DESIGN OF SHEAR WALL FOR BUILDINGS BOOK DOWNLOADS
DESIGN OF SHEAR WALL FOR BUILDINGS BOOK DOWNLOADS
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Wind loading
Lateral loading often dictates the proportions of a shear wall building. Wind loading is characterized by a design wind speed at a certain recurrence period. In the design of buildings, a 1-in-50-year recurrence wind is used to assess the ultimate strength of the structure. Including the load factor for wind, this is equivalent to a limiting strength condition under an extreme wind speed of recurrence about 1 in 500 years.
Both the steady and the gust components of the wind contribute to the structural response The increase in load over that of the building considered quasi-statically (i.e. as a rigid structure) is termed the 'dynamic magnification of load'.
In inner city areas, the dynamic component of load may be enhanced by buffeting of the wind between buildings, although the total loading is less than that for exposed sites.
The occupants of the building may also perceive dynamic motion induced by wind. Local cladding pressures (particularly suction at the corners of the building) can be considerably higher than those used in calculating the overall loads on the building. Wind flow at ground level around buildings can also be an important environmental factor, and some means of dissipating local gusts may need to be investigated.
Seismic loading In many parts of the world, ground accelerations from seismic activity can be the principal design condition
This is usually expressed in terms of an equivalent lateral loading (5 to 20% of total vertical load is typical). In more severe cases, the structure should be able to absorb the energy of the earthquake by elasto-plastic loading cycles. This imposes a need for careful detailing of the structure.
Movement Creep and shrinkage of concrete
These effects take place over a number of years, and they may be estimated using References 11 and 12. Both are a function of the environmental conditions, construction sequence, size of members and concrete mix proportions.
Creep of concrete is strongly affected by the age at loading. Relative vertical movements between elements with different properties, or which are differentially loaded, can be significant.
This is important where brick infills may be used within a reinforced concrete frame (see Section 6.2, page 65). Similarly, differential movement between a reinforced concrete core and a steel frame may be 2 to 3 mm per storey height. Because of the construction sequence, differential movement between a slip-formed concrete core and concrete frame may also be significant.
In buildings of more than 60 to 70 m length, movement joints are normally provided to reduce the out-of-plane deformation of the walls resulting from differential horizontal movement between the floors and the roof.
The positioning of movement joint also influences the overall structural action of shear wall buildings.
Temperature Differential temperature movement between the roof and internal floors and the basement, or between exposed and shaded sides of the building or exposed and insulated members can be significant, and it may need to be considered as for creep and shrinkage. Relative movement between elements such as mullions and windows should be taken into account in detailing of the joints.
Out-of-plumb walls Because of sway displacements, construction tolerances, and differential settlement, the enhanced, moment resulting from axial loading (P-6 effect) should be taken into account in design. Control of verticality should be maintained during construction by precise optical plumbing methods.
REINFORCED CONCRETE DESIGN OF WALLS
Walls are usually designed as compression elements under the combined action of in-plane bending and axial forces
1). Lateral restraints axe required at each floor level, and adequate tie reinforcement should be provided. The wall should be braced against relative translation of its ends. The compres-sive resistance of a wall element is a function of its slenderness — effective height/thickness (helt).
The effective height may be taken as 0.75 x storey height if the wall is fully restrained at its ends. Where the wall is connected to a flexible floor element, the use of the full storey height is more appropriate. In CP110(1), walls are defined as 'stocky' when helt is less than 12.
The design of walls which are more slender than this, should take into account out-of-plane moment transferred from the slabs and destabilising moment from eccentricity of axial load
The minimum amount of reinforcement varies with the design requirement. A minimum percentage of 0.25% high yield steel (HYS) or 0.3% mild steel both horizontally and vertically is usually required for shrinkage and temperature reasons. However, a larger percentage may be required for fire resistance (see Section 1.4) or seismic design
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