Thursday, February 09, 2017

What is use of SP:20?

This code is based on structural safety of buildings: Masonary Walls.

  • Masonary
An assemblage of masonary units properly bonded together with mortar.
  • Design by calculated masonary method 
The thickness of masonary for different spans, storey heights and openings given by Nomograms are worked out for three occupancies as given below:
a) Residential buildings 200 kg/m2
b) Office buildings 300 kg/m2
c) Office buildings 400 kg/m2

  • Structure of Nomograms
The Nomograms for thickness of brick wall consist of nine vertical lines. From left to right, the vertical lines represent:

  1. Basic stress: of Masonry depends on the crushing strength of masonry unit and mortar used.
  2. Storeys: In case of multi- storeyed buildings, the wall thickness at each floor is found by passing the line through the no. of storey above that section.
  3. Reference line 1: It fixes a point on the line for any combination of values for Basic stress and storey.
  4. Span point: Fourth line has a span point, through which all lines shall pass through for arriving at the thickness.
  5. Reference line 2: It fixes a point on the line for any combination of values for Basic stress and Storeys.
  6. Percentage of openings and thickness of walls for spans of 3m, 3.6m & 4.2m: The openings provided on the walls for windows, ventilators, doors etc are taken care of in the nomograms by this line.

  •  Permissible Compressive stresses:
The permissible compressive stress in masonary shall depend upon the following factors:
  1. Eccentricity of loading
  2. Slenderness ratio of masonary
  3. Strength of Masonary units
  4. Mix of mortar
  5. Cross- sectional area of masonary and
  6. Shape factor

  •  Design considerations:
  1. Load bearing walls are structurally efficient when the load is uniformly distributed and when structure is so planned that eccentricity of loading on the wall is as small as possible.
  2. Mortar selection: Mortar strength shall be not greater than that of the masonary unit.
  3. Where steel  reinforcing bars are provided, the bars shall be embedded in cement sand mortar not leaner than 1:4 or in cement concrete of grade M100.
  4. Ignoring tensile strength, free standing walls shall checked against overturning allowing for a factor of safety of 1:5.
  5.  Panel or filler walls in framed buildings shall be properly bonded to surrounding framing members by means of suitable mortar or dowels.
  6. Tops of openings in storey shall preferably be at the same level so that continuous band could be provided over them including the lintels throughout the building.
  7. The opening shall preferably be located away from the corner by clear distance equal to atleast one- eighth of the height of the opening where seismic coefficient is less than 0.08 and one - fourth of the height where seismic coefficient is 0.08 or more.
  8. If a window or ventilator is to be projected out, the projection shall be in reinforced masonary or concrete and well anchored.
  9. If an opening is tall, say for the full height of wall, dividing the wall into two portions, these portions shall be reinforced with horizontal reinforcement of 6mm diameter bars at not more than 60 cm intervals one on inner and one on the outer face, properly tied to vertical steel  at Jambs and corner or Junction of walls where used.
  10. The use of arches to span over the openings is a source of weakness and shall be avoided unless steel ties are provided.
  11. Openings in bearing walls shall be strengthened where necessary by providing reinforced concrete members or reinforcing the brickwork around them. 

  •  Mode of Failure (in case of wall and column)
The lower elasticity of mortar causes vertical compressive load to impart lateral strain movement to mortar, which produce tensile stresses in the brick by interface bondwhilst maintaining the bed- joint mortar in compression. The mortar is then in condition of compressive stress and the bricks carries vertical compression in combination with biaxial lateral tension. The greater is hg/ Lg ratio of wall, the greater the value of horizontal tensile stresses at the vertical Joints and the weaker the wall against vertical splitting under load.