WHY ARE BUILDINGS WITH SHEAR WALLS PREFERRED IN SEISMIC REGIONS?

What is Shear Wall Building?

Reinforced concrete (RC) buildings often have like a vertical plate RC walls are called shear walls (Figure 1) Except for slabs, beams and columns. These walls usually start at foundation level and are continuous throughout the height of the building. Their thickness can be as low as 150 mm, or as high as 400 mm in high-rise buildings. Shear walls are usually provided with Both The length and width of the buildings (Figure 1). Shear walls are as follows vertically oriented Wide beam Which carries the load of the earthquake down to the foundation.

Figure 1

Benefits of shear walls in RC buildings

Properly designed and elaborate buildings with shear walls have shown Very good Performance in past earthquakes. The enormous success of shear-walled buildings in resisting strong earthquakes is summarized in the quote:

“We cannot afford to build concrete buildings to resist severe earthquakes without shear walls.” By Mark Fintel, a renowned consulting engineer in the USA

Shear walls in high seismic areas require special detailing. However, in previous earthquakes, even buildings with sufficient number of walls not particularly wide for seismic performance (but had sufficiently well-distributed reinforcement) were protected from collapse. Shear wall buildings are a popular choice in many earthquake prone countries, such as Chile, New Zealand and the USA. Shear walls are easy to build, as the reinforcement details of the walls are relatively straightforward and are therefore easily implemented on site. Shear walls are efficient in terms of both construction cost and effectiveness in reducing earthquake damage to structural and non-structural elements (such as glass windows and building materials).

Architectural Aspects of Shear Walls

Most RC buildings with shear walls also have columns; These columns mainly carry gravity burden (In other words, due to self-weight and material of the building).

Shear walls provide buildings with great strength and rigidity in the direction of their orientation, which significantly reduces the lateral sway of the building and thus minimizes damage to the structure and its materials. Since shear walls carry Vishal Horizontal earthquake forces, the reversal effects on them are large. Thus, the design of their foundation requires special attention. Shear walls should preferably be provided with Both length and width. However, if they are provided in only one direction, a proper grid of beams and columns in the vertical plane (called a .) moment-resistant frame) should be provided in the other direction to resist strong earthquake effects.

Door or window openings may be provided in shear walls, but they must be small in size to ensure minimal obstruction to flow through the walls. In addition, the opening should be located symmetrically. Special design checks are needed to ensure that the net cross-sectional area of ​​a wall at an opening is sufficient to carry the horizontal earthquake force.

line drawing number 2

Shear walls in buildings should be located symmetrically in plan to reduce the side effects of turning in buildings (Figure 2). They can be placed symmetrically in one or both directions in the plan. Shear walls are more effective when located along the outer perimeter of the building – such a layout increases the building’s resistance to ramming.

tensile design of shear walls

Like reinforced concrete (RC) beams and columns, RC shear walls also perform better if they are made to be ductile. The wall’s overall geometric proportions, types and amounts of reinforcement, and relationships with the rest of the elements in the building help to improve the walls’ flexibility. Indian Standard tensile description code For RC members (IS:13920-1993) provides special design guidelines for tensile description of shear walls.

overall geometry of the walls

Shear walls are skewed in cross-section, In other words, One dimension of the cross-section is much larger than the other. While rectangular cross-section is common, L- and U-shaped sections are also used (Figure 3). Thin-walled hollow RC shafts around buildings’ elevator cores also act as shear walls, and must be taken advantage of to resist earthquake forces.

Fig-3

Reinforcement Bars in RC WallsImage-4

Regularly spaced steel reinforcing bars are to be provided in the walls Stand And horizontal grid (Figure 4A). Vertical and horizontal reinforcement in the wall can be laid in one or two parallel layers called curtains, Horizontal reinforcement needs to be anchored at the ends of the walls. The minimum area of ​​reinforcing steel to be provided is 0.0025 Multiplying the cross-sectional area, with each in horizontal and vertical directions. This vertical reinforcement should be distributed evenly across the cross-section of the wall.

border element

Under large reversal effects due to horizontal seismic forces, the edges of shear walls experience high compressive and tensile stresses. To ensure that the shear walls behave in a ductile manner, the concrete at the end zones of the wall must be reinforced in a special way to sustain these load reversals without losing strength (Figure 4B). The end regions of the wall having increased confinement are called border element, This special limiting transverse reinforcement in boundary elements is similar to that provided in columns of RC frames. Sometimes the shear wall thickness in these boundary elements also increases. RC Walls with border elements have significantly higher bending strength and ability to bear horizontal shear forces, and are therefore less susceptible to earthquake damage than walls without border elements,

reading material

IS 13920, (1993), “Indian Standard Code of Practice for Tensile Description of Reinforced Concrete Structures Subjected to Seismic Forces, “Bureau of Indian Standards, New Delhi”

Paule, T., & Priestley, M.J.N., (1992), “Seismic design of reinforced concrete and masonry buildings. John Wiley & Sons, USA

Author:

CVR Murthy, Indian Institute of Technology Kanpur, Kanpur, India

sponsored by:

Building Materials and Technology Promotion Council, New Delhi, India (www.bmtpc.org,



Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.