How to Calculate the Total Load on the Footing

How to calculate the total load on the footing? | Building Construction This article has been written on the request from my readers. Engineering students generally get confused when it comes to calculating loads for footings. They ask weird and inappropriate questions regarding the load calculations. This is because they haven’t understood what loads are to be calculated when footing/foundation for a building is designed. Calculation of loads is extremely simple. I hope after reading this article, the queries of many of my readers would get a satisfactory answer. Four loads are to be considered in order to measure total load on the footing: 1. Self load of the column x Number of floors 2. Self load of beams x Number of floors 3. Load of walls coming onto the column 4. Total Load on slab (Dead load + Live load) If you get well versed with load calculations, then calculating the size of the footing and following the procedure for foundation design wouldn’t be a problem. Foundations Foundation of a structure is like the roots of a tree without which the tree cannot stand. The construction of any structure, be it a residence or a skyscraper; starts with the laying of foundations. Before designing the foundation, the type of soil is determined. Depending on whether the soil is hard soil or soft soil, a specific type of foundation is adopted.

Shallow Foundations versus Deep Foundations Foundations are made in various materials… They could be reinforced cement concrete foundations or brick foundations or stone rubble masonry foundations etc. The choice of material to be used in the construction of foundations also depends on the weight of the structure on the ground. The bearing capacity of soil plays a major role in deciding the type of foundation. The safe bearing capacity of soil should be 180N/mm2 to 200N/mm2. Foundations are broadly classified into shallow foundations and deep foundations. The depth of the foundation means the difference of level between the ground surface and the base of the foundation. If the depth of the foundation is greater than its width the foundation is classified as a deep foundation. Shallow foundations are commonly used in smaller structures such as residences and small buildings whose floor height is limited to 10m whereas Deep Foundations are used in Skyscrapers…. Piles are the most commonly used Deep Foundations used in skyscrapers… Types of Shallow foundations Footings Footings are structural members used to support columns and walls and to transmit their load to the underlying soils. Mats or rafts Combined footings, strap and strip footings

Column Footing 

In this type of foundation the base of the column is sufficiently enlarged to act as the individual support. The widened base not only provides stability but is useful in distributing the load on sufficient area of the soil.

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Column footings are usually used in the foundations of residences and buildings where the soil is hard enough has has sufficient bearing capacity.

Pressure distribution Under a Foundation 

The law of distribution of pressure under a foundation depends on the homogeneity of the soil and flexibility of the base. If really the soil is homogeneous and the base of the foundation is flexible, the pressure distribution under the foundation will be uniform. On the contrary if the foundation base is absolutely rigid, the pressure distribution will not be uniform but may follow such pattern.

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In our designs it is usual to assume a flexible base and hence to regard the pressure distribution to be uniform. This can be achieved by gradually decreasing the thickness of the base towards the edges so that the base is only as much thick as it is regarded to resist the induced moments and shears.

General rules of Foundation Design While designing a foundation the following points must be borne in mind. 

When a soil is yielding soil, a certain amount of settlement must be reduced as much as possible by bringing down the pressure intensities.

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It is necessary that a foundation shall be designed so that if at all a settlement should occur, it will be uniform. In other words, the settlement of all the footings must be more or less the same.

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This is a very important point in reinforced concrete structures due to the rigid connection between the different components of the structure.

In our next article, we will discuss the procedure of designing an isolated foundation and also justify the foundation design rules mentioned above. Step 1: Calculation of loads on the footing Step 2:

RCC Column A column forms a very important component of a structure. Columns support beams which in turn support walls and slabs. It should be realized that the failure of a column results in the collapse of the structure. The design of a column should therefore receive importance. Supporting the slabs is the main function of the columns… Such slabs are called Simply Supported Slabs. Simply supported slabs could be either one way slab or a two-way slab. It depends on the dimensions of the slab.

Reinforced Cement Concrete Column Plan and Section A column is defined as a compression member, the effective length of which exceeds three times the least lateral dimension. Compression members whose lengths do not exceed three times the least lateral dimension, may be made of plain concrete. In this article, we are going to discuss in detail the basis of classification of columns and different types of reinforcement required for a certain type of column. A column may be classified based on different criteria such as: 1. Based on shape

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Rectangle

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Square

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Circular

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Polygon

2. Based on slenderness ratio 

Short column, ? ? 12

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Long column, ? > 12

3. Based on type of loading 

Axially loaded column

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A column subjected to axial load and unaxial bending

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A column subjected to axial load and biaxial bending

4. Based on pattern of lateral reinforcement 

Tied columns

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Spiral columns

Minimum eccentricity Emin > l/500 + D/30 >20 Where, l = unsupported length of column in ‘mm’ D = lateral dimensions of column Types of Reinforcements for columns and their requirements Longitudinal Reinforcement 

Minimum area of cross-section of longitudinal bars must be atleast 0.8% of gross section area of the column.

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Maximum area of cross-section of longitudinal bars must not exceed 6% of the gross crosssection area of the column.

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The bars should not be less than 12mm in diameter.

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Minimum number of longitudinal bars must be four in rectangular column and 6 in circular column.

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Spacing of longitudinal bars measures along the periphery of a column should not exceed 300mm.

Transverse reinforcement 

It maybe in the form of lateral ties or spirals.

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The diameter of the lateral ties should not be less than 1/4th of the diameter of the largest longitudinal bar and in no case less than 6mm.

The pitch of lateral ties should not exceed 

Least lateral dimension

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16 x diameter of longitudinal bars (small)

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300mm

Helical Reinforcement The diameter of helical bars should not be less than 1/4th the diameter of largest longitudinal and not less than 6mm. The pitch should not exceed (if helical reinforcement is allowed); 

75mm

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1/6th of the core diameter of the column

Pitch should not be less than, 

25mm

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3 x diameter of helical bar

Pitch should not exceed (if helical reinforcement is not allowed) Least lateral dimension 

16 x diameter of longitudinal bar (smaller)

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300mm

Also check out: Thumb rules for designing a Column Layout Example of a Column Layout of a Residence Consequences of Wrong Structural Design

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Related posts: 1. Column Bases | Design of Steel Structures Column bases are structural elements used in the design of steel structures to transfer the column load to the footings. Types of Column bases Slab base Gusseted base Slab Base... 2. Methods of Structural Design | Design of Steel Structures Structural design is the science of studying the Mechanics of a structure. Designing the skeleton of a building determines the real strength of the structure. If the structure is externally... This entry was posted on May 27, 2010, 9:55 am and is filed under Building Construction. You can follow any responses to this entry through RSS 2.0. You can leave a response, or trackback from your own site.