Concrete Grandstands. Part I. Experimental Investigation.
4.3.1 Rationale
This paper describes the experimental investigation of a set of three identical precast concrete terrace units and their behaviour under static loading-unloading. Three main objectives were set: First, to examine the structural behaviour of a family of RC terrace units supported at three predefined positions and undergoing static loading-unloading.
Second, to estimate the uncracked and fully cracked stiffness of the units and use them in a dynamic analysis. Third, to provide a firm platform on which a rigorous numerical model could be built and used for further analysis work. The initial design of the units was carried out by BISON Concrete Products Ltd. It was evident that for simplicity, the units were considered supported at the two ends only and designed accordingly. However, it was also evident that this approach did not depict realistic site conditions.
Support of these units is often provided by steel raker beams on which a series of steel plates (stools) are welded to provide the end bearings. The front edge of the tread rests on the „riser‟ of the lower unit and so on. The raker beams were not reproduced in the
laboratory due to limited space. Steel stools were inserted under the ends of the riser and a suitable UB-section was placed under the front end to replicate the line restraint by the lower unit. Loading was applied by means of 6 point loads along the tread, arranged at a distance of 100 mm from the riser in an incremented-decremented manner. The deflection
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at mid-span and the maximum strain in the main longitudinal and lateral reinforcement was recorded, as well as the strain at ten different positions, against the applied load.
4.3.2 Contributions
It is not considered to be standard practice to perform full size physical tests in any laboratory mainly due to the limited space available. However, the benefits of such tests are well understood and welcomed by academics and especially by practicing engineers who have in the past been slightly cautious of any new theoretical development and especially numerical analysis techniques. Tests of this kind are always regarded as an improvement and a contribution to the engineering community, as well as a valuable aid to numerical analysis work.
Specifically, the maximum displacement of the units measured at mid-span was found to be approximately equal for both uncracked and cracked units. This would demonstrate a similar behaviour before and after cracking, somehow emphasizing already established reinforced concrete section theories. Also, strain distribution at mid-span across the riser was found to be linear and similar in both cases. However, a more complex deformed shape of the units was revealed, when a „trough‟ developed at the central region. At the same region the tread, situated below the neutral axis of the riser, followed closely the behaviour of the latter developing tension at the top. This could have certain implications on the design of the units, as the tread did not seem to be appropriately reinforced in the locality. In fact, it is understood that considerable savings could be achieved if the units were designed using plate theory rather than being considered as simply supported beams.
The above is reviewed later when a finite element model is under consideration. It is expected that the published results will be utilised by a number of researchers who would like to build upon current strengths.
4.3.3 Benefits
The use of full-size, RC laboratory investigations has historically been associated with many practical and financial problems and has generally been evaded by researchers.
Hence, any type of related work can only be welcomed by all those interested. Owing to its predictable properties, it is now postulated and established that most numerical models
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featuring steel as the main material may need no further validation. The same cannot be said for reinforced concrete. Although its behaviour under complex stress conditions has been under investigation for years, there is as yet no universally accepted constitutive law established. Hence, any experimental „add-ons‟ are always welcome. This series of carefully planned and executed full-size laboratory tests proved to be invaluable for calibrating, fine-tuning and finally validating the numerical models developed later. The same tests may be useful to the wider academic and practising communities as they can provide useful information and experiences for similar types of work. Also, they have been used several times by the author as an aid to teaching reinforced concrete theory to his students of Civil and Structural Engineering.
4.3.4 Originality
The author is not aware of similar earlier attempts to assess the performance of full-size grandstand terraces in the laboratory. It is expected that this study will be very useful to precast concrete manufacturers and design consultants of grandstands and other similar structures. The results obtained are original and of good quality and portray the real structural behaviour of the terrace units, not the one assumed in their design.
4.3.5 Value, Impact
Article No. III has been published recently and therefore it is relatively early to assess its impact by the number of times it is quoted elsewhere. The author believes that other researchers may be using his findings to develop their own hypothesis and even utilise his experimental results to validate their own models. Also, the paper should be of value to precast concrete manufacturers who specialise in similar types of structures and essentially in the design and construction of grandstands. Living in a highly competitive technical world any savings in materials should be welcomed and regarded as a matter for survival.
Therefore, revising their design according to the findings of this investigation should be beneficial. Finally, these tests have become the „validation certificate‟ for the numerical model described below.
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