Document Type

Concrete structures paper

Publication details

Noble, D, Nogal, M, O'Connor, AJ & Pakrashi, V 2014, 'The effect of prestress force magnitude on the natural bending frequencies of prestressed concrete structures', in ST Smith (ed.), 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23), vol. I, Byron Bay, NSW, 9-12 December, Southern Cross University, Lismore, NSW, pp. 333-338. ISBN: 9780994152008.

Peer Reviewed



The effect of prestress force magnitude (N) and eccentricity (e) on natural frequencies (NFs, ωn) of prestressed concrete (PSC) structures has been widely debated. No agreement currently exists as to how prestress force magnitude and eccentricity affect natural frequencies of PSC structures. Some reports state that the NFs decrease with increasing N (“compression softening” effect); others suggest that NFs increase with increasing N. Finally, some claim that the NFs are unaffected by N. This paper describes and analyses impact hammer testing conducted on a post-tensioned concrete beam at various levels of post-tensioning force. The aim of the research is to investigate how the NFs of PSC structures change with prestress force magnitude. Experimental modal analysis has been conducted and the experimental natural frequencies were identified by employing the Fast Fourier Transform (FFT). This was repeated for varying values of post-tensioning force at different impact locations. The applications of the results are widespread. The Eurocode 2 (EC2) equation for prestress loss as a function of time is well established. If the relationship between prestress force and NF for PSC structures can be established, then the variation of NF over the design life of a PSC structure due to prestress loss can be estimated. This problem is particularly pertinent in the field of post-tensioned concrete wind turbine towers and in the field of pre and post-tensioned concrete bridge girders.