engineers who designed an all-glass suspended window as- sembly recently installed in the Standard Bank Centre, Johannesburg, South Africa.
The assembly hangs from the lowest of three 'suspended' nine-storey stages which make up the high rise tower. The articulated stages each hang from cantilevers built out from a central spine.
The continuous glass facade, square in plan with re-entrant corners, encloses the foyer at street level. It is the biggest in the world outside the USA.
Perimeter of the facade is 384 ft., height 34 ft. and glass surface area 13,100 sq.ft. Eighteen tons of Pilkington 12 mm Armourfloat toughened glass in 336 plates were shipped from England for the assembly which uses small metal patch fittings to join panels together. The unusual suspended floor design caused the most extreme differential de- flections on the vertical runs of glass ever experi-
enced by the Pilkington engineers who designed the assembly.
The 520 ft. high tower consists of a central reinforced-concrete spine carrying, in addition to elevators and services, the three 'mechanical' floors. These comprise three sets of eight rein- forced concrete cantilevers, spaced at 106 ft. inter- vals. Each set supports the outer ends of the nine floors below with two external vertical concrete hangers, each 105 ft. long, per elevation. The inner ends of the floor slabs are tied back to the central
core.
The glass assembly hangs from the coffer beams of the first floor slab. Both the lower nine floors and the glass assembly hang on cantilevers, and maximum relative deflection at cross beam points is 1.13 in. and total vertical movement approxi- mately 4 in.
Pilkington engineers had to modify conven- tional assembly design to reduce differential de- flections and to ensure each vertical 34 ft. run of four glass panels could move independently of the adjacent runs. The panels at ground level had to be joined to the three above, allowing for vertical movement and for the horizontal movement caused by the building moving under a lateral wind load of up to 30 lb/ft2.
Steel compression springs, designed by Pilking- ton and Tonks (Birmingham) Ltd., are used to sus- pend the glass and reduce deflections. Capable of carrying 650 lb. deadweight of glass, each spring is mounted on a channel iron connected to an angle iron which in turn is bolted to the cross beam.
Divided patch fittings were developed to secure
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Standard Bank Centre, Johannesburg
Far East BUILDER, September 1970
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Suspended glass assembly seen from basement level
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