column shortening-compensation in tall buildings - presentations

7/29/2019 Column Shortening-Compensation in Tall Buildings - Presentations

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Column Shortening

Shapour Mehrkar-Asl

[email protected]

September 2012


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Basic Questions

When does it happen?

What are the consequences?

How to predict it?

How to compensate it? Other considerations.


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When does it happen?

Height of the building

Construction sequence

Difference in axial stress

Difference in cross sectional shapes Reinforcement quantities


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What are the consequences?

Differential shortening between core &

columns.

Distortion of slab

Redistribution of vertical loads Additional moments in the slab

Effects on non-load bearing elements suchas Partitions, cladding, finishes & piping


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How to predict it? Estimated axial load in vertical elements based on

uncracked section properties for all the elements Construction Sequence is assumed

Material properties of concrete Elastic Modulus, ultimateshrinkage and specific creep (use experimental values if

not then use codified vales) Methods to estimate strength gain of concrete, shrinkage

and creep with time

Use Mark Fintel et al to calculate shortening

Difference between core and columns is shortening ofcolumns


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How to compensate it?

At design stage try to balance the loads

(equal stresses in core and columns) During construction phase:

Composite or steel framing For columns with steel cross sections either

fabricate them longer or use site steel shims

Alternatively put the beam connection points in thecore at a lower level

For concrete frame columns adjust formworkto the required length


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Other Considerations

Additional effects

Wind loading

Temperature effects

Differential foundation movements Performance Criteria

Material Testing


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Elastic Shortening


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Ec of High Strength Concrete

For normal strength concrete of

Ec = 33 w1.5 (fc) about 57,000 (fc)

w in lb/ft3 = 145 lb/ft3

fc in psi (valid to about 6,000 psi)

Ec in psi

For high strength concrete

Ec = 3320 (fc) + 1.0 x 106

3,000 < fc < 12,000 psi


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Creep


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Specific Creep

Low value = 1.5x10-3/fc

High value = 2.1x10-3/fcf c in psi


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Shrinkage


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Shrinkage


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Subsequent LoadElastic Shortening Creep


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Rules of thumb for Total

Shortening Steel Columns

Only Elastic Shortening about 1.5 to 2mm/floor

Concrete Columns

Elastic Shortening about 0.5 to 0.8mm/ floor Creep about 1 to 2 x Elastic Shortening

Shrinkage about 0.2 to 0.5 mm/ floor

Overall very similar but happening at differenttimes.


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Al Mas Tower

160,000 sq.m

5B+3Podiums+60Floors+3Plants71 levels

360m in Height

Office, retail & diamond exchange


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Al Mas Tower


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Al Mas Tower


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Al Mas Tower


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Al Mas Tower


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Al Mas Tower


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Al Mas Tower Elastic Shortening

Long term effects (creep and shrinkage) Principles (effects before and after casting a slab)

Self compensating effects

Adjustments have to be made

Theoretical methods Paper by Mark Fintel, et al.

ACI 209

ACI 363

Computer Programs SMA program


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Al Mas Tower - CoreCore 1

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0 20 40 60 80 100 120 140 160 180

E

C+S

Core Total


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Al Mas Tower - ColumnColum n 3TC3-1

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0 50 100 150 200 250

E

C+S

Column Total


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Al Mas TowerDifferential Shortening

0

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-50 0 50 100 150 200 250

Core Total

Column Total

Differential


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Al Mas TowerControl ling Factors:

Column size/ length Concrete strength Conc. properties Member sizes Reinforcing amount

Floor dead loads Superimposed loads Construction time Construction loads Humidity at curing Temperature

column shortening-compensation in tall buildings - presentations

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