CONSTRUCTION SEQUENCE ANALYSIS : AN ADVANCE ANALYSIS
While analyzing a multi-storied building frame,conventionally all the probable loads are applied after modeling the entire building .But in practical condition,the frame is constructed in various stages accordingly,the stability of frame varies at every construction stage.Even during construction freshly placed concrete floor is supported by previously cast floor by form-work. Thus,the loads assumed in conventional analysis will vary in transient situation.Obviously,results obtained by the traditional analysis will be unsuitable .Therefore,the frame should be analysed at every construction stage taking into account variation in loads.The phenomenon is known as Construction stage analysis Or, Construction Sequence Analysis which considers these uncertainties precisely.So,the close comparison between these two analysis always illustrates the building behavior elaborately to us.
The effect of the sequential application of dead load due to the sequential nature of construction is an important factor to be considered in the multistory frame analysis.Unfortunately,however this effect has been ignored by many engineers in practice in the past.One of the ways to include this effect properly in the analysis is to carry out the analysis through step by step procedures in accordance with the sequential application of dead loads as the construction proceeds for bridges & high rise buildings,time dependent effects make analysis even more complex.The effects develop at the early stages of the construction processs and continue to evolve considerably after the structures are built.Depending on the construction method,the time dependent effects can appear and induce important stress redistribution in the structures like bridges.To obtain real behavior of concrete bridges and high rise buildings segment-ally construction sequence analysis using time and loads may change during this period and after.
In the structural analysis of multistory buildings,there are two important factors that have very significant effects on the accuracy of the analysis but are usually ignored in practice.They are
A) The effect of sequential application of dead loads due to the sequential nature of construction.
B) Differential column shortening due to the different tributary areas that the exterior and interior column support.
The exterior column in a building is loaded with roughly one half of the gravity loads to which the interior column is subjected.In many design properties,however there is a tendency to design the exterior columns so as to have cross-sectional areas nearly equal to the interior ones ,since additional
cross-section are required in the exterior columns to resist the forces induced by overturning moments due to lateral loads such as winds and earthquake.Therefore there exist a substantial inequality between the the ratio of the applied load to the cross-sectional area of an exterior column and that of an interior one.This inequality may cause a differential shortening between the exterior and interior columns in exterior and interior columns in the frame.Moreover,this differential shortening is enlarged in RC frame structures since additional time dependent deformations of concrete,which have a magnitude of more than two times the elastic deformations are accompanied.
In conventional analysis dead loads ,live loads,wind loads and seismic loads are applied simultaneously to the entire complete structure.In construction sequence analysis ,dead loads are applied in a sequential manner .Since the difference exists only in the application of dead load,so only dead load and dead load along with live load conditions has significant effect on this implications.
Column Shortening:
The effect of column shortening is a major consideration in the design and construction of tall buildings,especially in concrete and composite structure system.Column shortening occurs due to various factors like elastic stresses due to gravity loads,creep caused by temperature variations of exposed columns.
*** Calculating Column Shortening is complex process but we can use this following simplified formula:
*** Calculating Column Shortening is complex process but we can use this following simplified formula:
*** Differential Axial Shortening (DAS) causes servicibility related problems,unacceptable cracking and deflection of floor plates,beams and scondary structural components,damage to facades,claddings,finishes,mechanical and plumbing installations and other walls can occur.
*** Accurate predictions and management of differential axial shortening in buildings help to minimize the reference problems.
*** The differential axial shortening is responsible for the development of shear and bending moments on the horizontal elements,the forces from the horizontal elements lead to a redistribution of the applied loads on the vertical members,with more loads applied on the elements that suffer less shortening effect(cores).
*** In high rise buildings perimeter columns tends to be more heavily stressed compared to shear walls of internal core .These perimeter columns there by to deform axially at higher rates compared to shear walls .This leads to differential axial shortening (DAS) between the columns and shear walls.
*** Compensation for differential Column Shortening :
--- Reinforced Concrete Structure
1) Pre-Slab installation shortenings has no importance.
2) Compensation by leveling the forms.
3) Post-Slab installation shortenings due to subsequent loads and creep/shrinkage.
--- Steel Structure
1) Columns are fabricated to exact length.
2) Attachments to support the slab.
3) Pre-Slab installations shortenings need to be known.
4) Compensation for the summation of Pre-installation and Post-installation shortenings.
*** Compensation for differential Column Shortening :
--- Reinforced Concrete Structure
1) Pre-Slab installation shortenings has no importance.
2) Compensation by leveling the forms.
