Floor Bends with he Beam but moves all columns at that level together

Role of Floor Slabs and Masonry

Floor slabs are horizontal plate like elements, which facilitate functional use of buildings. Usually, beams and slabs at one storey level are cast together. In residential multi-story buildings, thickness of slabs is only about 110-150mm. when beams bend in the vertical direction during earthquakes, these thin slabs bend along with them (fig2a). And, when beams move with columns in the horizontal direction, the slab usually forces the beams to move together with it. In most buildings, the geometric distortion of slab is negligible in the horizontal plane; this behavior is known as the rigid diaphragm action .

After columns and floors in a RC building are cast and the concrete hardens, vertical spaces between columns and floors are usually filled-in with masonry walls to demarcate a floor into functional spaces (rooms). Normally, these masonry walls, also called infill walls, are not connected to surrounding RC columns and beams. When columns receive horizontal forces at floor levels, they try to move in horizontal direction, but masonry walls tend to resist this movement. Due to their heavy weight and thickness, these walls attract rather large horizontal forces. However, since masonry is a brittle material, these walls develop cracks once their ability to carry horizontal load is exceeded. Thus masonry walls is enhanced by mortars of good strength, making proper masonry courses, and proper packing of gaps between RC frame and masonry infill walls.

Effects of Horizontal Earthquake Vibrations

Under gravity loads, tension in the beams is at the bottom surface of the beam in the central location and is at the top surface at the ends. The level of bending moment due to earthquake loading depends on severity of shaking and can exceed that due to gravity loading. Thus, under strong earthquake shaking, the beam ends can develop tension on either of the top and bottom faces. Since concrete cannot carry this tension, steel bars are required on both faces of beams to resist reversals of bending moment.

Strength Hierarchy

For a building to remain safe during earthquake shaking, columns should be stronger than beams, and foundations should be stronger than columns. If columns are made weaker, they suffer severe local damage, at the top and bottom of a particular storey.

Brittle and Ductile Building Materials

Construction Materials

I. Masonry

Masonry is made up of burnt clay bricks and cement or mud mortar. Masonry can carry loads that cause compression (i.e. pressing together) but can hardly take load that causes tension (i.e. pulling apart). Masonry is a brittle material, these walls develop cracks once their ability to carry horizontal load is exceeded. Thus infill walls act like sacrificial fuses in buildings: they develop cracks under severe ground shaking but they share the load of the beams and columns until cracking.

II. Concrete

Concrete is another material that has been popularly used in building construction particularly over the last four decades. Cement concrete is made of crushed stone pieces (called aggregate), sand, cement and water mixed in appropriate proportions. Concrete is much stronger than masonry under compressive loads, but again its behavior in tension is poor. The properties of concrete critically depend on the amount of water used in making concrete, too much and too little water both can cause havoc.

III. Steel

Steel is used in masonry and concrete buildings as reinforcement bars of diameter ranging from 6mm to 40mm. reinforcing steel can carry both tensile and compressive loads. Moreover steel is a ductile material. This important property of ductility enables steel bars to undergo large elongation before breaking. Concrete is used with steel reinforcement bars. This composite material is called as reinforced cement concrete. The amount and location of steel in a member should be such that the failure of the member is by steel reaching its strength in tension before concrete reaches its strength in compression. This type of failure is ductile failure, and is preferred over a failure where concrete fails first in compression. Therefore, providing more steel in R.C. buildings can be harmful even!!

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