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Footing System


Purpose of using footing and main types

Most building may have series of columns or walls which are part of its structural system. These columns or walls will carry two types of loads - live load and static load. The loaded columns or walls may have high concentration of stress due to its small sectional area. This may result in a punch-through effect which would cause the building to fail or to collapse. To prevent this from happening, the installation of footing help to reduce the stress by spreading the loads over a larger area.

There are three main types of footings. They are intergrated footing with slab, isolated footing and strip footing.


Isolated Footing / Pier Footing / Column Footing

Introduction

Isolated footings are those individual pier blocks which sit into the ground. They are used to support walls, columns and post where load is significant within the area. The spacing of the footing depends on the load they are supporting - the heavier the load, the closer they are placed and vice versa.







Isolated footings are used along with strip footings in this construction. Each isolated footings are equally spaced so that the load rom the building can be equally distributed through all isolated footing.

Benefits, facts and situations best to be used
  • Not much excavation or landfill activities are required as leveling a site is not a requirement. Hence more suitable on a slopy site.

  • Can be placed at some distance apart hence a huge saving in materials and costs. Usually they are placed at 1800 centers for both directions depending on the spacing of the bearer for timber floors.

  • More suitable for building of timber framed flooring as it is lighter than concrete which provide fewer stress hence less chance of failure due to puncture through earth.

  • Require about 1-2 working days to complete.

    Site Preparation

    Only a flat and compacted hole dug in the earth is required as site leveling is not a requirement. The depth will be governed by the earth structure.

    Materials
  • Steel reinforced concrete block - The advantage of this is that the reinforcement inside the concrete force the whole concrete block to act as a whole hence less chance of a failure.

  • Brick/masonry structure - The disadvantage is that the mortar used to join the bricks may not always up to the strength of the brick and hence not all bricks act as a whole. This may lead to failure should one of the brick suffer higher pressure than the others. However the failure is less likely than of brick strip footing because it cover less area.

  • Galvanised steel plate which is used as a stump footing.

    Generally, all these materials can withstand moist, pressure, termite and fungal attack for a long time.


    pic Typical isolated footings in cross section.

    General Size / Dimension

    The size of the footing depends upon how heavy the load it is supporting. Generally, for reinforced concrete footings, it must be at least 150mm deep and must be at least 50mm wider all round than the column they are supporting.


    Strip Footing

    Introduction

    Strip footing are footings than run in strips. They work much better that isolated footing as they can distribute stress applied from the load more efficiently.

    Benefits, facts and situations best to be used
  • Commonly used in buildings with a brick wall as the footing can spread higher stress evenly along its reinforced footing strip.

  • Can be use with isolated footing to gain the features of the two as well as to cut cost.

  • Leveling of site not required but excavation of trenches would be great.

  • Require about 2-3 working days to complete.

    Site Preparation

    Trenches need to be dug, compacted and level to provide a firm and stable support. The depth of the trench depends on the type of earth structure within the area. The centers of footing placement is dependant of the load supported and is determined by the structural engineer.

    Strip footing is used in this construction. Notice that they mainly run along the area where the wall are likely to be placed. This construction is more likely to use a slab with separate strip footing construction.

    Materials
  • Reinforced concrete strips - This is commonly used because of savings in labour as compared with brick or masonry which need to be laid brick by brick.

  • Bricks or masonry structure can't spread the stress that well because they don't have a strong bondage. Hence, you may sometimes find a failure may occur where the strip footing is supporting a load bearing column.

    All materials used must be able to withstand moist, pressure, termite and fungal attack for a long time.