3) Post-Slab installation shortenings due to subsequent loads and creep/shrinkage.
--- Steel Structure
1) Columns are fabricated to exact length.
2) Attachments to support the slab.
3) Pre-Slab installations shortenings need to be known.
4) Compensation for the summation of Pre-installation and Post-installation shortenings.
*** The differential axial shortening is responsible for the development of shear and bending moments on the horizontal elements,in an extreme case the differential axial shortening can lead to an inversion on the moment diagram.This can be noticed for the dead load case,where the staged construction was not considered.
*** Tensile creep in concrete is a phenomenon that has received relatively little attention by structural engineers because the tensile strength of concrete is not codified component of the strength of reinforced concrete structures.We reinforce concrete precisely it is weak in tension.It is expected to crack and its tensile strength is disregarded in design.As a result,when inclined cracks are observed in reinforced concrete shear walls after earthquake,tensile creep is not the first mechanism that most engineers would begin to investigate.Most of us might begin with the assumption that cracks were caused by the response of the structure to ground shaking .Tensile creep ,however can cause cracking in reinforced concrete structures and likelihood that it will cause cracking.
*** The allowable column axial shortening limitations are as below :
Restrictions for Steel Columns due to Shortenings :
--- Only Elastic shortening not more than 1.5 to 2 mm/floor.
Restrictions for Concrete Cloumns due to Shortenings :
1) Elastic shortening not more than 0.5 to 0.8 mm/floor.
2) Creep about 1 to 2 times of Elastic shortenings.
3) Shrinkage about 0.2 to 0.5 mm/floor.
4) Overall very similar but happening at different times.
Multi-storied buildings have been analyzed for years on the assumption that whole of the load is applied on the complete frame,Looking into the mode of incidence of the load,it is evident that part of the load is applied in stages as the construction of the frame proceeds,whereas the remaining part of it is imposed on completion of the frame.
The main factors affecting the limit state of serviceability of building are---
A) Creep and shrinkage.
B) Span and cross section of the structural members
C) Cycle time for floor to floor construction and strength of concrete.
Staged construction allows defining a sequence of stages wherein one can add or remove portions of the structure selectively apply load to portions of the structure and to consider time-dependent material behavior such as aging ,creep and shrinkage.Staged construction is variously known as incremental construction sequential construction or segment-all construction .Staged construction is considered a type of non-linear static analysis because the structure may undergo changes during the course of the analysis.However,consideration of material and geometric non-linearity is optional.
*** Column shortening for Tall building is critical specially building located in high wind zone.
*** Time dependent Concrete Properties:
Even if a building is constructed with conventional sequence, the accumulated differences in forces and deformations in structural members between construction stage analysis and ordinary structural analysis get bigger due to creep and shrinkage of concrete during the long-termed construction period of tall building. While elastic deformations are simply calculated from the applied load and modulus of elasticity, creep and shrinkage deformations are influenced by various factors such as member size and shape, reinforcement ratio, relative humidity, modulus of elasticity, duration of load application, and age of curing at the start of loading (Fintel 1986). Input values for concrete properties during construction stage analysis are usually based on provisions from codes of practices and published papers. Examples of codes of practices are report from ACI 318, 363 and 209 committees (2008) and Eurocode 2. B3 (Bazant 1995) and GL2000 (Gardner 2004) models are generally accepted models for the prediction of creep and shrinkage.
Long term construction period of high-rise building may induce axial shortening of the structure over several hundreds millimeters at higher levels of the building during construction.For buildings with mass eccentricity or irregularity,the differential shortening combined with applied moment could also make the building move laterally due to the mechanism similar to bimetallic strip.The amount of axial or lateral movement can be ignored for ordinary buildings but is very important to high rise buildings,since it causes adverse effects to the construction and performance of elevators to the construction and performance of elevators(Bast,2007) and facade,and also develop locked in forces in outriggers or belt trusses (Baker,2006) which are vital to the lateral resistance of the high rise buildings.
P-Delta effects in Construction Sequence Analysis :
Even if a building is constructed with conventional sequence, the accumulated differences in forces and deformations in structural members between construction stage analysis and ordinary structural analysis get bigger due to creep and shrinkage of concrete during the long-termed construction period of tall building. While elastic deformations are simply calculated from the applied load and modulus of elasticity, creep and shrinkage deformations are influenced by various factors such as member size and shape, reinforcement ratio, relative humidity, modulus of elasticity, duration of load application, and age of curing at the start of loading (Fintel 1986). Input values for concrete properties during construction stage analysis are usually based on provisions from codes of practices and published papers. Examples of codes of practices are report from ACI 318, 363 and 209 committees (2008) and Eurocode 2. B3 (Bazant 1995) and GL2000 (Gardner 2004) models are generally accepted models for the prediction of creep and shrinkage.
