The cross sections of columns are usually designed to be as small as possible so columns are generally more heavily stressed than core walls. As construction progresses, columns and core walls undergo different amounts and rates of elastic, creep, and shrinkage shortening. After completion of the building, creep and shrinkage shortening continue to develop differently in core walls and columns throughout the service life of the building.
In addition, differential shortening between adjacent columns is significant when their locations and corresponding stress levels are different, eg, among interior, exterior, and corner columns.
Differential shortening between core walls and columns is generally greatest at between two-thirds and three-quarters of the height of a tall building, but not at the top of the building, as the shortening is based on SUBTO movement (see Figure 11.1 and Section 11.1.4).
The following factors are known to have an effect on axial shortening and should, therefore, be considered in the calculation of the shortening:
Properties of the concrete (initial and time-dependent properties) z Compressive strength
z Nominal initial modulus of elasticity z Creep
Figure 11.1 Example of differential shortening between core walls and columns in a tall building.
Image: Daewoo Engineering and Construction.
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Design assumptions for columns and walls z Cross-sectional area
z Longitudinal reinforcement ratio
z Area of embedded steel section (for composite members) z Volume-to-surface ratio (effective thickness).
Loading assumptions z Construction sequence z Progress of occupancy
z Environmental conditions (relative humidity).
11.1.2 Deviation from
When a tall building has an eccentric or irregular plan or a large variation in shapeverticality
vertically, it could be subject to a considerable amount of differential shortening, which in turn may cause permanent leaning of the building or a deviation from verticality in the construction stage and after occupancy.Additionally, tall buildings with symmetric plans may deviate from verticality due to an asymmetric construction sequence. As a deviation from verticality is the extreme case of differential axial shortening, it has more significant adverse effects on tall buildings and, therefore, should be identified before construction so that proper countermeasures can be taken.
The major factors affecting deviation from verticality are similar to those affecting axial shortening.
11.1.3 Target time
Because the building movement develops with time, it is necessary to define a ‘target time’; the point in time for which the movement is estimated. The movement is the result of the combined effects of accumulated loading, creep and shrinkage during construction and the residual effect of creep and shrinkage after occupancy.Therefore, the ultimate target time is usually some time after completion of
construction. Based on the creep and shrinkage properties of the concrete, the target time could be at least three years after completion of construction, when more than 90 % of the creep and shrinkage expected to develop during the service life has already occurred. A different target time could be chosen to identify the building movement at a specific time, such as at the installation of lifts.
11.1.4 UPTO and SUBTO
Building movement is the sum of the deformations of all of the building elements aftermovement
they have been constructed. As the elements are constructed consecutively, generally floor-by-floor, the total building movement can be classified as the movement occurring prior to the completed construction of a certain floor (UPTO) or subsequent tocompletion of construction of that floor (SUBTO).
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UPTO movement at a specific floor refers to movement already developed or accumulated in the time from the start of building construction to completion of the floor under consideration. This movement is negligible if a building is constructed so that every floor conforms to its designed location at the time of construction. With regard to axial shortening, it is standard construction practice that every floor be made level and, hence, the UPTO differential shortening is always zero.
SUBTO movement at a specific floor refers to movement developed or accumulated at a target time subsequent to when the floor under consideration was constructed. It is usually greatest at between two-thirds and three-quarters of the height of a tall building, and gradually decreases above that height due to the lower weight of the remaining floors above and the shorter remaining construction time. SUBTO movement is more important than UPTO movement as it causes the differential movement of adjoining or adjacent building elements; thereby producing additional (locked-in) forces on structural members and adverse effects on non-structural elements, such as façades and lifts.
11.2 Adverse effects
11.2.1 Slabs – forces
Differential shortening of vertical structural elements will cause an associatedand deflections
movement in the floor slabs, which may cause cracking and a redistribution of internal forces. This problem is most likely to occur between the central core walls (where applicable) and the perimeter columns, inducing deflections in the floor framing between these vertical elements.As the central core generally has lower stresses than the perimeter columns and may be constructed in advance using climbing forms, the shortening of the core and SUBTO shortening, in particular, is significantly less than that of perimeter columns.
