constructing a moving target (worsnop, j)

8/11/2019 Constructing a Moving Target (Worsnop, J)

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CONSTRUCTING A MOVING TARGET

Elastic, Shrinkage and Creep Effects upon Tall Structures during

the Construction Period and Beyond.

J.G.Worsnop B.Eng., MBA, C.Eng., MICE, MiStructEBURO HAPPOLD CONSULTING ENGINEERS

INTRODUCTION

The drive to build tall structures has traditionally been to provide large facilities

where land availability is limited. In developed areas where the price of potentialdevelopment sites is high and availability few, tall buildings provide an excellentsolution.

In recent times tall buildings have been used more as a symbol of a nations economic

development and this is no where more evident than in the Middle East. Dubai in theUnited Arab Emirates lead the way with the Burj Al Arab, the tallest hotel in theworld at 321metres high, framed by reinforced concrete core and dividing walls. More

recently one of the Emirates Towers has surpassed this heady height.

Riyadh in Saudi Arabia, traditionally a city of low-rise buildings sees the horizonpierced by two massive monuments. The tallest, at 295metres, will be the KingdomTrade Center, an office, hotel and residential facility, now under construction and due

for completion in late 2001. On the 14th May 2000, the Al Faisaliah Tower will openas the first skyscraper in Saudi Arabia, at over 260 metres high.

The Middle East has developed industries upon the use of concrete as its primarybuilding material and steelwork remains an imported product and less prevalent.

Unlike steel, concrete shortens with age, firstly as drying shrinkage, then secondly ascreep under long term loads. A third aspect is its elastic behaviour which varies whilst

the concrete is in its infancy.

As concrete structures rise above 100 150 metres the study of the three becomes an

essential part of the design and construction process. However this detail of design

can only be carried out with the cooperation of the contractor since programming ofthe works is a primary input into the assessment.

MOVEMENTS

Buildings move as in response to changes imposed upon them. The imposition of

wind loads, live loads, temperature or moisture fluctuations occur during the life ofthe building and are generally short lived. These assessments of these can be madewithin the design stage.

This paper considers the movements due to drying shrinkage, creep and elastic

shortening during the construction period and beyond. The assessment traverses thedesign and construction periods as each of the strains are time dependent.


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Elastic shortening is the deformation from an applied load and is instantaneous at thetime of application. Further deformation from the residual stress in the concrete

occurs over the following months and years and is considered as creep.

Concrete is an inelastic material and its modulus of elasticity depends upon the age ofthe concrete, its strength and the applied stress. Fortuitously, the stress strainrelationship may be considered linear at low stresses, if all other parameters are equal.

The British Standards suggest that when the stress to ultimate stress ratio is below0.33 a linear relationship exists. The American Code suggests a maximum ratio of 0.4to be appropriate. These values lie within the limits of construction loads.

Drying shrinkage starts the moment the concrete begins its hardening process and

continues to a decreasing effect over the following 30 years. The effect is caused bythe evaporation of water from the concrete and is irreversible. The initial loss of freewater causes very little volume change to the concrete, but as drying continues, the

absorbed water evaporates and changes the unrestrained hydrated cement paste.Consequently, the proportion of cement will influence the degree of volume change,

the greater the cement content, the larger the shrinkage of the element.

The timing of shrinkage deformation also depends upon the ability of the structural

element to give up the absorbed water. The volume to surface area of the concretespecimen will determine the speed at which shrinkage takes place, thicker sections

being slower than thin elements. Also the humidity at the concrete surface influencesthe time taken for water to be given up through evaporation. High temperatures,winds over the surface and dry atmospheres will encourage the transpiration of water

and hence speed the shrinkage process.

Creep is the increase in strain under a sustained stress. It commences at the time ofloading and its rate of deformation is greatest in the early period, decreasing over thelonger term. The stress within the concrete element is directly proportional to the

creep strain. However, the strength of the concrete also influences the eventual strain.Concrete of higher strengths exhibit higher modulus of elasticity and lower strains.

But as the strength takes time to reach its peak so too does its modulus of elasticity.Therefore loading of a concrete member shows greater deformation in the earlyweeks.

