Civil Engineering

How to Calculate Design Load of a Concrete Slab (factored)

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In reinforced concrete design , there is what is called the factor of safety. The factor of safety is multiplied by the calculated load of the structural element. This is done to reduce the possibility of the structural element failing under its actual, or expected loadings.

When the factor of safety has been applied to the calculated load of a structural element, it is called design load. Note that Factored load is also the same as design load.

Determining the Design (factored) load of a Concrete Slab.

The formula for calculating the design load of a Concrete Slab is:

1.4gk + 1.6qk.

Where:

gk= dead load

qk= live load

1.4= factor of safety applied to dead load

1.6= factor of safety applied to live load.

Dead load is the same as the weight of the concrete slab element. 

Live load is the movable or variable load the slab will carry such as people, furniture,machinery and so on. 

The unit of the design load will be in kN/m².

The weight of concrete used in reinforced concrete design calculation is 24kN/m³

Example 1 calculating design load of slab

concrete slab panel

Let’s say a concrete slab panel is to be designed for a private dwelling. It  has a depth ( thickness) of 0.2m, a length of 5m, and a width of 3m as shown above, the design load can be calculated as follows:

The dead load of the slab panel is the weight of concrete multiplied by the slab thickness.

That is,

24kN/m³ × 0.2m = 4.8kN/m²

Therefore gk = 4.8kN/m²

Live loads for buildings are standardized. In page 2 of BS 6399 part 1, the live load for private dwellings is 1.5kN/m² (shown below)

Table minimum imposed floor loads.BS 6399 part 1

Therefore qk= 1.5kN/m²

Design load= 1.4gk+1.6qk=( 1.4×4.8kN/m²) + (1.6×1.5kN/m²) = 6.72kN/m² + 2.4kN/m²

=9.12kN/m²

Example 2. calculating design load of slab

Looking at part of a floor plan shown below, we will be calculating the design load for the floor plan labelled P8 as shown.Take live load for private dwellings as 1.5kN/m².

Part of general floor arrangement plan of a proposed building

Solution

The depth of the slab panel is 150mm or 0.15m

We know that the formula for  design load of slab panel is  1.4gk + 1.6qk and weight of concrete is 24kN/m³

Dead load, gk, = weight of concrete multiplied by panel thickness= 24kN/m³ × 0.15m = 3.6kN/m²

Live load , qk=1.5kN/m². (page 2 of BS 6399 part 1)

Design load is 1.4gk + 1.6qk=

(1.4×3.6kN/m²)+(1.6×1.5kN/m²)= 7.44kN/m²

Slab Design load is used for the calculation of the amount of steel reinforcement required for the slab panel.

How to calculate unfactored dead concrete slab load

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Slab panel

A slab in terms of its dimensions consists of a thickness(depth), length and width. Understanding this concept will correctly Enable you to calculate and apply slab loading.

In reinforced concrete design, the weight of concrete is usually taken as 24kN/m³. So let’s look at an example.

A reinforced concrete slab is to be constructed having a length of 5m, width of 3m and depth of 200mm. The slab loading can be calculated as follows.

Calculating slab load in kN/m²

Note that 200mm = 0.2m..

..and weight of concrete is 24kN/m³

In most design calculations the unit of the slab load is in kN/m². In this case the slab load is calculated by multiplying 24kN/m³ by thickness (depth) of slab which is 0.2m

So 24kN/m³ × 0.2m = 4.8kN/m²

(4.8kN/m² is the unfactored dead load of the slab)

Calculating slab load in kN

To represent the slab load in kN, just multiply the unit weight concrete which is 24kN/m³ by the volume of the slab. 

Note that the volume of the slab is length x width x depth

So 24kN/m³ × 0.2m × 5m × 3m = 72kN

In practice, you will be calculating slab loading from structural plan or floor panel arrangement. A typical structural plan is shown below.

Structural plan

The slab panels are label P and a number. Let’s take a look at one of the slab panels, panel P8.

The panel’s thickness is 150mm or 0.15m. Also it is a 2 way slab. You can also easily calculate the length and width of the slab from the dimensions shown.

To calculate the unfactored dead load of the slab in kN/m², ( which is what we have been doing so far), just multiply 24kN/m³ by the slab thickness which is 0.15m;

24kN/m³ × 0.15m = 3.6kN/m²

One and Two Way Slabs Simply Explained

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One Way Slab

In reinforced concrete design, a slab is said to be one way if the length of the longer side divided by the length of the shorter side is equal to or greater than 2. 

With reference to the diagram of the slab shown below, If the length of the longer side (Ly) = 5m, and the shorter side (Lx) is 2.5m then 

Ly/Lx = 5m/2.5m = 2

On structural plans, the symbol used to identify a one way slab panel is shown in the previous diagram (symbol that the arrow points at).

In a one way slab design, the main reinforcement steel spans along the shorter side of the slab, which is also the span of the slab

In a one way slab design, only the two long supporting beams along the the long side of the slab  is assumed to carry the slab load equally between them. The beams along the shorter side do not carry any slab load.

