Due to the unbonded nature of tendons to the slab within Unbonded Post Tensioned (UPT) concrete
structures, tendon stress relaxation under heating affects all regions of the slab spanned by the
tendon; not just in the locality of the fire. The numerical modelling of bonded and unbonded post
tensioned concrete structures in fire has been performed to some degree, notably by Bailey and
Ellobody. The consideration of elevated temperature creep to the relaxation of tendon prestress
however, has not been considered. This thesis attempts to incorporate a uniaxial creep strain rate
function of stress and temperature into the commercial FE software package Abaqus, compatible for
use within the in-built multiaxial metal plasticity constitutive framework. What follows is a validation
study of the Harmathy’s uniaxial creep strain accumulation function via the modelling of stress
relaxation in isolated, tensioned and heated prestressing steel tendons, against experimental data.
From here, UPT concrete slab models are analysed whilst exposed to a standard fire temperature-time
curve and subsequently allowed to cool. Tendon prestress relaxation and resulting UPT
concrete slab deflection is compared, where tendon creep is explicitly modelled, as opposed to
implicitly covered by Eurocode 2 determined temperature dependent stress-strain curves. Following
this, a large scale continuous one-way spanning UPT concrete structural model is developed to
consider global structural behaviour resulting from localised fire, where realistic boundary
conditions such as beam rotation and deflection are permitted. The ignorance of explicit elevated
temperature creep consideration, in prestressing steel tendons, is commonly justified through the
implicit accountability stated within Eurocode 2 temperature dependent stress-strain curves. This
however is not completely true; Eurocode 2 states implicit accountability only holds should the
tendon be heating at a rate within the bounds of 2⁰C/min to 50⁰C/min. Where only heating of a UPT
concrete slab is considered, evidence from this thesis suggests Eurocode 2 determined stress-strain
curves can implicitly account for accumulated creep strain up to limited temperatures. Prestressing
steel tendons are however embedded within a concrete slab through which thermal gradients build
up during fire. This means heat transfer can continue to the tendon, increasing its temperature postfire
at an ever decreasing rate until it reaches its peak. Should post-fire cooling behaviour not be
considered, continued tendon heating and subsequent creep strain accumulation will be ignored.
Further, during the transition from heating to cooling within the tendon, it will be exposed to
elevated temperatures with a rate of change below 2⁰C/min, whereby Eurocode 2, as stated cannot
implicitly account for creep. It is shown, a significant degree of subsequent relaxation of prestress,
UPT concrete slab deflection and concrete damage in hogging can occur during this phase of postfire
behaviour, where the tendon temperature peaks during its transition from heating to cooling. In
order to justify non consideration of creep, it should be shown tendon temperature will remain
suitably low throughout the entire heating-cooling regime to which the UPT concrete slab is
exposed. This must be achieved through adequate specification of minimum concrete cover to
tendons to limit tendon temperature exposure for a given parametric fire curve duration, including
the potential continued rise post-fire. Evidence within this thesis identifies 350⁰C as a critical
temperature whereby the explicit consideration of tendon creep does not significantly increase
predicted prestress relaxation and subsequent UPT concrete slab deformation, compared to implicit
creep consideration from Eurocode 2. The manufacturing standard to which prestressing steel
tendon strands are produced has been shown experimentally by Gales to significantly influence their
susceptibility to elevated temperature creep. This is reflected by Gales determining differing creep
parameters as a function of stress for incorporation in Harmathy’s uniaxial creep strain function.
Modelled prestress relaxation of isolated, tensioned and heated tendons within this thesis is
therefore significantly reduced when tendons are manufactured to a yield stress of 1860MPa
according to the BS 5896 standard, as opposed to the ASTM A416 standard. As a result Eurocode 2
determined stress-strain curves implicitly account for accumulated creep strain during heating, at
10⁰C per minute, up to approximately 400⁰C for grade 1860 ASTM A416 manufactured tendons and
500⁰C for grade 1860 BS 5896 standard tendons. The aforementioned critical temperature of 350⁰C
does not in actuality apply to necessary explicit creep consideration for UPT concrete slabs modelled
with grade 1860 BS 5896 standard tendons. This temperature however remains a design
temperature limit, owing to the potential onset of microstructural recrystallization beyond 400⁰C
and the associated degradation of mechanical properties that coincides. The reasons for such
differing elevated temperature creep and stress relaxation behaviour between the two
manufacturing standards of prestressing steel wires and strands has been postulated within this
thesis to be due to differing chemical compositions. This relates specifically to large relative
differences of phosphorus and sulphur found in wires manufactures to each standard as tested by
Gales