Shear Wall

download

of 37

  • date post

    16-Oct-2014
  • Category

    Documents
  • view

    273
  • download

    14

Embed Size (px)

transcript

Shear Walls

Earthquake Resistant Building Construction (ERBC)

y y

y

Earthquakes are devastating because they destroy infrastructure. Collapse of building structures in which they live during the earthquake kills people. Hence, the real culprits are our construction practices and poor quality of construction. The conventional buildings instantly collapse without any warnings during the unpredictable and sudden earthquakes.

Gujrath earthquake(2001) Occurred on January 26, 2001 and coincided with the 51st celebration of India's Republic Day. y Magnitude of between 7.6 and 8.1. Killed more than 20,000 people and injured another 1,67,000 and destroyed near a million homes. y Bhuj, situated only 20 km from the epicentre, was the most devastated town. The quake destroyed around 90% of the homes in Bhuj, eight schools, and flattened two hospitals. It partly destroyed the historic Swaminarayan temple in the city. It also destroyed 4 km of road in Bhuj. y In Ahmedabad, as many as 50 multi-storied buildings collapsed and several hundred people were killed.y

Latur / Killari earthquake (1993)y y y y y y y y

September 30, 1993 at 3.56 am. 6.4 on Richter scale. Epicentre near Killari, Latur District. 7,928 people killed. 16,000 people injured. 15,854 livestock killed. 52 villages collapsed. 30,000 houses collapsed. 2,11,000 houses in 13 districts suffered damages of varying degree.

Earthquake Resistant Building Construction (ERBC)y

y

It is not feasible and economical to design and construct totally earthquake proof building structures, particularly to withstand very severe earthquakes of magnitude above 8.0 (Richter Scale). But it is possible to design and construct ERB structures, though they may suffer huge damage under the influence of very severe earthquakes, they may not suddenly collapse and will not kill people.

Shear WallConcrete walls, which have high plane stiffness, placed at convenient locations are economically used to provide necessary resistance to horizontal forces. y The walls may be placed in the form of assemblies surrounding the lift shafts or stair wells.y

Shear wall concepty

y

The walls are not only designed to resist gravity / vertical loads (due to its self-weight and other living / moving loads), but they are also designed for lateral loads of earthquakes / cyclones. The walls are structurally integrated with roofs / floors and other lateral walls, thereby giving the three dimensional stability for the building structures.

Shear wall concepty y

Shear wall structural system of 3-dimensions can effectively withstand the earthquake forces. A railway compartment or a bus coach, which will be subjected to lot of vibrations and base excitations are built of stiffened plate system - a simple steel plate stiffened (strengthened / reinforced) by a grid of steel structural angles or channels.

Shear wall concepty

y

In shear wall system, it is made of concrete or masonry wall panels, which are reinforced, with a grid of steel rods. Shear wall systems are comparatively more stable to withstand earthquakes because their supporting area (total cross-sectional area of all shear walls) with reference to total plan area of building, is comparatively more, unlike in the case of RCC framed structures.

Shear wall structures vs Load bearing masonry structures and / or RCC framed structuresThe load bearing masonry is very brittle material. Due to different kinds of stresses such as shear, tension, torsion, etc., caused by the earthquakes, the conventional unreinforced brick masonry collapses instantly during the unpredictable and sudden earthquakes. y The RCC framed structures are slender, when compared to shear wall concept of box like three-dimensional structures. y It is possible to design the earthquake resistant RCC frame, but it requires extraordinary skills at design, detailing and construction levels, which can not be anticipated in all construction projects.y

Shear wall structures vs Load bearing masonry structures and / or RCC framed structures

y

y

y

Moderately designed shear wall structures not only more stable, but also comparatively quite ductile. In safety terms it means that, during very severe earthquakes they will not suddenly collapse causing death of people. They give enough indicative warnings such as widening structural cracks, yielding rods, etc., offering most precious moments for people to run out off structures, before they totally collapse.