The effect of differential shortening on a floor slab in terms of internal forces is a progressive alteration of the distribution of bending moments and shear forces in the structural elements of the slab due to the additional locked-in forces that develop over time. This redistribution must be recognised in the design of the floor structures and the horizontal members must be reinforced accordingly.
Furthermore, the differential settling of supports can affect the levelness of the floors and, if not carefully considered or compensated for in the design and construction, this unevenness may give rise to tolerance problems for floor finishes and cladding details.
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11.2.2 Beams/outriggers/
On the upper floors of a building cumulative differential shortening between verticalbelt trusses – locked-in forces
members can cause adjacent (deep) beams to tilt, resulting in locked-in (moment and shear) forces.These forces can be a significant design problem if certain lateral stability systems such as outriggers and/or belt trusses engage these vertical members, as the lateral stability systems are designed to have greater stiffness than other structural members. In extreme cases, the total member forces, including the locked-in forces, might exceed the capacity of the member; a problem that can be solved only through design changes or alternative construction processes (see Section 11.4).
In addition, the redistribution of internal forces in horizontal members in turn applies counter-forces on vertical members and this may become a concern when there are significant variations over time. These variations should be estimated in the analysis and their effect on safety must be properly considered.
11.2.3 Dimensional
Movement in tall buildings may give rise to construction and serviceability problems asincompatibility
a result of dimensional incompatibilities between the building structure and non-structural elements.With regard to axial shortening, the shortened vertical structure can transfer compressive forces to neighbouring non-structural elements such as partitions, cladding, piping and lift guide rails not designed to support vertical loads (see Figure 11.2). A slab that tilts because of differential shortening or deviation from verticality may cause cracking or bowing of partitions, unless joints sufficiently allowing for partition movement are provided.
Deviations from verticality mostly influence lifts because verticality is the main concern during installation and maintenance. The overall deflection of a tall building during construction will cause the lift shaft to deflect in the same direction and thus reduce the vertical space available for the installation of lifts.
Occasionally it becomes necessary to remove part of the core wall if a distortion of the lift shaft occurs during the installation of the lifts. If a deviation from verticality occurs after installation of the lifts, other problems such as degradation in performance or durability may arise. In a building with excessive deviation from verticality, the lifts will come into contact with deflected guide rails, reducing the lifts’ maximum speed and causing premature wear in their mechanical components such as clips and rollers.
The effect of a dimensional incompatibility is usually greater in upper floors due to the accumulated differential movement between structural and non-structural elements. However, occasionally incompatibility is greatest at the bottom of the building, where both elastic and time-dependent stresses increase significantly over the construction period.
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Figure 11.2 Bond failure between sealant and slab (upper) and scrape in lift guide rail (lower) due to downward movement of the building structure (90 mm) relative to non-structural elements (Bast et al. 2003)[25].
Details for attaching non-structural elements to the structure must be planned so their displacement or deformation relative to the structure will not cause stresses. All tall building movements and deformations should be understood and considered in conjunction with other members of the design team to ensure adequate detailing provision and to avoid adverse effects on the building services and architecture.
11.3 Prediction and verification
11.3.1 Movement analysis One-column shortening analysis
Axial shortening of a tall building can be predicted relatively easily during the preliminary design stage. The prediction is used to evaluate the approximate effect of axial shortening and to guide the design of the structure. Axial shortening is the sum of elastic, creep and shrinkage deformations, and depends on the construction sequence (Fintel et al. 1986)[27].
A one-column shortening analysis can be performed using repetitive spreadsheet calculations with quantities such as member geometries obtained from drawings, material properties from code provisions, applied loads on each member from the calculation of tributary areas, environmental conditions from meteorological records, and stages of construction from construction schedules.
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This analysis method has been widely used for decades, but over recent years there has been a move towards construction stage analysis and time-history analysis of a 3D model. As single column analysis is of connected structural members it cannot consider restraining effects against differential shortening of the beams or slabs connected to the column or wall. When these restraining effects are significant it becomes necessary to conduct additional structural analyses based on the tentative results from a one-column shortening analysis. Construction-stage analysis and time-history analysis of a 3D model provide a more direct method of accounting for these effects.