The serviceability requirements of the SAA Concrete Structures Code include recommendations for maximum span to depth ratios, intended to avoid excessive long-term deflections of floor slabs.
Deflection may exceed Code recommendations and not be perceived by some building occupants.
A study of a 20 year old building with normally reinforced flat slab floors designed by others revealed span to depth ratios which exceeded the Code recommendations, excessive ‘sag’ in a typical floor slab, good correlation between measured and calculated deflections, and acceptance of the floor slope by the occupants.
Case Study
Although structural engineers are concerned about reinforced concrete floor slab deflections, building occupants may find the perception of sagging floors difficult to assess.
In a normal office building with carpeted typical floors, a floor surface has different slopes in different areas, because of surface finish irregularities, initial and long-term deflections, and for high-rise buildings, prop settlement.
The perception of a camber or a sag in the floor depends on the slope of the floor, and the magnitude of deflection must be significant to produce a slope that is perceptible to an occupant walking on a carpet covered floor. A floor slope of 1/100 or less would usually not be detected by walking across the floor, and up to 1/75 may not be detected by some occupants (Ref. 1).
These slopes suggest that significant deflections can occur and not be detected. It is more likely that occupants perceive floor slopes from the behaviour of furniture and fittings, such as the appearance of cracks in walls, opening of joints in partitions, sliding of desk drawers, jamming of cupboard and partition doors, sliding of compactus units, and the movement of utensils on sloping surfaces.
The relationship between the above floor slopes and the magnitudes of deflection are shown in Table 1.
For floor spans of from 6m to 8m, floor slopes give the following deflections (mm): * Code limiting deflection value at the mid-span of a column strip (Ref. 5).We were recently involved in the refurbishment of a five storey normally reinforced concrete building designed by others and built in Sydney in 1986. The typical floor was a flat slab 220mm thick with 400mm overall deep drop panels, and spans of 8.0m x 9.5m approximately, discontinuous as shown in Fig. 1.
The structural design should have been carried out in accordance with the SAA Concrete Structures Code AS1480-1982 which recommended a span to depth ratio of 31 for continuous flat slabs not supporting partitions likely to be damaged by significant deflection (Ref 3). Hence the minimum slab thickness should have been 9500/31 = say 305mm. The use of a 220mm thick slab gave a span to depth ratio of 43, which was excessive and not in compliance with the Code. A 15mm upwards pre-camber at the middle of the 9.5m span was specified on the structural drawings.
The unusually long end span of from 9.3m to 9.9m and the large span to depth ratio indicated that excessive long-term deflection of the typical floor slab was likely. So we carried out deflection calculations and arranged for a survey of the top of slab levels.
Our structural calculations were carried out in accordance with RAPT 6 (Ref. 4). Assuming long-term superimposed permanent loading of 1 kPa and a live load (Q) of 0 kPa the long-term column strip deflection at mid-span of the 9.5m span was expected to be approximately 63mm (± 20%). Assuming that the specified pre-camber of +15mm was achieved, the actual deflections from a level survey were 55 and 65mm, as shown in Table 2. The calculated column strip deflection for the load case combination using a live load (Q) of 3 kPa was 79mm.
Note: Actual deflections = measured deflections +15mm (pre-camber) Q = design live load (kPa)
Only the typical floor level 3 was surveyed. We have no confirmation of the concrete compressive strength, the slab thickness, or the achieved pre-camber, and have assumed the values specified on the drawings as shown on Figure 1. Structural calculations indicated that the flexural and shear capacities of the slab are adequate for the design loads required for office use. From typical long-term deflection graphs in Reference 1 we have inferred the plots of deflection shown in Figure 3.
In the above circumstances, the building owner had the options of:
a. Topping the sagging floor to achieve a level surface, or
b. Using partial topping to infill the mid-span areas, or
c. Continuing to use the floors in their sagged state.
The building owner decided not to top the floors because of cost considerations.
Comments:
The original structural design did not comply with the SAA Concrete Structures Code guidelines for serviceability, resulting in excessive long-term sag or deflection of the typical floor slab.
The calculations carried out using RAPT 6 indicated good correlation between the measured and calculated values of the assumed long-term deflection.
The perceptions of floor sag experienced by the occupants were not sufficiently severe to overcome concern about the cost of rectification.
References
Taylor, P.J., ‘The Initial and Long-Term Deflections of Normally Reinforced Concrete Flat Slabs and Plates’, a special projects report for the ACSE, June, 1997.
American Concrete Institute, ‘Deflections of Reinforced Concrete Flexural Members’, Jour. Proc. Vol. 63, No. 6, June 1966.
Standards Association of Australia, ‘SAA Concrete Structures Code’, AS1480 – 1982.
www.raptsoftware.com
Standards Association of Australia, ‘SAA Concrete Structures Code’, AS3600-2001.
