Axial shortening in an 80-storey concrete building

Transcription

1 Southern Cross University 23rd Australasian Conference on the Mechanics of Structures and Materials 2014 Axial shortening in an 80-storey concrete building S Fragomeni Victoria University H Whaikawa Victoria University S Boonlualoah PBL Group Ltd. Y C. Loo Griffith University Publication details Fragomeni, S, Whaikawa, H, Boonlualoah, S, Loo, YC 2014, 'Axial shortening in an 80-storey concrete building', in ST Smith (ed.), 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23), vol. II, Byron Bay, NSW, 9-12 December, Southern Cross University, Lismore, NSW, pp ISBN: epublications@scu is an electronic repository administered by Southern Cross University Library. Its goal is to capture and preserve the intellectual output of Southern Cross University authors and researchers, and to increase visibility and impact through open access to researchers around the world. For further information please contact epubs@scu.edu.au.

2 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23) Byron Bay, Australia, 9-12 December 2014, S.T. Smith (Ed.) AXIAL SHORTENING IN AN 80-STOREY CONCRETE BUILDING S. Fragomeni* College of Engineering and Science, Victoria University, Footscray Park Campus PO Box 14428, Melbourne Vic, 8001 Australia. (Corresponding Author) H. Whaikawa College of Engineering and Science, Victoria University, Footscray Park Campus PO Box 14428, Melbourne Vic, 8001 Australia. S. Boonlualoah Director, PBL Group Ltd, Bangkok, Thailand Y.C. Loo Science, Environment, Engineering and Technology Group, Griffith University Director, Internationalisation and Professional Liaison. ABSTRACT An 80-storey reinforced concrete building located on the Gold Coast was instrumented to monitor axial shortening behavior in load-bearing elements. Axial shortening of selected columns and core walls across a number of levels were taken up to 3 year duration, from initial construction period to the completion of the structure with full service loads applied. This paper presents an extensive set of measurements of selected basement columns. Comparisons of columns between basement levels, as well as symmetrically related and adjacent columns, are made. KEYWORDS Axial shortening, differential shortening, concrete columns, high strength concrete, tall building. INTRODUCTION Axial shortening is experienced in load-bearing concrete columns and walls. It can be expressed as the summation of elastic strain caused by load application, shrinkage strain caused by drying of concrete and creep strain induced by sustained stress over a long-term period. Axial shortening is particularly relevant in tall buildings as cumulative loadings on vertical structural members result in significant stresses. Concrete columns and walls can potentially shorten at different rates within the same floor resulting in differential shortening. This becomes an issue when the difference becomes significant resulting in serviceability problems with building services, partitions, finishes and facades. Assumed load paths can also change resulting in both added moments and excessive deflections occurring that result in design actions exceeding design allowances on some members, (Boonlualoah et al. 2005). Current cumulative shortening estimation methods, such as the Age-Adjusted Effective Modulus Method (AEMM) proposed by Bazant (1972) given in Eq. 1, are extensively used by practitioners. Without considering thermal deformations, the AEMM equation for total strain (t,s) can be written as; (1) This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this license, visit

