AdSec for prestress design Ian Feltham, R+D, London

AdSec principles AdSec is a section analysis program considering the loads and moments acting on that section It was originally written for reinforced concrete, but it has been extended for use with other materials AdSec uses an iterative routine that, for a trial strain distribution across the section, integrates the stresses to calculate the resulting axial force and bending moments The solution is obtained when the resulting axial force and bending moments are within acceptable limits

AdSec principles stress strain NM

Prestress in AdSec - basics Prestress can be applied to bars or tendons either as a force or as a strain in ‘General Reinforcement Definition’; in all cases the prestress must not exceed the elastic limit of the material Note that since AdSec takes compression as positive, a negative prestress will result in tension in the steel

Prestress - options in AdSec There are two ways in which engineers consider prestressing: –situations where the external loads do not include the effects of anchorages and tendon curvature (the traditional prestress option that has been available in AdSec) –situations where the external loads include the effects of anchorages and tendon curvature (an option available in AdSec version 8.0, now in beta testing)

w L stress P e 8Pe/L 2 N=0M=wL 2 /8 external loads do not include prestress effects N=0M=wL 2 /8 external loads do not include prestress effects N=PM=wL 2 /8 - Pe external loads include prestress effects strain prestrain prestress

Prestress - options in AdSec

For structural elements such as prestressed precast floor and bridge units with straight tendons, usually stressed before the concrete is cast, it is often more convenient to consider only external forces and moments applied to the element Situations where the external loads do not include the effects of anchorages and tendon curvature

AdSec will generate the prestress force in the concrete. Applying a prestress this way is equivalent to shifting the origin of the stress-strain diagram of the tendon or bar along the tensile strain axis, as shown in red on the diagram to the right origin for steel without prestress concrete compressive strain compressive stress origin for steel with prestress where loading does not include effects of anchorages and curvature prestrain (-ve) prestress (-ve) Situations where the external loads do not include the effects of anchorages and tendon curvature

resulting curvature and shortening for no applied external load strain prestrain strain in concrete Situations where the external loads do not include the effects of anchorages and tendon curvature

Situations where the external loads include the effects of anchorages and tendon curvature For structural elements such as post-tensioned floors, bridge beams and offshore structures, it is usually more convenient to consider the forces exerted on the structure by the prestressing tendons or bars in addition to the forces and moments applied to the element as a whole The forces exerted by the tendons or bars are applied: –at their anchorages –where they deflect or curve –where frictional losses occur

AdSec will not generate any additional prestress force in the concrete. Applying a prestress this way is equivalent to shifting the origin of the stress-strain diagram of the tendon or bar along the tensile stress and strain axes, as shown in blue on the diagram to the right origin for steel without prestress concrete compressive strain compressive stress origin for steel with prestress where loading does not include effects of anchorages and curvature prestrain (-ve) prestress (-ve) origin for steel with prestress where loading includes effects of anchorages and curvature Situations where the external loads include the effects of anchorages and tendon curvature

section remains straight and unchanged in length for no applied external load strain strain in concrete effective strain in tendon equals strain in adjacent concrete Situations where the external loads include the effects of anchorages and tendon curvature

Losses Shrinkage losses can be modelled directly by applying strains to the concrete only

Losses For situations where the external loads do not include the effects of anchorage and tendon curvature: – the force or strain specified in the bar or tendon should be that when the concrete is unstressed, usually during the casting process, with no losses – elastic losses are accounted for automatically with this option – it may be convenient to model relaxation losses using ‘Prestress Factors’ in ‘Analysis Cases’

Losses For situations where the external loads include the effects of anchorage and tendon curvature: –the external applied loads should be those that the prestressed system apply to the structure; they should therefore include elastic, anchorage, frictional and relaxation losses –to be completely correct, the force or strain specified in the bar or tendon should have the elastic losses removed; it is the force or strain that would be in the steel were the surrounding concrete to be unstressed; however, this adjustment will not affect the results if the bar or tendon remains linear elastic (usually so for SLS)

What do you need to do? Be aware if your applied forces include prestress effects Select the appropriate prestress option Remember that, irrespective of the bar or tendon strains and stresses used internally during the analysis, total strains and stresses are always output Consider losses, remembering that the program automatically includes elastic losses