Natural Frequencies Reduction of RC Slab Subjected to Incremental Concentrated Loads


  • Mezgeen S. Ahmed College of Engineering, University of Duhok
  • Abdulhameed A. Yaseen College of Engineering, University of Duhok
  • Fouad A. Mohammad School of the Architecture, Design and the Built Environment, Nottingham, Trent University, UK



Reinforced concrete slabs are frequently used in many engineering structures such as buildings and bridges. Damages in an engineering structure including reinforced concrete slab are classified as light, moderate, and severe cracks. A special treatment is required for every typical crack-damage to strengthen its structure. The reinforced concrete slab experiencing cracks reduce stiffness as well as its strength. An experimental program was set up to establish a relation between cracks and changes of a dynamic system characteristics of the slab through natural frequencies. The aim is to perform experimental modal analysis (EMA) as a non-destructive vibration testing method for diagnosing the structural integrity of the slab. For this purpose, two RC square slabs specimens of dimensions 1200mm x 1200mm x 40mm were used.  As a first part of the project, a reinforced concrete slab was subjected to an increasing partially concentrated loads at the slab’s center to introduce cracks. After each incremental load steps (5, 10, 12kN) the slab was unloaded. Then, a free-free set-up for the slab was established by using the flexible elastic ropes (bungee cords). After the dynamic test of 5, 10 and 12kN loads, the slab was strengthened using external CFRP sheets (type TR30S). Next, experimental modal analysis was performed to establish a relation between cracks and changes in natural frequencies of the slab. The overall trend of frequency decrease after occurring damage that would be expected for all modes. However, it is not guaranteed all modes have same sensitivity to the applied load that inflicts severe cracks.


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1. AL-Ghalib, A., A. (2013). Damage and repair identification in reinforced concrete beams modelled with various damage scenarios using vibration data. PhD thesis. Nottingham Trent University.
2. American Concrete Institute (ACI318 : 11), (2011). Building Code Requirements for Structural Concrete and Commentary, Farmington Hills, MI, USA
3. Blevins, R. D. (2001). Formulas for natural frequency and mode shape. Krieger Pub. California 95327.
4. Carden, E. and Fanning, P., (2004), “Vibration Based Condition Monitoring : A Review”. Structural Health Monitoring, 3(4) : PP.355–377.
5. Chen, G., Yang, X., Ying, X. and Nanni, A. (2006). Damage detection of concrete beams using nonlinear features of forced vibration. Structural Health Monitoring vol 5 (2), PP. 125–141.
6. Daneshjoo F, Gharighoran A. (2008). Experimental and theoretical dynamic system identification of damaged RC beams, Electronic Journal of Structural Engineering. 8, PP. 29-39.
7. Doebling, S.W., Farrar, C.R., and Prime, M.B., (1998) “A Summary Review of Vibration-Based Damage Identification Methods, ” The shock and Vibration Digest, vol. 30, No. 2, March 1998, 91-105.
8. Ewins, D.J. (2000). Modal Testing : Theory, Practice and Application. 2nd ed. Baldock, England : Research Studies Press. 562.
9. Labonnote, N. (2012). Damping in Timber Structures. A thesis submitted for the Degree of Doctor of Philosophy in Engineering, Norwegian University of Science and Technology Faculty of Engineering Science and Technology Department of Structural Engineering, Norway.
10. Liu, Y., Chandra, J., and Teng, S. (2011). Shear analysis of reinforced concrete slabs with effective moment of inertia. Proceedings of the 36th Conference on Our World in Concrete & Structures, Singapore.
11. Maia N., Silva J., He J, Lieven N., Lin R., Skingle G., To W-M And Urgueira A. (1997). Theoretical and Experimental Modal Analysis. Editors : Maia and Silva, UK : Research Studies Press Ltd., Taunton, Somerset, England.
12. McConnell K and VAROTO P (2008). Vibration Testing : Theory and Practice. John Wiley & Sons, Inc., USA.
13. Meruane, V. and Heylen. W., (2008). Damage detection on a multi-cracked beam by parallel genetic algorithms using modal characteristic. Proceedings of the International Conference on Noise and Vibration Engineering, ISMA, Leuven, Belgium.
14. Ozdoganlar, O. B., Hansche, B. D., and Carne, T. (2000). Experimental modal analysis for microelectromechanical systems. Experimental Mechanics, 45(6) : PP.498–506, 2005.
15. Peeters B., Abdel Wahab, M., De Roeck, G., De Visscher, J., De Wilde, W.P., Ndambi, J.-M. and Vantomme, J. (1996). Evaluation of structural damage by dynamic system identification.Proceedings of ISMA 21, the 21th International Seminar on Modal Analysis
16. Raj, B., Jayakumar, T., Rao, B. P. C., (1995).Non-destructive testing and evaluation for structural integrity. Sadhana. Volume 20, Issue 1, PP. 5-38.
17. Reynolds, P. and Pavic, A. (2000). Impulse hammer versus shaker excitation for the modal testing of building floors. Experimental Techniques. 24(3) : PP. 39-44.
18. Saatcı, S., (2007). Behaviour and modelling of reinforced concrete structures subjected to impact loads. A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy to civil engineering department, University of Toronto.
19. Salawu O. (1997). Detection of structural damage through changes in frequency : a review, Engineering Structures, 19. 718-23.
20. Saleh F, Supriyadi B, Suhendro B. (2004). Damage Detection in Non-Prismatic reinforced Concrete Beams Using Curvature Mode Shapes, Proceedings of the International Conference on Structural Integrity and Fracture. PP. 331-338.
21. Schwarz, B.J. and M.H. Richardson (1999). Experimental Modal Analysis. in CSI Reliability Week.. Orlando.
22. Vecchio, F.J. and Tata, M. (1999). Approximate analysis of reinforce concrete slabs”, Structural Engineering and Mechanics, 8(1), 1-18.
23. Zonta, D., (2000). Structural damage detection and localization by using vibrational measurements. PhD dissertation, University of Bologna, Gennaio.



How to Cite

Ahmed, M. S., Yaseen, A. A., & Mohammad, F. A. (2019). Natural Frequencies Reduction of RC Slab Subjected to Incremental Concentrated Loads. Academic Journal of Nawroz University, 8(3), 17–26.