3 where σ 0 is the initial concrete stress, E c (s) is the elastic modulus of concrete at age s, (t,s) is the creep coefficient, (t,s). The shrinkage component is defined by sh, (t) and σ(t) is accounts for stress increments applied over time. Finally χ(t,s) is the aging coefficient taking account the restraining effects of the reinforcement. In Eq. 1 the shrinkage and creep components can be estimated using a reliable code model, such as that given in the Australian standard, AS3600 (Fragomeni and Baweja 2008). Differential shortening is cumulative over the height of the structure therefore it has become an important consideration for taller buildings; particularly since the increased use of high strength concrete (HSC) in tall structures. The effects of axial and differential shortening are therefore both structural and architectural. It is vital that accurate estimation of axial shortening in tall buildings occur in order to control differential shortening through design and construction. Tall concrete buildings instrumented to observe axial shortening using demountable strain gauges (DEMEC) include Lake Point Tower in Chicago, USA, Water Tower Place also in Chicago, USA, and World Tower in Sydney, Australia. Measurements in these cases provided a reasonable database of short time period measurements. However, comprehensive measurements over significant durations for axial shortening are rare across a number of levels and columns. Recently an 80-storey concrete building on the Gold Coast was instrumented to measure axial shortening. An extensive set of shortening measurements was documented (of up to 3 years) on selected columns at various levels, (Boonlualoah, 2010). In this paper an overview of the measured building is presented along with the shortening measurement procedure used. More specifically axial shortening measurements of selected columns located in basement levels were analysed and results are discussed. METHOD OF SOLUTION Description of Instrumented Structure The 80-storey residential building is located on the Gold Coast (see Figure 1). It comprises of a twostorey basement for car parking capacity of 800, a two-storey lower section for retail and a 76-storey tower that contains 500 residential apartments. Above the rooftop at level 78 there is a steel frame glass crown of which a 50 metre mast rises from its peak resulting in a total height of 325 m. The structure has predominately large rectangular/circular columns and walls. Inclusive is a central concrete core wall system coupled with concrete flat slabs partially post tensioned in various locations, (Boonluloah, (2010)). The column layout of the main tower was designed to provide lateral stability, architectural layout and achieve typical floor slab depth requirements. Figure 2 displays the basement floor plan. Columns depicted will be referred to throughout this paper. Dimensions of columns in basement level 2 (B2) are 3200mm wide x 800mm thick, 4000mm wide x 900mm thick, 3400mm wide x 1000mm thick and 2000mm diameter denoted as TC12, TC10, TC08 and TC06 respectively. Related columns on the opposite side of the structure are geometrically symmetrical. Respective locations of all columns are shown in Figure 2. Figure 1 depicts specified nominal concrete strength of the levels measured in this study over the 80- storey concrete building. The grade of concrete specified for the structure varies from 65MPa (B2 to L9), 50MPa (L10 to L39) and 40MPa (L40 to L78). Concrete supplied was sampled in accordance with Australian standards (AS1012, AS1379 and AS3600) for elastic modulus, standard compression, drying shrinkage and creep. In levels B2 and B1 actual sample concrete specimens were found to be greater than 75MPa, which is considered to be high strength concrete (HSC), (Boonlualoah 2010). Details of the Measurements The strain monitoring, to measure axial shortening was undertaken using a demountable strain gauge (DEMEC) with gauge length of 200mm. Manufacturer specifications indicate that the smallest deformation that could be measured was 10 microstrains. The device was considered accurate, portable and reliable thus deemed suitable for taking measurements across a number of levels and columns. Two stainless steel mechanical gauge markers were attached for each separate gauge point ACMSM

4 with heavy-duty epoxy glue (separated by the specific gauge length, refer to Figure 3). Larger rectangular columns were generally designated 6 (in some cases 9) symmetrically placed sets of gauge points on the column face and a single gauge line of 3 points on the side face. Cylindrical columns were designated up to 9 sets of gauge points in 3 lines evenly distributed around the circumference. Figures 3 and 4 depict gauge point orientations for cylindrical and rectangular columns respectively. Figure storey building, (Boonlualoah 2010). Figure 2. Plan view of level B2 (Boonlualoah 2010) Figure 3. Cylindrical column gauge point layout Figure 4. Rectangular column gauge point layout. MEASURED AXIAL SHORTENING Basement Level 2 (B2) Measurements Axial shortening of selected B2 columns measured onsite are presented in Figures 5 to 8. The series numbers correspond to locations of gauge points as depicted in Figures 3 and 4. The solid dark line represents the average of gauge point measurements across the column. Some Peaks and troughs are evident in this average line. This is due in part to temperature variations, with measurements taken at different times of the day and seasons. Even though individual data is relatively scattered, the average cumulative strain follows a log curve as depicted by the dotted log average trend line. Interestingly, some columns experienced individual gauge readings upwards of 800 microstrain (see Figures 5, 6 and 8). This equates to a maximum B2 column axial shortening of 1.8 mm over 900 days for floor height of 2250mm. Related Symmetrical Column Comparison Figures 9 to 12 compare symmetrical axial shortening on level B2 using the average measured trend line on opposite sides of the structure. In some cases it was necessary assume that the columns instrumented at a later date share the same axial shortening path as their related columns until ACMSM

