Friday, January 24, 2014

WALLS and Buildings

A wall is a barrier system, used for dividing or enclosing a space, usually in a position that is perpendicular to the gravity, but not necessarily. A wall can also be defined as a planner structure, generally vertical, with a proportionately narrow thickness in comparison to its height and length. A wall is required to resist besides its own weight -the self load, the dead load of super structures, and live loads of people and their objects. In addition to these loads a wall is required to transmit lateral forces from arches, vaults, and side pressures like wind, vibrations and earthquakes. Loads are transmitted along its section and often across the section of the wall. However, beyond the gravitational zone, in outer space structures, super imposed loads on the ‘wall’ are converted into stresses and ultimately in some form of kinetic energy, so an equilibrium must to be maintained. Loads on walls primarily occur as super imposed loads, and as reactions from the supporting elements. Loads are concentrated at a point, or distributed. Concentrated loads induce local stresses and failures, and under excessive distributed loads the wall fails locally at its weakest section or gets crushed or deformed wholly depending on the homogeneity.

Wall loads ultimately accumulate at a point within the section that is closest to the earth. (often requiring heavier bearing area) and are transmitted ultimately to another structural member or base (foundation).

When a wall has width equal to or less than its length, it becomes a column and loses its meaning. Moreover, a wall that has a height equal or less than its width, remains an infill course or layer only. Walls without any external down bearing loads are called partition walls.

A wall, due to its mass, specific gravity, constitution, shape, size, extent, position, surroundings, is capable of absorbing, reflecting, refracting the incident energy like heat, light, sound, vibration etc.

Walls also carry loads as side thrusts across the section. Such loads include wind and sound (sonar) pressures, retained liquids or gases, pressures caused by a mass of loose particles such as grains, sand, soil etc., air displacement pressures of blasts, eddy currents, tides, waves, etc. Additional depth throughout or intermittent supports are required in such walls along the length and height.

A wall carrying only side thrust is called a retaining wall. A wall which carries the load of upper structure (bridge) and also retains earth is an abutment wall. A gravity wall resists the side thrust of retained material by its dead weight. Gravity walls primarily have a trapezoidal section, with wider part forming the base. Cantilever retaining walls have ‘L’ or inverted ‘T’ section. A buttressed wall has additional intermittent strengthening mass, on the open face, to buttress the wall, whereas a counter-fort wall has a similar system (in tension) on the inside or loading face.

Walls with very thin width are called membranes. Membranes structures are formed by stretching a thin and pliable material along its plane, into structural forms. Membranes ‘walls’ or surfaces can carry well-distributed loads, but are incapable of taking any pointed loads. Boat sail and circus tents are examples of membrane structures. When the wall material is hard, homogeneous and rigid, it is called a plate. A plate wall functions like a wall, as in the case of folded plate structure forming side walls (Tagore Hall, Ahmedabad, India).

Openings like doors and windows weaken a wall. Logically openings at lower level have to be fewer and with restricted widths, because lintels bear greater loads. However, openings are placed one above the other so as to leave uninterrupted vertical wall masses to transfer loads directly to the ground.

Walls are likely to be less stable if:
  • are designed as partition walls,
  • do not touch any rigid or secured member on either side ends,
  • do not intermittently turn around or have additional wide stiffening members
  • are free at the top that is do not touch a structural member
  • are very tall for their sectional width
  • face high temperature difference across the faces
  • have a centre of gravity very high or out of the section
  • stand on a vibratory, compressible or unstable member or earth surface
  • are not consistently homogeneous
  • have not been designed for side thrusts, but have to bear it
  • have not been designed for down bearing loads, but are required to carry excessive self, dead or live loads
  • have carry pointed loads or unequally distributed loads
  • the constituents are eroded.

Walls are composed of not only the usual ‘engineering’ or building materials, but also of air, water, ice, particles, netting materials. A wall may consist of a single material (earth, stone), modules of single material (dressed stones, timber planks, bricks), composition of many materials (brick-cement-reinforcement masonry), or could be a multiplex system of many composite materials (particle board coated with multi layered coating system).

Load bearing (downward or sideways) walls are fixed -supported on a base (foundation), beam, lintel etc. However, non load bearing walls -partition walls could be removable, demountable, movable, collapsible, shrinkable, expandable, relocatable, inclinable.

Non-bearing walls often called curtain walls. Curtain walls are generally very thin or even transparent, but always suspended on the outer face of a framed structure. Such walls besides resisting side thrusts of wind etc. also accommodate building movements and vibrations. A curtain wall is a partition wall but extends over many floors. 


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