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strap footing

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by on 30 April 2013

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Transcript of strap footing

Cantilever or Strap Footings Historical Development of Footing General Considerations CHOICE OF FOUNDATION TYPE AND PRELIMINARY SELECTION THE CANTILEVER PRINCIPLE DESIGN OF STRAP
(OR CANTILEVER) FOOTINGS Strap footings Footing usually supports the following loads:
•Dead loads from the substructure
•Live loads resulting from material or occupancy
•Weight of material used in backfilling
•Wind loads
Each footing in a building is designed to support the maximum load that may occur on any column due to the critical combination of loadings using allowable soil pressure. The type of foundation most appropriate for a given structure depends upon several factors:
(i)the function of the structure and the loads it must carry,
(ii)the subsurface conditions,
(iii)the cost of the foundation in comparison with the cost of the superstructure. This is also called ‘cantilever footing’ or ‘pump-handle foundation’. A strap comprises two or more footings connected by a beam called ‘strap’. This may be required when the footing of an exterior column cannot extend into an adjoining private property. The strap beam is usually designed and constructed so that it does not bear on the soil. In the earliest attempts to enlarge the areas of footing without increasing weight, timber grillages were constructed and conventional masonry footings built on them
increasing weight, timber grillages were constructed and conventional masonry footings built on them.
In 1891 a grillage consisting of steel railroad rails embedded in concrete was devised as improvement over the timber grillage.
The steel I beam proved admirably suited for the construction of cantilever footings these were introduced in 1887 almost simultaneously in two building in Chicago. TYPES OF FOOTING The various types of structural foundations may be grouped into two broad categories – shallow foundations and deep foundations. The classification indicates the depth of the foundation relative to its size and the depth of the soil providing most of the support. According to Terzaghi, a foundation is shallow if its depth is equal to or less than its width and deep when it exceeds the width. In temperature latitudes footings are commonly located at a depth not less than that normal frost penetration. In warmer climates and especially in semiarid regions, the minimum depth of footing may be governed by the greatest depth at which seasonal changes in moisture cause appreciable shrinkage and swelling of the soil. The elevation at which a footing is established depends on the character of the subsoil, the loss to be supported and the cost of the foundation. Ordinarily the footing is located at the highest level where adequate supporting material may be found. In some instances, if an especially firm layer in encountered at greater depth, it may be more economical to establish the footing at a lower elevation because the area required for the footing is smaller. The excavation for reinforced-concrete footing should be kept dry ao that the reinforcement can be set and held in its proper position while the concrete is being placed. To do this in water bearing soil it may be necessary to pump either from sumps or from a previously installed system of drains. Forms are usually required around the sides of the footing. The necessity for pumping and for supporting the sides of the excavation in which the footing are placed may add appreciably to cost of a footing foundation. The following are the essential steps involved in the final choice of the type of foundation:
1.Information regarding the nature of the superstructure and the probable loading is required, at least in a general way.
2.The approximate subsurface conditions or soil profile is to be ascertained.
3.Each of the customary types of foundation is considered briefly to judge whether it is suitable under the existing conditions from the point of view of the criteria for stability – bearing capacity and settlement.
4.More detailed studies, including tentative designs, of the more promising types are made in the next phase.
5.Final selection of the type of foundation is made based on the cost. The cantilever principle is largely concealed in actual footings of this type. This principle is illustrated in the figure: BASIS FOR DESIGN OF STRAP FOOTINGS
Strap footings are designed based on the following assumptions;
(i)The strap footing is considered to be infinitely stiff. It serves to transfer the column loads onto the soil with equal and uniform soil pressure under both the footings.
(ii)The strap is a pure flexure member and does not directly take soil reaction. The soil below the strap will loosened up in order that the strap does not rest on the soil and exert pressure. Design Problem
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