5 measurements commenced. Figures 9 to 12 indicate measurements were very consistent and constant over the 900 days of measurement. Also minimal differences in shortening exist between related symmetrical columns thus indicating that measurements were reasonably reliable and accurate. Figure 5. Cumulative strain of TC12 B2 Figure 6. Cumulative strain of TC10-B2 Figure 7. Cumulative strain of TC08-B2 Figure 8. Cumulative strain of TC06-B2 ACMSM

6 Figure 9. Ave. cumulative strain TC12/TC11-B2 Figure 10. Ave. cumulative strain TC10/TC09-B2 Figure 11. Ave. cumulative strain TC08/TC07-B2 Figure 12. Ave. cumulative strain TC06/TC05-B2 Basement Level Comparison Figures 13 to 16 show shortening comparison of the same column at B2 and B1 levels Again, shortening measurements followed similar paths. As expected, columns in the lower B2 level showed slightly greater shortening values than those at B1. The most significant differences in shortening are in the early column ages as seen in Figures 13 and 15. For example, a differential shortening value of 200 microstrain can be observed in Figure 13; equating to 0.45mm which is relatively small. Columns instrumented at 370 days show minor differentials as depicted in Figures 14, 16 and 17. Early shrinkage (particularly in first 7 days) is difficult to measure due to unexposed columns (formwork). This may help explain some variations in early parts of axial shortening measurements. This segment is theoretically defined as sh in Eq. 1, and if not predicted correctly may lead to erroneous results. Figure 13. Ave. cumulative strain TC12-B2/B1 Figure 14. Ave. cumulative strain TC11-B2/B1 Figure 15. Ave. cumulative strain TC10-B2/B1 Figure 16. Ave. cumulative strain TC09-B2/B1 ACMSM

7 Figure 17. Ave. cumulative strain of TC08-B2/B1 Figure 18. Ave cumulative strain TC12/TC10-B2 Figure 19. Ave. cumulative strain TC10/TC08-B2 Figure 20. Ave. cumulative strain TC08/TC06-B2 Adjacent Column Comparison Figures 18 to 20 provide comparisons of adjacent columns on level B2 displaying differential shortening. Differential shortening can be best observed in Figure 18 with the majority of differential shortening occurs in the first 150 days of column age for TC12 and TC10. During this stage a difference of 150 microstrains (0.34 mm) between adjacent columns was observed, which is small. CONCLUSIONS This paper provided comprehensive axial shortening measurements of basement levels in an 80-storey concrete building on the Gold Coast, Australia. Overall results conclude that: i) measurements were consistent and accurate, ii) measured cumulative strains followed an ideal logarithmic path, even though small peaks and troughs were noted in actual curves, iii) maximum axial shortening in basement columns reached 1.8mm over 900 days which is small, iv) maximum differential shortening was 0.45mm which is not significant, and v) HSC does not seem to present strain shortening issues. ACKNOWLEDGMENTS The authors acknowledge Readymix Australia (now Holcim) for providing support for this research. REFERENCES Bazant, Z.P. (1972): Prediction of creed effects using age adjusted effective modulus method, ACI Journal Vol. 69, April, pp Boonlualoah, S. (2010): Experimental and Analytical Study of Reinforced Concrete Column and Core Shortening in a Tall Building, MEng Thesis (2010), Griffith University, 353pp. Boonlualoah, S. Fragomeni, S. Loo, Y.C. and Baweja, D (2005): Differential deformations of columns and cores in tall buildings - assessment, monitoring and correction Measures, Proceedings of the 6th international conference on Tall Buildings, Hong Kong, Dec 2005, pp Fragomeni, S. and Baweja, D (2008): Performance of shrinkage prediction method given in AS3600, Proceedings of 20th ACMSM Conference, Toowoomba, Queensland, 2-5 Dec 2008, pp ACMSM