Masonry pilaster wall consist of pilaster, which is rectangular projectile column constructed of masonry blocks in the face of the wall and provide extra strength to the wall, filler wall, and foundation.
The pilaster is cantilevered up from the foundation and built at determine interval along the wall length. Filler is masonry units that spanned horizontally between pilasters.
By and large, dimensions of pilaster might be 40.64 cm width x 40.64 cm length. Moreover, using spaced pilaster with masonry block filler might be an economical option for freestanding wall or low height retaining walls.
Various components of pilaster masonry wall are illustrated in Figure-1. In this article design of filler, pilaster, and nominal footing is discussed in the following sections.
Figure-1: Typical Masonry Pilaster Retaining Wall
Masonry Pilaster Filler Wall Design
Filler wall is extended horizontally between pilasters and this wall controls the spacing between pilasters. Freestanding wall is designed for wind force and if applicable, seismic force. Lateral loads might be from either direction of the wall that is why it is required to provide and install steel reinforcement in the wall center as shown in Figure-2.
Figure-2: Installing Steel Reinforcement in the Center of the Pilaster Column
Moreover, in the case where the wall supports earth fill, the largest load will be at the bottom of the wall and this load controls wall thickness. In this condition, it is obvious that steel reinforcement is necessary at outside face of filler wall between pilasters.
However, for practical and economical reasons, reinforcements can be installed at the wall center as per either controlling positive moment at the middle of the span or negative moment at the pilasters.
Generally, ultimate applied moment can be calculated using the following equation:
Applied moment = (wl2/8) –> Equation-1
Applied moment is declined with at higher places or courses of the wall so reinforcement ratio can be decreased.
The first stage of the filler wall design is computing lateral pressure at the base of wall and how they will be transmitted to the pilasters by the adjacent wall panels, after that determining filler wall thickness, and finally calculating reinforcement areas.
It is recommended to employ minimum amount of vertical and horizontal reinforcement. Total reinforcement area for both directions is specified to be 0.002bd with no smaller than 0.007 in either direction.
Furthermore, vertical reinforcement is 13 mm diameter at spacing of around 81 mm or 122 mm.
Masonry Pilaster Wall Design
A masonry pilaster is a uniform vertical member cross section built as an integral part of a wall that may function as either a vertical beam or a column or both and it is considerably influential in increasing masonry wall capacity.
Generally, pilasters are constructed from 40.64 cm by 40.64 cm masonry units or smaller size units for lower wall height and the spacing is between 15.24 cm to 20.32 cm.
Pilasters are designed similar to columns except that pilasters are laterally supported in the direction of the wall; however, columns are typically unsupported in both directions.
The distribution of the lateral loads for earth retaining is either triangular or trapezoidal and uniform in the case of wind or earthquake forces.
In masonry pilasters, structural requirements control number and size of vertical steel reinforcement. Moreover, reinforcement must be placed in a way that provide adequate clearance for permitting flowing of grout around bars to achieve good bond.
As per ACI 530-11 the distance between parallel bars should not be less than nominal diameter of the bar or 25.4 mm. Additionally, grout thickness between reinforcement and masonry units should not be smaller than 6.4 mm for fine grout or 12.7 mm for coarse grout.
Special Masonry Pilaster Unit
Pilasters could be built by assembling standard configuration masonry units. However, there are different types of special units that manufacture specifically for pilaster construction.
There are number of advantages which might be achieved by using special units such as ease of construction, providing more space for reinforcements, and avoiding threading of block over reinforcements.
The availability of special units should be checked with suppliers when considering special unit pilasters. Figure-3 shows number of special masonry units.
Figure-3: Special Masonry Pilaster Units
Masonry Pilaster Footing Design
For filler wall, nominal footing is adequate and this type of foundation is employed solely but pilaster foundation could be either conventional rectangular footing or cast in place piers in drilled holes. Detailed design of these foundation types will be included in other article.