January_1965 — Page 17

SUPERLOAD

- LBS. SQ. FT.

500-

400+

FIRST VISIBLE CRACKS

LOAD MAINTAINED FOR 20 HOURS

HELICOIDAL STAIRCASE LOAD_DEFLECTION CURVE

DEFLECTION AT MID HEIGHT OUTER EDGE

300-

FOUNDATION MOVEMENT NOTED

200+

SUPPORTING_BEAM_DEFLECTED 0.3

100+

DEAD WEIGHT OF STRUCTURE – 110 LBS. SQ. FT.

DESIGN LOAD (SUPER IMPOSED)

SUPPORTING BEAM DEFLECTED 0,15

TOTAL DESIGN LOAD 210 LBS. SQ. FT.

Test staircase after test load has been removed. It has a residual distortion of about 1 in. Finishes are applied to the middle half but chipped away for mounting the gauges. There are slight cracks due to bending in the mosaic tiling. Tiles which have broken away were damaged while the load was being placed

mezzanine floor levels.

Although

this flight springs from a foundation block and not a landing, the larger relevent end deflections which can occur in this section represent a cri- tical loading condition,

The stair design was based on the use of concrete of minimum cube strength 6,000 lb./sq. in. at 28 days and deformed high tensile reinforce- ment having an ultimate tensile strength of 37 to 43 tons/sq. in. The amount of reinforcement provided represented two per cent of the waist cross section chosen.

The works concrete cube strengths at 28 days averaged 6,300 lb./sq. in.

The model was supported at its up- per end by a steel structure and the actual support condition simulated by a concrete shape equivalent to the cross section of the supporting beam and adjacent corridor slab, resting on six rubber pads each 3in. thick.

Bolts of 14in. passed through the pads and beam. By tightening these bolts, the upper end of the structure could be deflected to represent the behaviour of the supporting beam un- der load.

Load Testing

Prior to carrying out the test prop- per, sections of the model were tiled and other sections plastered to check the proposed finishes and to observe their behaviour under load.

The test was carried out 28 days after concreting. Fifteen tons of con- crete blocks had been cast as kent-

ledge and bundles of reinforcing steel were available nearby should additional load be required. Timber trestles were placed under the stair, but clear of the soffit, as a safety measure in case of collapse.

Thirteen gauges were mounted on

independent

supports to measure radial and vertical deflections under the test loads and a series of check levels were also taken on the stair- case and its supporting structure by dumpy level.

On the first day of testing the de- sign load of 100 lb. per sq. ft. was not exceeded. The loading was car- ried out in increments of 25 lb. per sq. ft. under a programme designed to demonstrate the elastic behaviour of the structure under various ar- rangements of the design load.

To Destruction

On the second day, testing to de- struction was commenced. The model was again loaded to its design condi- tion and the bolts in the supporting structure then tightened to simulate a beam deflection of 0.15in. Load- ing continued uniformly, gauge read- ings being taken at each further over- all load increment of 25 lb. per sq. ft.

At the end of the second day the staircase and landing had been load- ed to 225 lb. per sq. ft. and the de- flection bolts were again tightened to increase the beam deflection to 0.3in. Under this load, equivalent to a total super load of 13.4 tons, the maximum deflection was 0.554in. at mid-height on the outer edge of the stair and there were no signs of cracking.

On the third day it was not pos- sible to continue the gauge readings but loading continued and deflections were measured by dumpy level against a scale fixed to the side of the stair at its point of maximum de- flection.

Shortly after further load had been added it was noted that the founda- tion block was moving and eventual- ly the foundations to the supporting

Far East Architect & Builder January, 1965

2

DEFLECTION

INS.

3

structure also began to settle. Load- ing continued however and the first cracks were seen at a load of 530 lb. per sq. ft. in the outer edge of the stair close to the landing.

At this stage the deflection at mid- height of the staircase was 2.4 in. and the rate of deflection was show- ing a marked increase.

Further load was added up to 575 lb. per sq. ft., or a total super load of 34.5 tons, when the deflection was 2.8 in. and a substantial number of cracks were visible on both sides of the model. This load was maintain- ed for 20 hours but the deflection increased by not more than 0.3in. during this period.

When the load was removed fur- ther cracks were found in the treads and risers where they had been hid- den by the applied load. A later check showed that the staircase re- covered by about lin. after the load was removed but the foundations had all been substantially displaced and the maximum permanent distor- tion of the structure after the test was about in.

Conclusion

From the results it is apparent that the safety factor of the staircase de- sign was at least 3.0 and that the thin flat section chosen is a stronger heli- coidal element than standard methods of design suggest.

The test data is now being analysed in greater detail with the intention of publishing a technical report and conclusions likely to be of assistance to engineers designing similar struc- tures in the future.

The general contractors for the building, Ahong Construction Co. (M) Ltd., co-operated in carrying out the tests.

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