No_9_September_1968 — Page 33

ling

sds to be

stressing

bothways, m. s. mesh

1/2"

reinforcement]

!

Lettering horiz.

Fig. 7.

I's point

¡k point

Elevation of prestressed beam

basement was rejected in favour of water-proofing of the structural con- crete. The basement slabs and walls are only 9in. Pressure grouting was kept in mind as a contingency against random leaks.

R. C. frame

The basement beams and columns also were designed to act with the structure up to first floor to take normal wind and other sway loadings. (Fig. 1 and 2).

Main column spacing to the front elevation was 13ft. 6in. and at the rear the architect requested 27ft. spacing. The clear span across the building varied from 37ft. column centres to 47ft. due to the curved front facade. The 13ft. 6in, column spacing dimension was dictated by the centres of the load bearing 41⁄2 in. reinforced concrete walls over first floor forming the hotel rooms.

The first floor framing was com- plicated as there are vertical service piping ducts

across at alternate column positions and the services piping (water, sewer, etc.) dictated the beam layout.

The loading disposition of the first floor beams required some thought as arching of the load bearing 41⁄2 in. reinforced concrete walls was a pro- bability yet these walls were them- selves interrupted by the corridor, service duct, and suite connecting doors. Of the 26 framing cross- sections not two were identical. So it was necessary to produce 26 in- dividual frame calculations.

Prestressed concrete beam

A complication occurred when during piling a "feature" column in the centre of the foyer was required to be omitted. As this column had a loading of some 900 tons it was ecessary to redispose this loading nd the bearing piling was redesigned accordingly.

However, the beam at first floor spanned obliquely 47ft. and occurred at a service duct position. Also on the rear the loading fell midway onto the Tongitudinal beam due to the 27ft. column spacing.

Total load on the first floor at this. position was some 750 tons with limited headroom, Fabrication of the beam in structural steel was first con- sidered but the plating was so heavy that fabrication overseas would have been necessary.

A reinforced con- crete beam was designed but due to the congestion of reinforcing steel this design was discarded.

The only solution available locally

Far East BUILDER, September 1968.

Fig. 8.

point

of span

(27 20:29;3031!

* symmetry

abg waring!

bars

x'distance

ms. bars Kwelded jo 2* £ reinforcement

14 15 16 17 18 19.2023 2724 25 26 11.23.430MENTS 1013 17:13.

--+

cable

2 wide slot to be filled.

with cement|mortar on completion

of beam.

reinforcement

floor slab

arrangement of 31 cables: are similar to left section but opposite hand.

Section at centre of span

was to prestress the beam (which is really a double beam due to the ser- vice ducts. (Fig. 7 and 8). Due to limitations of depth of the beam it was not possible to prestress once and a method of construction was adopted whereby one or more floors and walls were constructed and the prestressing carried out a part at a time.

This section of the building was kept separate from the rest by straight joints and the relatively long period of constructing the walls and floors over, ensured that the creep and shrinkage of the prestressed concrete beam took place without restraint from the adjacent structural parts.

The PSC beam was hinged at the front end to the column through a steel hinge which was designed to slide under the prestressing forces. This prevented

an undesirable bending moment in the front column which was required to be the same size as the other lesser loaded columns.

First floor beams

One of the problems connected with concreting the large first floor beams. was the fact that the new concrete and shuttering would have overloaded the ground floor. Shoring right through to the basement was first contemplat- ed before a method was evolved whereby the bottom section of the first floor beams was cast and the shuttering removed. Specially design- ed shuttering was then fixed to the part-beam by in-cast bolts and the

floor and beams balance of the first were cast using the part-beam as sup- port. This left the ground floor and basement areas clear of propping.

'Steel beams

At certain parts of the structure there was insufficient depth, due to architectural requirements, to cast

these sections in reinforced concrete. Steel beams were fabricated to suit and cast integrally with the reinforced concrete frame to overcome this problem. (Fig. 4).

Steel hinges

There were numerous revisions during construction: the first floor (short block) was revised to offices, and the consultants were required to provide 27ft. width in two places instead of the 13ft. 6in. width. This was achieved by inserting a secondary ribbed structure at second floor level and increasing the wall thickness of the remaining adjacent walls to 9in.

Due to the other changes the wind and sway became a problem with the four main columns which support the short block. Construction had com- menced in the basement and it became necessary to control the location of the wind shear below first floor to prevent undue bending moments in the wrong places.

This was achieved by inserting four large steel hinges (similar to large bridge hinges) at pre-calculated posi- tions. Hence the whole of this block rests on only four hinges, each of 3in. diameter and 30in. long (Fig. 5). These hinges were fully fabricated in Singapore and the welding supervised by Industrial Inspection (F.E.) Ltd.

Load bearing R.C, walls

The walls above first floor generally are 41⁄2 in, thick reinforced concrete with one layer of BRC No. 10 in each face. The transverse wires of the mesh were increased to 6 gge, at 6in. c/c.

:

The slabs generally from 2nd floor and above are 4in. reinforced with BRC No. 10 bottom,-(No. 2..in end

37