Shear Wallsy

y y y y

Reinforced concrete (RC) buildings have vertical plate-like RC walls called Shear Walls in addition to columns, beams and slabs. These walls generally start at foundation level and are continuous throughout the building height. Their thickness can be as low as 150mm, or as high as 400mm in high rise buildings. Shear walls are usually provided along both length and width of buildings. Shear walls are like vertically-oriented wide beams that carry earthquake loads downwards to the foundation.

Shear WallsProperly designed and detailed buildings with shear walls have shown very good performance in past earthquakes. y A noted consulting engineer in USA, Mark Fintels quote: We cannot afford to build concrete buildings meant to resist severe earthquakes without shear walls.y

y y

y

y

y

Shear walls in high seismic regions require special detailing. However, even buildings with sufficient amount of walls that were not specially detailed for seismic performance (but had enough well-distributed reinforcement) were saved from collapse. Shear walls are easy to construct, because reinforcement detailing of walls is relatively straight-forward and therefore easily implemented at site. Shear walls are efficient, both in terms of construction cost and effectiveness in minimizing earthquake damage in structural and non-structural elements (like glass windows and building contents). Shear wall buildings are a popular choice in many earthquake prone countries, like Chile, New Zealand and USA.

Architectural Aspects Most RC buildings with shear walls also have columns; these columns primarily carry gravity loads due to self-weight and contents of building. y Shear walls provide large strength and stiffness to buildings in the direction of their orientation, which significantly reduces lateral sway of the building and there by reduces damage to structure and its contents.y

Architectural AspectsShear walls carry large horizontal earthquake forces, the overturning effects on them are large. Thus, design of their foundations requires special attention. y Shear walls should be provided along preferably both length and width. y If they are provided along only one direction, a proper grid of beams and columns must be provided along the other direction to resist strong earthquake effects.y

Architectural AspectsDoor or window openings can be provided in shear walls, but their size must be small to ensure least interruption to force flow through walls. y Openings should be symmetrically located. y Design checks are required to ensure that the net cross-sectional area of a wall at an opening is sufficient to carry the horizontal earthquake force.y

Architectural AspectsShear walls in buildings must be symmetrically located in plan to reduce ill-effects of twist in buildings. y They could be placed symmetrically along one or both directions in plan. y Shear walls are more effective when located along exterior perimeter of the building such a layout increases resistance of the building to twisting.y

Ductile Design of Shear Wallsy

y

y

Like reinforced concrete (RC) beams and columns, RC shear walls also perform much better if designed to be ductile. Overall geometric proportions of the wall, types and amount of reinforcement, and connection with remaining elements in the building help in improving the ductility of walls. The Indian Standard Ductile Detailing Code for RC members (IS:13920-1993) provides special design guidelines for ductile detailing of shear walls.

Overall Geometry of WallsOverall Geometry of Walls: Shear walls are oblong in cross-section, i.e., one dimension of the cross-section is much larger than the other. y While rectangular cross-section is common, L- and U-shaped sections are also used. y Thin-walled hollow RC shafts around the elevator core of buildings also act as shear walls, and should be taken advantage of to resist earthquake forces.y

Reinforcementy

y

y y

y

Steel reinforcing bars are to be provided in walls in regularly spaced vertical and horizontal grids . The vertical and horizontal reinforcement in the wall can be placed in one or two parallel layers called curtains. Horizontal reinforcement needs to be anchored at the ends of walls. The minimum area of reinforcing steel to be provided is 0.0025 times the cross-sectional area. This vertical reinforcement should be distributed uniformly across the wall crosssection.

Boundary Elementsy

y

y y y

Under the large overturning effects caused by horizontal earthquake forces, edges of shear walls experience high compressive and tensile stresses. To ensure that shear walls behave in a ductile way, concrete in the wall end regions must be reinforced to sustain these load reversals. End regions of a wall with increased confinement are called boundary elements. Sometimes, the thickness of the shear wall in these boundary elements is also increased. RC walls with boundary elements have substantially higher bending strength and horizontal shear force carrying capacity, and are therefore less susceptible to earthquake damage than walls without boundary elements.

Shear wall constructiony