eBusiness Weekly
Engineering and construction matters with Innocent Chatikobo
Introduction
In last week’s issue, we discussed about the different types of foundations. After a foundation has been constructed, we now erect walls for the building. Therefore, in this week’s edition we shall be touching on the walls of the building and how we select a particular wall for a particular application. The article, though not exhaustive, is meant to furnish the readers with the basic aspects that enable them to make informed decisions and get value for money.
Types of building walls
A wall provides a covering against adverse weather conditions as well as providing security to the inhabitants of a building. Walls can exist under two categories:
a) Load bearing walls — which support loads from floors and roof in addition to their own weight
b) Non load bearing walls — they carry no floor or roof loads
There are various types of walls which can be constructed using different materials. The walls can be divided into four main groups
i. Masonry walls: This is whereby a wall is built of individual blocks of materials and bonded together with properly mixed mortar. Examples of materials include brick, clay or concrete blocks, stone.
ii. Monolithic wall: This is when the wall is built of material placed in forms during construction.
Examples include concrete walls, water tanks, earth dam walls and, mud and pole walls as in our local traditional dwelling units.
iii. Frame wall: Whereby a wall is constructed as a frame of materials such as timber, wood, steel, or aluminium, with facing or sheeting on one or both sides of the frame. This method is widely used in partitioning of offices in commercial buildings. This is also referred to as cladding and its done where a structure needs to be covered without the use of bricks e.g. in industrial structures. Greenhouses in agriculture and horticulture also fall into this category.
iv. Membrane walls: This is where a wall is constructed as a sandwich of two thin skins or sheets of plastic, metal or asbestos cement. In between they are bonded to a core of materials like kaylite or foamed plastic to produce a thin wall. They are normally used where high insulation is required e.g. cold-rooms.
In these four groups, that’s where we find different walls such as partition walls; shear walls; cavity walls; retaining walls; core walls, etc.
Factors determining the type of wall
Good quality walls should provide strength and stability; weather resistance; fire resistance; thermal insulation and sound insulation –
Materials available at reasonable cost
Availability of tradesmen capable of using the materials in the best way to produce high quality work e.g. in stonework which needs a highly skilled builder and in framed walls which will also need a skilled welder
Climate and weather patterns
The function of the building
Design and size of the building
Nature and type of soil
Whether load bearing and non-load bearing
Building materials
There are different types of materials used in construction. There are two main categories – naturally occurring and man-made products.
a) Naturally occurring: e.g. plant parts like logs, bark, leaves; ice and snow; mud and clay; sand; stones or rock; thatch/grass; wood and timber.
b) Man-made substances: e.g. fired brick; clay blocks; cement blocks; concrete; fabric e.g. tent; foam as in membrane walls; glass; metal; plastics; ceramics.
Among all these different materials, bricks are the most widely used than all other materials in what is called Brick Masonary. Brick masonary is unique for fire resistance, durability, workability and is economical. Brick work can be done with mud or with cement and sand.
Advantages of brick masonary
It is cheaper compared to most materials e.g. stone masonary
Bricks are of uniform size, hence brickwork can also be done by less skilled labour
They are very workable and light in weight
They are readily and easily available
Brickwork is durable since it doesn’t rust, corrode and rot
Allows a wide range of design options and personal expression
Can withstand adverse weather and climatic conditions
Qualities of good bricks
i. Should be uniform in shape and standard size
ii. Should give a clear metalling ringing sound when struck together
iii. Should show a uniform and bright colour
iv. Should be free from voids, well-shaped and possess sharp edges
v. Should be hard – no mark must be left on the brick surface when scratched with a finger nail
vi. Should not break when dropped from a height of about 1m
vii. Should not absorb water more than 20 percent by weight when soaked in cold water for 24 hours
viii. Should not show any deposits of white salts when allowed to dry in shade (efflorescence)
ix. Should have crushing strength above 5.50 N/mm2
We have witnessed a lot of disregard for minimum standards required for bricks and this has led to substandard structures being built. It is, therefore, imperative that one chooses wisely when purchasing bricks.
Tests for bricks
Bricks can undergo tests to ascertain whether they are the correct grade needed to come up with a very strong building. Some of the tests that bricks can be subjected to are the following:
1. Absorption: The brick should not absorb water more than 20 percent of dry weight when soaked for 24 hours. Therefore, if brick weighs 2kg, when soaked, its weight should not increase by more than 400g, therefore brick weight must not exceed 2,4kg. We are now witnessing bricks that dissolve / crumble when soaked in water for a few hours
2. Crushing strength: According to standards, minimum crushing strength of bricks is 3.5N/mm2
3. Hardness: A brick is considered sufficiently hard if no mark / impression can be made on the brick surface by a finger nail
4. Soundness / sonorosity: When struck together, the bricks should emit a metallic ringing sound. Today’s cheap bricks instead of emitting a sound, they actually break when struck with each other
5. Structure, shape and size: The brick should be homogeneous, compact and free from any defects. It should have standard size and truly rectangular with sharp edges and smooth surface.
Bonding of bricks
Bonding is simply the pattern in which bricks are laid or the arrangement of bricks in a wall to create a strong and stable structure. There is need to select the proper bonding that is applicable to the building being erected.
For good bonding, bricks must be uniform in size and arranged uniformly throughout the wall. The bonds are classified based on laying and bonding style of bricks in walls. The most commonly used bonds in brick masonary are the following
i. Stretcher bond: Where bricks are laid with only their longer narrow face / stretcher showing. This is the most beautiful but weakest bond. This is commonly used because it can be used to build a half brick thick (41/2”) wall
ii. English bond: Has successive alternating courses of headers and stretchers. This is the strongest bond but very ugly.
iii. Flemish bond: Created by laying alternate headers and stretchers in a single course
iv. English garden wall bond: The wall is composed of three or more courses of stretchers alternating with one header course. The number of stretcher courses is always an odd number and the number of stretcher courses determines the strength of the wall. The more the stretcher courses, the weaker the wall
Wall binding and strengthening methods
A wall needs to be strengthened during the construction process and after building. This is to make the wall stronger and more durable and be able to withstand natural and man-made forces exerted upon the building.
It is believed that a building / structure makes movement and takes about three rainy seasons to stabilise. Therefore, during construction, it is imperative that due diligence is carried out to strengthen the wall. Some of the ways a wall can be strengthened include
A good and suitably done footing
Plastering – also protects wall against rain damage especially when substandard brick has been used
Use of reinforced concrete ring beam which bind the whole structure together
Use of brick force embedded in mortar during the construction process which reinforces the wall
Covering the wall with steel mesh sometimes before plastering to increase wall strength
Use of header courses which strengthens the walls.
This week’s question
1. What are some of the tests that can be carried out to test the strength of the wall and of the materials used? (Hints: knock test; moisture absorption test; etc.)
Innocent Chatikobo is an entrepreneur and engineer by profession with AtroServe Engineering Zimbabwe. He has extensive knowledge and experience in structural engineering and construction. For your comments, views, questions and feedback he can be contacted on the following platforms. Cell: +263 777 950 224; +263 712 376 037; +263 782 502 732; Email: [email protected]; Facebook: AtroServe Engineering Zimbabwe
