Introduction
Reinforced concrete walls are in many ways similar to columns. They support vertical loads and are vulnerable to minor axis bending. They often act as primary elements of a structure’s lateral stability systems and by doing so are subjected to high in-plane bending forces.
This technical learning note explains how reinforced concrete walls are designed to withstand these forces. It describes how the geometry of the reinforced concrete wall affects its capacity to act as a vertical support element, as well as explaining its role as a key component of a lateral stability system.
By definition a wall is a vertical or near vertical element with a breadth/length that
is greater than 4 times its thickness. While
a minimum thickness of 150mm for a wall
is possible, 180-250mm is recommended
to aid detailing and construction. Walls
thinner than 250mm can incur concrete
compaction problems due to the congestion
of reinforcement, particularly near openings.
A wall’s thickness is determined based on the forces it is being subjected to, required
fire resistance, durability and buildability. There are three types of action a reinforced
concrete wall can be subjected to: axial forces, minor axis bending and shear, and
major axis bending and shear. The action to which walls are most vulnerable, is bending, and this technical learning note principally addresses this form of stress. The manner by which walls resist the effects of these actions is dependent on the wall’s geometry shown in the above figure.
Other aspects that may impact the design of a wall are its design life and fire protection. Both of these set parameters for the concrete strength of the wall, its thickness
and cover to reinforcement.
The stages of design for a reinforced concrete wall can be summarised as follows:
Establish design life of the structure to which the wall is to form a part.
Determine actions on the wall and their combinations.
Select a concrete strength based on durability requirements.
Determine cover to reinforcement and thickness based on fire protection requirements.
Calculate shear force and bending moments acting on the wall
Check for slenderness
Calculate required amount of steel reinforcement based on applied forces
Review minimum and maximum areas of reinforcement and bar spacing
This technical learning note is concerned with last four stages, which encompass the design of the wall itself. The earlier 4 stages occur during concept and scheme design phases.
Determining stresses within a wall
When calculating the forces being applied to the wall, certain assumptions need to be made concerning how they are modelled. When considering bending in the major axis, all elements that are framing into it need to be taken to be simply supported. Bending moments that are applied due to lateral forces such as wind, must be based on the assumption that the wall is acting as a cantilever from its foundations. With respect to minor axis bending, all elements framing into the wall should be considered monolithic with the appropriate level of full fixity, e.g. if these elements are also concrete, then the full bending moment it transmitted from the horizontal element to the wall.
Major axis bending
Before establishing the required amount of reinforcement, the magnitude of stresses due
to the applied actions must be calculated. It is common practice to split the wall into 1m strips, which simplifies the design of the reinforcement within the wall. These can each be considered subject to axial compression and minor axis bending only, with the design of reinforcement based on the extreme fibre stresses (ft) in each idealised 1m segment of the wall. All relevant axial forces and bending moments are applied to each segment under consideration and each unit length is treated like a column for design purposes (see figure on right).
The extreme fibre stresses can be
calculated using the following formula:
where:
N is the design axial force in the 1m strip of wall
L is the overall length of the wall
h is the overall thickness of the wall
M is the applied design major axis bending moment in the segment of the wall
The resulting stress is then multiplied by the thickness of the wall (h) to give a stress per metre length. The reinforcement can then be determined using a column design methodology where minor axis bending, as well as the axial stresses due to both major
axis bending and axial forces, is applied.
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