Differential Column Shortening in Tall Buildings

Differential Column Shortening in Tall BuildingsA Project Presentation for Behavior of Reinforced Concrete CIV_ENV 425

Oluwatobi BABARINDEOverview

To avoid unexpected damage in structural and nonstructural elements, differential shortening between vertical members resulting from differing stress levels, loading histories, volume-to-surface ratios and other factors in a high-rise building must be properly considered in the design processShortening is due to elastic deformation, creep and shrinkageBecause factors affecting this causes (i.e the level of stress, volume-to-surface ratio, percentage reinforcement etc) is different, there is differential shortening.Different structural systems have different behaviors

Types of Column ShorteningShortening taking place up to the time of slab installation (before slab casting)Shortening taking place after slab installation (after slab casting)

Factors Affecting Column ShorteningElastic ShorteningCreepShrinkageCREEP AND SHRINKAGE IN CONCRETE

Total Strain: This is the total change in length per unit length observed on a specimenShrinkage: is the strain measured on a load-free concrete sample Basic shrinkageDrying shrinkageCarbonation shrinkage: this is due to the reaction of Ca(OH)2 in the concrete with CO2 of the atmospherePlastic shrinkage: refers to the shrinkage concrete experiences before settingSwelling: is the expansion of concrete due to the reduction of capillary forces when immersed in water.Creep strain: This is the strain concrete experiences due to a constant stress level. It consists of:Basic creep: Is the creep occurring without water content reduction i.e. a sealed sample.Drying creep: as the name implies, is the creep strain accompanying moisture loss.

Mechanism of Creep and ShrinkageAging of concreteShort term chemical aging due to the formation of new hydration products in a stress-free state on the wall of the capillary poresLong term aging due to non-chemical aging but due to relaxation of the microprestressDrying creep effect: due to the Pickett effect, the total strain during drying creep is lesser than the sum of the basic creep and drying shrinkage. This is due to:Microcracking or due to strain-softening damage. This is an apparent effect occurring in a companion sample and not the drying creep specimen itselfMechanism in the nanostructure related to the rate of shear slip and restoration of bonds. This is affected by the microprestress.Transitional thermal creep: This is due to heating or cooling of concrete. The governing mechanisms are:Microcracking due to thermal effectsNanoscale mechanisms related to the microprestress (Bazant, Cusatis, & Cedolin, 2004)

Prediction of Creep and ShrinkageACI ModelB3 ModelGL2000 and so many othersVariable stress formulation: Integral formulationRate-type formulationOPTIMAL COMPENSATION OF DIFFERENTIAL COLUMN SHORTENINGPark (2003)

Before compensationAfter compensation

N- No of lumped floors, c correction neededCorrection for a set of lumped floorsReducing shortening

Source: Kim and Shin 2014Amount of reinforcementType of connecting horizontal membersType of columnLoading stagesAdmixturesConclusionUsing the optimal compensation method, the differential shortenings in high-rise buildings are to be compensated for in a systematic manner without a time-consuming trial-and-error compensation procedure.The shortening can be reducedThank you