Post-tensioned (PT) concrete slabs with unbonded tendons have been widely used in residential and commercial buildings. However, fire may cause irrecoverable damages to such slabs including rupture of prestressing steel tendons, concrete spalling, large downward deflection and even progressive collapse. Therefore, thorough investigations of the slabs under fire condition are desirable to formulate performance-based fire resistance design guidance.
Numerical modelling of PT concrete slabs with unbonded tendons under fire condition was carried out based on available test results in the literature. Finite element (FE) models were established employing the commercial package ABAQUS. Results from the preliminary study achieved acceptable agreement with the test results. Parametric study was further conducted for better understanding of the structural responses and failure modes of the slabs.
The behaviour of prestressing steel tendons at elevated temperature was then investigated with emphasis on thermal creep, where a three-dimensional (3D) model incorporating Harmathy’s thermal creep model was proposed and verified. As there was limited data of thermal creep, thermal creep tests of prestressing steel to GB/T 55224 and BS 5896 were conducted, from which parameters of Harmathy’s creep model were obtained. Making use of the parameters obtained and the proposed 3D model, the thermal relaxation of prestressing steel tendons could be accurately predicted as compared with the results of thermal relaxation tests of the both types of prestressing steel conducted in the present study.
The proposed 3D model for prestressing steel was further incorporated into the FE models of PT concrete slabs to investigate the effects of thermal creep of prestressing steel tendons on the structural responses of the slabs. Numerical results show that thermal creep has slight effect on the downward deflections of slabs, but has obvious effect on the tendon stresses. Moreover, the transient creep strain of concrete was further incorporated into the FE models of PT concrete slabs, and the results showed that the transient creep strain had obvious effect on the downward deflections of slabs, and hence the transient creep strain should not be ignored.
Tests and numerical modelling of PT two-way slabs with high-strength self-compacting concrete and unbonded tendons under ISO 834 standard fire condition were conducted. Useful results were obtained from four fire tests including the vertical and horizontal deflections, crack patterns, possible tensile membrane action and explosive concrete spalling. Based on the tests, numerical modelling was further improved in consideration of tendon distributions, levels of prestressing and areas exposed to fire, where the thermal creep of prestressing steel and the transient creep strain of concrete were explicitly considered. Better understanding of the structural responses and failure modes was obtained.
The results obtained were then examined for more insight into the structural fire performance of PT concrete slabs and formulation of performance-based fire resistance design guidance. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy
| Identifer | oai:union.ndltd.org:HKU/oai:hub.hku.hk:10722/208022 |
| Date | January 2014 |
| Creators | Wei, Ya, 魏亞 |
| Contributors | Au, FTK |
| Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
| Source Sets | Hong Kong University Theses |
| Language | English |
| Detected Language | English |
| Type | PG_Thesis |
| Rights | The author retains all proprietary rights, (such as patent rights) and the right to use in future works., Creative Commons: Attribution 3.0 Hong Kong License |
| Relation | HKU Theses Online (HKUTO) |
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