Efficient and economical multi-storey buildings for Sri Lanka pt.2 : guidelines for framed buildings braced with brick wall infills

Abstract

In the design of low to medium rise reinforced concrete buildings, it is possible to use two options. The building can be designed as a rigid frame structure where the frame resists both vertical and lateral loads. Therefore, the columns should be designed as unbraced. When brick walls are used as partition walls, it is possible to use them to provide the lateral stability so that the reinforced concrete frame can be designed to resist only the vertical loads. In this case, the columns can be designed as braced thus leading to a smaller cross section and lesser amount of steel.

However, it is quite important to ensure that the infill walls are able to resist the design lateral loads acting on the building without failure. Thus, the infill panels also should be designed. These panels act in composite with the reinforced concrete frame.

The behaviour of frames with brick wall infills is highly complex because of the influence of various parameters like the quality of bricks, the workmanship, magnitudes of space left between the frame and the infill, moisture expansion of brick walls and axial shortening of the concrete frame due to creep and shrinkage.

Axial shortening of reinforced concrete columns due to creep and shrinkage is usually ignored for low to medium rise buildings. It is shown with detailed calculations that the axial shortening of columns can be beneficial either in closing any minute gap left between the brickwork and the frame or in transferring certain amount of precompression that can enhance the lateral load resistance of the infill brick panel.

When brick wall infills provide the lateral resistance, it is possible to model the infill panel as a bracing element with two hinges. This will enable the accurate determination of moments in columns and beams. The beams and columns should also be designed for some additional moments and shears that arise due to the existence of an infill panel.

Reinforced concrete structures with infill brick walls have performed well and survived many earthquakes of moderate magnitude. However, when infill brick walls are used with openings, as in external walls, the formation of a short column at the opening level has led to the early failure of the reinforced concrete frames. Thus, guidelines presented for the construction of brick wall infills in reinforced concrete structures to enhance the composite action and to minimise the unacceptable failures.

In low to medium rise reinforced concrete frame buildings with infill brick walls, it is usually assumed that the reinforced concrete frame can be designed as a braced frame. This implies that the lateral loads are resisted by the brick walls acting as shear walls.

The design of a brick shear wall interacting with the reinforced concrete frame pose a considerable challenge to the design engineer due to the variability of brickwork properties and difficulties in selecting a reliable design technique. In this report, the design data and methods, which can be used for the design of brick shear walls using locally available bricks, are given. Guidelines have been developed for the selection of the required number of shear wall panels to suit the grid selected and the floor height. The beneficial effects of axial shortening of reinforced concrete columns has also been highlighted. Additional guidelines are given to enhance the earthquake resistance when openings occur in infill panels.