Similar to shrinkage, creep is a function of the volumetric content of cement paste inthe concrete, the role of the aggregates is that of restraint. The degree of restraint isinfluenced greatest by the modulus of elasticity of the aggregates. The greater themodulus the greater the restraint and hence the smaller the creep strain.

The magnitudes of elastic, shrinkage and creep movements of the total structure may

be considered as both time and event dependent. The rate of overall deformationoccurs early and predominately within the construction period. Elastic strain is driven

by events, for example, each time a higher level floor is applied there is an

instantaneous load increase and thus movement. Beyond the construction completiondate only strains from operational or live loads are imposed.


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Shrinkage of an individual concrete member is driven by time initiated as the concrete

starts to harden. The rate of shortening is greatest in the first few months, declining toless than 20% beyond the construction period.

Creep is dependent upon both event and time. As the construction event progresses,

new loads are applied which start a new time dependant movement. Similar toshrinkage the rate is greatest in the concretes infancy, i.e. during the construction

period.

THE PROJECT

In late 1993 the King Faisal Foundation, an Islamic philanthropic organisation in

Saudi Arabia commissioned Architects Sir Norman Foster and Partners, andConsulting Engineers Buro Happold from the United Kingdom, as a joint designventure to develop a mixed use facility in Olaya, a business district of Riyadh. The Al

Faisaliah Center would feature a landmark office tower, a 224 bedroom, a five star

hotel, residential accommodation, a retail mall, banqueting facilities, car parking,landscaping and associateed infrastructure. The jewel of the development was theTower, but the project can boasts many other state of the art achievements.

It was at the Al Faisaliah Centre, Riyadh, where Buro Happold Consulting Engineers,joined with the local contractor, Saudi Bin Laden, to analyse the movements of the

structure, taking full account of the construction sequence. The information generatedwas planned finalise such considerations as preseting the formwork, cladding joints,second order bending in beams and jointing in the vertical pipework.

THE TOWER

The Al Faisaliah Tower is the landmark feature of the development, which soarsabove the surrounding city. Saudi Arabias first sky scraper has recently been topped

out with a stainless steel finial, 30 months after pouring the 6000 m3 concretefoundation, the largest pour in the region.

Through those 30 months the concrete structure has been progressively loaded with 1giga-newton of permanent weight, compressing its own columns and walls. Coupled

with deformation as the concrete shrinks the building has shortened by 150mm,although 2/3 of this has been compensated for within the construction process.

The Tower is square on plan with a central reinforced concrete core forming a centralspine throughout its height. The core is the primary lateral load stability system and


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occupies 25% of the floor plate. It also accounts for approximately 80% of the gross

cross-sectional area of concrete available to carry the vertical loads, yet carries onlyapproximately 65%.

At the top of the Tower, the core continues upward through the observation sphere

and gives support to a light weight steel mast structure which also gains support fromthe corner columns.

The floor plates are single span concrete ribbed slabs supported at the core and at thefaade column/beam structure. The slabs are pre-tensioned to limit their floor depthand minimise the buildings weight.

The faade structure tapers as it rises with the 4 No. corner columns running

continuously throughout the height of the building. Between the corners, smallercross-sectional area perimeter columns are positioned at 9 metre centres which collectthe load of each floor plate from the perimeter beam and slab. In order to keep their

size small, 400-600mm diameter, these columns are stacked for a maximum of 11storeys. Each of the stacks are supported by macro frames which are in turn supported

by the corner columns. The k brace macro frames consist of reinforced concrete topchords, post-tensioned concrete bottom chords, with the diagonals fabricated fromgrade 50 steel rectangular sections.

Only the corner columns and the cores extend to the 4 metres deep monolithic

foundation which had been cast upon the underlying rock strata.

CALCULATION PROCESS

Each of the vertical load carrying components, the core and columns are imposed with

different stresses and these impose short and long deformations. It is important topredict the movement of the concrete structure through the construction period andmake a final assessment of the 30 year position.

The process must establish the parameters for which the study is being undertaken. In

a tall structure there are a vast number of vertical elements and the management ofeach component of shortening increases with the number of floors, the number ofelements and the periods when deformation is required to be known.

At Al Faisaliah the analysis identified the following considerations:

The Main Contractor required the magnitude of the presets for each floor plate.