For instance with reference to this diagram of a one way slab, beam 1 carries half of the slab load and beam 2 takes the remaining half, none of the slab load is carried by beams 3&4. This makes beam 1&2 the critical beams. Beams 3&4 will not be carrying any slab load, but may carry wall load and part of roof load. 

Two Way Slab

If the length of the longer side divided by that of the shorter side is less than 2, the slab should be designed as a two way.

With reference to the diagram shown, the length of the longer side, Ly, =6, and that of the shorter side Lx = 4.75. Therefore, Ly / Lx = 6 / 4.75 = 1.26. 1.26 is less than 2 hence the slab should be designed as 2 way.

The symbol of a two way slab is as shown above (the symbol that the arrow points at);

In a 2 way slab design, the main steel reinforcement spans both directions.

All 4 supporting beams carry some part of the slab load but the beams along the longer side carry more of the slab load than the beams along the shorter side.

The loading on beams for a 2 way slab is as shown. Note that the beams 1& 2 carry more slab load than beams 3&4

How to determine the realistic cost of workmanship for a building construction work

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Sometimes paying high cost for workmanship doesn’t usually result in a good quality job. And too low a cost for workmanship can be an excuse for a shoddy job. You don’t also want to feel cheated by overpaying for workmanship.

There are standard rates of pay for workmanship in the building industry. It is determined by economic forces and so the rate changes from time to time.

Here are tips to help you get a good economy rate and quality workmanship.

You can ask a building contractor or someone who usually gets building construction jobs on  a regular basis. 

You can also look for or place an advert for professionals to come and access the work to be done and give their quotations. Make sure you ask each one of  them questions on areas of the work you need more clarity. Their individual quotations and how satisfied you are on how they will undertake the job will enable you to choose economically, the right professional for the job.

Beware of middle men, they pose as professionals but they are not. A middle man may charge incredibly higher fees so that if he gets the job, he passes it on to a professional who he pays the normal rates or even much lower. 

 Also, if you know a friend or someone who has done a similar project , you can ask him to get you the professional he or she used.

What to plaster your House?, these are the materials you will need

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house plastering

In many places, the term, plaster In building construction means cement sand rendering of block constructed walls.

Materials you will need for this type of works are;

Ordinary Portland cement.

Fine Sand.

Clean water.

Sand required should not contain too much clay, or too cohesive. It must also not be too loose. Water used must be clean water of drinking quality. Using dirty water can affect the quality of the plaster or render.

Workmanship.

Skilled renderers and  unskilled labourers are required. A skilled renderer is usually a mason. Good quality rendering depends on the skill of the renderer, so get a good one to do the work for you. The unskilled labour, hand mixes the plastering material and transports it to the point of application.

Mixing ratio of Cement Sand rendering

The mixing ratio is usually 1:6, that is one part of cement to 6 parts of sand. For stronger rendering the mixing ratio can be as high as 1:3.

Calculation of the amount of cement and sand  required.

A skilled mason can easily estimate the amount of cement and sand required. You can also do it yourself. Click on the article link below for details;

https://specsfinder.info/2021/10/12/how-to-calculate-the-number-of-cement-bags-and-quantity-of-sand-required-for-plastering/.

How Ceramic Floor tiles are made

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ceramic floor tiles

The main material used in the production of ceramic tiles is clay. Clay is readily available in different types and in large quantities. Different types of clay and other similar binding materials can be combined in a particular proportion to create ceramic tiles.

The 3 major types of clay available are earthenware, stoneware and porcelain. 

earthenware clay
stoneware clay

The earthenware clay is red in colour while the stoneware and porcelain are classified as white clay. All the 3 above mentioned types of clay are suitable for making ceramic floor and wall tiles.

The process of making ceramic tiles industrially will be discussed under the following stages.

Grinding the raw material

Moulding

Initial heating

Painting/ glazing

Final heating

Making of ceramic floor tiles in a factory

Grinding the raw material

clay and other binding and waterproofing substances are grinded in required proportions to smooth power. This is done using an industrial grinder.

Moulding 

To the powder mixture a very little amount of water is added and thoroughly mixed to make it plastic. The powdery mixture is then placed into a mould and compressed under very high pressure and temperature to form a flat solid mass that has been shaped into the required time size.

Initial heating 

Before the tiles can be glazed it needs most of the moisture to be taken out. This can be done by heating the tiles in an oven. The tiles are heated up to a temperature of over 900°C and then cooled. 

Painting/glazing

Next the tiles are painted and or glazed.

Painting gives the tiles its waterproofing properties and aesthetic look. Glazing does similar work in addition to design and engravings.

Final Heating 

After the tiles have been given its waterproof and aesthetic look. It is heated in an oven for the final time to dry up all the moisture and make the tiles stronger in the process. Temperatures in the oven are above 1000°C.The heating process can last for several hours, after which it is allowed to cool completely. After this final process, the ceramic tiles are ready for packaging and distribution to sale outlets.