*** Although
these theoretical models for creep and shrinkage give a good guideline at the
preliminary stage of construction stage analysis, rigorous tests on concrete
used in the actual construction should be performed as early as the mix design
of the concrete is established due to the following reasons:
First,
laboratory database for the theoretical models is relatively outdated in terms
of current progress of technology development in concrete industry. Concrete
with high strength and multi-performance has quite different creep and
shrinkage properties from ordinary normal strength concrete;
Second, measured database for the theoretical models is
mainly from bridges, not from building. The effect of creep and shrinkage on
bridge girders is increased deflection, while that on building is regarding the
axial shortening of concrete column, which is in compression during its entire
life. Since the structural behavior of bending and compression is different,
the creep behaviour under sustained loading should also be discriminated. The
material tests for creep and shrinkage are conducted at least three months long
in climate chamber where the temperature and relative humidity can be
maintained at levels specified in the codes. Values of specific creep and
ultimate shrinkage are derived from nonlinear regression on the results of
tests. However, these values are for specimens of standard size and in standard
environment, which can be quite different from those in the construction field.
Measurement of member deformation in the course of construction is therefore a
good practice to compare the predicted values based on material tests with
actual deformation (Russell 1989). The measurement is better to be conducted in
the early stage of construction for the predicted values to be revised
according to the measured values. If preset were scheduled to compensate the
structural deformation, it can also be rescheduled based on the revised
prediction.
Long term construction period of high-rise building may induce axial shortening of the structure over several hundreds millimeters at higher levels of the building during construction.For buildings with mass eccentricity or irregularity,the differential shortening combined with applied moment could also make the building move laterally due to the mechanism similar to bimetallic strip.The amount of axial or lateral movement can be ignored for ordinary buildings but is very important to high rise buildings,since it causes adverse effects to the construction and performance of elevators to the construction and performance of elevators(Bast,2007) and facade,and also develop locked in forces in outriggers or belt trusses (Baker,2006) which are vital to the lateral resistance of the high rise buildings.
P-Delta effects in Construction Sequence Analysis :
It is
observed from the analysis that the results obtained for the moment due to
sequential analysis with P-Delta are most significant than that obtained due to
linear static analysis since during construction phase itself the sequential
effect counts the load.
*** Moment and shear force results observed are greater in the supporting
beam due to construction sequence analysis.
*** Regarding displacement results, structure considered sequential effects shows the worst part than that of structure which have not considered these effects i.e. Linear static analysis which makes it significant to consider the effects of sequence and time-dependent.
*** Therefore, the outcomes obtained from the analysis shows that much moment and shear force is taken by the steel structure than the RCC structure which proves that steel structure resists maximum moment and shear force but with lesser displacements than that of RCC.
*** The results taken from the analysis turns to be worst part for constructional sequence analysis than linear static analysis, therefore for the construction of high-rise involving many floor constructions along with time consuming sequential effects consideration is must.
*** Hence preference is drawn first for the steel structure than the RCC structures for construction sequence analysis for the loading effects of long term.
*** Regarding displacement results, structure considered sequential effects shows the worst part than that of structure which have not considered these effects i.e. Linear static analysis which makes it significant to consider the effects of sequence and time-dependent.
*** Therefore, the outcomes obtained from the analysis shows that much moment and shear force is taken by the steel structure than the RCC structure which proves that steel structure resists maximum moment and shear force but with lesser displacements than that of RCC.
*** The results taken from the analysis turns to be worst part for constructional sequence analysis than linear static analysis, therefore for the construction of high-rise involving many floor constructions along with time consuming sequential effects consideration is must.
*** Hence preference is drawn first for the steel structure than the RCC structures for construction sequence analysis for the loading effects of long term.
Construction Sequence Analysis (CSA) is important for the following reasons:
A) Construction Sequence Analysis (CSA) in structures of RCC is necessary to improve the analysis accuracy in terms of displacements,axial,moment and shear force in supporting beam and column near of it and also for the whole structure overall.
B) Regarding displacement results,structure considered sequential effect shows the worst path that of structure.
C) Inclusion of sequential load case in the analysis of multi-storied RCC structure provides more realistic design than the conventional design.
( THANKS TO ALL OF YOU )
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