The core concrete was under lower stresses from permanent loads than the corner

and perimeter columns. The consequence was that the perimeter would drooprelative to the central core so the differential deflection would determine the preset

level. It was important that the levels moved within the specified tolerance boundsbetween Contract Completion and the 30 years condition. It is worth noting thatsome floors were consciously cast outside the tolerances, knowing that during the

construction period the movement would satisfy the overall objective.

The resulting differential movement between corner columns and first internalcolumns induced secondary moments into the connecting perimeter beams. As the


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corner columns taper inwards the beams become shorter, inducing greater

moments. The predicted differentials were required for each to ensure thatadequate reinforcement had been provided.

The connection of the corner columns at the concrete steel interface was designed

for compression in the short term and tension resulting from the 30 yearshortening of the concrete below. The long term redistribution of loads was aconcern for the whole of the upper steel mast structure.

The cladding contractor required information about the relative movement

between floors in order to design an adequate joint. The cladding system extendedbetween each of the K braces, and the top chords were considered as the relativehard spot where floor movement would be greatest.

The cladding contractor also required the differential movement between the

corner column and the first internal faade column. The corner columns were

highly stressed and susceptible to shortening. In comparison the internal columns

under the K braces had little stress and consequently no shortening. Theresulting large differential movement created warping in the panel which neededto be accommodated.

The mechanical service engineers were interested in the shortening of the vertical

service duct in order to set the pipework joints. It was realised that as the Towercompressed the joints in the pipework would constricted. As chilled water flowed

the joints would expand as the pipework contracted. Continued compression of theTower would again close the pipework joints. The final scenario to be considered

was a potential maintenance problem when the chilled water would be drained,causing the pipework to expand and the joints to constricted further. The see-sawing of joint movement had to be accommodated within very small tolerances.

The consequence of a joint failure implies 300 000 litres of chilled watercascading down the Tower.

Although not encountered on the Al Faisaliah Tower, cladding difficulties can

arise when a core forms part of the faade. The junction with a higher stressed

column is worthy of consideration.

Each tall building will have its own areas for consideration and these will be found in

zones where there are abrupt discontinuities in the building structure. As engineeringstrives to accommodate modern architecture these regions of discontinuity become

numerous and investigation essential.

INPUT INTO THE MATRIX

The reference used in the calculations for the Al Faisaliah Study followed the

recommendations of British Standard, Structural Use of Concrete, BS8110 Part 2Section 7. The manipulation and summing of all the individual shortenings weremanaged through a matrix spreadsheet. Separate calculations including bending

deflections were undertaken upon the K brace to find the cumulative effects.


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For the Al Faisaliah Tower Study, four main types of vertical structural elements were

considered;

The core

The corner columns

The inner perimeter columns ( spacing at 9 metres) The outer perimeter columns (varying spacing from the corner columns)

For each of the elements at each floor level the cross sectional area was calculated and

its height above datum defined.

The loading was separated into three categories per floor level, the self weight of the

concrete construction, the superimposed dead load, including finishes and thesuperimposed live load. For plant room areas the superimposed live load (equipment

weights) were assigned to the superimposed dead load so that the long term effectswere considered. Creep effects were not deemed appropriate upon the superimposedlive load due to their short term nature, but plant room loads are long term and creep

needed to be included within the calculation.

Against each of the three loads for each floor and for each vertical structural membera programme date was assigned. All the superimposed live loads were given theConstruction Completion date. The construction dates were monitored and significant

deviations resulted in rerunning the matrices and new movement profiles generated.

The loads, programme dates, cross sectional areas and heights above datum wereinputted into 4 matrices, one for each vertical structural element. Each matrix had a

Defined Date for which it would calculate each incremental deformation at eachfloor. In order to determine the deformations with time the Define Date was changedand the matrix rerun.

The strain for the elastic, shrinkage and creep are calculated within the matrices. Asdiscussed previously, these are time dependant and equations have been generated

with age as the underlying parameter.

The instantaneous elastic shortening is dependant upon the 28 day characteristicstrength of concrete and corresponding Modulus of Elasticity. These vary with timeand the formulae allow for a linear rate of increase within the first 12 months and a

constant value beyond.

