Brick walls are subjected to compression, and IS 1905 — 1987, "Code of Practice for Structural Use of Unreinforced Masonry Walls", and SP 20 deal with their design. In this article, we will examine the fundamental aspects of design. Details of the design can be obtained from the above references.
EFFECTIVE HEIGHT, EFFECTIVE LENGTHS OF WALLS AND
SLENDERNESS RATIO
Designs
of all compression members such as columns and walls are based on the
slenderness ratio of the member, which is defined as the effective length
divided by the thickness of the walls. Unlike in a column (which is defined as
a member with a breadth less than four times its thickness), for calculating the effective length of a wall, we have to consider not only its dimension along with the height but also the dimension along its length between supports such as
cross walls.
Effective height
= 0.75 H to 0.8 H, for
walls restrained at both the ends
= 1.5 H, for walls
restrained only at one end such as in compound walls and parapets
Effective Length
= L, for walls restrained
at both the ends
= 1.5 L, for walls
restrained at one end
= 2 L, for walls with no
restraints
Where,
H = The height of the wall
between the restraints
L = The length of the wall
between the cross walls, pilasters and other constraints
The larger of the above
values with respect to H and L is taken for calculation of the slenderness ratio
(for details refer to IS 1905).
Slenderness ratio |
Larger the values of effective length and
effective height of the wall |
The thickness of the wall |
Brick
walls with a slenderness ratio of up to 6.0 are considered short walls which fail
under compression. Walls with a slenderness ratio greater than 24 are unstable
and are prohibited. To find the reduced load that can be carried on walls with a slenderness ratio greater than 6.0, we use a reduction factor explained in the Section above
FAILURE LOAD ON SHORT WALLS
The
crushing strength of common bricks varies from 5 N/mm2 to 35 N/mm2 depending on
the different regions in India. It is recommended that the mortar to be used in
brickwork should have a strength very nearly equal to the strength of brick If
such a short-brick pillar is loaded, the failure load of the pillar will be
much less than that of the individual brick.
This
is because when we load the pillar, each brick with its mortar bed behaves as a
beam on an elastic foundation and the failure is in bending by tension
corresponding to its modulus of rupture. Hence, the maximum allowable strength
A_{max} of a short column is as follows:
p_{max} = 1/10 x compressive
strength of the bricks
REDUCTION FACTOR FOR SAFE STRESS IN BRICK WALLS
The reduction factors given
in Table A.1 are recommended in IS 1905, 1987 and SP20 for central loading (e =
0). It will be further reduced for eccentricity of load as shown in the table below
Stress reduction factors for
slenderness ratio and eccentricity for brick walls (SF)
AN EXAMPLE OF THE APPROXIMATE DESIGN OF BRICK WALLS
Problem. A brick wall of a two-storey
building with each floor 3 m high has a thickness of 200 mm. Let the distance
between cross walls in the room be 3 m. Assume that the load from each floor is
10 kN per metre length of the wall. What should be the minimum strength of bricks
that can be used for this building? Assume that there is no eccentricity of
load.
Solution:
1. Find the total load on the base.
We have
Total
weight of two-storey high wall @ 20 kN/m^{3} for brickwork for 0.2 m
thick wall = 0.2 x 20(2 x 3) = 24 kN/m (DL)
Live
load from the floors = 2 x 10 = 20 kN/m (for two floors)
Total
load = 44 kN/m
2. Find effective height, length and
slenderness ratio.
Effective
height = 0.75 x H = 0.75 x 3 = 2.25 m
Effective
length = L = 3 m (larger value = 3 m)
Larger
slenderness ratio = 3/0.2 =15
3. Find stress factor SF for
eccentricity, e = 0.
From Above Table for
slenderness ratio = 15, S_{F} = 0.625 (by interpolation)
4. Find the necessary strength of brickwork at the base of
brickwork (Ïƒ).
We have Load/Area = Stress
= S_{F} x strength of masonry
or
(44 x1000)/(200x1000) = S_{F}
Ïƒ N/mm^{2}
or
Ïƒ = 44/(200 x 0.625) = 0.4
N/mm^{2 }(approx)
5. Find brick strength for the
brickwork of strength 0.4 N/mm^{2}.
We
have strength of brick = 10 x strength of the brickwork
=
10 x 0.4 = 4 N/mm^{2}
Note: Ordinary well-burnt
country bricks with a minimum strength of 5 N/mm2 will be suitable for this
building. Further details of the design can be obtained from IS 1905.
Conclusion: The design of masonry walls is important when we build buildings more than one
storey in height. The slenderness of the walls and the required strength of the
masonry units are important items to be checked in these constructions.
Must read: How to
calculate dead load of brick wall or block wall?
Must read: Method of Laying Brickwork in Masonry Construction
FAQ
What are the reference codes used for brick wall design?
IS: 4326, IS 1905 — 1987, "Code of Practice for Structural Use of Unreinforced Masonry Walls", and SP 20 deal with their design
No comments:
Post a Comment