The British Standard tabulates shrinkage strain values for varying thickness ofelements exposed to different humidity climates. Riyadh, Saudi Arabia has a very dryatmosphere and the concrete under consideration is behind cladding in the final state,

therefore indoor exposure was chosen as the appropriate climatic condition.

Three member thickness, 150mm, 300mm and 600mm, have tabulated strain valuesand these were converted to best fit hyperbolic equations. The 30 year values wereused for the upper strain values and the 6 month value was used to ensure an accurate

value during construction.


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The British Standard graphically estimates the creep coefficient for ambient relative

humidity, age of loading and section thickness. Again indoor exposure, equivalent to45% relative humidity was taken and section thickness 600mm assumed. From valves

at 28 day, 90 day and 365 day intervals, a best fit hyperbolic equation was generatedto determine the coefficient at a specified time of loading. With the time dependent

modulus of elasticity and creep coefficient, the creep strain was determined.The difference between the Date Load Applied and the Date Concrete Cast definesthe age of the concrete and the loaded period can be determined from the difference

from the Defined Date. These allow the calculation of the appropriate elastic,shrinkage and creep strains for each load case and resulting deformation. The studyconsidered deformation at 3 month intervals and the final 30 year case.

Concrete structures are cast to level therefore the deformation that has occurred prior

to casting is compensated for. It was therefore necessary to find the movement at thedate of casting each floor level and deducted from the quarterly figures.

The resulting movements were presented both graphically and within a table for thedesign of the presets, cladding, beams and pipework.

MOVEMENTS AND LEVELS CALCULATED

The typical information generated within the Al Faisaliah Study is shown in the table

below.

TABLE FOR LEVELS 17, 18 & 19End of 30 year

Contract M'mentQ2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q1

Level 19

98 98 98 99 99 99 99 0 0 28

Core 0.00 0.00 0.00 6.59 18.59 25.32 31.22 37.08 44.31 74.41

Corner 0.00 0.00 0.00 15.76 45.82 55.44 63.04 71.44 82.37 125.97

Out. Perimeter 0.00 0.00 0.00 22.89 43.29 52.22 60.51 67.86 78.91 120.92

Int. Perimeter 0.00 0.00 0.00 25.64 46.43 55.70 64.95 72.60 84.86 129.08

Level 18Core 0.00 0.00 0.00 7.80 19.36 25.89 31.63 37.30 44.34 73.44

Corner 0.00 0.00 0.00 16.03 44.48 53.81 61.15 69.24 79.82 121.79

Out. Perimeter 0.00 0.00 0.00 22.35 42.50 51.23 59.08 66.25 76.92 117.40

Int. Perimeter 0.00 0.00 0.00 25.12 45.60 54.62 63.27 70.69 82.39 124.74

Level 17

Core 0.00 0.00 0.00 8.88 20.00 26.34 31.90 37.38 44.23 72.31

Corner 0.00 0.00 0.00 16.20 43.04 52.07 59.16 66.92 77.14 117.48

Out. Perimeter 0.00 0.00 0.00 22.03 41.97 50.53 58.04 65.04 75.39 114.56

Int. Perimeter 0.00 0.00 0.00 24.76 44.99 53.80 61.98 69.20 80.43 121.20


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GRAPHICAL REPRESENTATION OF DIFFERENTIAL MOVEMENT

LEVEL 11

-80.00

-70.00

-60.00

-50.00

-40.00

-30.00

-20.00

-10.00

0.00

10.00

Time

Core

Corner column

Inner Perimeter Column

Outer Perimeter Column

Headline information figures are:

The largest movement over the life of the building is 154mm in a faade

column .

The largest core movement is 85mm

The largest difference between core and the faade is 70mm

With the long term deformations calculated and combined with bending deflections ofthe K braces the presets for the concrete floors were determined. A final table was

issued to the construction and design team identifying the predicted levels for the fourvertical structural elements at:

Structural Slab Level DesignConcrete Cast Level

Cladding Fixing Level (assumed date of fixing)

Contract End Level30 Year Level

The information then allowed the detailed design and fabrication of the subsequent

building components and their fixings, taking account of the continuing movement ofthe concrete frame.

constructing a moving target (worsnop, j)

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